Week4: Meiosis

Question 1

What is Meiosis?

Answer 1

The mechanism in which haploid gametes are formed from diploid precursor cells.

This is accomplished with two rounds of cell division, and 1 DNA replication

Question 2

How many cell divisions occur during Meiosis?

Answer 2

Two:

• In Meiosis 1: Two haploid cells are formed from a diploid cell
• After meiosis 1, a second meiosis called “equational division” in which each haploid cell is replicated

Question 3

What happens in Prophase?

Answer 3

• Prophase begins as the chromatin strands coil and condense, causing them to become visible as chromosomes
• During the process of synapsis, the homologous chromosomes pair up side by side, lying together in perfect alignment
• As prophase continues, chromatids of the two chromosomes intertwine
• Chiasma form, which indicate a point where the homologous chromosomes exchange genetic information (crossing over)
• At the end of prophase 1, the bivalents begin to move towards the equatorial plane, a spindle apparatus begins to form in the cytoplasm, and the nuclear membrane dissipates.

Question 4

Why is crossing over important?

Answer 4

Crossing over produces chromosomes which consist of combinations of part of the original chromosomes

This is important because it increases the possinle combinations of genes in each gamete, and thereby increases combinations of human traits

Question 5

What happens in Metaphase 1?

Answer 5

• Completion of spindle formation and allignment of bivalents, which are still attached at the chiasma, in the equatorial plane
• The two centromeres of each bivalent now lie on opposite sides of the equitorial plane

Question 6

What happens in Anaphase 1?

Answer 6

• The chiasmata dissapear, and the homologous chromosomes are pulled by the spindle fibers to opposite sides of the cell.
• The centromeres do not duplicate and divide, so only half of the original number of chromosomes migrate towards each pole
• The chromosomes migrating towards each pole thus consist of one member of each pair of autosomes and one of the sex chromosomes

Question 7

What happens in Telophase 1?

Answer 7

• begins when each chromosome reach opposite side of the cell
• The chromosomes uncoil slightly, and new nuclear membrane begins to form
• The 2 daughter cells contain the haploid number of chromosomes, and each chromosome has two sister chromatids
• In humans, cytokinesis occurs, and the cytoplasm is divided equally in male gametes, and in female gametes, nearly all cytoplasm goes into one daughter cell, which will later form an egg. The other female gamete becomes a polar body and degenerates

Question 8

What happens in prophase II?

Answer 8

The cell nucleus contains only the haploid number of chromosomes

-The chromosomes thicken as they coil, the nuclear membrane dissapears, and new spindle fibers form

Question 9

What happens in metaphase II?

Answer 9

The spindle fibers pull the chromosomes into allignment at the equitorial plane

Question 10

What happens in Anaphase II?

Answer 10

-The centromeres split and each carry a single chromatid towards a pole of the cell.

The chromatids have now separated, but because of chiasma formation and crossing over, the newly seperated sister chromatids might not be identical

Question 11

What happens in Telophase II?

Answer 11

• Begins when the chromosomes reach opposite ends of the cell
• They begin to uncoil, new nuclear membranes are formed around each group of chromosomes and cytokinesis occurs.
• In gametes formed as males, the cytoplasm is divied equally.
• The end result of male meiosis, is 4 daughter cells
• In female gametes, unequal cytoplasm division, forming egg cell and polar body