Crossing over(from English crossing–over– crossover) is an exchange of homologous sections of homologous chromosomes (chromatids).
The “break-reunion” crossing-over mechanism
According to the Janssens–Darlington theory, crossing over occurs in prophase of meiosis. Homologous chromosomes with chromatid haplotypes AB And ab form bivalents. In one of the chromatids in the first chromosome there is a break in the area A–B, then in the adjacent chromatid of the second chromosome there is a break in the area a–b. The cell seeks to correct the damage using repair-recombination enzymes and attach chromatid fragments. However, in this case it is possible to join crosswise (crossing over), and recombinant haplotypes (chromatids) Ab And aB . In anaphase of the first division of meiosis, the divergence of two-chromatid chromosomes occurs, and in the second division, the divergence of chromatids (single-chromatid chromosomes) occurs. Chromatids that did not participate in crossing over retain their original combinations of alleles. Such chromatids (single-chromatid chromosomes) are called non-crossover; with their participation, non-crossover gametes, zygotes and individuals will develop. Recombinant chromatids that were formed during crossing over carry new combinations of alleles. Such chromatids (single-chromatid chromosomes) are called crossover, with their participation, crossover gametes, zygotes and individuals will develop.
Thus, due to crossing over, recombination– the emergence of new combinations (haplotypes) of hereditary inclinations in chromosomes.
Note. According to other theories, crossing over is associated with DNA replication: either in pachytene of meiosis or in interphase (see below). In particular, it is possible to change the matrix at a replication fork.
Interference is the suppression of crossing over in areas immediately adjacent to the point of the exchange that occurred. Consider the example described in one of early works Morgana. He studied the frequency of crossing over between genes w (white- white eyes) at (yellow– corpus luteum) and m(miniature - small wings), localized on the X chromosome D. melanogaster. Distance between genes w And at the percentage of crossing over was 1.3, and between genes at And m– 32.6. If two acts of crossing over are observed by chance, then the expected frequency of double crossing over should be equal to the product of the crossover frequencies between genes at And w and genes w And m. In other words, the double crossover rate would be 0.43%. In fact, only one double crossing over per 2205 flies was detected in the experiment, i.e. 0.045%. Morgan's student G. Moeller proposed to determine the intensity of interference quantitatively by dividing the actually observed double crossing-over frequency by the theoretically expected (in the absence of interference) frequency. He called this indicator co-incident coefficient, i.e. coincidences. Möller showed that in the Drosophila X chromosome interference is especially strong at short distances; as the interval between genes increases, its intensity decreases and at a distance of about 40 morganids or more, the co-incidence coefficient reaches 1 (its maximum value).
Types of crossing over:
1.Double and multiple crossing over
2.Somatic (mitotic) crossing over
3. Unequal crossing over
The evolutionary significance of crossing over
As a result of crossing over, unfavorable alleles, initially linked to favorable ones, can move to another chromosome. Then new haplotypes arise that do not contain unfavorable alleles, and these unfavorable alleles are eliminated from the population.
Biological significance crossing over
Thanks to concatenated inheritance successful combinations alleles turn out to be relatively stable. As a result, groups of genes are formed, each of which functions as a single supergene, controlling several traits. At the same time, during crossing over, recombinations occur - i.e. new combinations of alleles. Thus, crossing over increases the combinative variability of organisms.
It means that…
a) during natural selection“useful” alleles accumulate in some chromosomes (and carriers of such chromosomes gain an advantage in the struggle for existence), while undesirable alleles accumulate in other chromosomes (and carriers of such chromosomes drop out of the game - are eliminated from populations)
b) during artificial selection, alleles of economically valuable traits accumulate in some chromosomes (and the carriers of such chromosomes are retained by the breeder), while undesirable alleles accumulate in other chromosomes (and the carriers of such chromosomes are discarded).
Crossing-over: exchange of genetic material between chromosomes as a result of “breaking” and joining of chromosomes; the process of exchanging sections of chromosomes during the crossing of chromosomes (Fig. 118, B4).
During pachytene (thick filament stage), homologous chromosomes are in a state of conjugation a long period: in Drosophila - four days, in humans - more than two weeks. All this time, individual sections of chromosomes are in very close contact. If in such a region a break in the DNA chains occurs simultaneously in two chromatids belonging to different homologs, then when the break is restored, it may turn out that the DNA of one homologue will be connected to the DNA of another, homologous chromosome. This process is called crossing-over.
Since crossing over is mutual exchange homologous sections of chromosomes between homologous (paired) chromosomes of the original haploid sets - individuals have new genotypes that differ from each other. In this case, a recombination of the hereditary properties of the parents is achieved, which increases variability and provides richer material for natural selection.
