Molecular Basis of Crossing Over | Cell Division

In this article we will discuss about the molecular basis of crossing over.

Any molecular model of crossing over must accommodate the explanation of the following phenomena:

(a) Gene conversion,

(b) Post-meiotic segregation,

(c) Polarized conversion and

(d) Negative interference.

a. Gene Conversion:

It is the unequal recovery of alleles in the region of exchange during genetic recombination in cases where all the products of a meiotic event can be recovered. Gene conversion results into 3A : la or 1A : 3a ratios instead of the normal 2A : 2a ratio in the meiotic products.

b. Post-meiotic Segregation:

In fungi of the class Ascomycetes, the four products of meiosis undergo one mitotic division to produce 8 ascospores. The ratio of these products in normal cases in 4A : 4a, but in cases of gene conversion, it becomes 6A : 2a or 2A : 6a. However, in some cases, 5A : 3a or 3A : 5a ratios are also obtained which indicates the segregation of conversion products during the post-meiotic mitosis.

c. Polarized Conversion:

Conversion may occur in directional, polarized fashion as has been observed in some fungi. Thus the frequency of gene conversion depends on the position of a mutation in the region of conversion.

These influences are reflected in the intragenic polarity patterns which are of the following two types:

(1) Unidirectional, and

(2) Bidirectional.

In case of unidirectional polarity, pairwise interallelic crosses show that the allelic pair located at one side always converts more frequently than the one located on the other side. In contrast, in the case of bidirectional polarity, the alleles giving the lowest conversion frequency in two-factor crosses lie between the two ends of the gene involved in conversion.

d. Negative interference:

When the occurrence of crossing over in one chromosomal section does not restrict, or even enhances the occurrence of other exchanges in the adjacent sections, it is called negative interference. This phenomenon has been observed in some fungi, such as, Aspergillus.

Negative interference is an unusual event and cannot be explained on the basis of the classical models of crossing over. Investigators have suggested that some DNA is synthesized during the course of recombination and hybrid DNA molecules are formed.

There are two main models or recombination:

(1) Hybrid DNA, and

(2) Polaron hybrid DNA exchange models.

Hybrid DNA Model:

This model of recombination was proposed by Holliday in 1964. According to this model, each chromatid is regarded as a single DNA molecule, while a pair of non-sister chromatids is viewed as a pair of DNA molecules. A primary breakage occurs in one strand each of the two DNA molecules paired together; subsequently, non-sister strands join together forming a section of “Hybrid DNA” (Fig. 11.8).

The breakage (nick) is caused by a specific endonuclease in one strand of each DNA molecule; the two strands involved in nicking have the same polarity (Fig. 11.8 II). The two strands separate from their complementary strands, the separation extending from the point of nick to some distance in either direction.

The free separated strands pair reciprocally and the gap (nick) is sealed by the enzyme DNA ligase to form a “half’ chiasma” (Fig. 11.8 III). The half chiasma moves towards one end (branch migration) forming the regions of hybrid DNA on both the molecules (Fig. 11.8 IV). The lower part of this X-shaped configuration (Fig. 11.8 V) rotates by 180° (Fig. 11.8 VI).

Now secondary breakage is caused by the endonuclease in the two normal (not involved in exchange) strands. Breakage in the original intact Strand (vertical breakage) and rejoining by ligase results into recombination between the outside markers a and b and two recombinant DNA molecules (Ab, aB) are produced (Fig. 11.8 VII, VIII).

If the secondary breakage occurs in the same strands that are involved in the exchange (Fig. 11.8 II) (horizontally), subsequent joining by ligase leads to no recombination between the outside markers, although hybrid DNA regions would have been formed (Fig. 11.8 IX, X).

In case of mismatch (due to the presence of mutation) between the nucleotides in the hybrid DNA region, repair enzymes excise the nucleotides and synthesize DNA complementary to the un-excised strand; this leads to gene conversion either from b⁺ to b or from b to b⁺ (Fig. 11.9) depending on which of the two strands is excised.

Gene conversion may occur both with and without crossing over. Thus this model explains well the phenomenon of gene conversion without crossing over.

Polaron-Hybrid DNA Exchange Model:

This model of genetic recombination was developed by Whitehouse in 1963, 1965 and Whitehouse and Hastings in 1965.

It also leads to hybrid DNA formation during recombination. But it differs from the Holliday model in the following aspects:

(i) The polynucleotide chains of opposite polarity are subject to endonuclease nicking (Fig. 11.10); the nicked strands separate and unwind up o some distance from the point of nick,

(ii) New polynucleotide chains are synthesized using the intact strands as templates; the newly synthesized segments separate and pair with the separated segments of the nicked strands,

(iii) The unpaired portions of the intact polynucleotide chains are now degraded,

(iv) This model requires a small amount of additional DNA synthesis during zygotene-pachytene.

According to the Polaron-hybrid DNA model, the two non-sister chromatids are represented by two DNA molecules and the DNA strands involved in exchange are of opposite polarity.

A specific endonuclease enzyme induces a nick at the Polaron point in one polynucleotide chain of a DNA molecule; the enzyme makes another nick at the same Polaron point in the polynucleotide chain of opposite polarity belonging to the homologous DNA molecule (Fig. 11.10 II).

The nicked strands unwind and separate from the intact strands. Now DNA synthesis occurs using the unpaired regions of the intact strands as templates. The newly synthesized strands also unwind and separate from their templates (Fig. 11.10 III, IV).

Now the strand synthesized against one DNA molecule pairs with the nicked and separated strand of the homologous DNA molecule, this forms a stretch of double-stranded DNA (Fig. 11-10 V, VI). The two stretches of double-stranded “DNA produced in this way consist of strands of different origin; therefore, they are called “hybrid DNA.”

The single-stranded regions of the intact strands of the two DNA molecules are now degraded, and recombination is completed when the hybrid segment of one DNA molecule becomes joined to the non-hybrid segment of the homologous molecule (Fig. 11.10 VII, VIII). The nucleotides of the hybrid DNA may not match completely due to allelic differences.

This mismatch of the nucleotides is repaired through excision repair process. The repair enzymes excise the mismatched nucleotides and incorporate in their place the nucleotides complementary to those present in the other strand. As a consequence of this repair, one of the allele is converted into the other; this generates the phenomenon of “gerie conversion”.

DNA synthesis has been reported during zygotene and pachytene stages; this amounts to 0.3-0.4% of the total nuclear DNA synthesis, and has a high content of G = C residues. Inhibition of DNA synthesis during zygotene results in the prevention of chromosome pairing and fragmentation of chromosomes.

The DNA synthesis during zygotene-pachytene is required to repair the breakage and degradation of DNA during crossing over. At the end of zygotene, protein synthesis is required for chiasma formation. A DNA binding protein isolated from Lilium was found similar to “gene-32 protein” that is necessary for gene recombination in T4 bacteriophage.

It is also similar to a nuclear protein isolated from the early prophase of various mammalian cells. It indicates similarity of the mechanism of recombination between prokaryotes and eukaryotes.