Cell Division Mitosis and Meiosis

Terminology At the beginning of G1, there is a single strand of chromosome called Chromatin. In the S stage it doubles when the DNA replicates. Each chromosome now consists of 2 strands of chromatin – now called Chromatids. A chromatid is one sub-unit of a double chromosome.

Terminology As it enters prophase it is still a chromosome consisting of 2 chromatids, held together by a Centromere.

Eukaryote Chromosomes Chromosome means “coloured body” but chromosomes are no coloured. They bind easily to certain dyes so when they were first observed they were coloured. Chromosomes are strands of DNA bound to proteins.

Eukaryote Chromosomes Usually exist in pairs. Typically 2 of each kind in somatic cells. The members of the pair are called homologous chromosomes. Each chromosome in a homologous pair is usually the same size, has a centromere in the same place, and has gene loci in the same sequence.

Chromosome Terms

Eukaryote Chromosomes In humans, there are 23 pairs of chromosomes. One member of each pair is derived from the mother and one from the father. A single set of chromosomes is called Haploid (n). The 2 sets of chromosomes are called Diploid (2n)

Eukaryote Chromosomes The exception to the rule that the homologous pairs are the same are the sex chromosomes. The females of most species have 2 identical X chromosomes. Males have an X chromosome and a smaller Y chromosome.

Eukaryote Chromosomes Human males have 22 pairs of autosomes, plus an X and Y chromosome. Human females have 22 pairs of autosomes, plus X and X. The total of an individual’s genetic material is called the genome. Organised into chromosomes it is called a Karyotype.

Normal Male Karyotype

Normal Female Karyotype

Mitosis Used for growth and repair. Each new cell has a complete set of chromosomes identical to the parent cells. A cell division process called Cytokinesis that actually divides the cytoplasm into two cells occurs.

Mitosis

Meiosis This is the formation of gametes in animals and the spores that form the gametophyte generation in plants. Gametes are different from each other. Meiosis halves the chromosome number so that each is haploid. Takes place in 2 steps: Meiosis I and Meiosis II.

Meiosis At the end of cell division there are 4 daughter cells. Crossing over may occur during synapsis.

Meiosis

Gametogenesis This is the formation of the gametes and is different in males and females.

Oogenesis