Name some of the theories that explain process of crossing over

Crossing over is a very important process that takes place during prophase-I of meiosis, especially during the pachytene sub-stage, where homologous chromosomes exchange genetic material. This process increases genetic variation and is a key part of sexual reproduction. To explain how crossing over happens, many scientists have proposed different theories over time. These theories developed step-by-step, with later theories giving more accurate and detailed explanations. There are mainly four important theories that explain the process of crossing over. These are:

1. Copy Choice Theory

Copy choice theory was proposed by J. Belling in 1931. This is one of the oldest theories. It suggested that new chromosomes are formed by copying some parts from one chromosome and some from the other during DNA replication. According to this theory, the copying machinery switches templates while making the new DNA strand. However, this theory was later rejected because no physical exchange between chromosomes happens as per this idea, which is not true according to cytological evidence.

2. Break and Exchange Theory

Break and exchange theory proposed by C. D. Darlington in 1937. This theory suggested that homologous chromosomes break at the same point and then exchange the broken parts with each other. This theory was better than the previous one because it involved actual physical exchange. But still, it lacked proper details about how exactly the exchange takes place at the molecular level.

3. Breakage and Reunion Theory

Breakage and reunion theory proposed by Stern and Hotta in 1969. According to this theory, crossing over happens due to the breakage of chromatids at identical locations. Then, the broken ends of non-sister chromatids of homologous chromosomes join with each other. This theory gave more support to the idea that exchange happens during the pachytene stage of meiosis. It could be observed under a microscope and this made the theory more acceptable than the earlier ones.

4. Double-Strand Break Repair Model (DSBR Model)

DSBR model proposed by Resnick in 1976 and improved by Szostak et al. in 1983. This is the most accepted and modern theory of crossing over. It explains the process at the molecular level. According to this model, crossing over starts with a double-strand break (DSB) in one chromatid. The broken DNA ends are then processed and a strand from the homologous chromosome invades the broken site, forming a Holliday junction. This structure helps in the proper exchange and later gets resolved to complete the crossing over. This model is supported by genetic and biochemical experiments and is used in modern textbooks.






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