Chromosomal mutations (also called rearrangements, aberrations) are caused by abnormal cell division and change the structure of the chromosome itself. Most often this happens spontaneously and unpredictably under the influence of external factors. Let's talk about the types of chromosomal mutations in genes and their causes. We will tell you what a chromosomal mutation is and what consequences arise for the body as a result of such changes.
Chromosomal mutation- This is a spontaneously occurring anomaly with a single chromosome or with the participation of several of them. The changes that have taken place are:
- inside a single chromosome, they are called intrachromosomal;
- interchromosomal, when individual chromosomes exchange certain fragments with each other.
What can happen to the carrier of information in the first case? As a result of the loss of a chromosomal region, a violation of embryogenesis occurs and various anomalies arise, leading to mental underdevelopment of the child or physical deformities (heart defects, a violation of the structure of the larynx and other organs). If a chromosome break occurs, after which the torn fragment is built into its place, but already turned over by 180 ° - they speak of inversion. The order of the genes changes. Another intrachromosomal mutation is a duplication. In its process, a portion of the chromosome is doubled or it is duplicated several times, which leads to multiple malformations of mental and physical development.
If two chromosomes exchange fragments, the phenomenon is called “reciprocal translocation”. If a fragment of one chromosome is inserted into another, this is called a “non-reciprocal translocation”. “Centric fusion” is the connection of a pair of chromosomes in the region of their centromeres with the loss of neighboring regions. With a mutation in the form of a transverse gap adjacent chromosomes are called isochromosomes. Such changes do not have external manifestations in the born offspring, but make it a carrier of abnormal chromosomes, which may affect the occurrence of abnormalities in future generations. All types of chromosomal mutation are fixed in the genes and are inherited.
Main Causes of Chromosome Mutations
Exact Causes of Chromosomal Mutations cannot be determined in any particular case. In general, DNA mutations are a tool of natural selection and a sine qua non of evolution. They can have a positive neutral or negative value and are inherited. All mutagens that can lead to changes in chromosomes are usually divided into 3 types:
- biological (bacteria, viruses);
- chemical (heavy metal salts, phenols, alcohols and other chemicals);
- physical (radioactive and ultraviolet radiation, too low and high temperatures, electromagnetic field).
Spontaneous chromosomal rearrangements can also occur, without the influence of aggravating factors, but such cases are extremely rare. This happens under the influence of internal and external conditions (the so-called mutational pressure of the environment). Such randomness leads to a change in genes and their new distribution in the genome. The further viability of organisms with the resulting changes is determined by the ability to adapt to survival, which is part of natural selection. For a person, for example, mutation processes often become a source of various hereditary diseases, sometimes incompatible with life.
What is the difference between gene, genomic and chromosomal mutations
Mutations in chromosomes, genes, and the genome are often associated with each other. A mutation is called a gene. occurring inside the gene, chromosomal - inside the chromosome. Mutations that change the number of chromosomes are called genomic mutations.
These changes are combined into the general concept of "chromosomal abnormalities", they have a common classification that divides them into aneuploidy and polyploidy.
In total, about a thousand chromosomal and genomic anomalies are known to science, including various syndromes (about 300 species). These are chromosomal diseases.(a striking example is Down's syndrome), and intrauterine pathologies leading to miscarriages, and somatic diseases.
Chromosomal diseases
Their manifestation is spoken of when severe congenital genetically determined diseases are detected, manifested by congenital malformations. Such diseases testify to the most extensive changes that have occurred in DNA.
Failure can occur at any stage, even at the moment of conception, with the fusion of normal parental cells. Scientists have not yet been able to influence this mechanism and prevent it. This question has not been fully studied.
For humans, chromosomal mutations are more often negative, which manifests itself in the occurrence of miscarriages, stillbirths, the manifestation of deformities and deviations in intelligence, the appearance of genetically determined tumors. All such diseases conditionally divided into 2 groups:
Can chromosomal abnormalities be cured or prevented?
In the future, science sets the task of learning how to interfere with the structure of cells and change human DNA if necessary, but at the current moment this is impossible. As such, there is no treatment for chromosomal diseases; only methods of perinatal diagnosis (prenatal examination of the fetus) have been developed. Using this method, it is possible to identify Down and Edwards syndromes, as well as congenital malformations of the organs of an unborn baby.
According to the examination, the doctor, together with the parents, decides on the extension or termination of current pregnancy. If the pathology suggests the possibility of intervention, the fetus can be rehabilitated even at the stage of intrauterine development, including an operation that eliminates the defect.
Future parents at the stage of pregnancy planning can visit a genetic consultation, which exists in almost every city. This is especially necessary if there are relatives in the family of one or both with severe hereditary diseases. The geneticist will compile their pedigree and recommend a study - a complete set of chromosomes.
Doctors believe that such a gene analysis is necessary for every couple planning the appearance of a baby. This is a low-cost, universal and fast method that allows you to determine the presence of most chromosomal diseases of any type. Future parents all you need to do is donate blood. Those who already have a child with a genetic disease in the family must do this without fail before re-pregnancy.
This brochure provides information about what chromosomal disorders are, how they can be inherited, and what problems they can cause. This booklet cannot replace your conversation with your doctor, but it can help you discuss your concerns.
In order to better understand what chromosomal disorders are, it will be helpful to first know what genes and chromosomes are.
What are genes and chromosomes?
