Cell Cycle and Chromosomes

In the process of meiosis, four haploid daughter cells are formed. In humans, cells that go through meiosis are the sperms and egg cells. The reason why DNA does not duplicate between meiosis I and meiosis II is because only half the number of chromosomes is necessary in a sperm or egg cell. That's why, when a sperm fertilizes an egg, each cell has half the number of chromosomes necessary to create an organism, so together, they have the correct amount of chromosomes.

It is described as a phase where the chromosomes at each pole uncoil and enlarges to form chromatin. Nucleolus and nuclear membrane are re- formed surrounding each chromatin network. Asters and spindle fibres disappear and centrioles divide. Daughter nuclei are formed.

This is described as the process when a segment of chromosome splits and relocates within a non- homologous chromosome.

This type of sex determination occurs in insects and mammals including human beings. Here, the females bear two identical homologous sex chromosomes designated as XX and the males bear two dissimilar sex chromosomes designated as X and Y. Thus, females are homogametic, producing only one type of ovum, and males are heterogametic, producing two types of gametes.

These are defined as the chromosomes which are found in the salivary gland cells of the fruitfly Drosophila. They are many times bigger than the normal chromosomes attaining a length of 2000 micro meters and are visible even under a compound microscope.

Here the centromere is located far away from the centre of the chromosome so that the two arms are highly unequal. The chromosome acquires the shape of 'J' during anaphase.

It leads to the new genetic combinations as a result of crossing over. This situation brings about variations.

 It is described as the stage where each chromosome breaks into two chromatids forming four chromatids; two paternal and two maternal which are known as tetrads. Chiasma is the site of an exchange of genetic material between the two chromatids. It occurs due to breakage and reunion between the two non-sister chromatids. This process is called genetic recombination.

It is described as the phase where chromosomes become short & visible as single structure. It contains the dense stained chromomeres.

Here DNA replication takes place. Protein molecules namely histones are synthesised and cover each DNA strand. 

XXY is Klinefelter's syndome. It is described as an inequality in X chromosomes where a male (dubbed by inheriting a X chromosome) also inherits an extra X chromosome, giving them female qualities. This can include a more slender shape, more refined hips, breast development, absence of facial hair, etc.

DNA replication occurs several times without cell division, thereby increasing cell volume in case of polytene chromosomes.

The chromosomes continue to become denser. The homologous pairs have also found each other and begin to initially align with one another, referred to as 'rough pairing'. Lateral elements also form between the two homologous pairs, forming a synaptonemal complex.

During this stage, the centromeres that contain two kinetochores attach to the spindle fibers at each pole from the centrioles. The equatorial plate formed here is rotated by 90 degrees, compared to meiosis I and is perpendicular to the previous metaphase plate.

Also known as the monarchial type of chromosomes, they have a centromere that is located towards the end of the chromosome. Thus, telocentric chromosomes have a 'rod' shaped appearance. In some cases, the telomeres extend from both the chromosome ends. The telocentric chromosome is not present in humans.