Meiosis (College Board AP® Biology): Study Guide

Meiosis

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

  • This means that daughter cells contain half the number of chromosomes compared to the parent cells

  • When two haploid gametes fuse the result is a diploid zygote, therefore meiosis helps maintain the correct number of chromosomes in offspring

• Meiosis 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

  • telophase

Meiosis I

Prophase I

• Homologous chromosomes pair up, condense and become visible as chromosomes

• DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere

• The chromosomes are arranged side by side in homologous pairs

  • As the homologous chromosomes are very close together, synapsis occurs

• The crossing over (genetic recombination) of non-sister chromatids may occur, and chiasmata may form

• Spindle fibers form, and centrosomes move to opposite poles of the cell

• The nuclear membrane breaks down

Metaphase I

• The pairs of homologous chromosomes line up along the equator of the cell, with the spindle fibers attached to the centromeres

• The maternal and paternal chromosomes in each pair position themselves independently of the others; this is independent assortment

  • This means that the proportion of paternal or maternal chromosomes that end up on each side of the equator is due to chance

Anaphase I

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

• The centromeres do not divide

• Sister chromatids remain attached

Telophase I

• The chromosomes arrive at opposite poles

• Spindle fibers start to break down

• Nuclear envelopes form around the two groups of chromosomes

  • In animal cells, a cleavage furrow is formed

  • In plant cells, a cell plate forms

• Nucleoli reform

Cytokinesis

• Cytokinesis is the division of cytoplasm

• Cell organelles also get distributed between the two developing cells

• The end product of cytokinesis in meiosis I is two haploid cells

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 and become visible

• Meiotic spindle fibers form and attach to sister chromatids - they are connected at the centromere

Metaphase II

• Chromosomes align in a single file along the metaphase plate

  • This is similar to metaphase during mitosis

• The kinetochore of each chromatid is attached to a microtubule extending from the poles

Anaphase II

• Proteins at the centromeres break down

• Sister chromatids are pulled to opposite poles of the cell

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

Telophase II

• Meiotic spindle breaks down

• New nuclear envelope forms around each group of chromosomes

• A cleavage furrow (animal cell) or a cell plate (plant cell) forms

• Chromatids begin to decondense

Cytokinesis

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

  • Each has an unduplicated chromatid

Comparing mitosis & meiosis

• Mitosis and meiosis are both processes of cell division that result in chromosome segregation

• Mitosis supports an organism’s growth and replaces damaged or dying cells by producing two daughter cells genetically identical to each other and the parent cell

  • This consistency allows for continuous growth and tissue maintenance in all body cells (except gametes)

• In contrast, meiosis generates genetically different gametes for sexual reproduction

  • Meiosis creates four daughter cells, all of which contain half the genetic material of the parent cell and are all unique from each other and the parent

  • This variation is essential for genetic diversity within families and populations, reducing the likelihood of inheriting genetic disorders

Similarities between mitosis & meiosis

• Both processes start from a diploid cell

• Both consist of four sequential stages

  • prophase

  • metaphase

  • anaphase

  • telophase

• In metaphase, chromosomes line up along the cell's equator

• In anaphase, chromosomes separate to the poles of the cell

• Both end with cytokinesis

Differences between mitosis & meiosis