Our body is made up of millions of cells. Most cells contain a complete set of genes. Humans have thousands of genes. Genes can be compared to instructions that are used to control growth and coordinate the work of the whole organism. Genes are responsible for many traits of our body, such as eye color, blood type, or height.
Genes are located on thread-like structures called chromosomes. Normally, most body cells contain 46 chromosomes. Chromosomes are passed on to us from our parents - 23 from mom and 23 from dad, so we often look like our parents. So we have two sets of 23 chromosomes, or 23 pairs of chromosomes. Since genes are located on chromosomes, we inherit two copies of each gene, one copy from each parent. Chromosomes (hence genes) are made up of a chemical compound called DNA.
Figure 1: Genes, chromosomes and DNA
Chromosomes (see Figure 2), numbered 1 to 22, are the same in males and females. Such chromosomes are called autosomes. Chromosomes of the 23rd pair are different in women and men, and they are called sex chromosomes. There are 2 variants of sex chromosomes: X-chromosome and Y-chromosome. Normally, women have two X chromosomes (XX), one of them is transmitted from the mother, the other from the father. Normally, males have one X chromosome and one Y chromosome (XY), with the X chromosome inherited from the mother and the Y chromosome from the father. So, in Figure 2, the male chromosomes are shown, since the last, 23rd, pair is represented by the XY combination.
Figure 2: 23 pairs of chromosomes distributed by size; chromosome number 1 is the largest. The last two chromosomes are the sex chromosomes.
Chromosomal changes
The correct chromosome set is very important for normal human development. This is due to the fact that the genes that give “instructions for action” to the cells of our body are located on the chromosomes. Any change in the number, size, or structure of our chromosomes could mean a change in the amount or sequence of genetic information. Such changes can lead to learning difficulties, developmental delays, and other health problems in the child.
Chromosomal changes can be inherited from parents. Most often, chromosomal changes occur at the stage of egg or sperm formation, or during fertilization (newly occurring mutations, or de novo mutations). These changes cannot be controlled.
There are two main types of chromosomal changes. Change in the number of chromosomes. With such a change, there is an increase or decrease in the number of copies of any chromosome. Change in the structure of chromosomes. With such a change, the material of any chromosome is damaged, or the sequence of genes is changed. Perhaps the appearance of additional or loss of part of the original chromosomal material.
In this brochure, we will look at chromosomal deletions, duplications, insertions, inversions and ring chromosomes. If you are interested in information about chromosomal translocations, please refer to the brochure "Chromosomal Translocations".
Change in the number of chromosomes.
Normally, each human cell contains 46 chromosomes. However, sometimes a baby is born with either more or fewer chromosomes. In this case, there is, respectively, either an excess or an insufficient number of genes necessary for regulating the growth and development of the organism.
One of the most common examples of a genetic disorder caused by an excess number of chromosomes is Down syndrome. In the cells of people with this disease, there are 47 chromosomes instead of the usual 46, since there are three copies of the 21st chromosome instead of two. Other examples of diseases caused by an excess number of chromosomes are Edwards and Patau syndromes.
Figure 3: Chromosomes of a girl (the last pair of XX chromosomes) with Down syndrome. Three copies of chromosome 21 are visible instead of two.
Change in the structure of chromosomes.
Changes in the structure of chromosomes occur when the material of a particular chromosome is damaged or the sequence of genes is changed. Structural changes also include an excess or loss of part of the chromosomal material. This can happen in several ways, described below.
Changes in the structure of chromosomes can be very small, and it can be difficult for specialists in laboratories to detect them. However, even if a structural change is found, it is often difficult to predict the effect of this change on the health of a particular child. This can be frustrating for parents who want comprehensive information about their child's future.
Translocations
If you would like to learn more about translocations, please refer to the Brochure Chromosomal Translocations.
Deletions
The term "chromosomal deletion" means that part of the chromosome is missing or shortened. A deletion can happen on any chromosome and throughout any part of the chromosome. The deletion can be of any size. If the material (genes) lost during the deletion contained important information for the body, then the child may experience learning difficulties, developmental delay and other health problems. The severity of these manifestations depends on the size of the lost part and localization within the chromosome. An example of such a disease is Joubert's syndrome.
Duplications
The term "chromosomal duplication" means that part of the chromosome is doubled, and because of this, an excess of genetic information occurs. This excess chromosome material means that the body is receiving too many "instructions" and this can lead to learning difficulties, developmental delays and other health problems in the baby. An example of a disease caused by a duplication of a portion of chromosomal material is type IA motor sensory neuropathy.
Insertions
Chromosomal insertion (insert) means that part of the material of the chromosome was "out of place" on the same or on another chromosome. If the total amount of chromosomal material has not changed, then such a person is usually healthy. However, if such a movement leads to a change in the amount of chromosomal material, then the person may experience learning difficulties, developmental delay and other health problems for the child.
Ring chromosomes
The term "ring chromosome" means that the ends of the chromosome are connected, and the chromosome has acquired the shape of a ring (normally, human chromosomes have a linear structure). This usually happens when both ends of the same chromosome are shortened. The remaining ends of the chromosome become "sticky" and join together to form a "ring". The consequences of the formation of ring chromosomes for an organism depend on the size of the deletions at the ends of the chromosome.
Inversions
Chromosomal inversion means a change in the chromosome in which part of the chromosome is unfolded, and the genes in this region are in reverse order. In most cases, the carrier of the inversion is healthy.
