Cellular Reproduction Cell Division cellular material is divided between new daughter cells Nuclear division: Mitosis or Meiosis Cytoplasmic division: Cytokinesis

Mitosis and Meiosis in the Human Life Cycle 2n = diploid = 2 sets of chromosomes n = haploid = 1 set of chromosomes

Cell Cycle with Mitosis P M Mitosis P = prophase M = metaphase A = anaphase T = telophase A G2 T assembly of components for division cytokinesis Interphase G1 = growth phase 1 S = synthesis phase G2 = growth phase 2 S G1 cytoplasm doubles chromosomes replicate Interphase

Applying Your Knowledge S G2 Mitosis Cytokinesis When does nuclear division occur? When are the chromosomes duplicated? When do the cytoplasmic contents double?

Interphase nucleus nucleolus Chromatin is diffuse, individual chromosomes cannot be distinguished.

Prophase Chromosomes condense. Spindle forms. Nucleolus disappears. Nuclear envelope breaks down.

centromere chromatid chromatid Sister Chromatids are held together at their centromeres. Each chromosome appears as a pair of chromatids attached at the centromere.

Metaphase Chromatid pairs align at the cell equator.

Anaphase Sister chromatids separate and are drawn to opposite poles by spindle fibers.

Telophase Chromosomes have reached poles of cell. Spindle disperses. Nuclear envelope surrounds chromosomes. Chromosomes become diffuse, nucleolus reforms.

Telophase + Cytokinesis New Daughter Cells These phases can overlap in time.

Cytokinesis in animal cells Cytokinesis in plant cells

Applying Your Knowledge Prophase Metaphase Anaphase Telophase When do chromosomes line up at the cell equator? When are sister chromatids first visible? When do sister chromatids separate from each other?

Outcome of Meiotic Divisions Human Cell with 46 chromosomes (Diploid) Duplication of Cell Contents Two Chromosomal Divisions (Meiosis I and Meiosis II) 4 Genetically-Different Human Cells , Each with 23 chromosomes (Haploid)

Homologous Chromosome Pairs Diploid cells have pairs of chromosomes Each homologous chromosome pair is genetically matched so that the genes are arranged in the same order Figure: 11.UN06b Title: Formation of daughter chromosomes Caption:

Cell Cycle With Meiosis M1 = meiosis I M2 = meiosis II M1 G2 M2 assembly of components for division cytokinesis S G1 Interphase G1 = growth phase 1 S = synthesis phase G2 = growth phase 2 cytoplasm doubles chromosomes replicate Interphase

Applying Your Knowledge G1 phase S phase G2 phase Meiosis I and II When does chromosome duplication occur? When do the cytoplasmic contents double? When does chromosomal division occur?

Meiotic Divisions Meiosis I: reduction division Separation of homologous chromosomes into two different nuclei Chromosome number reduced to haploid Meiosis II: equational division Duplicated chromosomes separate No change in chromosome number

Phases of Meiosis I Prophase I Metaphase I Anaphase I Telophase I --Chromosomes Shorten and Thicken (condense) --Homologous Chromosomes Pair with Each Other and Exchange Genetic Information Metaphase I Chromosome Pairs Line up at Cell Equator Anaphase I Homologous Chromosomes Separate Telophase I Separated Homologues Reach Opposite Poles

Centromeres Chromatid (duplicate) Non-sister chromatids Chromosomes condense. Synapsis (pairing) of homologs occurs. Crossing-over occurs at chiasma (contact points).

Crossing Over Homologous Pairs Exchange Genetic Information Figure: 11.15 Title: The mechanism of crossing over Caption: 1) Homologous chromosomes pair up side by side. 2) One end of each chromosome binds to the nuclear envelope. Protein strands “zip” homologous chromosomes together. 3) Homologous chromosomes are fully joined by protein strands. 4) Recombination enzymes bind to the chromosomes. Recombination enzymes snip chromatids apart and reattach the chromatids. Chiasmata are formed when one end of a chromatid of a paternal chromosome (yellow) is attached to the other end of a chromatid of a maternal chromosome (violet). 5) The protein strands and recombination enzymes leave as the chromosomes condense. The chiasmata remain as locations where homologous chromosomes are twisted around each other, helping to hold homologues together. Chromosomes with combinations of maternal and paternal traits

Chromosome pairs align at the center of the spindle. Homologues move to opposite poles of the cell. I

Cytokinesis along the central plane yields two haploid cells with duplicated chromosomes. Prior to the start of Meiosis II, there may be a pause called interkinesis but the chromosomes do not duplicate a second time. Homologues reach opposite poles.

Phases of Meiosis II Prophase II Metaphase II Anaphase II Telophase II --Chromosomes Shorten and Thicken (condense) --No Chromosome Pairing Occurs Metaphase II Chromosomes Line up at Cell Equator Anaphase II Sister Chromatids Separate Telophase II Separated Chromosomes Reach Opposite Poles

Chromosomes condense. Chromosomes align at the cell equators on the new spindles. Sister chromatids separate at centromeres and move to opposite poles.

Daughter chromosomes reach opposite poles. After cytokinesis, four genetically-different haploid cells are formed.

Sperm formation Equal divisions of cytoplasm Four functional products per meiosis Egg formation Unequal divisions of cytoplasm Small polar bodies formed One functional product per meiosis

Applying Your Knowledge Prophase I Metaphase I Anaphase I Metaphase II Anaphase II When do sister chromatids separate? When does crossing over occur? When do homologous pairs line up at the cell equator?

Comparing Mitosis and Meiosis Cell type at start Cell type at end Are products identical? (Y/N) Number of cells produced Number of chromosome duplications Pairing of homologues (Y/N) Crossing-over (Y/N) Number of chromosome divisions

Comparing Mitosis and Meiosis Cell type at start Diploid Cell type at end Haploid Are products identical? (Y/N) Yes No Number of cells produced Two Four Number of chromosome duplications prior to One Pairing of homologues (Y/N) Crossing-over (Y/N) Number of chromosome divisions