Chromosome basis of genetic variation

Eukaryotic chromosomes are composed of chromatin. Chromatin is  complex of DNA and basic proteins called histones.

The five types of histones fold the DNA into bead-like structures caled nucleosomes. Each structure cotains about 200 nucleotide pairs of DNA. The role of the nucleosome is first order folding of DNA.

The complement of genetic material in one chromosome set is called the genome. The haploid human genome consists of 23 chromosomes, about 10^5 functional genes and has a DNA size of 3,000 million nucleotide pairs.

Each chromosome is replicated once per cell division cycle.

The two landmarks of the cell cycle are: DNA replication (S) and mitosis (M). S is separated from M by gap periods G1 and G2. The length of the cell cycle varies according to the organism and the tissue. In culture the cycle may take 16-42 hours.

Cells that have stopped proliferating are arrested before the S stage in a phase called G0.

Late interphase: the chromosomes have replicated but are not condensed.

Late prophase: the longitudinal bipartite natire of each chromosome is clearly visible in the form of two sister chromatids joined at the centromere.

Metaphase: the chromosomes are maximally condensed and positioned on a "plate" equidistant from the poles of the spindle.

Anaphase: the centromere have divided and sister chromatids of each chromosome move to opposite poles.

Telophase: the proces has ended with the formation of two daughter nuclei each containing the diploid number of chromosomes. The chromosomes uncoil to become long diffues structures.

Late interphase: The chromosomes are not condensed and have replicated.

Late prophase I: Homologous chromosomes have paired forming a bivalent.

Anaphase I: Homologous chromosomes separate to opposite poles of the cell.

End of first divsion of meiosis. Two diploid cells formed.

Anaphase II: Centromeres divide and sister chromatids separate.

End of second division of meiosis. There are four daughter cells each containing the haploid number of chromosomes.