Cell and nuclear division

Biology · D Continuity and change. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology topics

The idea

Before a nucleus divides, all the DNA is replicated. Each chromosome then consists of two identical sister chromatids. The chromosomes condense by supercoiling, and microtubules of the spindle move them.

Mitosis separates the sister chromatids. It gives two nuclei that are genetically identical and have the same chromosome number as the parent, so it is used for growth, repair and asexual reproduction. Meiosis is a reduction division. Meiosis I separates homologous chromosomes and halves the chromosome number. Meiosis II separates the chromatids. The result is four haploid nuclei that are all different.

Cytokinesis is the division of the cytoplasm that follows.

Rules to remember

Phases of mitosis in order: prophase, metaphase (chromosomes at the equator), anaphase (chromatids pulled to the poles), telophase.
Mitosis: 1 division, 2 nuclei, diploid (2n), identical. Meiosis: 2 divisions, 4 nuclei, haploid (n), different.
After replication the chromosome number is unchanged, but the number of DNA molecules is doubled.
Cytokinesis: animal cells are pinched in by a ring of actin and myosin. Plant cells build a new membrane and wall from vesicles.
Unequal cytokinesis: oogenesis in humans and budding in yeast.
Variation from meiosis: crossing over and random orientation of bivalents. Combinations from random orientation = 2ⁿ, where n is the haploid number.
Non-disjunction: chromosomes fail to separate. Three copies of chromosome 21 (47 chromosomes in total) cause Down syndrome.

Common mistake

Students think the chromosome number is halved in meiosis II. It is halved in meiosis I, when homologous chromosomes separate. Meiosis II only separates sister chromatids.

Worked example

A cell with a diploid number of 12 enters meiosis. How many chromosomes and DNA molecules are in the cell at the start, in each cell after meiosis I and in each cell after meiosis II?

Start (after replication): 12 chromosomes, each of two chromatids, so 12 × 2 = 24 DNA molecules.
After meiosis I: homologous pairs have separated, so 12 ÷ 2 = 6 chromosomes, each still of two chromatids, so 12 DNA molecules.
After meiosis II: chromatids have separated, so 6 chromosomes and 6 DNA molecules.

Answer: 12 chromosomes and 24 DNA molecules, then 6 and 12, then 6 and 6.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

D2.1 Cell and Nuclear Division

Keep going

← Mutations and gene editing Water potential →Take a Biology test