Genes are mixed due to the fusion of gametes of two different individuals, but genetic changes are not carried out only in this way. No two offspring of the same parents (unless they are identical twins) will be exactly alike. During meiosis, two different types of gene reassortment occur.
One type of reassortment is the result of the random distribution of different maternal and paternal homologues between daughter cells during the first meiotic division, each gamete receiving its own different selection of maternal and paternal chromosomes. It follows from this that the cells of any individual can, in principle, form 2 to the power of n genetically different gametes, where n is the haploid number of chromosomes. However, in fact, the number of possible gametes is immeasurably greater due to crossing over (crossover) - a process that occurs during the long prophase of the first division of meiosis, when homologous chromosomes exchange sections. In humans, in each pair of homologous chromosomes, crossing over occurs on average at 2 - 3 points.
When crossing over there is a gap double helix DNA in one maternal and one paternal chromatid, and then the resulting segments are reunited “crosswise” (the process of genetic recombination). Recombination occurs in prophase of the first meiotic division, when the two sister chromatids are so closely packed together that they cannot be seen separately. Much later in this extended prophase, the two separate chromatids of each chromosome become clearly distinguishable. At this time, it is clear that they are connected by their centromeres and are closely aligned along their entire length. The two homologs remain linked at the points where crossing over occurred between the paternal and maternal chromatids. At each such point, which is called a chiasm, two of the four chromatids intersect. Thus, this is the morphological result of the crossing over that has occurred, which in itself is not observable.
Name the type and phase of cell division shown in the pictures. What processes do they illustrate? What do these processes lead to?
Explanation.
1) Type and phase of division: Meiosis - prophase1.
2) Processes: crossing over, exchange of homologous regions of chromosomes. Mutual exchange of sections between homologous (paired) chromosomes.
3) Result: a new combination of gene alleles, hence combinative variability
Note:
in paragraph 2, the “conjugation” process was indicated, but was removed from the criteria, because
Chromosome conjugation is a pairwise temporary bringing together of homologous chromosomes, during which an exchange of homologous regions may occur between them (or may not occur).
Explanation from site “user” Evgeniy Sklyar- clarifications to point 2. Will also be counted by the inspectors “as correct”
2) Processes: conjugation (synapsis) - approaching and contact of homologous chromosomes, crossing over - exchange of homologous regions of chromosomes.
3) Result: a new combination of gene alleles, consequently increasing the genetic heterogeneity of chromosomes and, as a consequence, the resulting gametes (spores).
Without combinative variability, because One can speak about variability only judging by a new generation of organisms.
Synapsis- conjugation of chromosomes, pairwise temporary bringing together of homologous chromosomes, during which an exchange of homologous regions can occur between them... (textbook for specialized classes edited by Shumny)
Therefore, crossing over is part of conjugation, at least in terms of time.
Source: Unified State Examination in Biology 05/30/2013. Main wave. Siberia. Option 4., Unified State Exam 2017
Guest 19.08.2015 17:20
There is an error in the explanation. The figure shows the crossing over process: 1. bivalent before crossing over, 2. bivalent after crossing over.
THERE IS NO CONJUGATION IN THE FIGURE.
Gulnara 01.06.2016 13:49
Crossing over is the exchange of homologous sections of chromosomes, why write crossing over, exchange of sections of homologous chromosomes separately, separated by commas???
Natalia Evgenievna Bashtannik
no, these are three different processes:
conjugation, crossing over, exchange of homologous chromosome regions
Svetlana Vasilyeva 17.11.2016 02:56
Can crossing over occur without conjugation???? Conjugation (bringing together homologous chromosomes) always occurs, but crossing over does not always happen, only in 30%! Crossing over is a contact of homologous chromosomes, after which an exchange occurs between their identical sections..... or not?
Natalia Evgenievna Bashtannik
What is the essence of the question?
Crossing over is cross, mutual exchange of homologous sections of homologous chromosomes as a result of the break and connection in a new order of their threads - chromatids; leads to new combinations of alleles of different genes.
Why 30%??? Crossover probability different, depends on the distance between genes. 1% crossing over = 1M (Morganide).
If crossing over has occurred, this does not mean that an exchange will take place.
CROSSINGOVER CROSSINGOVER
(English crossingover - cross), crossover, mutual exchange of homologous sections of homologous chromosomes as a result of the break and connection in a new order of their threads - chromatids; leads to new combinations of alleles of different genes. The most important mechanism that ensures combinative variability in populations and thereby provides material for natures. selection. Occurs in meiotically, less often mitotically dividing cells. Can lead to recombination of large sections of a chromosome with several. genes or parts of one gene (intrigene K.), both strands of the DNA molecule or just one. The frequency of crossover between genes reflects the distance between them in the chromosome and is defined as the frequency of crossover (with a non-parental combination of alleles) individuals in an analyzing cross, that is, as the frequency of crossover gametes; may change under the influence of certain physical, chemical. and physiol. factors. Molecular mechanism K. is not completely clear. K. is used in genetics. analysis for solving plural problems of genetics. (see RECOMBINATION, GENETIC MAP OF A CHROMOSOME).
