The cell life cycle
A chromosome is made of DNA and proteins; after DNA doubles before division, each chromosome consists of two chromatids joined at the centromere. The set of the number, size and shape of a species’ chromosomes is the karyotype; a human somatic cell has 46 chromosomes (23 pairs, diploid) and a gamete 23 (haploid). The cell cycle consists of interphase (G1, S, G2; about 90% of the cycle) and division; DNA replicates in S phase. Mitosis: in prophase chromosomes condense and the nuclear envelope disappears, in metaphase they line up on the equatorial plane, in anaphase chromatids are pulled to the poles (the number of chromosomes is temporarily doubled), in telophase nuclei re-form and cytokinesis occurs. The result is two genetically identical diploid cells: the basis of growth, regeneration and asexual reproduction. Meiosis consists of two successive divisions; in prophase I homologous chromosomes pair (conjugation) and crossing over (exchange of segments) occurs, and in anaphase I homologues separate; the result is 4 haploid, genetically different cells – a source of combinative variation. Prokaryotes reproduce by binary fission.
“Put the phases in order”. Mixed pictures are given; students order the phases of mitosis and meiosis and write the numbers of chromosomes and chromatids in each.