|
Band and Exchange Theory of Crossing Over |
|
|
Break and Exchange Theory
The break and exchange theory originally forwarded by Muller and is a most widely accepted theory about the mechanism of crossing over. It suggests that (i) prior to crossing over, the chromosomes of each bivalent get duplicated to form a tetrad; (ii) crossing over occurs only in non-sister chromatids of a tetrad; and (in the crossing over involves the mechanical breaks in non-sister chromatids, mutual exchange of chromosomal segments between the broken non-sister chromatids and reunion or recombination of chromatids during the early part of meiotic prophase I. Most geneticists are agreed with this theory, both ever, there exists a lot of controversy about the manner and time of the break of chromatids during the crossing over. Different cytologists and geneticists have forwarded different views regarding breakage process of chromatids and these views are following:
Band and Exchange Theory of Crossing Over
|
(i) Serebrovsky's contact first theory: According to this theory two chromatids of a tetrad first touch and cross each other and then at the point of cross the breakage of chromatids occur. The chromosome segments then unite to form new combinations.
(ii) Darlingtons’ strain or torsion on theory: Darlington (1935) propounded that chromosomal breaks during crossing over occur as a result of strain during pairing. According to this theory during meiotic prophase I when homologous chromosomes twist around each other in synapsis, then each of the two pairing chromosomes itself becomes spirally wound up. This is called spiralization. The chromosomes are thus under heavy strain due to tight coiling or twisting, and are capable of bearing this strain so long as they are not split. After splitting, each chromosome forms two weaker chromatids, so that one chromatid breaks at one point. This releases the strain on the sister chromatid but somehow, it increases the strain on the non-sister chromatids causing a break in one of them at a point opposite to the first break. The broken chromatids are now unwind near their breaking points and during this, they come into contact and unite, thus forming a chiasma.
|
(iii) Differential contraction theory: Huskins and Newcombe (1941)suggested that tension set up by differential contraction after pairing may cause breaks at overlaps. This hypothesis is based on measurements which indicate that contraction is greater in paired regions than in those not yet paired.
(iv) White’s Frontier theory : White (1951) suggested that the chromosome does not split simultaneously along its entire length. Further, he suggested that the unsplit parts of both homologous chromosomes while remain together by pairing force, the split regions of chromosomes are being forced apart, by some kind of repulsion and thus, consequently a localised strain is developed at the meeting point of the split and unsplit regions, causing a breakage in the chromosome.
|
|
Diagramatic Representation of Copy Choice Theory and Break and Exchange Theory of Crossing Over
A - Copy Choice, a - Synthesis of New Chromatids, b - Switching of synthesis to copy other chromids, c - Reciprocal recombinant copies |
A - Breakage and Reunion, a - Pairing and coiling, b - Breakage of two chromatids, c - Crosswise reunion of broken chromids |
|
|
|
(v) Muller's breakage first theory: This theory holds that before the occurrence of chiasma and crossing over two chromatids break in two fragments and then reunion of chromosomal segments occur in new arrangement.
(vi) The polaron-hybrid DNA exchange model of Whitehouse and Hastings: According to the model of the Whitehouse and Hastings (1965) for crossing over, chromatid replication is essentially completed prior to crossing over. When the homologous chromosomes pair at zygotene or have completed pairing at pachytene, crossing-over can occur at several sites along the chromosomes and generally involves non-sister chromatids. Crossing-over requires the breakage of one polynucleotide strand in each double helix of the chromatids. A special endonuclase enzyme is found to cause such a single strand breakage (see Sybenga, 1972 and Goodenough and Levine, 1974).
|
According to Whitehouse (1965), the broken stands wind loose perhaps over the, length of an entire gene and immediately new DNA is synthesized along the remaining strand and although it remains attached at the beginning point, it unwinds from its template, Now the pair of strands freed first associate with the complementary, newly formed strands of the opposite chromosome, forming hybrid DNA. New end attachments are made and the redundant DNA of the original unbroken strand is removed. The amount of DNA removed equals the amount newly synthesized: there is no net synthesis. At the end of this process,' the double strands have experienced a complete exchange. Neither of the two chromosomes has lost or gained material but the exchange, results in recombination of the genes in the two chromosomes.
The Polaron hybrid DNA exchange model according to Whitehouse and Hastings (1965). Two double DNA helices are shown (A) between which DNA exchange or crossing over occurs. The black dot represents the limit of the polaron, which is initiation point of DNA Synthesis, and where single DNA Strands start separating and Unwinding from their sister strands as a preparation for exchange (B). Along the remaining strand DNA is synthesized (C) but this new DNA also peels off or unwinds from its template, now starting at the other end (D). These new strands associate with the first released strand of the other DNA double helix to form a stretch of double stranded DNA (E,F). These two pairing strands have a different origin and may even be slightly different in base composition: it is hybrid DNA. After new end attachments have been made and the now redundant stretch of old connecting DNA has been removed (G,H) the exchange has been completed (after J.Sybenga, 1972).
|
|
