The following points highlight the four theories proposed for the mechanism of crossing over. The theories are: 1. Janssen’s Classical Theory 2. Belling’s Copy Choice Theory 3. Uhl’s Theory 4. Darlington’s Theory of Crossing Over.
Theory # 1. Janssen’s Classical Theory:
Janssen (1909) believed that prior to the formation of chiasmata the homologous maternal and paternal chromosomes come in pair and in pachytene stage they become coiled round each other and become doubled (Fig. 16.1).
He further suggested that in order to derive chiasmata from such a coil, the paternal and maternal chromatids made contacts at intervals and then one chromatid of chromosome penetrated that of the other until they were broken, where upon they rejoined in new ways; paternal to maternal and vice versa, forming the typical chiasmata between them.
The theory did not suggest a satisfactory mechanism by which two chromatids break at precisely equivalent points.
Theory # 2. Belling’s Copy Choice Theory:
In 1931, a cytologist named J. Belling proposed “the copy choice theory”. According to this theory, the paired chromosome in first meiotic prophase duplicates their genes before the fibres that join them in tandem are developed.
During the process, if the chromosomes are twisted around each other, the connecting fibres may connect genes of one chromosome at some points and adjacent genes produced by other chromosome at the other.
In brief, the theory assumes that the crossing over is the direct result of the new chromatids copying partly from one strand and partly from other homologous strand (Fig. 16.2).
There are two main defects in the copy choice theory:
(i) The theory does not properly account for the fact that crossing over can involve all the four chromatids and not just the two chromatids, though three strands and four strands double cross overs (i.e., successive cross-overs involving three or four different strands) are also known to occur (Fig. 16.3).
(ii) The second shortcoming of this hypothesis is that it requires singleness of leptotene threads and duplication of genes must occur during meiosis. But the synthesis of new chromosome material, at least the DNA, occurs during the interphase and thus the gene duplication takes place before the prophase of first meiosis begins.
Hence, copy choice theory must assume that crossing over occurs in the interphase rather than in pachytene, the only known time at which most chromosomes regularly pair and duplicate themselves.
Theory # 3. Uhl’s Theory:
Uhl (1965) proposed a mechanism some-what similar to that of Belling. According to him, a chromosome consists of many small strands of DNA which are joined successively end to end by linkers. At the time of DNA replication, the linkers remain single and go with one or the other complementary strand.
Formation of linkers to fill the vacant spaces after synapsis, sometimes results in the linking of segments from homologous chromosomes rather than linking the sister chromatids and thus it eventually results in crossing over and chiasmata. The basic sequence is shown in Fig. 16.4. The theory has only a few defects but many merits.
(a)The linkers should be so close that telophase chromosomes should appear single, but it is not so.
(b)The cistron (constitutional gene) must be interrupted by linkers or be unreasonably long since intragenic crossing over does take place, unless this kind of crossing over has different
(a) The theory goes far toward reconciling the multi-stranded nature of the chromosomes with unitary behaviour of chromatids.
(b) In this theory, stress has been given on the bond exchange between the adjacent DNA strands of the two homologues and no stress is laid on the physical breakage and reunion.
(c) The theory restricts rejoining to successive segments and does not allow for lateral exchange. Presumably, very specific bonds are involved in reunion.
(d) Broken chromosome and chromatid ends resulting from the action of radiation or chemicals have apparently free choice and they can rejoin with any other broken end regardless of the direction.
Other important points in the theory are:
I. Exchange takes place between chromatids without any breakage.
ii. Precision in the process.
iii. Positive interference, i.e., successive chiasmata will be minimal distances apart.
iv. Three and four strands double cross-overs and chiasmata can be explained.
v. The synthesis of almost all DNA is supposed to occur prior to synapsis.
Theory # 4. Darlington’s Theory of Crossing Over:
A more probable and easy hypothesis of mechanism of crossing over has been advanced by C. D. Darlington in 1935. This is called “breakage and reunion theory.” Darlington postulated that the homologous chromosomes are intertwined during the four stranded stage of first meiosis. The twisting exerts strain on the chromatids.
If the stresses are great enough one or more chromatids of the homologues will be broken at one or more points. If more than one chromatid break, there is possibility that the broken chromatids of one chromosome will unite with broken ends of different chromatid forming a chiasma.
If the union takes place between sisters chromatid parts (i.e., paternal to paternal and maternal to maternal) no genetic consequence is anticipated. However, if the break and reunion occur between non- sister chromatids (i.e., paternal to maternal or vice versa) recombinants would result.
This hypothesis represents the best explanation to date to account for the formation of recombinants. Diagrams showing the stages involved in this hypothesis are given in (Fig. 16.5).
Previously it was thought that the mechanical stress produced the breaks before the recombination but now scientists believe that some enzymes are involved in the processes of breakage and reunion. The idea that some enzymatic processes bring about breakage and rejoining of DNA strands was first put-forth by Howard Flanders and Boyce in 1964.
Reproduction of bacterial chromosome, for instance, during conjugation indicates that the ring structure of DNA may be broken without any known mechanical stress.
There is also no known twisting of bacterial chromosomes in the recombination process. While considering the reunion of broken DNA strands, regardless of breaks occurred, one must hypothesize an enzymatic process in the formation of covalent bonds necessary to repair the sugar phosphate backbone of the DNA molecule.
Stem and Hotta (1969) are of the opinion that two nuclear enzymes namely endonuclease and ligase bring about crossing over. The enzyme endonuclease helps in the breaking of the chromatids while the ligase helps in the reunion of the chromatid segments. They have also reported the synthesis of a small amount of DNA during the period of crossing over which is used in the repairing of the broken chromatids.
Somatic Crossing Over:
The pairing and crossing over take place not only in gonad cells but also in somatic cells. The process of somatic crossing over is known to occur in a number of organisms including Drosophila and maize. In the Diptera insect, the homologous chromosomes have been observed to pair in the early mitotic prophase.
The crossing over takes place when the chromosomal threads are plectonemically arranged. Although the somatic crossing over is difficult to detect, there is no doubt that it occurs. The exact mechanism is not yet clears (Fig. 16.6).
The somatic crossing over may bring several changes in the structure and physiology of the organisms. Majority of individuals are heterozygous for many genes. Somatic crossing over may result in the malfunctioning of parts or organs on account of presence of recessive genes in homozygous condition. The influence of this process may sometimes be very serious and fatal.
Factors Affecting Crossing Over:
It has been proved experimentally that the frequency of crossing over between two genes is not completely dependent on their distance but several other physiological and environmental factors can also influence it. Temperature, x-ray and chemical composition of food may change the frequency of crossing over. In Drosophila melanogaster it may be reduced with the increase of the age of flies.
Significance of Crossing Over:
Crossing over is a widespread phenomenon which is known to occur in all the higher organisms, as well as in most bacteria and viruses. The exchange of genes between non sister chromatids of homologous chromosomes in this process leads to the production of recombinants. Hence, it is of great importance in the evolution.
The phenomenon of crossing over is of paramount importance in plant breeding because new varieties with valuable characters can be evolved by eliminating undesirable genes through this process. Study of crossing over is very much helpful in mapping of genes on the chromosomes.