If a parent has an unusual chromosomal rearrangement, how might this affect the child?
There are several possible outcomes of each pregnancy:
- A child can get a completely normal set of chromosomes.
- A child can inherit the same chromosomal rearrangement that the parent has.
- The child may have learning difficulties, developmental delays, or other health problems.
- Spontaneous abortion is possible.
Thus, healthy children can be born to a carrier of a chromosomal rearrangement, and in many cases this is exactly what happens. Since each rearrangement is unique, your specific situation should be discussed with a geneticist. It often happens that a child is born with a chromosomal rearrangement, despite the fact that the chromosome set of the parents is normal. Such rearrangements are called newly arisen, or arisen “de novo” (from the Latin word). In these cases, the risk of re-birth of a child with a chromosomal rearrangement in the same parents is very small.
Diagnosis of chromosomal rearrangements
It is possible to conduct a genetic analysis to identify the carriage of a chromosomal rearrangement. A blood sample is taken for analysis, and the blood cells are examined in a specialized laboratory to detect chromosomal rearrangements. This analysis is called karyotyping. It is also possible to perform a test during pregnancy to evaluate the chromosomes of the fetus. Such an analysis is called prenatal diagnosis, and this issue should be discussed with a geneticist. For more information on this subject, see the brochures Chorionic Villus Biopsy and Amniocentesis.
How it affects other family members
If a chromosomal rearrangement is found in one of the family members, you may want to discuss this issue with other family members. This will enable other relatives, if desired, to undergo an examination (analysis of chromosomes in blood cells) to determine the carriage of a chromosomal rearrangement. This may be especially important for relatives who already have children or are planning a pregnancy. If they are not carriers of a chromosomal rearrangement, they cannot pass it on to their children. If they are carriers, they may be asked to be screened during pregnancy to analyze the fetal chromosomes.
Some people find it difficult to discuss chromosomal rearrangement problems with family members. They may be afraid of disturbing family members. In some families, people experience difficulties in communication because of this and lose mutual understanding with relatives. Geneticists are usually experienced in dealing with such family situations and can help you discuss the problem with other family members.
What is important to remember
- Chromosomal rearrangement can either be inherited from parents or occur during fertilization.
- Perestroika cannot be corrected - it remains for life.
- Restructuring is not contagious, for example, its carrier can be a blood donor.
- People often feel guilty about the fact that their family has such a problem as chromosomal rearrangement. It is important to remember that this is not anyone's fault or a consequence of anyone's actions.
- Most carriers of balanced rearrangements can have healthy children.
Mutations are changes in the DNA of a cell. Arise under the influence of ultraviolet, radiation (X-rays), etc. They are inherited and serve as material for natural selection.
Gene mutations- change in the structure of one gene. This is a change in the sequence of nucleotides: dropout, insertion, replacement, etc. For example, replacing A with T. Causes - violations during doubling (replication) of DNA. Examples: sickle cell anemia, phenylketonuria.
Chromosomal mutations- change in the structure of chromosomes: loss of a site, doubling of a site, rotation of a site by 180 degrees, transfer of a site to another (non-homologous) chromosome, etc. Causes - violations during crossing over. Example: cat cry syndrome.
Genomic mutations- change in the number of chromosomes. Causes - violations in the divergence of chromosomes.
- Polyploidy- multiple changes (several times, for example, 12 → 24). It does not occur in animals, in plants it leads to an increase in size.
- Aneuploidy- changes on one or two chromosomes. For example, one extra twenty-first chromosome leads to Down syndrome (while the total number of chromosomes is 47).
Cytoplasmic mutations- changes in the DNA of mitochondria and plastids. They are transmitted only through the female line, because. mitochondria and plastids from spermatozoa do not enter the zygote. An example in plants is variegation.
Somatic- mutations in somatic cells (cells of the body; there may be four of the above types). During sexual reproduction, they are not inherited. They are transmitted during vegetative propagation in plants, during budding and fragmentation in coelenterates (in hydra).
The following terms, except for two, are used to describe the consequences of a violation of the arrangement of nucleotides in a DNA region that controls protein synthesis. Define these two concepts that “fall out” from the general list, and write down the numbers under which they are indicated.
1) violation of the primary structure of the polypeptide
2) divergence of chromosomes
3) change in protein functions
4) gene mutation
5) crossing over
Answer
Choose one, the most correct option. Polyploid organisms result from
1) genomic mutations
3) gene mutations
4) combinative variability
Answer
Establish a correspondence between the characteristic of variability and its type: 1) cytoplasmic, 2) combinative
A) occurs with independent divergence of chromosomes in meiosis
B) occurs as a result of mutations in the DNA of mitochondria
B) occurs as a result of chromosome crossing
D) manifested as a result of mutations in plastid DNA
D) occurs when gametes meet by chance
Answer
Choose one, the most correct option. Down syndrome is the result of a mutation
1) genomic
2) cytoplasmic
3) chromosomal
4) recessive
Answer
1. Establish a correspondence between the characteristic of a mutation and its type: 1) gene, 2) chromosomal, 3) genomic
A) a change in the sequence of nucleotides in a DNA molecule
B) a change in the structure of chromosomes
C) change in the number of chromosomes in the nucleus
D) polyploidy
E) change in the sequence of genes
Answer
2. Establish a correspondence between the characteristics and types of mutations: 1) gene, 2) genomic, 3) chromosomal. Write down the numbers 1-3 in the order corresponding to the letters.
