Meiosis and Sexual Reproduction Chapter 6 part 2

Mini-Quiz (not graded)  Name the following steps in the cell cycle: a d b e c

Impacts, Issues: Why Sex?

Introducing Alleles  Asexual reproduction produces genetically identical copies of a parent (clones)  Sexual reproduction introduces variation in the combinations of traits among offspring  Meiosis Process that enables organisms to make special reproductive cells (gametes)

Halving the Diploid Number  A diploid cell has 2 nonidentical copies of every chromosome pairs are homologous chromosomes  In germ cells meiosis changes diploid haploid, producing gametes Eggs and sperm have 23 unpaired chromosomes

Somatic cell Diploid # Homologous chromosomes

 23 unpaired chromosomes  Haploid # Gamete

Two Divisions, Not One MEIOSIS  DNA is replicated once and divided twice (meiosis I and meiosis II), making 4 haploid nuclei  In meiosis I, each duplicated homologous chromosome is separated from its partner  In meiosis II, sister chromatids are separated

Tour of Meiosis I

Tour of Meiosis II

How Meiosis Introduces Variation in Traits

Crossing Over in Prophase I  Crossing over The process by which a chromosome and its homologous partner exchange heritable information in corresponding segments

Fig. 10-6b, p. 160 Crossing Over in Prophase I

Fig. 10-6c, p. 160 Crossing Over in Prophase I

Fig. 10-6d, p. 160 Crossing Over in Prophase I

Random Assortment Random assortment produces (8,388,608) possible combinations of homologous chromosomes!!!

Sperm Formation in Animals

Egg Formation in Animals

Mitosis Fig b, p. 165 one diploid nucleus two diploid nuclei ProphaseMetaphaseAnaphaseTelophase Chromosomes condense. Chromosomes align midway between spindle poles. Sister chromatids separate as they are pulled toward spindle poles. Chromosome clusters arrive at spindle poles. Bipolar spindle forms; it attaches chromosomes to spindle poles. New nuclear envelopes form. Nuclear envelope breaks up. Chromosomes decondense.

Meiosis I Fig c, p. 164 one diploid nucleus two haploid nuclei Prophase I Chromosomes condense. Homologous chromosomes pair. Metaphase I Anaphase I Telophase I Crossovers occur. Chromosomes align midway between spindle poles. Homologous chromosomes separate as they are pulled toward spindle poles. Chromosome clusters arrive at spindle poles. Bipolar spindle forms; it attaches chromosomes to spindle poles. New nuclear envelopes form. Nuclear envelope breaks up. Chromosomes decondense.

Fig d, p. 165 two haploid nucleifour haploid nuclei Prophase II Metaphase IIAnaphase IITelophase II Chromosomes condense. Chromosomes align midway between spindle poles. Sister chromatids separate as they are pulled toward spindle poles. Chromosome clusters arrive at spindle poles. Bipolar spindle forms; it attaches chromosomes to spindle poles. New nuclear envelopes form. Nuclear envelope breaks up. Chromosomes decondense. Meiosis II

Table of Differences Kind of cellKind of Reproduction ProphaseMetaphaseAnaphaseTelophase Mitosis Meiosis