.(Source: Biological encyclopedic Dictionary." Ch. ed. M. S. Gilyarov; Editorial team: A. A. Babaev, G. G. Vinberg, G. A. Zavarzin and others - 2nd ed., corrected. - M.: Sov. Encyclopedia, 1986.)
crossing overMutual exchange of sections between homologous (paired) chromosomes. Occurs during cell division - meiosis and (much less often) mitosis at the prophase stage, when paired homologous chromosomes already contain two sister chromosomes chromatids. At this four-chromatid stage, homologous sections of chromatids are exchanged: in each homologous chromosome, one chromatid is broken, and then the resulting fragments of neighboring chromatids are reunited, but in a criss-cross manner (English: “crossing over” - cross). During crossing over, genes from one homologous chromosome are moved to another, resulting in new combinations. alleles genes, i.e. is happening recombination genetic material. Crossing over is one of the mechanisms of hereditary variability.
.(Source: “Biology. Modern illustrated encyclopedia.” Chief editor A. P. Gorkin; M.: Rosman, 2006.)
Synonyms:
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Crossing over... Spelling dictionary-reference book
- (English crossing over) mutual exchange of sections of homologous (paired) chromosomes, leading to redistribution (recombination) of genes localized in them. Occurs during cell division; one of the mechanisms of hereditary variability. IN… … Big Encyclopedic Dictionary
Crossing over (English crossing-over - crossing of chromosomes) - the process of exchange of homologous chromosomes in sections during their conjugation in prophase I of meiosis. Crossing over is one of the mechanisms of genetic recombination (gene exchange). Its frequency depends on the distance between genes: the further the genes are located from each other, the more often crossover occurs between them. 1% crossing over is taken as a unit of distance between genes. She is named Morganida in honor of T. Morgan, who developed principles of genetic mapping. The cytological sign of crossing over is chiasmata - χ -shaped figures bivalents during the exchange of sites. Crossing over is usually meiotic, but sometimes occurs in mitosis (somatic crossing over). It can also occur within a gene.
Crossing over is one of the most important processes providing combinative variability and thus providing material for natural selection.
The essence of this process consists in the exchange of sections of homologous chromosomes. This occurs by breaking and then joining the chromatids in a new order. Crossing over can lead to the recombination of large sections of a chromosome with several genes or parts of one gene (the so-called intragenic crossing over), both strands of the DNA molecule or just one. Crossing over occurs during conjugation in phase I meiosis. Crossing over can also be observed when mitotic division, but less often. When asexual organisms mitotic crossing over is the only way genetic recombination. Mitotic crossing over can lead to mosaic expression recessive traits in a heterozygous individual. This expression has important in oncogenesis and in the study of lethal recessive mutations.
The phenomenon of crossing over was discovered by F. Janssens in 1909 while studying the meiosis of salamander cells, but theoretically the phenomenon of crossing over was predicted earlier. In particular, the American cytologist W. Sutton suggested in 1903 that several genes may be located on one chromosome, and then linked inheritance of traits should be observed, i.e. some different signs can be inherited as if they were controlled by a single gene. Such a set of genes on one chromosome forms a linkage group. Actually, the study of crossing over and linkage groups made it possible to create chromosome maps. The first chromosome map was created for the fruit fly Drosophila.
Types of crossing over
Depending by cell type, in which crossing over occurs:
- meiotic - occurs in the prophase of the first division of meiosis, during the formation of germ cells,
- mitotic - during division somatic cells, mainly embryonic. Leads to a mosaic pattern in the manifestation of symptoms.
Depending on the molecular homology of chromosome regions, entering crossing over:
- normal (equal) – exchange takes place different areas chromosomes.
- unequal - there is a gap in non-identical sections of chromosomes.
Depending on the number of formed chiasmata and chromosome breaks with subsequent recombination of genes:
- single,
- double,
- multiple.
The biological significance of crossing over is extremely great, since genetic recombination makes it possible to create new, previously non-existent combinations of genes and thereby increase hereditary variability, which provides ample opportunities for the organism to adapt to different conditions environment. Crossover value:
- leads to an increase in combinative variability,
- leads to an increase in mutations.
A person specifically carries out hybridization in order to obtain the necessary combinations for use in breeding work.