A) deletion of a segment of a chromosome
B) a change in the sequence of nucleotides in a DNA molecule
C) a multiple increase in the haploid set of chromosomes
D) aneuploidy
E) change in the sequence of genes in the chromosome
E) loss of one nucleotide
Answer
Choose three options. What is a genomic mutation characterized by?
1) a change in the nucleotide sequence of DNA
2) loss of one chromosome in the diploid set
3) a multiple increase in the number of chromosomes
4) a change in the structure of synthesized proteins
5) doubling a section of a chromosome
6) a change in the number of chromosomes in the karyotype
Answer
1. Below is a list of characteristics of variability. All but two of them are used to describe the characteristics of genomic variability. Find two characteristics that "drop out" of the general series, and write down the numbers under which they are indicated.
1) limited by the norm of the reaction of the sign
2) the number of chromosomes is increased and a multiple of haploid
3) an additional X chromosome appears
4) has a group character
5) there is a loss of the Y chromosome
Answer
2. All but two of the characteristics below are used to describe genomic mutations. Identify two characteristics that “fall out” of the general list, and write down the numbers under which they are indicated.
1) violation of the divergence of homologous chromosomes during cell division
2) destruction of the fission spindle
3) conjugation of homologous chromosomes
4) change in the number of chromosomes
5) an increase in the number of nucleotides in genes
Answer
3. All but two of the characteristics below are used to describe genomic mutations. Identify two characteristics that “fall out” of the general list, and write down the numbers under which they are indicated.
1) change in the sequence of nucleotides in a DNA molecule
2) a multiple increase in the chromosome set
3) decrease in the number of chromosomes
4) duplication of a chromosome segment
5) nondisjunction of homologous chromosomes
Answer
Choose one, the most correct option. Recessive gene mutations change
1) the sequence of stages of individual development
2) composition of triplets in a DNA segment
3) a set of chromosomes in somatic cells
4) the structure of autosomes
Answer
Choose one, the most correct option. Cytoplasmic variability is associated with the fact that
1) meiotic division is disturbed
2) mitochondrial DNA is able to mutate
3) new alleles appear in autosomes
4) gametes are formed that are incapable of fertilization
Answer
1. Below is a list of characteristics of variability. All but two of them are used to describe the characteristics of chromosomal variation. Find two characteristics that "drop out" of the general series, and write down the numbers under which they are indicated.
1) loss of a chromosome segment
2) rotation of a chromosome segment by 180 degrees
3) decrease in the number of chromosomes in the karyotype
4) the appearance of an additional X chromosome
5) transfer of a chromosome segment to a non-homologous chromosome
Answer
2. All but two of the following features are used to describe a chromosomal mutation. Identify two terms that "fall out" from the general list, and write down the numbers under which they are indicated.
1) the number of chromosomes increased by 1-2
2) one nucleotide in DNA is replaced by another
3) a section of one chromosome is transferred to another
4) there was a loss of a section of the chromosome
5) a segment of the chromosome is turned 180°
Answer
3. All but two of the characteristics below are used to describe chromosomal variation. Find two characteristics that "drop out" of the general series, and write down the numbers under which they are indicated.
1) multiplication of a segment of a chromosome several times
2) the appearance of an additional autosome
3) change in the nucleotide sequence
4) loss of the terminal section of the chromosome
5) turn of the gene in the chromosome by 180 degrees
Answer
WE FORM
1) doubling the same part of the chromosome
2) a decrease in the number of chromosomes in germ cells
3) an increase in the number of chromosomes in somatic cells
Choose one, the most correct option. What type of mutation is a change in the structure of DNA in mitochondria
1) genomic
2) chromosomal
3) cytoplasmic
4) combinative
Answer
Choose one, the most correct option. The variegation of the nocturnal beauty and snapdragon is determined by variability
1) combinative
2) chromosomal
3) cytoplasmic
4) genetic
Answer
1. Below is a list of characteristics of variability. All but two of them are used to describe the characteristics of genetic variation. Find two characteristics that "drop out" of the general series, and write down the numbers under which they are indicated.
1) due to the combination of gametes during fertilization
2) due to a change in the sequence of nucleotides in the triplet
3) is formed during the recombination of genes during crossing over
4) characterized by changes within the gene
5) is formed when the nucleotide sequence changes
Answer
2. All of the following characteristics, except for two, are the causes of gene mutation. Define these two concepts that “fall out” from the general list, and write down the numbers under which they are indicated.
1) conjugation of homologous chromosomes and exchange of genes between them
2) replacement of one nucleotide in DNA with another
3) change in the sequence of the connection of nucleotides
4) the appearance of an extra chromosome in the genotype
5) loss of one triplet in the DNA region encoding the primary structure of the protein
Answer
3. All but two of the characteristics below are used to describe gene mutations. Identify two characteristics that “fall out” of the general list, and write down the numbers under which they are indicated.
1) replacement of a pair of nucleotides
2) the occurrence of a stop codon within the gene
3) doubling the number of individual nucleotides in DNA
4) an increase in the number of chromosomes
5) loss of a chromosome segment
Answer
4. All but two of the characteristics below are used to describe gene mutations. Identify two characteristics that “fall out” of the general list, and write down the numbers under which they are indicated.
