Meiosis Chapter 11.4

Function of Meiosis Produces gametes (sex cells) Males – sperm Females – egg or ovum Only occurs in gonads (testes or ovaries) Sperm + egg → zygote (fertilized egg)

Chromosome Count Chromosome number = ploidy number (n) Haploid: Half the normal number of chromosomes (1 set = n) Diploid: Normal number of chromosomes (2 sets = 2n) Sperm (n) + Egg (n) → Zygote (2n)

Human Chromosomes Humans have 2 sets of 23 chromosomes n = 23 2n = 46 22 pairs of autosomes 1 pair of sex chromosomes XX = female XY = male Meiosis makes haploid gametes from diploid cells

Sex chromosomes

Homologous Chromosomes Each pair of chromosomes are said to be homologous One from mom and one from dad Carry the same genes, but different forms of each gene Ex – both chromosomes may have eye color gene, but one has blue and one has brown

Meiosis Two cell divisions Meiosis I (PMAT I) Meiosis II (PMAT II) Homologous chromosomes separate Results in 2 haploid cells Meiosis II (PMAT II) Sister chromatids separate Results in 4 haploid cells

Meiosis I Prophase I Metaphase I nuclear env. disappears homologous chromosomes pair up Form a tetrad (tetra = four) Metaphase I tetrads line up along cell’s equator

Meiosis I Anaphase I Telophase I Tetrad splits, homologous chromosomes separate Telophase I Cell splits into two haploid cells

Meiosis II Similar to mitosis Prophase II uneventful Metaphase II Replicated chromosomes line up at equator Anaphase II Sister chromatids separate Telophase II Cells divide = 4 haploid gametes

Gametogenesis Spermatogenesis Oogenesis 4 tiny haploid sperm 1 large haploid egg, 3 small polar bodies

Creating Variation Meiosis creates genetic variation in two ways: Random shuffling of chromosomes in Metaphase I More than 8 million possible gametes Crossing over in Prophase I Homologous chromosomes swap pieces of DNA when tightly paired as tetrad

Crossing Over