Biology, 9th ed,Sylvia Mader

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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 Essential Knowledge Chapter 10 Meiosis & Sexual Reproduction 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. c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms. 1. Meiosis ensures that each gamete receives one complete haploid (1n) set of chromosomes. 2. During meiosis, homologous chromosomes are paired, with one homologue originating from the maternal parent and the other from the paternal parent. Orientation of the chromosome pairs is random with respect to the cell poles.

Biology, 9th ed,Sylvia Mader Essential Knowledge Chapter 10 Meiosis & Sexual Reproduction 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. c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms. 3. Separation of the homologous chromosomes ensures that each gamete receives a haploid (1n) set of chromosomes composed of both maternal and paternal chromosomes. 4. During meiosis, homologous chromatids exchange genetic material via a process called “crossing over,” which increases genetic variation in the resultant gametes.

Biology, 9th ed,Sylvia Mader Essential Knowledge Chapter 10 Meiosis & Sexual Reproduction 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. c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms. 5. Fertilization involves the fusion of two gametes, increases genetic variation in populations by providing for new combinations of genetic information in the zygote, and restores the diploid number of chromosomes.

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 Becomes the next diploid generation

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 Diploid cells have two of each type Chromosomes of the same type are said to be homologous They have the same length Their centromeres are positioned in the same place 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 Chromosomes Biology, 9th ed,Sylvia Mader Homologous Chromosomes Chapter 10 Meiosis & Sexual Reproduction

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 Many genes exist in several variant forms in a large population 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)

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

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 Physically align themselves against each other end to end End view would show four chromatids – Tetrad 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 One (internally duplicate) chromosome of each type (1n, haploid)

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 Similar to mitotic interphase Usually shorter No replication of DNA

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 meiosis I At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues Holds homologues together Aligns DNA of nonsister chromatids Allows crossing-over to occur 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 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 Causes random mixing of blocks of alleles into gametes

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

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

Phases of Meiosis II: Similar to Mitosis Biology, 9th ed,Sylvia Mader Phases of Meiosis II: Similar to Mitosis Chapter 10 Meiosis & Sexual Reproduction 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 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 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 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 In plants, algae, & fungi, gametes produced by haploid individuals

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 All four cells become sperm Production of eggs Oogenesis Only one of four nuclei get cytoplasm Becomes the egg or ovum Others wither away as polar bodies

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 Has genetic makeup determined when zygote was formed

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

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