Meiosis Hand-out Answers

• Sexual reproduction involves two key processes. The first is the formation of haploid sex cells, or gametes. The second process is fertilization, which occurs when two sex cells fuse to form a zygote.

In animals, female gametes, called ova, are formed in the ovaries and male gametes, called sperm, are formed in the testes.

In plants, the male gametes are found in the pollen grains, and the sex cells develop in special structures called flowers, in angiosperms, and cones in gymnosperms.

Meiosis involves 2 stages of cell division. Unlike mitosis, where the daughter cells have the same number of chromosomes as the parent cell, meiosis produces gametes with half the number of chromosomes. In humans body cells have 46 chromosomes whereas sex cells have 23 chromosomes.

Mitosis and meiosis

•Meiosis l•Meiosis l is often called

reduction division because the diploid number is reduced to the haploid number of chromosomes.

During prophase l, the nuclear membrane begins to dissolve, the centriole splits, and its parts move to opposite poles within the cell, and spindle fibres are formed. The chromosomes come together in homologous pairs. Each chromosome of the pair is a homologue and is composed of a pair of sister chromatids.

• The whole structure (2 homologous chromosomes is called a tetrad because there are 4 chromatids.

• During this process (pairing up of homologues) called synapsis, the chromatids often intertwine, swapping pieces of the chromosome, in an exchange called crossing over. This breaking off and reattaching fragments of DNA results in the recombination of genetic material.

• During Metaphase l the tetrads migrate to the centre of the cell and their centromeres form a line across the cell.

In Anaphase l the homologous chromosomes move toward opposite poles. The reduction division is accomplished as one member of each homologous pair will be found in each of the new cells. Each chromosome in the new cell consists of two sister chromatids.

During Telophase l, a membrane begins to form around each set of chromosomes. Unlike in mitosis, the chromosomes in the two nuclei are not identical.

Meiosis llMeiosis ll usually begins immediately after telophase l. No duplication of DNA occurs between meiosis l and meiosis ll.

During Prophase ll the nuclear membrane dissolves and spindle fibres begin to form.In Metaphase ll the chromosomes are arranged across the middle of the cell.

In Anaphase ll the sister chromatids separate and move to opposite poles of the cells. The nuclear membrane begins to form around the chromatids, now called chromosomes.

During Telophase ll the second nuclear division is complete and the second division of the cytoplasm is complete.

The formation of sex cells is called gametogenesis. The production of sperm cells is called spermatogenesis and the production of egg cells is called oogenesis.

During egg production the cytoplasm does not divide equally so only 1 egg and 3 polar bodies are produced. During sperm production the cytoplasm divides equally and 4 sperm are produced.

Sperm must be highly motile and being large and heavy would slow them down. Egg cells do not need to be motile. Having a large supply of nutrients in their cytoplasm to fuel future cell divisions in the event the egg cell becomes fertilized.

Sex chromosomesOne of the pairs of chromosomes are known as the sex chromosomes. In humans, females have a combination of XX and males have a combination of XY. All the chromosomes that are not sex chromosomes are called autosomes.