Meiosis in Animal & Plant cells (AQA A Level Biology): Revision Note

Meiosis in animal & plant cells

• Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells

• It produces gametes in plants and animals that are used in sexual reproduction

• It has many similarities to mitosis however, it has two divisions: meiosis I and meiosis II

• Within each division, there are the following stages: prophase, metaphase, anaphase and telophase

Meiosis I

• Meiosis I is the first division in meiosis

• It separates homologous chromosomes and reduces the chromosome number by half, producing two haploid cells

Prophase I

• DNA replication has already occurred

• DNA condenses and becomes visible as chromosomes

• The chromosomes are arranged side by side in homologous pairs

• Crossing over of non-sister chromatids may occur

• The spindle is formed by centrioles

• The nuclear envelope breaks down, and the nucleolus disintegrates

Metaphase I

• Homologous pairs of chromosomes line up randomly along the equator of the spindle

• Independent segregation occurs in metaphase I

Anaphase I

• The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle

  • The centromeres do not divide

Telophase I

• The chromosomes arrive at opposite poles

• Spindle fibres start to break down

• Nuclear envelopes form around the two groups of chromosomes, and nucleoli reform

• The cell then divides by cytokinesis, forming two haploid daughter cells — each with half the original chromosome number (but still with duplicated chromosomes)

  • These cells are haploid as they contain half the number of centromeres

Meiosis II

• There is no interphase between meiosis I and meiosis II, so the DNA is not replicated

• The second division of meiosis is almost identical to the stages of mitosis

Prophase II

• The nuclear envelope breaks down and chromosomes condense

• A spindle forms at a right angle to the old one

Metaphase II

• Chromosomes (still made of two sister chromatids) line up in a single file along the equator of the spindle

Anaphase II

• Centromeres divide, and individual chromatids are pulled to opposite poles

  • Each chromatid is now an individual chromosome

• This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell

Telophase II

• Nuclear membranes form around each group of chromosomes

• Cytokinesis occurs

  • Cytoplasm divides as new cell surface membranes are formed, creating four haploid cells