Meiosis 10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis. Chromosomes, genes, alleles & mutations.This approaches helps to characterized the role of enzymes and protein in DNA synthesis, chromosome pairing and in crossing over (recombination).In this approach of characterizing biochemical processes in cell during meiosis, it was found that several enzymes and protein are responsible for chromosome pairing as well as DNA replication and other events.In genetic approach, from the study of several mutants such as yeast, Neurospora etc, it was found that several genes are responsible for meiosis process.So, it is now very clear that these protein structures are an integral part of meiosis all living organisms.From the behavior study of chromosome during prophase stage, it was found that chromosome comes together forming homologous pair which is brought together by a protein called synaptonemal complex.

In this approach, morphological and cytological investigation of dividing cell was carried out and from these studies it was clear that chromosome behaves in certain ways during meiosis.

Biochemical approaches: Characterization of the biochemical processes of meiosis.ġ.

Morphological and cytological approaches.

Historically, there have been three general approaches for the study of meiosis, (gametogenesis) to produce gametes with only one chromosome with either dominant Thus itīecomes clear that both alleles of heterozygote separate during meiosis

In the meiosis-II, the two chromatids of Achromosomes and those of a chromosome separate.

In meiosis-I, allele Aand allele afrom homologouschromosome will segregate or separate.

In the prophase stage the two chromosomes will.

On two different homologous chromosomes i.e., A on one chromosome and aon the second chromosome.

The cytological basis of segregation of allelesĬan be explained if we take the example of an individual which possesses two.

Sister chromatids are separated at meiosis II, and four recombined haploid gametes are formed.Ĭytological basis of segregation of alleles: During meiosis, replicated homologous chromosomes pair and engage in recombination and then segregate to opposite poles.With the separation of the parental chromosome pairs in the subsequent stages, the first meiotic division results in the reduction of chromosomes by half, which is followed by another division, analogous to mitosis to produce haploid gametes.Telophase II: chromosomes decondense, nuclear membrane and nucleolus forms, centrioles move back together.Anaphase II: paired chromatids separate and are pulled to alternate poles.Metaphase II: microtubules connect from the centrioles to the centromeres, and the paired chromatids line up along the equator of the cell.Prophase II: chromosomes condense and centrioles move to the poles.Telophase I: chromosomes decondense, nuclear membrane and nucleolus forms, centrioles move back together.Anaphase I: paired chromosomes separate and are pulled to alternate poles.Metaphase I: microtubules connect from the centrioles to the centromeres, and the paired chromosomes line up along the equator of the cell.Prophase I: chromosomes condense, undergo duplication and crossing over, and centrioles move to the poles.In diplotene and diakinesis stages, nuclear membrane disappears and spindle fibres formations begins.The pachytene is the longest stage of the meiotic prophase, during which crossing over and genetic recombination takes place between the paired homologous chromosomes.These Synaptonemal complex have a tripartite structure, comprising a central element bounded by two lateral elements. During the zygotene stage, the parental homologous chromosomes pair with the help of a structure known as the Synaptonemal complex (SC).In leptotene stage, the chromosomes condense along their axial elements.This prophase-I is divided into five distinct stages which are termed as (a) leptotene, (b) zygotene, (c) pachytene, (d) diplotene, and (e) diakinesis.Before the onset of meiotic division, the chromosomes of the pre-gametic cell are replicated to yield two pairs of sister chromatids after which they enter a long meiotic prophase interval.The phases of meiosis is divided into two meiosis-I and meiosis-II.Meiosis consists of two rounds of chromosome segregation following a single replication.The events of meiosis of a cell includes duplication of chromosomes which is followed by one reductional division and other equational division resulting into four haploid cells, each containing half the number of chromosomes. Meiosis division occurs during gametogenesis in higher organism (eukaryotic cells).

Prophase 1 meiosis