1) adding one triplet to DNA
2) an increase in the number of autosomes
3) change in the sequence of nucleotides in DNA
4) loss of individual nucleotides in DNA
5) multiple increase in the number of chromosomes
Answer
5. All of the following characteristics, except for two, are typical for gene mutations. Identify two characteristics that “fall out” of the general list, and write down the numbers under which they are indicated.
1) the emergence of polyploid forms
2) random doubling of nucleotides in the gene
3) loss of one triplet in the process of replication
4) the formation of new alleles of one gene
5) violation of the divergence of homologous chromosomes in meiosis
Answer
SHAPING 6:
1) a segment of one chromosome is transferred to another
2) occurs in the process of DNA replication
3) there is a loss of a section of the chromosome
Choose one, the most correct option. Polyploid wheat varieties are the result of variability
1) chromosomal
2) modification
3) gene
4) genomic
Answer
Choose one, the most correct option. The production of polyploid wheat varieties by breeders is possible due to the mutation
1) cytoplasmic
2) gene
3) chromosomal
4) genomic
Answer
Establish a correspondence between characteristics and mutations: 1) genomic, 2) chromosomal. Write the numbers 1 and 2 in the correct order.
A) a multiple increase in the number of chromosomes
B) rotation of a segment of the chromosome by 180 degrees
C) exchange of sections of non-homologous chromosomes
D) loss of the central region of the chromosome
D) duplication of a section of a chromosome
E) repeated change in the number of chromosomes
Answer
Choose one, the most correct option. The appearance of different alleles of one gene occurs as a result of
1) indirect cell division
2) modification variability
3) mutation process
4) combinative variability
Answer
All but two of the terms listed below are used to classify mutations by changes in genetic material. Identify two terms that "fall out" from the general list, and write down the numbers under which they are indicated.
1) genomic
2) generative
3) chromosomal
4) spontaneous
5) gene
Answer
Establish a correspondence between the types of mutations and their characteristics and examples: 1) genomic, 2) chromosomal. Write down the numbers 1 and 2 in the order corresponding to the letters.
A) loss or appearance of extra chromosomes as a result of a violation of meiosis
B) lead to disruption of the functioning of the gene
C) an example is polyploidy in protozoa and plants
D) doubling or loss of a chromosome segment
D) Down syndrome is a prime example.
Answer
Establish a correspondence between the categories of hereditary diseases and their examples: 1) gene, 2) chromosomal. Write down the numbers 1 and 2 in the order corresponding to the letters.
A) hemophilia
B) albinism
B) colorblindness
D) "cat's cry" syndrome
D) phenylketonuria
Answer
Find three errors in the given text and indicate the numbers of sentences with errors.(1) Mutations are random, persistent changes in the genotype. (2) Gene mutations are the result of "mistakes" that occur in the process of doubling DNA molecules. (3) Mutations are called genomic, which lead to a change in the structure of chromosomes. (4) Many cultivated plants are polyploids. (5) Polyploid cells contain one to three extra chromosomes. (6) Polyploid plants are characterized by stronger growth and larger size. (7) Polyploidy is widely used in both plant breeding and animal breeding.
Answer
Analyze the table "Types of variability". For each cell marked with a letter, select the appropriate concept or the appropriate example from the list provided.
1) somatic
2) gene
3) replacement of one nucleotide with another
4) duplication of a gene in a region of the chromosome
5) addition or loss of nucleotides
6) hemophilia
7) color blindness
8) trisomy in the chromosome set
Answer
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A change in the number of chromosomes in a cell means a change in the genome. (Therefore, such changes are often called genomic mutations.) Various cytogenetic phenomena associated with changes in the number of chromosomes are known.
Autopolyploidy
Autopolyploidy is the repeated repetition of the same genome, or basic number of chromosomes ( X).
This type of polyploidy is characteristic of lower eukaryotes and angiosperms. In multicellular animals, autopolyploidy is extremely rare: in earthworms, some insects, some fish and amphibians. Autopolyploids in humans and other higher vertebrates die in the early stages of intrauterine development.
In most eukaryotic organisms, the main number of chromosomes ( x) matches the haploid set of chromosomes ( n); while the haploid number of chromosomes is the number of chromosomes in the cells formed in the chord of meiosis. Then in diploid (2 n) contains two genomes x, and 2 n=2x. However, in many lower eukaryotes, many spore and angiosperms, diploid cells contain not 2 genomes, but some other number. The number of genomes in diploid cells is called the genomic number (Ω). The sequence of genomic numbers is called polyploid near.
For example, in cereals x = 7 the following polyploid series are known (the + sign indicates the presence of a polyploid of a certain level)
Distinguish between balanced and unbalanced autopolyploids. Balanced polyploids are called polyploids with an even number of chromosome sets, and unbalanced - polyploids with an odd number of chromosome sets, for example:
|
unbalanced polyploids |
balanced polyploids |
|||
|
haploids |
1 x |
diploids |
2 x |
|
|
triploids |
3 x |
tetraploids |
4 x |
|
|
pentaploids |
5 x |
hexaploids |
6 x |
|
|
hectaploids |
7 x |
octoploids |
8 x |
|
|
enneaploids |
9 x |
decaploids |
10 x |
|
Autopolyploidy is often accompanied by an increase in the size of cells, pollen grains and the overall size of organisms, an increased content of sugars and vitamins. For example, the triploid aspen ( 3X = 57) reaches gigantic dimensions, is durable, its wood is resistant to decay. Among cultivated plants, both triploids (a number of varieties of strawberries, apple trees, watermelons, bananas, tea, sugar beets) and tetraploids (a number of varieties of rye, clover, and grapes) are widespread. Under natural conditions, autopolyploid plants are usually found in extreme conditions (in high latitudes, in high mountains); moreover, here they can displace normal diploid forms.
