Biology, 9th ed,Sylvia Mader Chapter 10 Chapter 10 Meiosis & Sexual Reproduction Meiosis & Sexual Reproduction

Biology, 9th ed,Sylvia Mader Outline Chapter 10 Meiosis & Sexual Reproduction Reduction in Chromosome Number Meiosis Overview Homologous Pairs Genetic Variation Crossing-Over Independent Assortment Fertilization Phases of Meiosis Meiosis I Meiosis II Meiosis Compared to Mitosis Human Life Cycle

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Meiosis: Used only for sexual reproduction Halves the chromosome number prior to fertilization Parents diploid Meiosis produces haploid gametes Gametes fuse in fertilization to form diploid zygote Becomes the next diploid generation

Homologous Pairs of Chromosomes Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Homologous Pairs of Chromosomes In diploid body cells chromosomes occur in pairs (two of each type) Humans have 23 different types of chromosomes Chromosomes of the same type are said to be homologous They have the same length Their centromeres are in the same position One came from the father (the paternal homolog) the other from the mother (the maternal homolog)

Homologous Chromosomes Biology, 9th ed,Sylvia Mader Homologous Chromosomes Chapter 10 Meiosis & Sexual Reproduction

Homologous Pairs of Chromosomes Biology, 9th ed,Sylvia Mader Chapter 10 Homologous Pairs of Chromosomes Meiosis & Sexual Reproduction Homologous chromosomes have genes controlling the same trait at the same position Many genes exist in several variant forms in a large population The variants that exist for a gene are called alleles An individual may have: Identical alleles for a specific gene on both homologs (homozygous for the trait), or A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)

Biology, 9th ed,Sylvia Mader Overview of Meiosis Chapter 10 Meiosis & Sexual Reproduction

Phases of Meiosis I: Prophase I Biology, 9th ed,Sylvia Mader Chapter 10 Phases of Meiosis I: Prophase I Meiosis & Sexual Reproduction Meiosis I (reductional division): Prophase I Each chromosome internally duplicated (consists of two identical sister chromatids) Physically align themselves against each other end to end End view would show four chromatids – Tetrad

Phases of Meiosis I: Metaphase I Spindle is fully formed Homologous pairs arranged onto the metaphase plate Kinetochores are seen Protein complex right outside the centromere that attach to spindle fibers

Phases of Meiosis I: Anaphase I & Telophase I Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Phases of Meiosis I: Anaphase I & Telophase I Meiosis I (cont.): Anaphase I Synapsis breaks up Homologous chromosomes separate from one another Homologues move towards opposite poles Each is still an internally duplicate chromosome with two chromatids

Phases of Meiosis I: Telophase I Spindle disappears Daughter cells have one internally duplicate chromosome from each homologous pair One (internally duplicate) chromosome of each type (1n, haploid) May or may not form a nuclear envelope or undergo cytokinesis before meiosis II

Phases of Meiosis I: Cytokinesis I & Interkinesis Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Phases of Meiosis I: Cytokinesis I & Interkinesis Meiosis I (cont.): Cytokinesis I Two haploid daughter cells Both with one internally duplicate chromosome of each type Meiosis I is reductional (halves chromosome number) Interkinesis Similar to mitotic interphase, but shorter No replication of DNA

Genetic Variation: Crossing Over Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Genetic Variation: Crossing Over Meiosis brings about genetic variation in two key ways: Crossing-over between homologous chromosomes, and Independent assortment of homologous chromosomes 1. Crossing Over: Exchange of genetic material between nonsister chromatids during meiosis I At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues Holds homologues together and aligns DNA of nonsister chromatids (allows crossing-over) Then homologues separate and are distributed to different daughter cells

Biology, 9th ed,Sylvia Mader Chapter 10 Crossing Over Meiosis & Sexual Reproduction

Genetic Variation: Independent Assortment Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Genetic Variation: Independent Assortment 2. Independent assortment: When homologues align at the metaphase plate: They separate in a random manner The maternal or paternal homologue may be oriented toward either pole of mother cell Causes random mixing of blocks of alleles into gametes

Independent Assortment Biology, 9th ed,Sylvia Mader Independent Assortment Chapter 10 Meiosis & Sexual Reproduction

