Outline for today’s lecture (Ch. 13) Sexual and asexual life cycles Meiosis Origins of Genetic Variation Independent assortment Crossing over (“recombination”)

Heredity Transmission of traits between generations Molecular basis of heredity is DNA replication Gene is a specific segment of DNA Physical location on the chromosome is called a genetic LOCUS (plural = “loci”) e.g., the “eye-color locus”, Adh locus

Asexual Life Cycles Single (diploid) individual is the parent Parent passes copies of ALL its genes to its offspring (reproduces “clonally”) Various mechanisms Mitotic cell division in unicellular Eukaryotes Vegetative reproduction, e.g., plant cuttings, hydra budding Parthenogenesis

Sexual Life Cycles Two (diploid) parents give rise to offspring Offspring differ genetically from their parents and their siblings GAMETES are haploid reproductive cells that transmit genes across generations

Sexual Life Cycles Key Point: Sexual reproduction → Genetic variation MOST eukaryotes reproduce sexually at least sometimes Most prokaryotes (e.g., bacteria) exchange genes at least occasionally

Sexual Life Cycles – Human Example 46 Chromosomes 22 Homologous pairs, called “autosomes” Same length Same centromere position Same sequence (+/-) SAME GENES!!

Sexual Life Cycles – Human Example One pair of “sex chromosomes” i.e., “sex-determining gene(s)” reside on these chromosomes Females are XX Males are XY Only small region of homology (= same genes) between X, Y X Y

Schematic drawing of a chromosome

Diploid cell, n=3 BEFORE DNA replication 3 Homologous Pairs 2 autosomes 1 sex chromosome (XX) One homologous chromosome from each parent DNA content = 2C Ploidy = 2n X 2 1

Diploid cell, n=3, AFTER DNA replication 3 Homologous Pairs One homologous chromosome from each parent = TWO SISTER CHROMATIDS DNA content = 4C Ploidy = 2n X 1 2

Sexual Life Cycles - animals Free-living stage is diploid Gametes formed by meiosis Haploid gametes merge genomes to form diploid zygote (“syngamy”)

Sexual Life Cycles - Plants Diploid sporophyte forms haploid spores by meiosis Spores form gametophyte by mitosis Gametophyte forms gametes by mitosis Gametes merge to form diploid zygote

Sexual Life Cycles - Fungi Free-living, multicellular organism is haploid Gametes formed by mitosis Gametes merge to form diploid zygote Zygote undergoes meiosis to form haploid cells

Meiosis RECALL: Function of MITOSIS is to faithfully replicate the parental genome in each daughter cell with no change in information content Function of MEIOSIS is to produce haploid cells from diploid cells Necessary for the formation of gametes Necessary for sexual reproduction

Meiosis – an overview Interphase 1 – Begin with two homologous chromosomes, DNA content = 2C Ploidy = 2n (diploid)

Meiosis – an overview Interphase 1 – Chromosomes replicate DNA content = 4C Ploidy = 2n

Meiosis – an overview “Meiosis I” Homologous chromosomes separate Cell Division #1 Result is TWO haploid (ploidy = n) cells with TWO SISTER CHROMATIDS of one of the two homologs

Meiosis – an overview “Meiosis II” Sister chromatids separate Cell Division # 2 Result is FOUR haploid daughter cells, each with an unreplicated chromosome (= 1C) Half as many chromosomes as the parent cell

Meiosis I – early Prophase I Tetrad Chiasmata Homologous chromosomes pair Synaptonemal complex (proteins) attaches homologs “synapsis” Homologs form tetrad

Meiosis I – late Prophase I Chiasmata Spindle fiber Chromosomes cross over, form “chiasmata” Exchange of DNA between homologs occurs at chiasma Spindles form and attach to kinetochores as in mitosis Tetrad

Meiosis I – Metaphase I Chromosomes lined up on metaphase plate in homologous pairs Spindles from one pole attach to one chromosome of each pair Spindles from the other pole attach to the other chromosome of the pair Kinetochore

Meiosis I – Anaphase I Homologous chromosomes separate and move along spindle fibers toward pole Sister chromatids remain attached at centromeres Note that recombination has occurred!

Meiosis I – Telophase and cytokinesis Homologous chromosomes reach (opposite) poles Each pole has complete haploid complement of chromosomes Each chromosome consists of two sister chromatids

Meiosis II – Prophase II Spindle forms Chromosomes move toward metaphase plate

Meiosis II – Metaphase II Chromosomes reach metaphase plate, as in mitosis Kinetochores of sister chromatids attach to spindle fibers from opposite poles

Meiosis II – Anaphase II Centromeres of sister chromatids separate Sister chromatids move toward opposite poles

Meiosis II – Telophase and cytokinesis Mechanism as before Note that now FOUR HAPLOID DAUGHTER CELLS formed from each parent cell Note that some chromosomes are recombinant, some are not

Meiosis I - Summary Chiasma (site of crossing-over) Tetrad formed by synapsis of homologs

Meiosis I - Summary Tetrads align at metaphase plate

Meiosis I - Summary Homologous chromosomes separate Sister chromatids remain paired

Meiosis II - Summary Sister chromatids separate Haploid daughter cells result

Origins of Genetic Variation Independent Assortment of Chromosomes Recombination among chromosomes Crossing over Recombination within chromosomes Random fertilization

Independent Assortment of Chromosomes

Independent Assortment of Chromosomes Number of possible combinations of chromosomes within a gamete Two homologs A, B: Mom = A1B1, Dad = A2B2 Four combinations: A1B1, A1B2, A2B1, A2B2 Three homologs: Mom = A1B1C1, Dad = A2B2C2 Eight combinations: A1B1C1, A1B1C2, A1B2C1, A1B2C2, A2B1C1, A2B2C1, A2B1C2, A2B2C2 n homologs: 2n combinations

Crossing-over – Recombination within chromosomes Averages ≥ 2 per chromosome per meiosis in humans, flies If no crossing-over, genes on same chromosomes would always be inherited together

Crossing-over – Recombination within chromosomes Human genome has ~20K genes. Suppose each gene assorts independently. How many combinations?

Review: Mitosis vs. Meiosis Event Mitosis Meiosis DNA Replication Interphase Interphase I # Cell Divisions 1 2 # Daughter cells 2 4 “Ploidy” of daughters 2n (diploid) n (haploid) Synapsis of homologs? No Yes Crossing-over No Yes (recombination) Biological Purpose Duplicate cells Generate faithfully gametes

Meiosis, Genetic variation, and Evolution Role of segregation Role of crossing-over What about LIMITS to evolution? E.g., body size

For Thursday: Introduction to Mendelian Genetics Read Chapter 14 through p. 260