Test: Mieosis and its Significance (June 11) - NEET MCQ

In diplotene stage of meiosis I, the chromosomes may unfold to nearly form and start transcription of mRNA and rRNA to build up food reserves in the cytoplasm. This process is most pronounced in the primary oocytes of amphibians, reptiles and birds. In some species, the chromosomes enlarge greatly, assuming lampbrush form.

Leptotene also known as leptonema is the first stage of prophase I during which individual chromosomes begin to condense into long strands within the nucleus which are loossely interwoven. However, the two sister chromatids are still so tightly bound that they are indistinguishable from one another. They possess a string of swollen areas called chromomeres.

Meiosis takes place in reproductive organs. It results in formation of gametes with half the normal chromosome number. Young anthers are the best material to study meiosis.

The lampbrush chromosomes occuring in prophase of meiosis II are highly elongated special kind of synapsed mid-prophase or diplotene chromosome bivalents which have already undergone crossing over. Lampbrush chromosomes occur in pairs. Each chromosome of a pair has a double main axis due to presence of two elongated chromatids. Both the adjacent chromatids bear rows of large number of chromomeres. Two adjacent chromomeres are separated by interchromomeric stretches. Many of the chromomeres give out lateral projections or loops. The lateral loops provide a test tube or lampbrush-like appearance to the chromosome pair. Lateral loops take part in rapid transcription of DNA to mRNA meant for synthesis of yolk and other substances required for growth and development of Meiocyte.

Meiosis of one Pollen Mother Cell (PMC) produces 4 haploid pollen grains. Thus, for producing 1200 pollen grains 1200/4=300 PMCs are required.

Meiosis I consists of separation of the homologous chromosomes, each made up of two sister chormatids into two cells. Entire haploid content of chomosomes is contained in each of the resulting daughter cells. The first meiotic division therefore reduces the ploidy of the original cell by a factor of 2. Meiosis II consists of decoupling of each chormosome sister strands (chromatids) and segregating the individual chromatids into haploid daughter cells. The two cells resulting from meiosis I divide during meiosis II, creating 4 haploid daughter cells.

During pachytene sub-stage of meiosis I, crossing over i.e. exhcnage of genetic material takes place between homologous, which results in recombinations. The two homologous chromosomes are contributed by different parents. One of them is partnerl chromosome and the other one is maternal chormosome.

Reduction divison is known as meiosis, gametes foremed from this divison are known as meiogametes.