Cell Cycle and Cell Division Test - 1

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.

Meiosis is the type of cell division by which gamete cells (eggs and sperm) are produced. Meiosis involves two successive nuclear divisions and one cytoplasmic division that produce four haploid cells. The first division (meiosis 1) is the reductional division during which number of chromosomes is reduced to half. The second division (meiosis II) separates the chromatids. It is called equational division as chromosome number remains the same as produced after meiosis-l.

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.

Crossing over is a process of exchange of genetic material between non-sister chromatids of two homologous chromosomes and and leads to recombination of genes. It is an enzyme mediated process.

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.

All the stages are of the meiosis 1 because here we can see the bivalent or tetrad stages. Metaphase is the stage where chromosome comes on the equator and bivalent arrange themselves on the equatorial plate. Then anaphase 1 involves the separation of a homologous chromosome which start moving up to opposite poles and each tetrad is divided into two daughter cells. So anaphase involves the production of chromosome number and is also known as disjunction. Telophase 1 shows the formation of two daughter nuclei but the chromosome number is half then the chromosome number of mother cell and nuclear membrane reappears.

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

During anaphase of mitosis, each chromosome arranged at metaphase plate is split and two daughter chromatids are formed. Centromeres split and chromatids move to opposite poles. During anphase I of meiosis I, the homologous chormosomes separate, while sister chromatids remain associated at their centromeres. Anaphase II of meiosis II begins with simultanoeus splitting of the centromere of each chromosome allowing them to move towards oppostie poles of the cell.

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.

Homologous chormosomes are chormosomes having same structural features. In dipilod nuclei, pairs of homologous chromosomes can be identified at the start of meiosis. One member of each pair comes from the female parent and other from the male parent. Homologous chromosomes have the same pattern of genes along the chromosome but the nature of the genes may differ.

The given figure is showing diplotene stage of prophase I of meiosis I. Next stage would be metaphase I.

Pairing of homologous chromosomes occurs in zygotene and crossing over occurs in pachytene stage of prophase I of meiosis I. Paired chromosomes line up on equator in metaphase I. Then there is complete separation of chromatids in anaphase I of meiosis I.

Meiosis is a double division that occurs in the reproductive cell in which nucleus divide but chromosome replicate only once to form four haploid cells, each cell having half the number of chromosomes present in a parent cell. It causes a reduction in the number of chromosomes. It is also known as reduction division. Meiosis in a cell occurs only once to form gametes.

The homologous chromosomes break their connections and separate out during anaphase I of meiosis I. This process of separation is named as disjunction.

Anaphase II, where the centromeres are cleaved, allows the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes and they are pulled towards opposte poles.

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