Question

List and describe in your own words the stages of mitosis and meiosis and include a description of interphase and cytokinesis.

Answer 1

Eukaryotic cell division involves two kinds of division

• Mitosis: Mitosis usually takes place in somatic cells and results in two daughter cells with equal number of chromosomes. Walther Flemmming gave the term "Mitosis" in 1882.
• Meiosis: Meiosis takes place in reproductive cells and results in 4 daughter cells with half the number of chromosomes.

Both the mitosis and meiosis are part of cell cycle, which is divided in to interphase and M-phase.

Interphase: The duration between end of M-phase to the beginning of another M-phase is called as interphase. Interphase generally is subdivided in to

• G1 (Gap-1) phase : in this phase cells grow
• S-phase (synthesis phase): in this phase DNA replication takes place (thus doubling the genetic material to be divided in to two daughter cells or 4 in case of meiosis).
• G2 (Gap-2) phase: in this phase cell prepares for division.

Mitosis completes in following stages

Prophase: Chromatin condensation begins with the start of prophase. By end of prophase, chromosomes with two sister chromatids are formed (as DNA replication has been performed in S-phase). Nuclear envelop disappear and mitotic spindle is formed.

Metaphase: Metaphase is marked by chromosome alignment along the center plate of the cell. Kinetochore microtubules play role in bringing this arrangement of chromosomes.

Anaphase: Anaphase begins as the sister chromatid start separating and move towards the opposite poles. Cohesin protein is suddenly disrupted which results in sister chromatid separation and spindle fiber shortening cause sister chromatids to move towards opposite poles.

Telophase: Telophase is marked by disappearance of kinetochore microtubule after chromosomes have arrived at two opposite poles. Chromosomes begin to decondense and nuclear envelop reappears.

Cytokinesis: Mitosis culminates as the cytoplasm divide in cytokinesis. Cytokinesis (i.e.., division of cytoplasm) actually start since the end of anaphase and continue through telophase and completes by end of mitosis (i.e.., cytokinesis) and results in beginning of next interphase.

Meiosis completes in following stages:

MEIOSIS I: Meiosis I is also called as reduction division because number of chromosomes become half in resulting daughter cells.

Prophase I : Prophase I of meiosis is further divided in to five different sub-stages:

1. Leptotene: Chromatin condensation starts. Although sister chromatids are present they are not apparent as they are closely apposed. Hence, each chromosome looks like a single chromatid.
2. zygotene: Zygotene stage is marked with synapsis between two homologs. this intimate pairing results in the formation of synaptonemal complex.
3. Pachytene: Pachytene is marked by crossing over between homologous chromosomes. Crossing over is a kind of physical exchange between non-sister chromatids. This results in recombination.
4. Diplotene: Chiasmata formation takes place in diplotene stage. Chiasmata represent a cross like structure signifying the place where crossing over has taken place.
5. diakinesis: This is a transitions stage of prophase and metaphase. Chromosomes become thick and visible with two sister chromatids.

Metaphase I: Chromosomes become arranged on the center plate.

Anaphase I: Chromosomes become separate and moves to opposite poles. In should be noted that chromosome number become half after anaphase. Sister chromatids are still together.

Telophase I and cytokinesis : Telophase is marked by gathering of chromosomes at opposite poles and then the cytoplasm divides resulting in two daughter cells each with half the number of chromosomes.

MEIOSIS II: Meiosis II is similar to mitosis.

• Prophase II: Prophase II is similar to the prophase of mitosis and is not sub divided in to different stages as in prophase I. Chromosomes become condensed.
• Metaphase II: Chromosomes arranged on to the equatorial plate.
• Anaphase II: Sister chromatids now move to opposite poles.
• Telophase II: Chromosomes are gathered at opposite poles and cytoplasm begins to divide. Nuclear envelop reforms.
• cytokinesis: cytoplasm division complete via cytokinesis and results in a total of 4 daughter cells each with half the number of chromosome as compared to the Parent cell.