Crosing-over:-Crossing-over takes place during prophase I of meiosis. Crossing-over is another name for recombination or physical exchange of equal pieces of adjacent non-sister chromatids. During the process of crossing-over one of the paired chromosome arms exchanged physically at one or more locations .The two homologous chromosomes are connected at a certain point called chiasma (pl chiasmata). When crossing-over occurs chromatids break at chiasma and reattached to a different homologous chromosome . The chromatids resulting from the interchange of segments are known as the cross over recombinants and the chromatids that remain intact are called non-crossover parental chromatids.When these chromosomes segregate in meiosis, they form gametes that have completely new combinations of alleles. Generally, the longer the chromosome, the greater the number of chiasmata. The ability of genes to recombine is called recombination frequency.
For example ,If the alleles on tetrad are :-
- 1. A B C D E F G
- 2. A B C D E F G
- 3. a b c d e f g
- 4. a b c d e f g
After crossing- over a new associations of genes and alleles are formed if crossing over takes place between 2nd and 3rd chromosomes
- 1. A B C D E F G- non-crossover parental chromatids
- 2. A B c d e f g - cross over recombinants
- 3. a b C D E F G - cross over recombinants
- 4. a b c d e f g - non-crossover parental chromatids
Types of crossing over crossing overs are of many types depending on number of chiasma:-
- Single crossing over:- when the chiasma formation takes place at a single point of The chromosome pair this type of crossing over is known as single crossing over. In this types two crossed over chromatids and two non crossed over chromatids are formed.
- Double crossing over:- When the chiasmata occur at two places in the same chromosomes known as double crossing over In the double crossing over formation of each chiasma is independent of the other and in it four types of recombination is possible.
Two types of chiasma may be formed in double cross over:-
- Reciprocal chiasma in this type both the chiasma are formed on two same chromatids. So, the second chiasma restores the order which was changed by the first Chiasma, and as a result two non- cross over chromatids are formed.
In this type out of four chromatids only two are involved in the double crossing over.
- Complimentary chiasma When both the chromatids taking part in the second chiasma are different from those chromatids involved in the first. In this type four single cross overs are produced but no non cross over. Complimentary chiasma occurs when three or four chromatids of tetrad undergo crossing over.
- Multiple crossing over When crossing over take place at more than two point in the same chromosome pair it is known as multiple crossing over. It occurs rarely.
There are two theories on the physical nature of the process:-
- Classical theory or two plane theory ( L. W. Sharp ):- proposes that cross-over and formation of the chiasma occur first, followed by breakage and reunion with the reciprocal homologues. According to this theory, chiasma formation need not be accompanied by chromosome breakage.But this theory was not accepted.
- Chiasmatype theory or one plane theory This theory was proposed by F.A. Janssens (1909) breakage occurs first, and the broken strands then reunite. Chiasmata are thus evidence, but not the causes, of a cross-overs.
Recombination During Meiosis:- John Belling( 1928) suggested that no break was necessary and proposed the copy choice model.He believed that crossing over might occur during duplication of homologous chromosomes and might brought about due to novel attachments formed between newly synthesized genes. While studying meiosis in some plant species. He visualized genes as beads (described as chromomeric), connected by non-genic interchromomeric regions. The newly synthesized daughter chromatids is derived due to copying of one chromosomes upto certain region and then switching on to the other homologous chromosome for copying the remaining portion or region of the chromosomes. The new chromatid would have a new arrangement, but no breaks and rejoining need be involved. This was such an attractive idea that the hypothesis in some form held center stage for nearly thirty years.
New chromosome arrangements were associated with recombinant genes, using chromosomes marked by mutant genes and morphological differences at each end demonstrated in Drosophila melanogaster (Stern 1931 ) and Zea mays ( H. S. Creighton and B. McClintock 1931). That crossing over was correlated with segmental interchange between homologous chromosomes.
DNA models during 1960s,DNA models had become very popular, the widely accepted model for DNA crossover was first proposed by Robin Holliday in 1964.
Homologous recombination during meiosis has following important features.
Two homologous DNA molecules line up (e.g., two nonsister chromatids line up during meiosis).
Cuts in one strand of both DNAs, a double-strand break in a DNA molecule is enlarged by an exonuclease, such that the single-strand extension with a free 3‘-hudroxyl group is left at the broken end.
The exposed 3‘ ends invade the intact duplex DNA, and this is followed by branch migration and/or replication to create a pair of crossover structures,( The cut strands cross and join homologous strands)called Holliday junctions(Holliday structure).
Heteroduplex region is formed by branch migration,cleavage of the two crossover creates two complete recombinant products
In this double-strand break repair model for recombination, the 3‘ ends are used to initiate the genetic exchange. Once paired with the complementary strand on the intact homolog, a region of hybrid DNA is created containing complementary strands from two different parental DNA . Each of the 3‘ ends can then act as a primer for DNA replication.
The structure that are formed, called Holliday Intermediates are a feature of homologous genetic recombination pathways in all organisms. Homologous recombination can vary in many ways from one species to another, but most of the process are same.
DNA strands may be cut along either the vertical line or horizontal line and break the Holliday intermediate so that the two recombinant products carry genes in the same linear order as the original, unrecombined chromosomes.
If cleaved occurs on vertical line , the DNA flanking the region containing the hybrid DNA is not recombined; if cleaved occurs on the horizontal line. , the flanking DNA is recombined.