The positive effects of polyploidy are associated with an increase in the number of copies of the same gene in cells, and, accordingly, in an increase in the dose (concentration) of enzymes. However, in some cases, polyploidy leads to inhibition of physiological processes, especially at very high levels of ploidy. For example, 84 chromosome wheat is less productive than 42 chromosome wheat.
However, autopolyploids (especially unbalanced ones) are characterized by reduced fertility or complete infertility, which is associated with impaired meiosis. Therefore, many of them are only capable of vegetative reproduction.
Allopolyploidy
Allopolyploidy is the repeated repetition of two or more different haploid chromosome sets, which are denoted by different symbols. Polyploids obtained as a result of distant hybridization, that is, from crossing organisms belonging to different species, and containing two or more sets of different chromosomes, are called allopolyploids.
Allopolyploids are widely distributed among cultivated plants. However, if somatic cells contain one genome from different species (for example, one genome BUT and one - AT ), then such an allopolyploid is sterile. The infertility of simple interspecific hybrids is due to the fact that each chromosome is represented by one homologue, and the formation of bivalents in meiosis is impossible. Thus, with distant hybridization, a meiotic filter arises that prevents the transmission of hereditary inclinations to subsequent generations sexually.
Therefore, in fertile polyploids, each genome must be doubled. For example, in different wheat species, the haploid number of chromosomes ( n) is equal to 7. Wild wheat (einkorn) contains 14 chromosomes in somatic cells of only one doubled genome BUT and has the genomic formula 2 n = 14 (14BUT ). Many allotetraploid durum wheats contain 28 chromosomes of duplicated genomes in somatic cells. BUT and AT ; their genomic formula 2 n = 28 (14BUT + 14AT ). Soft allohexaploid wheats contain 42 chromosomes of doubled genomes in somatic cells BUT , AT , and D ; their genomic formula 2 n = 42 (14 A+ 14B + 14D ).
Fertile allopolyploids can be obtained artificially. For example, a radish-cabbage hybrid, synthesized by Georgy Dmitrievich Karpechenko, was obtained by crossing radish and cabbage. The radish genome is symbolized R (2n = 18 R , n = 9 R ), and the cabbage genome as a symbol B (2n = 18 B , n = 9 B ). Initially, the resulting hybrid had the genomic formula 9 R + 9 B . This organism (amphiploid) was sterile, since 18 single chromosomes (univalents) and not a single bivalent were formed during meiosis. However, in this hybrid, some gametes turned out to be unreduced. When such gametes were fused, a fertile amphidiploid was obtained: ( 9 R + 9 B ) + (9 R + 9 B ) → 18 R + 18 B . In this organism, each chromosome was represented by a pair of homologues, which ensured the normal formation of bivalents and the normal divergence of chromosomes in meiosis: 18 R + 18 B → (9 R + 9 B ) and ( 9 R + 9 B ).
Currently, work is underway to create artificial amphidiploids in plants (eg, wheat-rye hybrids (triticale), wheat-couch hybrids) and animals (eg, hybrid silkworms).
The silkworm is an object of intensive selection work. It should be noted that in this species (as in most butterflies), females have a heterogametic sex ( XY), while males are homogametic ( XX). For the rapid reproduction of new silkworm breeds, induced parthenogenesis is used - unfertilized eggs are removed from females even before meiosis and heated to 46 ° C. Only females develop from such diploid eggs. In addition, androgenesis is known in the silkworm - if the egg is heated to 46 ° C, the nucleus is killed by X-rays, and then inseminated, then two male nuclei can penetrate the egg. These nuclei fuse together to form a diploid zygote ( XX), from which the male develops.
The silkworm is known to be autopolyploidy. In addition, Boris Lvovich Astaurov crossed the silkworm with the wild handicap of the tangerine silkworm, and as a result, fertile allopolyploids (more precisely, allotetraploids) were obtained.
In the silkworm, the yield of silk from male cocoons is 20-30% higher than from female cocoons. V.A. Strunnikov, using induced mutagenesis, brought out a breed in which males in X- chromosomes carry different lethal mutations (system of balanced lethals) - their genotype l1+/+l2. When such males are crossed with normal females ( ++/ Y) only future males hatch from eggs (their genotype l1+/++ or l2/++), and females die at the embryonic stage of development, because their genotype or l1+/Y, or + l2/Y. To breed males with lethal mutations, special females are used (their genotype + l2/++ Y). Then, when such females and males with two lethal alleles are crossed in their offspring, half of the males die, and half carry two lethal alleles.
There are breeds of silkworms in which Y-chromosome has an allele for dark egg color. Then dark eggs ( XY, from which females should hatch), are discarded, and only light ones are left ( XX), which later give male cocoons.
Aneuploidy
Aneuploidy (heteropolyploidy) is a change in the number of chromosomes in cells that is not a multiple of the main chromosome number. There are several types of aneuploidy. At monosomy one of the chromosomes of the diploid set is lost ( 2 n - 1 ). At polysomy one or more chromosomes are added to the karyotype. A special case of polysomy is trisomy (2 n + 1 ), when instead of two homologues there are three of them. At nullisomy Both homologues of any pair of chromosomes are missing ( 2 n - 2 ).
