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

Essential knowledge 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization. a. The cell cycle is a complex set of stages that is highly regulated with checkpoints, which determine the ultimate fate of the cell. b. Mitosis passes a complete genome from the parent cell to daughter cells. c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms.

Essential knowledge 3.C.2: Biological systems have multiple processes that increase genetic variation. a. The imperfect nature of DNA replication and repair increases variation. b. The horizontal acquisitions of genetic information primarily in prokaryotes via transformation (uptake of naked DNA), transduction (viral transmission of genetic information), conjugation (cell-to-cell transfer) and transposition (movement of DNA segments within and between DNA molecules) increase variation. c. Sexual reproduction in eukaryotes involving gamete formation, including crossing-over during meiosis and the random assortment of chromosomes during meiosis, and fertilization serve to increase variation. Reproduction processes that increase genetic variation are evolutionarily conserved and are shared by various organisms.

Essential knowledge 3.A.4: The inheritance pattern of many traits cannot be explained by simple Mendelian genetics. Many traits are the product of multiple genes and/or physiological processes. b. Some traits are determined by genes on sex chromosomes. c. Some traits result from nonnuclear inheritance.

Essential knowledge 3.B.2: A variety of intercellular and intracellular Signal transmissions mediate gene expression. Signal transmission within and between cells mediates gene expression. b. Signal transmission within and between cells mediates cell function.

Essential knowledge 3.A.1: DNA, and in some cases RNA, is the primary source of heritable information. Genetic information is transmitted from one generation to the next through DNA or RNA. b. DNA and RNA molecules have structural similarities and differences that define function. c. Genetic information flows from a sequence of nucleotides in a gene to a sequence of amino acids in a protein. d. Phenotypes are determined through protein activities. e. Genetic engineering techniques can manipulate the heritable information of DNA and, in special cases, RNA.

Meiosis: Halves the Chromosome Number Biology, 9th ed,Sylvia Mader Meiosis: Halves the Chromosome Number Chapter 10 Meiosis & Sexual Reproduction Special type of cell division 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

Homologous Pairs of Chromosomes Biology, 9th ed,Sylvia Mader Homologous Pairs of Chromosomes Chapter 10 Meiosis & Sexual Reproduction In diploid body cells chromosomes occur in pairs Humans have 23 different types of chromosomes 22 pairs of autosomes 1 pair of sex chromosomes Diploid cells have two of each type Chromosomes of the same type are said to be homologous They have the same length One came from the father (the paternal homolog) the other from the mother (the maternal homolog) When stained, they show similar banding patterns Because they have genes controlling the same traits at the same positions

Homologous Pairs of Chromosomes Biology, 9th ed,Sylvia Mader Homologous Pairs of Chromosomes Chapter 10 Meiosis & Sexual Reproduction Homologous chromosomes have genes controlling the same trait at the same position Each gene occurs in duplicate A maternal copy from the mother A paternal copy from the father Homologous copies of a gene may encode identical or differing genetic information 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)

Genetic Variation: Crossing Over Biology, 9th ed,Sylvia Mader Genetic Variation: Crossing Over Chapter 10 Meiosis & Sexual Reproduction 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 prophase of meiosis I At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues Holds homologues together forming a tetrad Aligns DNA of nonsister chromatids Chiasmata are points where crossing over occurs Allows crossing-over to occur

Crossing Over Crossing over Produces recombinant chromosomes that carry genes derived from two different parents

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

In independent assortment Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

Genetic Variation: Random Fertilization Biology, 9th ed,Sylvia Mader Genetic Variation: Random Fertilization Chapter 10 Meiosis & Sexual Reproduction 3. Random Fertilization When gametes fuse at fertilization: Chromosomes donated by the parents are combined (genetic recombination) in a random manner 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

4. Mutation produces genetic variation Crossing over, independent assortment and random fertilization produce only new combinations of existing alleles

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

Phases of Meiosis I: Prophase I & Metaphase I Biology, 9th ed,Sylvia Mader Phases of Meiosis I: Prophase I & Metaphase I Chapter 10 Meiosis & Sexual Reproduction Meiosis I (reductional division): Prophase I Each chromosome internally duplicated (consists of two identical sister chromatids) Homologous chromosomes pair up – synapsis and crossing over occur Physically align themselves against each other end to end Metaphase I Homologous pairs arranged onto the metaphase plate

Phases of Meiosis I: Anaphase I & Telophase I Biology, 9th ed,Sylvia Mader Phases of Meiosis I: Anaphase I & Telophase I Chapter 10 Meiosis & Sexual Reproduction 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 Telophase I Daughter cells have one internally duplicate chromosome from each homologous pair

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

Phases of Meiosis II: Similar to Mitosis Biology, 9th ed,Sylvia Mader Phases of Meiosis II: Similar to Mitosis Chapter 10 Meiosis & Sexual Reproduction Meiosis 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

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

Life Cycle Basics: Plants Biology, 9th ed,Sylvia Mader Life Cycle Basics: Plants Chapter 10 Meiosis & Sexual Reproduction Haploid multicellular “individuals” alternate with diploid multicellular “individuals” The haploid individual: Known as the gametophyte May be larger or smaller than the diploid individual The diploid individual: Known as the sporophyte May be larger or smaller than the haploid individual Mosses are haploid most of their life cycle 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 Life Cycle Basics: Animals Chapter 10 Meiosis & Sexual Reproduction In familiar animals: “Individuals” are diploid; produce haploid gametes Only haploid part of life cycle is the gametes The products of meiosis are always gametes Meiosis occurs only during gametogenesis Production of sperm Spermatogenesis Production of eggs Oogenesis Only one of four nuclei get cytoplasm Becomes the egg or ovum Others wither away as polar bodies

Oogenesis begins prior to birth At birth, primary oocyte cells are resting in prophase of meiosis I During ovulation one primary oocyte completes meiosis I to become a secondary oocyte

Males produce primary spermatocytes by mitosis throughout their reproductive years, females are born with all the primary oocytes they will ever have (100,000+), oocytes stopped in prophase I

Biology, 9th ed,Sylvia Mader The Human Life Cycle Chapter 10 Meiosis & Sexual Reproduction 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

Biology, 9th ed,Sylvia Mader Review 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