Category: Biology Second Paper

In anaphase-1, homologous chromosomes separate. Chromosomes run in opposite poles. Polar movement of chromosomes occurs due to contraction of chromosome bases and elongation of stem bodies. This is called anaphagic movement. During polar movement, the centromere is the leader and the armature is the follower. When the chromosomes reach the poles, they look like the letters V, L, J or I. As undivided complete chromosomes of bivalents reach the poles, the chromosome number becomes half of the chromosome number of the maternal cell. Siddique Publications

In metaphase-1, the chromosomes become thicker and shorter. Each centromere of a bivalent is poleward, equidistant from the equator. Centromeres are associated with traction fibers. Loops are formed between chromosomes. Towards the end of this stage, the bivalent chromosomes separate under the pull of traction fibers.

Dia means opposite and kinesis means insertion. At this stage the chromosomes are shorter and thicker. The maximum marginalization of kiazmata occurs. Chromatids cannot be identified separately due to the accumulation of matrix on each chromosome in bivalents. Bivalents move from the center of the nucleus to the periphery. At this point the exchanged part is visible through the crossover. Towards the end of this stage, the nucleolus disappears and the nuclear envelope is removed.

Diplos means two or two and tene means thread. At this stage the chromosomes become shorter and thicker through continuous contraction. In bivalents, the attraction between the homologous chromosomes decreases and repulsion begins instead. Repulsion is usually observed first and most extensively within the centromere. As a result of repulsion, the chromosomes move away from each other. However, homologous chromosomes may not be completely separate. Chromosomes moving apart is called disjunction. A loop or loop is formed in the bivalent in the presence of two atoms. The clay gradually moves towards the edges. Moving towards the edge of the material is called terminalization. At this time two or more arms are rotated by 90 degree angle. But if there is a kayazma, the arms can rotate through 180 degree angle. Towards the end of this stage, the number of kayazmata decreases. Absence of the nucleolus is indicated although the nuclear membrane is intact.
Oocytes in human oocytes are arrested in the diplotene stage during embryonic stage. This condition is called dicotyledonous condition. Under the influence of luteinizing hormone, the diktyoten phase ends. Dr. Siddique Publications

The Greek word pachys means thick or thick and tene means thread. Pachytin is a relatively long-lasting phase. At this stage the chromosomes are more compressed, shorter and thicker. Each chromosome in a bivalent divides longitudinally without the centromere into two chromatids. As a result, four chromatids are formed in each bivalent. This condition is called tetrad. Two chromatids of the same chromosome are called sister chromatids and two chromatids of different chromosomes are called non-sister chromatids. Two non-sister chromatids come close together and form an X-shaped structure. It is called chiasma (chiasma=cross) or kai structure. If bivalent chromatids are shorter in length, zygoma may not form. Again, if the length of the chromatids is longer, the chiasmata may be formed at more than one place. With the help of endonuclease enzymes, non-sister chromatids break into two at the chiasma segment and exchange segments. The exchanged chromatids are then joined with the help of ligase enzyme. In this way, the exchange of parts between two non-sister chromatids is called crossing over or crossover. Qualitative changes occur between chromosomes due to zygomatic crossing over. At this stage, the nuclear membrane and nucleolus are intact. Dr. Siddique Publications

Zygos means pair and tene means thread. At this stage the homologous chromosomes are arranged in pairs. Attraction begins between homologous chromosomes. Chromosomes come close to each other and pair up (one from the father and the other from the mother). The process of pairing of chromosomes is called synapsis and a pair of chromosomes is called bivalent. Bivalent chromosomes are linked by two synaptonemal complexes. The pairing process may start from one end to the other end, or may begin at the centromere and gradually spread to both sides, or may begin at a site. Bivalents form half the number of chromosomes in each cell. At this stage the chromosomes are more compressed, thicker and shorter. Nuclear membrane and nucleolus are clear. Dr. Siddique Publications
[Chromosomes having the same structure are called homologous chromosomes. One of the homologous chromosomes comes from the father and the other from the mother]

Leptos means thin and tene means thread. At this stage, the nucleus of the cell begins to increase in size. Chromosomes look like thin threads. Dehydration begins between the chromosomes. It increases the color or dye capacity of chromosomes. Chromosomes contain granular clusters called chromomeres. DNA duplicates by making its counterpart. Towards the end of this stage, the chromosomes are compressed, short, thick and visible. Nuclear membrane and nucleolus are evident. In animal cells, the ends of chromosomes and the nuclear membrane join towards the aster rays to form the attachment plate. The attachment plate looks a lot like a bouquet of flowers. Scientist Darlington called this state of chromosomes Boke stage or bouquet. When chromosomes in plant cells form bouquet-like structures, it is called synogenesis.

Chromosomes contain one or more other sites of contraction other than the centromere. This is called secondary shrinkage. A secondary contraction called SAT (Sine Acid Thymonucleic) forms the nucleolus. Each diploid chromosome has a pair of secondary chromosomes. It is called Nucleolar Organizer Region-NOR. Human chromosomes 13, 14, 15, 21 and 22 have a total of five pairs of NORs.

The disc or disk like structure at the centromere of chromosome is called kinetochore. The kinetochore contains DNA, tubulin protein and microtubules. Each kinetochore is composed of two plates. The outer plate is associated with microtubules and the inner plate with centromeric heterochromatin. It regulates chromosomal movement.

The word Centromere is formed from the Greek word Kentron meaning center and meros meaning part. The round, colorless and constricted area in the chromosome is called centromere. Each chromosome has only one centromere. However, some chromosomes may have two or more centromeres. The position of the centromere creates a groove in the chromosome. This is called Mukhyakunchan or Mukhyakhan. Scientist Darlington named it centromere.