explain crossing over mechanism
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The Mechanism of Crossing-over
The DNA strands must be broken to exchange their segments. The portions of the chromosome that undergo crossing over contain the same gene loci. Crossing-over leads to the recombination of the genes on the chromosomes. The point of crossover is visible as a cross-shaped chiasma.
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Answer:
Crossing Over Definition :-
Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis, which results in new allelic combinations in the daughter cells.
Each diploid cell contains two copies of every chromosome, one derived from the maternal gamete and the other from the paternal gamete. These pairs of chromosomes, each derived from one parent, are called homologous chromosomes. When diploid organisms undergo sexual reproduction, they first produce haploid gametes through meiosis. During prophase I of meiosis, homologous chromosome align with each other and exchange genetic material, so that some of the resultant chromosomes are recombinants – containing a mixture of genes derived from the maternal as well as the paternal chromosomes.
Homologous Chromosomes:-
Most organisms that undergo sexual reproduction contain two types of cells in their body – haploid and diploid cells. Diploid cells are seen in most parts of the body and contain two copies of every chromosome. Therefore, they contain two genes to determine the sequence of nearly every RNA or protein. Haploid cells are usually male or female gametes formed as a result of meiosis and are seen in sexual organs. They contain only one complete set of chromosomes and are designed to fuse with another haploid gamete to produce a diploid zygote – with one paternal and one maternal set of chromosomes. Though they code for the same genes, their DNA sequences can vary slightly from one another. For instance, the paternal chromosome may contain the genetic information to result in antigen A being present on red blood cells, while the maternal chromosome may code for antigen B. These two antigens are similar in function, but differ from each other marginally in their amino acid sequence.
All the cells of an adult organism are derived from the initial zygote and are therefore genetically identical to each other. When this adult prepares for reproduction, some specialized cells undergo meiosis, producing four haploid daughter cells. These daughter cells can then undergo maturation to form male or female gametes.
During meiosis, chromosomes derived from the maternal and paternal gamete can assort independently from one another. In other words, chromosomes from the mother or the father can be chosen randomly for each gamete. A human sperm can contain chromosomes 1, 3, 5, 7, 9, 21 and 23 from his female parent, and the rest can arise from the male parent. Similarly, an egg can have only two chromosomes from the female parent and the rest from the male parent.
In addition, each of these chromosomes could be a recombinant mixture of genes arising from the two parents. One of the four haploid daughter cells arising from meiosis could have a chromosome with 80% of its sequence from the male parent and the remaining 20% from the female parent. Correspondingly, there will be another daughter cell with the reverse ratio. This occurs through crossing over, when replicated homologous chromosomes are placed close to one another and exchange segments with each other.
Chromosomal Crossing Over
The image is a representation of one set of homologous chromosomes, with genes being represented by different letters of the alphabet. The genes on one chromosome are shown using capital letters and that on the homologous pair using small letters. Two of the four daughter cells formed after this crossing over event have a recombinant chromosome that is neither completely derived from the mother nor the father. The image also demonstrates that genes that are in close physical proximity to one another on the chromosome are likely to be inherited together, while those that are farther away might get independently assorted during meiosis.