In humans, aneuploidy leads to the development of severe hereditary diseases. Some of them are associated with a change in the number of sex chromosomes (see Chapter 17). However, there are other diseases:
Trisomy on the 21st chromosome (karyotype 47, + 21 ); Down syndrome; the frequency among newborns is 1:700. Slowed physical and mental development, wide distance between the nostrils, wide bridge of the nose, development of the fold of the eyelid (epicant), half-open mouth. In half of the cases, there are violations in the structure of the heart and blood vessels. Immunity is usually lowered. The average life expectancy is 9-15 years.
Trisomy on the 13th chromosome (karyotype 47, + 13 ); Patau syndrome. The frequency among newborns is 1:5.000.
Trisomy on the 18th chromosome (karyotype 47, + 18 ); Edwards syndrome. The frequency among newborns is 1:10,000.
haploidy
Reducing the number of chromosomes in somatic cells to the main number is called haploidy. There are organisms haplobionts, for which haploidy is a normal state (many lower eukaryotes, gametophytes of higher plants, male Hymenoptera insects). Haploidy as an anomalous phenomenon occurs among sporophytes of higher plants: in tomato, tobacco, flax, Datura, and some cereals. Haploid plants are characterized by reduced viability; they are practically sterile.
Pseudopolyploidy(false polyploidy)
In some cases, a change in the number of chromosomes can occur without a change in the amount of genetic material. Figuratively speaking, the number of volumes changes, but the number of phrases does not change. Such a phenomenon is called pseudopolyploidy. There are two main forms of pseudopolyploidy:
1. Agmatopolyploidy. It is observed if large chromosomes break up into many small ones. Found in some plants and insects. In some organisms (for example, in roundworms), fragmentation of chromosomes occurs in somatic cells, but the original large chromosomes are preserved in germ cells.
2. Fusion of chromosomes. It is observed if small chromosomes are combined into large ones. Found in rodents.
Chromosomal mutations (otherwise they are called aberrations, rearrangements) are unpredictable changes in the structure of chromosomes. Most often they are caused by problems that occur during cell division. Exposure to initiating environmental factors is another possible cause of chromosomal mutations. Let's see what the manifestations of such changes in the structure of chromosomes can be and what consequences they have for the cell and the whole organism.
Mutations. General provisions
In biology, a mutation is defined as a permanent change in the structure of the genetic material. What does "persistent" mean? It is inherited by the descendants of an organism that has mutant DNA. It happens in the following way. One cell receives the wrong DNA. It divides, and two daughters copy its structure completely, that is, they also contain altered genetic material. Further, there are more and more such cells, and if the organism proceeds to reproduction, its descendants receive a similar mutant genotype.
Mutations usually do not go unnoticed. Some of them change the body so much that the result of these changes is a fatal outcome. Some of them make the body function in a new way, reducing its ability to adapt and leading to serious pathologies. And a very small number of mutations benefits the body, thereby increasing its ability to adapt to environmental conditions.
Allocate mutations gene, chromosomal and genomic. Such a classification is based on the differences that occur in different structures of the genetic material. Chromosomal mutations thus affect the structure of chromosomes, gene mutations - the sequence of nucleotides in genes, and genomic mutations make changes to the genome of the whole organism, adding or taking away a whole set of chromosomes.
Let's talk about chromosomal mutations in more detail.
What are chromosomal rearrangements?
Depending on how the changes occurring are localized, the following types of chromosomal mutations are distinguished.
- Intrachromosomal - transformation of genetic material within one chromosome.
- Interchromosomal - rearrangements, as a result of which two non-homologous chromosomes exchange their sections. Non-homologous chromosomes contain different genes and do not meet during meiosis.
Each of these types of aberrations correspond to certain types of chromosomal mutations.
Deletions
A deletion is a separation or loss of a portion of a chromosome. It is easy to guess that this type of mutation is intrachromosomal.
If the extreme part of the chromosome is separated, then the deletion is called terminal. If there is a loss of genetic material closer to the center of the chromosome, such a deletion is called interstitial.
This type of mutation can affect the viability of the organism. For example, the loss of a portion of the chromosome encoding a certain gene provides a person with immunity to the immunodeficiency virus. This adaptive mutation arose about 2000 years ago, and some people with AIDS managed to survive only because they were lucky to have chromosomes with an altered structure.
Duplications
Another type of intrachromosomal mutations is duplications. This is a copying of a section of the chromosome, which occurs due to an error in the so-called crossover, or crossing over, in the process of cell division.
The region copied in this way can maintain its position, rotate 180°, or even repeat several times, and then such a mutation is called amplification.
In plants, the amount of genetic material can increase precisely through multiple duplications. In this case, the ability of the whole species to adapt usually changes, which means that such mutations are of great evolutionary importance.
Inversions
Also refer to intrachromosomal mutations. Inversion is a rotation of a certain section of the chromosome by 180 °.
The part of the chromosome inverted as a result of inversion can be located on one side of the centromere (paracentric inversion) or on opposite sides of it (pericentric). The centromere is the so-called region of the primary constriction of the chromosome.
Usually, inversions do not affect the external signs of the body and do not lead to pathologies. There is, however, an assumption that in women with an inversion of a certain part of the ninth chromosome, the probability of miscarriage during pregnancy increases by 30%.