Biology, 9th ed,Sylvia Mader Chapter 10 Recombination Meiosis & Sexual Reproduction

Genetic Variation: Fertilization Biology, 9th ed,Sylvia Mader Chapter 10 Genetic Variation: Fertilization Meiosis & Sexual Reproduction When gametes fuse at fertilization: Chromosomes donated by the parents are combined In humans, (223)2 = 70,368,744,000,000 chromosomally different zygotes are possible If crossing-over occurs only once (423)2, or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible

Genetic Variation: Significance Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Genetic Variation: Significance Asexual reproduction produces genetically identical clones Asexual reproduction is advantageous when environment is stable Sexual reproduction causes novel genetic recombinations If environment changes, genetic variability introduced by sexual reproduction may be advantageous

Phases of Meiosis II: Similar to Mitosis Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Phases of Meiosis II: Similar to Mitosis Metaphase II Overview Unremarkable Virtually indistinguishable from mitosis of two haploid cells Prophase II – Chromosomes condense Metaphase II – chromosomes align at metaphase plate Anaphase II Centromere dissolves Sister chromatids separate and become daughter chromosomes Telophase II and cytokinesis II Four haploid cells All genetically unique

Meiosis I & II in Plant Cells Biology, 9th ed,Sylvia Mader Meiosis I & II in Plant Cells Chapter 10 Meiosis & Sexual Reproduction

Meiosis versus Mitosis Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis versus Mitosis Meiosis & Sexual Reproduction Meiosis Requires two nuclear divisions Chromosomes synapse and cross over Centromeres survive Anaphase I Halves chromosome number Produces four daughter nuclei Produces daughter cells genetically different from parent and each other Used only for sexual reproduction Mitosis Requires one nuclear division Chromosomes do not synapse nor cross over Centromeres dissolve in mitotic anaphase Preserves chromosome number Produces two daughter nuclei Produces daughter cells genetically identical to parent and to each other Used for asexual reproduction and growth

Meiosis Compared to Mitosis Biology, 9th ed,Sylvia Mader Meiosis Compared to Mitosis Chapter 10 Meiosis & Sexual Reproduction

Meiosis I Compared to Mitosis Biology, 9th ed,Sylvia Mader Meiosis I Compared to Mitosis Chapter 10 Meiosis & Sexual Reproduction

Meiosis II Compared to Mitosis Biology, 9th ed,Sylvia Mader Meiosis II Compared to Mitosis Chapter 10 Meiosis & Sexual Reproduction

Life Cycle Basics: Plants Biology, 9th ed,Sylvia Mader Chapter 10 Life Cycle Basics: Plants Meiosis & Sexual Reproduction Haploid multicellular “individuals” alternate with diploid multicellular “individuals” The haploid individual: Known as the gametophyte and may be larger or smaller than the diploid individual In plants, algae, & fungi, gametes produced by haploid individuals Mosses are haploid most of their life cycle The diploid individual: Known as the sporophyte and may be larger or smaller than the haploid individual Ferns & higher plants have mostly diploid life cycles In fungi and most algae, only the zygote is diploid

Life Cycle Basics: Animals Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Life Cycle Basics: Animals In familiar animals: “Individuals” are diploid; produce haploid gametes The products of meiosis are always gametes Meiosis occurs only during gametogenesis Production of sperm Spermatogenesis - all four cells become sperm Production of eggs Oogenesis - only one of four nuclei get cytoplasm and becomes the egg or ovum (Others wither away as polar bodies)

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction The Human Life Cycle Sperm and egg are produced by meiosis A sperm and egg fuse at fertilization Results in a zygote The one-celled stage of an individual of the next generation Undergoes mitosis Results in multicellular embryo that gradually takes on features determined when zygote was formed All growth occurs as mitotic division As a result of mitosis, each somatic cell in body Has same number of chromosomes as zygote Has genetic makeup determined when zygote was formed

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Review Reduction in Chromosome Number Meiosis Overview Homologous Pairs Genetic Variation Crossing-Over Independent Assortment Fertilization Phases of Meiosis Meiosis I Meiosis II Meiosis Compared to Mitosis Human Life Cycle