Sexual Reproduction SBI3U

Meiosis allows: 1. The formation of gametes 2. Genetic reduction: reduces the number of chromosomes to half (n=23) 3. Genetic Recombination: different combinations of alleles are formed. Contributes to genetic variation in population.Due to independent assortment of chromosomes and crossing over.

MEIOSIS Meiosis occurs in progenitor diploid cells (cells with 46 chromosomes) and produces 4 gametes; haploid cells. Unlike mitosis, these gametes do not divide further, but instead fuse with another gamete to make a diploid zygote. (i.e. sperm fuses with the egg) There are two main phases: Meiosis I and Meiosis II

Begins as a diploid cell (2n) and becomes a haploid(n) cell.) Meiosis is subdivided into 2 main phases: Meiosis I, Meiosis II

Phases of meiosis Meiosis I: Involves the separation of homologous chromosomes, One of each pair is given to two new daughter cells. REDUCTIVE DIVISION Meiosis II: Involves the separation of sister chromatids into 4 new daughter cells. (Each of the previous daughter cells separate into 2) Similar to mitosis.

Stage 1: Interphase The cell goes through the G1, S and G2 stages. (similarly to Mitosis) DNA replicates and is in the form of chromatin Centrosome replicates (2 centrioles)) The cell is diploid (46 chromosomes)

Meiosis i Stage 1: Prophase I Nuclear envelope disapears and chromatin condenses and becomes chromosomes Homologous chromosomes pair (tetrads)***** Synapsis occurs, a protein holds homologous chromosomes together Crossing Over, non sister chromatids exchange genetic information. ******* Centrioles move to opposite poles and spindle fibers form

Crossing over Nonsister chromatids Each gene of one homologue is aligned with the other homologue The non sister chromatids – one maternal and one paternal chromatid of a homologous pair – are broken at the same place then rejoined to each others DNA Genetic information is exchanged

Meiosis i Stage 2: Metaphase I Homologous chromosomes (tetrads) line up on the metaphase plate of the cell One chromosome of each pair is facing each pole Spindle fibers attach to each homologue at the centromere

Meiosis i Stage 3: Anaphase I Homologous chromosomes (tetrads) separate and go to opposite poles Cenromeres stay intact because the sister chromatids do not separate Chromosome number is now haploid (n) .

Meiosis i Stage 4: Telophase I Homologous chromosomes uncoil (forming chromatin) Spindle fibers disappear Nuclear membrane forms around each nuclei Cytokenesis occurs simultaneously Formation of haploid daughter cells (each with 23 chromosomes)

Meiosis II

Meiosis ii Stage 6: Prophase II Nuclear envelope disappears Spindle apparatus forms

Meiosis ii Stage 7: Metaphase II Chromosomes are positioned on metaphase(equatorial) plate Due to crossing over in meiosis I, the sister chromatids are no longer identical The spindle fibers are attached to the centromeres

Meiosis ii Stage 8: Anaphase II The centromere of each chromosome separates Sister chromatids come apart Individual chromosomes move to opposite poles of the cell

Meiosis ii Stage 9: Telophase II Chromosomes uncoil into chromatin Nuclei form (nuclear membrane reforms) Cytokenesis occurs simultaneously amd separates the cytoplasm and forms two new,variable haploid daughter cells