This DNA amount is utilised in the repairing of broken DNA molecules of the chromatids during the chiasmata formation and crossing over. Stern and Hotta (1969) have reported that during the pachytene and zygotene stages, synthesis of small amount of DNA takes place. This process of interchange of chromatin material between non-sister chromatids of each homologous chromosome is known as the crossing over which is accompanied by the chiasmata formation. The broken chromatid segments are united with the chromatids due to the presence of an enzyme, the ligase (Stern and Hotta, 1969). The crossing over involves reshuffling, redistribution and mutual exchange of hereditary material of two parents between two homologous chromosomes.Īccording to recent views, one chromatid of each homologous chromosome of a bivalent may divide transversely by the help of an enzyme, the endonuclease which is reported to increase in the nucleus during this stage by Stern and Hotta (1969).Īfter the division of chromatids, the interchange of chromatid segments takes place between the non-sister chromatids of homologous chromosomes. The pachynema chromosome, thus, consists of four chromatids closely joined together in one complex unit called a bivalent, because it actually contains a pair of chromosomes.ĭuring pachynema stage, an important genetic phenomenon called “crossing over” takes place. In the pachynema stage, this is now changed, the two chromatids of each chromosome containing half of the DNA present in the chromosome at start, become partially independent of one another although they still continue to be linked together by their common centromere. Through the earlier part of the meiotic prophase, however, the DNA molecule in each chromosome behaves as a single body. The chromosomes at this stage take up a specific orientation inside the nucleus the ends of the chromosomes converge toward one side of the nucleus, that side where the centrosome lies (the bouquet stage).Īctually, the doubling of the DNA molecule strands, which is necessary for the subsequent duplication of the chromosomes, occurs earlier, before the beginning of meiotic prophase. In the leptotene stage, the chromosomes become more uncoiled and assume a long thread-like shape. In this stage, the chromosomes are extremely thin, long, uncoiled, longitudinally single and slender thread-like structures. The preleptotene stage closely resembles with the early mitotic prophase. ![]() Most of the synthesis of DNA occurs at the beginning of this phase. During this stage, the amount of DNA becomes double. The first prophase is the longest stage of the meiotic division. After these changes, the cell passes to the first stage of first meiotic division which is known as prophase. This increase in the volume of the nucleus causes modification of nuclear components. In the beginning of the first meiotic division, the nucleus of the meiocyte starts to swell up by absorbing the water from the cytoplasm and the nuclear volume increases about three folds. Heterotypic Division or First Meiotic Division : ![]() The successive meiotic sub stages can be represented as follows: The prophase is the longest meiotic phase, therefore, for the sake of convenience, it is divided into six sub stages, viz., preleptonema (proleptotene), leptonema (leptotene), zygonema (zygotene), pachynema (pachytene), diplonema (diplotene) and diakinesis. The prophase of first meiotic division is very significant phase because the most cytogenetical events such as synapsis, crossing over, etc., occur during this phase. In the homotypic division pairing of chromosomes, exchange of the genetic material and reduction of the chromosome number do not occur.īoth the meiotic divisions occur continuously and each includes the usual stages of the mitosis, viz., prophase, metaphase, anaphase and telophase. The second meiotic division is also known as the homotypic division. In the second meiotic division, the haploid cell divides mitotically and results into four haploid cells. The first meiotic division is also known as the heterotypic division.
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