Topic 3 - Lesson 8

Question 1

Meiosis

Answer 1

Meiosis is the type of cell division which produces gametes for sexual reproduction. Unlike mitosis, the daughter cells are genetically different from the parent cell and contain just half the number of chromosomes (i.e. they are haploid). When two haploid gametes join during fertilisation, a diploid cell called a zygote is formed. Meiosis involves two rounds of cell division which are referred to as meiosis I and meiosis II.

Question 1

meiosis 1

Answer 1

Prophase I: chromatids condense and arrange themselves into homologous pairs (called bivalents). Crossing over occurs (see below). The nuclear envelope disintegrates and spindle fibres form.

Metaphase I: homologous chromosomes line up along the equator and attach to the spindle fibre by their centromeres.

Anaphase I: homologous chromosomes are separated

Telophase I: chromosomes reach opposite poles of the cell. Nuclear envelope reforms around the chromosomes. Cytokinesis results in the formation of two daughter cells.

Question 2

Meiosis 2

Answer 2

Prophase II: chromosomes condense, nuclear envelope disintegrates and spindle fibres form.

Metaphase II: chromosomes attach to the spindle fibre by their centromeres.

Anaphase II: sister chromatids are separated.

Telophase II: chromatids reach opposite poles of the cell. Nuclear envelope reforms and cytokinesis takes places. Four genetically unique daughter cells are produced.

Question 3

Crossing Over

Answer 3

During prophase I of meiosis, a process called crossing over occurs. This is when the homologous chromosomes move towards each other and exchange genetic material. A chromatid from the maternal chromosome becomes twisted around the paternal chromosome and they connect through a structure called the chiasmata. Pieces of chromosomes are exchanged and the chromatids separate, forming chromosomes with different combinations of alleles.

Question 4

Independent Assortment

Answer 4

Depending on the order in which chromosomes line up along the equator of the cell during metaphase, different combinations of chromosomes will end up in each gamete. The way in which the chromosomes align themselves on the spindle fibre is completely random, resulting in a huge number of possibilities of chromosomal combinations in the gametes.