Chapter 13:Meiosis and Sexual Life Cycles

Heredity Heredity: the transmission of traits from one generation to the next Genes- hereditary units Transmitted by sex cells

Chromosomes: Where genes are located (locus). Every species has a characteristic number of chromosomes Humans = 46 chromosomes (23 pairs)

Asexual vs. Sexual Reproduction Asexual Reproduction: clones (mitosis ) Example: budding(hydra) Sexual Reproduction: MeiosisVariation Life cycle- begins with fertilization of the egg by the sperm

Sets of Chromosomes in Human Cells: Somatic cells: body cells or any cell that is not a gamete Karyotype Homologous chromosomes One exception?? Autosomes??

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Representing Chromosomes: n = number of chromosomes 2n = diploid ( two sets of chromosomes) n = haploid (one set of chromosomes) Human somatic cells = diploid = 23 pairs (46 total) Human sex cells(gametes) = haploid = 23 chromosomes 22 autosomes and a sex chromosome

Meiosis: Cell division that reduces the number of sets of chromosomes Formation of gametes What would happen if gametes are formed by mitosis?

Chromosome Behavior: Human life cycle = Fertilization: haploid sperm + haploid egg Diploid zygote- 46 chromosomes is restored

Alteration of life cycles: Alternation of generations: All sexually reproducing organisms will alternate between haploid cells and diploid cells.

Alteration of life cycles: Most fungi and some protists : Haploid gametes form diploid zygote Diploid zygote divides by meiosis, then by mitosis to produce a haploid multicellular organism Haploid cells divide by mitosis to produce gametes

Alteration of life cycles: Plants and Some Algae: sporophyte produces a gametophyte as its offspring, which then produces the next sporophyte generation

Alteration of life cycles: Animals: Diploid organism produces haploid gametes by meiosis Fertilization produces a diploid zygote Divides by mitosis to form a multicellular organism

Meiosis: Reduces number of chromosome sets from diploid to haploid Two consecutive cell divisions Meiosis I: separates homologous chromosomes Meiosis II: separates sister chromatids similar to mitosis Four daughter cells distinct from each other and the parent cell

Meiosis

Meiosis I: Prophase I: pairing of homologous chromosomes, synapsis, chiasmata, tetrads Metaphase I: tetrads line up on metaphase plate Anaphase I: chromosomes migrate to respective poles Telophase I: haploid chromosome, sister chromatids intact Cytokinesis: cleavage furrow(cell plate)

Meiosis II: Prophase II: spindle apparatus, chromosomes start to migrate Metaphase II: chromosomes line up on metaphase plate Anaphase II: sister chromatids migrate towards their respective poles Telophase II: de-condensing of chromosomes, nuclear envelop re- assembles Cytokinesis: cleavage furrow

Meiosis vs. mitosis

Genetic variation: Prokaryotes vs. Eukaryotes: Prokaryotes: Transformation- uptake of DNA Transduction- viral transmission of genetic information Transposition- movement of DNA segments between and within DNA molecules Eukaryotes: Crossing over Independent assortment of alleles Fertilization

Origins of Genetic Variation Genetic variation c0ntributes to evolution: Mutations are responsible for change Reshuffling of genes

Origins of Genetic Variation Independent assortment: homologous pairs of chromosomes are randomly arranged (metaphase I)

Origins of Genetic Variation 50% chance offspring will get their mothers chromosomes 50% chance offspring will get their fathers chromosomes Chromosomes sort independently during meiosis I

Origins of Genetic Variation Each daughter cell represents one possible outcome Combinations possible: n = haploid number of organisms 2 n = the number of possible combinations when chromosomes sort independently

Example: Example: what are the total number of combinations when n = 3 Example: what are the total number of combinations when n = 23

Origins of Genetic Variation Crossing over (prophase I): the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes)

Origins of Genetic Variation Random fertilization: 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!

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Chapter 12 reveiw Pg. 220, #’s: 1 and 2 Pg. 228, #’s: 1, 4 and 5 Pg. 233, #’s: 1, 3, 5 and 6