The Stages of Meiosis (College Board AP Biology)

• Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
  • This means that daughter cells contain half the number of chromosomes compared to the parent 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

Prophase I

• DNA condenses and becomes 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 the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
• In this stage centrioles migrate to opposite poles and the spindle is formed
• The nuclear envelope breaks down and the nucleolus disintegrates

Metaphase I

• The pairs of homologous chromosomes line up along the equator of the spindle, 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 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 fibers start to break down
• Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
• Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Cytokinesis

• This is when the division of the cytoplasm occurs
• Cell organelles also get distributed between the two developing cells
• In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
• In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
• The end product of cytokinesis in meiosis I is two haploid cells

Second division of Meiosis : 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 90° to the old one
• Metaphase II
  • Chromosomes line up in a single file along the equator of the spindle
• Anaphase II
  • Centromeres divide and individual chromatids are pulled to opposite poles
  • 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
  • Cytoplasm divides as new cell surface membranes are formed creating four haploid cells

The Stages of Meiosis I Diagram

The different stages of Meiosis I in an animal cell

 

Meiosis II Stages Diagram

Prophase II, Metaphase II and Anaphase II in Meiosis II of an animal cell

 

Cytokinesis Diagram

Telophase II and cytokinesis in Meiosis II of an animal cell

The need for meiosis in a sexual life cycle

• The life cycles of organisms can be sexual or asexual (some organisms are capable of both)
  • In an asexual life cycle, the offspring are genetically identical to the parent (they have exactly the same chromosomes)
  • In a sexual life cycle, the offspring are genetically distinct from each other and from each of the parents (their chromosomes are different, causing them to be genetically distinct)
• The halving of the chromosome number during meiosis is very important for a sexual life cycle as it allows for the fusion of gametes
• Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote (fertilized egg cell) and the production of offspring that are genetically distinct from each other
• This fusion of gamete nuclei is known as fertilization
  • Fertilization doubles the number of chromosomes each time it occurs
  • This is why it is essential that the chromosome number is also halved at some stage in organisms with a sexual life cycle, otherwise the chromosome number would keep doubling every generation
  • This halving of the chromosome number occurs during meiosis
  • In animals, this halving occurs during the creation of gametes

Sexual Life Cycle Overview Diagram

The sexual life cycle