10.1 Meiosis Learning Targets: Describe chromosomes in the phases of meiosis. Outline chiasmata in crossing over. Explain how meiosis results in genetic variety.

What are some advantages and disadvantages of both asexual and sexual reproduction? Asexual – only requires one parent (advantage), but offspring are clones (disadvantage for adaptation) Asexual – only requires one parent (advantage), but offspring are clones (disadvantage for adaptation) Sexual – requires two parents (disadvantage), produces unique combinations and genetic diversity (advantage for adaptation) Sexual – requires two parents (disadvantage), produces unique combinations and genetic diversity (advantage for adaptation)

Define: Diploid – a cell with two sets of chromosomes – symbolized as 2n In humans, n=23, so diploid cells have two sets of 23 chromosomes (2n=46) In humans, n=23, so diploid cells have two sets of 23 chromosomes (2n=46) Examples: somatic (body) cells, a zygote (fertilized egg) Examples: somatic (body) cells, a zygote (fertilized egg) Haploid – a cell with a single chromosome set – symbolized as n In humans, haploid cells have one set of 23 chromosomes In humans, haploid cells have one set of 23 chromosomes Examples: gametes (egg and sperm cells formed by meiosis) Examples: gametes (egg and sperm cells formed by meiosis)

Homologous chromosomes – chromosomes carrying genes for the same traits Examples: your diploid cells contain chromosome 1 from your mother, and chromosome 1 from your father Examples: your diploid cells contain chromosome 1 from your mother, and chromosome 1 from your father Gene – a segment of DNA in which the sequence carries the code for making an enzyme or protein (in most cases) Each chromosome carries a version of a gene for the same trait (ex. blue vs. brown eyes) Sister chromatids are identical; Homologous chromosomes are not

Label:

Label: Karyotype Homologous chromosomes Sister chromatid Chromosome Sex chromosomes Autosomes 1-22

Review: In what phase of the cell cycle is DNA synthesized to create identical sister chromatids? Interphase (S phase of interphase) BEFORE mitosis or meiosis Interphase (S phase of interphase) BEFORE mitosis or meiosis

Why is it important that meiosis reduces a cell’s chromosome number to produce haploid gametes? Otherwise a fertilized egg would have double the normal chromosomal material – it would be 4n, the next generation would be 8n, etc. Otherwise a fertilized egg would have double the normal chromosomal material – it would be 4n, the next generation would be 8n, etc.

Meiosis I: Prophase I: Chromosomes condense; homologous chromosomes form tetrads; crossing over occurs at chiasmata Prophase I: Chromosomes condense; homologous chromosomes form tetrads; crossing over occurs at chiasmata Metaphase I: Tetrads line up at midline Metaphase I: Tetrads line up at midline Anaphase I: Homologous pairs separate Anaphase I: Homologous pairs separate Telophase I and cytokinesis: 2 haploid cells form (chromosomes still consist of 2 non-identical sister chromatids) Telophase I and cytokinesis: 2 haploid cells form (chromosomes still consist of 2 non-identical sister chromatids)

Meiosis II: similar to mitosis Prophase II, Metaphase II: chromosomes line up at midline Prophase II, Metaphase II: chromosomes line up at midline Anaphase II: sister chromatids separate Anaphase II: sister chromatids separate Telophase II and cytokinesis: 4 haploid cells form Telophase II and cytokinesis: 4 haploid cells form

MitosisAll Cell DivisionMeiosis  One division  Forms 2 diploid daughter cells  Daughter cells are identical to parent cell  Functions in growth, repair  Chromosomes replicate once in interphase before division  Prophase, metaphase, anaphase, telophase  Two divisions  Forms 4 haploid cells  Crossing over occurs in prophase I  Cells are genetically unique  Produces gametes

Sexual reproduction reshuffles genes to increase variation (but mutation is the original source of all variation) Sources of genetic variation: 1. Independent assortment of homologous chromosomes in Meiosis I, and non- identical sister chromatids in Meiosis II 2 n possibilities, so 2 23 options for each human gamete 2 n possibilities, so 2 23 options for each human gamete

In example below, n=2, so 2 2 = 4 different possible gametes In example below, n=2, so 2 2 = 4 different possible gametes

2. Crossing over – genetic exchange creates recombinant chromosomes with genes from each parent Crossing over Crossing over Tetrad Recombinant gametes Chiasmata

3. Random fertilization – creates unique combinations of parental genes 2 23 x 2 23 options for each human zygote! 2 23 x 2 23 options for each human zygote!