Answer to Question #346126 in Zoology for B blessed

Question

Differentiate between mitosis and meiosis with respect to their phases involved.

Answer 1

Meiosis and mitosis both have a prophase, metaphase, anaphase, telophase and cytokinesis.

In meiosis, prophase, metaphase, anaphase and telophase occur twice. The first round of division is special, but the second round is more like mitosis.

In mitosis, prophase, metaphase, anaphase and telophase occur once.

Prophase: Chromosomes condense and the centrosomes begin to form an early spindle.

• Meiotic prophase I is much longer that mitotic prophase.
• During prophase I homologous chromosomes make contacts with each other called chiasmata and “crossing over” occurs. This is where chromosomes exchange sections of DNA. This is important for generating genetic diversity but is also crucial mechanically to hold homologous chromosomes together.

• Mitotic prophase is much shorter that meiotic prophase I.
• There is no crossing over in mitosis.

Metaphase: In metaphase II of meiosis, and metaphase of mitosis, chromosomes line up along the metaphase plate due to the action of microtubule spindle fibres emanating from the centrosomes located at opposite cell poles. These fibres are attached to the chromosomes by kinetochores at the centromeres of the chromosomes.

• In meiotic metaphase I pairs of homologous chromosomes line up along the metaphase plate.
• The way in which the homologous pairs are oriented randomly with respect to the cell poles is referred to as the law of independent assortment and ensures a random and independent distribution of chromosomes to the daughter cells of meiosis I and ultimately to the haploid gametes at the end of meiosis II.
• In mitotic metaphase a single chromosome/ pair of chromatids line up along the metaphase plate.
• Sister chromatids are identical and so the orientation of the chromosome doesn’t carry any meaning.

Anaphase: In anaphase, chromosomes are split to opposite poles of the cell.

• In anaphase of meiosis I cohesin at the centromeres of the chromosomes is not cleaved and it therefore continues to hold sister chromatids together as the homologous chromosomes are segregated to opposite cell poles.
• In anaphase of mitosis (and meiosis II), cohesin protein holding the centromeres of the sister chromatids together is cleaved, allowing the sister chromatids to segregate to opposite poles of the cell, at which point they are called chromosomes.

Telophase: A nuclear membrane reforms around the newly separated chromosomes, which begin to uncoil, becoming less condense. The spindle microtubules disassociate. Each daughter cell will inherit one centrosome.

Cytokinesis: The cell plasma membrane pinches, to leave two daughter cells with separate plasma membranes.

• In meiosis, cytokinesis must occur twice: once after telophase I and again, after telophase II.
• In mitosis, cytokinesis does not always occur, some cells divide and are multinucleate, like muscle cells.