Making cells without changing the chromosome count

By UK A Level Biology · independent study · myAudience

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Making cells without changing the chromosome count

A cell cycle includes growth, DNA replication and division. During mitosis, copied chromosomes are distributed so daughter nuclei normally receive the same chromosome complement. Cytokinesis divides the cell contents; it is distinct from division of the nucleus.

Count chromosomes using centromeres, rather than counting every replicated chromatid as a separate chromosome. Bacteria divide by binary fission without a mitotic spindle. Viruses reproduce through host processes rather than by ordinary cell division.

In prophase chromosomes condense; at metaphase they align at the spindle’s equator, at anaphase sister chromatids separate, and telophase accompanies formation of new nuclei. Cytokinesis divides the cytoplasm. A mitotic index is dividing cells divided by all cells counted, so 18 dividing among 120 gives 0.15 or 15%. It samples the observed population, not an exact duration without additional assumptions about cycling and sampling. Binary fission separates bacterial copies without these eukaryotic mitotic phases.

Worked example: An organism has 8 chromosomes in a body-cell nucleus. After DNA replication, count chromosomes and chromatids before separation.

Each chromosome has replicated into two chromatids.

The centromere-based chromosome count remains 8.

Answer: 8 chromosomes and 16 chromatids.

Check your understanding: Does mitosis normally halve chromosome number?

Answer: No.

Why: Mitosis preserves the chromosome complement; meiosis has a reduction division.