Translocations
Translocation is the movement of a section of one chromosome to another. These mutations are of the interchromosomal type. There are two types of translocations.
- Reciprocal - this is the exchange of two chromosomes in certain areas.
- Robertsonian - the fusion of two chromosomes with a short arm (acrocentric). In the process of Robertsonian translocation, short sections of both chromosomes are lost.
Reciprocal translocations lead to fertility problems in humans. Sometimes such mutations cause miscarriage or lead to the birth of children with congenital developmental pathologies.
Robertsonian translocations are quite common in humans. In particular, if the translocation occurs with the participation of chromosome 21, the fetus develops Down syndrome, one of the most frequently recorded congenital pathologies.
isochromosomes
Isochromosomes are chromosomes that have lost one arm, but at the same time replaced it with an exact copy of their other arm. That is, in fact, such a process can be considered a deletion and inversion in one vial. In very rare cases, such chromosomes have two centromeres.
Isochromosomes are present in the genotype of women suffering from Shereshevsky-Turner syndrome.
All the types of chromosomal mutations described above are inherent in various living organisms, including humans. How do they manifest themselves?
Chromosomal mutations. Examples
Mutations can occur in the sex chromosomes and in autosomes (all other paired chromosomes of the cell). If mutagenesis affects the sex chromosomes, the consequences for the organism, as a rule, are severe. Congenital pathologies arise that affect the mental development of the individual and are usually expressed in changes in the phenotype. That is, outwardly mutant organisms differ from normal ones.
Genomic and chromosomal mutations are more common in plants. However, they are found in both animals and humans. Chromosomal mutations, examples of which we will consider below, are manifested in the occurrence of severe hereditary pathologies. These are Wolff-Hirschhorn syndrome, "cat's cry" syndrome, partial trisomy disease along the short arm of chromosome 9, and some others.
Syndrome "cat's cry"
This disease was discovered in 1963. It arises due to partial monosomy on the short arm of chromosome 5, due to a deletion. One in 45,000 babies is born with this syndrome.
Why is this disease so named? Children suffering from this disease have a characteristic cry that resembles a cat's meow.
With the deletion of the short arm of the fifth chromosome, its different parts may be lost. The clinical manifestations of the disease directly depend on which genes were lost during this mutation.
The structure of the larynx changes in all patients, which means that the "cat's cry" is characteristic of everyone without exception. Most of those suffering from this syndrome have a change in the structure of the skull: a decrease in the brain region, a moon-shaped face. The auricles in the syndrome of "cat's cry" are usually located low. Sometimes patients have congenital pathologies of the heart or other organs. Mental retardation is also a characteristic feature.
Usually patients with this syndrome die in early childhood, only 10% of them survive to the age of ten. However, cases of longevity with the "cat's cry" syndrome have also been recorded - up to 50 years.
Wolff-Hirshhorn Syndrome
This syndrome is much less common - 1 case per 100,000 births. It is caused by a deletion of one of the segments of the short arm of the fourth chromosome.
The manifestations of this disease are varied: delayed development of the physical and mental spheres, microcephaly, a characteristic beak-shaped nose, strabismus, cleft palate or upper lip, small mouth, and malformations of internal organs.
Like many other human chromosomal mutations, Wolff-Hirschhorn disease is classified as semi-lethal. This means that the viability of the organism with such a disease is significantly reduced. Children diagnosed with Wolff-Hirschhorn syndrome usually do not live to be 1 year old, but one case has been recorded when the patient lived for 26 years.
Syndrome of partial trisomy on the short arm of chromosome 9
This disease occurs due to unbalanced duplications in the ninth chromosome, as a result of which there is more genetic material in this chromosome. In total, more than 200 cases of such mutations in humans are known.
The clinical picture is described by a delay in physical development, mild mental retardation, and a characteristic facial expression. Heart defects are found in a quarter of all patients.
In the syndrome of partial trisomy of the short arm of chromosome 9, the prognosis is still relatively favorable: most patients survive to old age.
Other syndromes
Sometimes, even in very small sections of DNA, chromosomal mutations occur. Diseases in such cases are usually due to duplications or deletions, and they are called microduplication or microdeletion, respectively.
The most common such syndrome is Prader-Willi disease. It occurs due to a microdeletion of a section of chromosome 15. Interestingly, this chromosome must be obtained by the body from the father. As a result of a microdeletion, 12 genes are affected. Patients with this syndrome are mentally retarded, obese, and usually have small feet and hands.
Another example of such chromosomal diseases is Sotos syndrome. A microdeletion occurs in the area of the long arm of chromosome 5. The clinical picture of this hereditary disease is characterized by rapid growth, an increase in the size of the hands and feet, the presence of a convex forehead, and some mental retardation. The frequency of occurrence of this syndrome has not been established.
Chromosomal mutations, more precisely, microdeletions in regions of chromosomes 13 and 15, cause Wilms' tumor and retinblastoma, respectively. Wilms' tumor is a kidney cancer that occurs predominantly in children. Retinoblastoma is a malignant tumor of the retina that also occurs in children. These diseases are treated if they are diagnosed in the early stages. In some cases, doctors resort to operative intervention.
Modern medicine eliminates many diseases, but it is not yet possible to cure or at least prevent chromosomal mutations. They can only be detected at the beginning of intrauterine development of the fetus. However, genetic engineering does not stand still. Perhaps soon a way to prevent diseases caused by chromosomal mutations will be found.
