Meiosis: Phases and Processes

Overview of Meiosis

• Definition: Meiosis is a type of cell division that produces gametes (sex cells), such as sperm in males and eggs in females.
• Stages: Meiosis is divided into two stages: Meiosis I and Meiosis II.

Meiosis I

• Phases: Comprises four phases: Prophase I, Metaphase I, Anaphase I, and Telophase I.

Prophase I

• Starts with a diploid cell containing two sets of chromosomes (one from each parent).
• Chromatin Replication: DNA in chromatin replicates and condenses into X-shaped chromosomes.
• Synapsis: Each chromosome pairs with its homologous chromosome, forming a tetrad (group of four sister chromatids).
• Genes and Alleles:
  • Genes: Inherited genetic information from parents.
  • Alleles: Different versions of a gene present on homologous chromosomes.
• Crossing Over:
  • Homologous chromosomes exchange segments of alleles (recombination).
  • Results in genetic variety among gametes, making offspring genetically distinct from parents and siblings.
• Cell Changes:
  • Nuclear membrane disappears.
  • Centrioles move to opposite ends, and spindle fibers fan out.

Metaphase I

• Homologous chromosomes line up at the equator of the cell.
• Attaches to spindle fibers from opposite poles.

Anaphase I

• Spindle fibers separate the homologous chromosomes, pulling them to opposite poles.

Telophase I

• Each pole has one chromosome from each homologous pair, still in the form of sister chromatids.
• Sister chromatids are genetically different due to crossing over.
• Spindle fibers disappear, and nuclear membrane reforms.
• Cytokinesis: Results in two genetically different haploid daughter cells (each with paired sister chromatids).

Meiosis II

• Unlike Meiosis I, DNA does not replicate before Meiosis II.
• Phases: Comprises four phases: Prophase II, Metaphase II, Anaphase II, and Telophase II.

Prophase II

• Nuclear membrane disappears.
• Spindle fibers fan out from two sets of paired centrioles.

Metaphase II

• Chromosomes line up at the equator and attach to spindle fibers from both poles.

Anaphase II

• Sister chromatids of each chromosome separate and move to opposite poles.
• After separation, sister chromatids are referred to as individual chromosomes.

Telophase II

• Spindle fibers disappear, nuclear membranes reform, and cytokinesis occurs in both cells.
• Results in four genetically different haploid daughter cells, each with one set of chromosomes.

Key Points to Remember

• Starts with: A diploid cell.
• Produces: Genetically different haploid cells (gametes).
• Stages: Two stages of cell division - Meiosis I and Meiosis II.
• Meiosis I: Separates homologous chromosomes to create two haploid cells (with paired sister chromatids).
• Meiosis II: Separates sister chromatids into individual chromosomes, resulting in four genetically different haploid gametes.

Prophase I Highlights

• Synapsis occurs (pairing of homologous chromosomes).
• Formation of tetrads (each pair consists of four chromatids).
• Crossing over results in genetic differences in gametes.
• All gametes produced are haploid.