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Biology Chapter 10 Powerpoint

Biology Chapter 10 Powerpoint - Chapter 10 Meiosis&...

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Unformatted text preview: Chapter 10 Meiosis & Sexual R 2 Meiosis & Sexual Reprod Outline 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 Meiosis: Halves the Chromosome Number 3 Meiosis & Sexual Reprod Special type of cell division Used only for sexual reproduction Halves the chromosome number prior to Halves fertilization fertilization Parents diploid Meiosis produces haploid gametes Gametes fuse in fertilization to form diploid Gametes zygote zygote Becomes the next diploid generation Homologous Pairs of Chromosomes 4 Meiosis & Sexual Reprod 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 Chromosomes homologous homologous They have the same length Their centromeres are positioned in the same place One came from the father (the paternal homolog) the One other from the mother (the maternal homolog) other When stained, they show similar banding patterns Because they have genes controlling the same traits Because at the same positions at Homologous Chromosomes 5 Homologous Pairs of Chromosomes 6 Meiosis & Sexual Reprod Homologous chromosomes have genes controlling Homologous the same trait at the same position the 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 Many population population Homologous copies of a gene may encode identical Homologous or differing genetic information or The variants that exist for a gene are called alleles An individual may have: Identical alleles for a specific gene on both homologs Identical (homozygous for the trait), or (homozygous A maternal allele that differs from the corresponding maternal paternal allele (heterozygous for the trait) paternal Overview of Meiosis 7 Phases of Meiosis I: Prophase I & Metaphase I 8 Meiosis & Sexual Reprod Meiosis I (reductional division): Prophase I ­ Each chromosome internally duplicated Each (consists of two identical sister chromatids) (consists ­ Homologous chromosomes pair up – synapsis ­ Physically align themselves against each other Physically end to end end ­ End view would show four chromatids – Tetrad Metaphase I ­ Homologous pairs arranged onto the Homologous metaphase plate metaphase Phases of Meiosis I: Anaphase I & Telophase I Meiosis I (cont.): 9 Meiosis & Sexual Reprod Anaphase I ­ Synapsis breaks up ­ Homologous chromosomes separate from one Homologous another another ­ Homologues move towards opposite poles ­ Each is still an internally duplicate chromosome Each with two chromatids with Telophase I ­ Daughter cells have one internally duplicate Daughter chromosome from each homologous pair chromosome ­ One (internally duplicate) chromosome of each One type (1n, haploid) type Phases of Meiosis I: Cytokinesis I & Interkinesis 10 Meiosis & Sexual Reprod Meiosis I (cont.): Cytokinesis I ­ Two daughter cells ­ Both with one internally duplicate chromosome Both of each type of ­ Haploid ­ Meiosis I is reductional (halves chromosome Meiosis number) number) Interkinesis Similar to mitotic interphase Usually shorter No replication of DNA Genetic Variation: Crossing Over 11 Meiosis & Sexual Reprod Meiosis brings about genetic variation in two key Meiosis ways: ways: Crossing-over between homologous chromosomes, Crossing-over and and Independent assortment of homologous chromosomes 1. Crossing Over: Exchange of genetic material between nonsister Exchange chromatids during meiosis I chromatids At synapsis, a nucleoprotein lattice (called the At synaptonemal complex) appears between homologues synaptonemal ­ Holds homologues together ­ Aligns DNA of nonsister chromatids Aligns ­ Allows crossing-over to occur Then homologues separate and are distributed to Then different daughter cells different Crossing Over 12 Genetic Variation: Independent Assortment 13 Meiosis & Sexual Reprod 2. Independent assortment: When homologues align at the When metaphase plate: metaphase ­ They separate in a random manner ­ The maternal or paternal homologue may The be oriented toward either pole of mother cell cell Causes random mixing of blocks of Causes alleles into gametes alleles Independent Assortment 14 Recombinatio n 15 Genetic Variation: Fertilization 16 Meiosis & Sexual Reprod When gametes fuse at fertilization: Chromosomes donated by the parents are Chromosomes combined combined In humans, (223)2 = 70,368,744,000,000 chromosomally different zygotes are possible chromosomally If crossing-over occurs only once (423)2, or 4,951,760,200,000,000,000,000,000,000 or genetically different zygotes are possible genetically Genetic Variation: Significance 17 Meiosis & Sexual Reprod Asexual reproduction produces genetically Asexual identical clones identical Sexual reproduction cause novel genetic Sexual recombinations recombinations Asexual reproduction is advantageous when Asexual environment is stable environment However, if environment changes, genetic However, variability introduced by sexual reproduction may be advantageous may Phases of Meiosis II: Similar to Mitosis 18 Meiosis & Sexual Reprod Metaphase II Overview ­ Unremarkable ­ Virtually indistinguishable from mitosis of two haploid Virtually cells cells Prophase II – Chromosomes condense Metaphase II – chromosomes align at metaphase Metaphase plate plate Anaphase II ­ Centromere dissolves ­ Sister chromatids separate and become daughter Sister chromosomes chromosomes Telophase II and cytokinesis II ­ Four haploid cells ­ All genetically unique Meiosis I & II in Plant Cells 19 20 Meiosis & Sexual Reprod Meiosis versus Mitosis Meiosis Requires two nuclear Requires two divisions divisions Chromosomes synapse and Chromosomes synapse cross over cross Centromeres survive Centromeres survive Anaphase I Anaphase Halves chromosome number number Produces four daughter Produces four nuclei nuclei Produces daughter cells Produces genetically different from different parent and each other parent Used only for sexual Used sexual reproduction reproduction Mitosis Requires one nuclear Requires one division division Chromosomes do not Chromosomes do synapse nor cross over synapse cross Centromeres dissolve in Centromeres dissolve mitotic anaphase mitotic Preserves chromosome number number Produces two daughter Produces two nuclei nuclei Produces daughter cells Produces genetically identical to identical parent and to each other parent Used for asexual Used asexual reproduction and growth growth Meiosis Compared to Mitosis 21 Meiosis I Compared to Mitosis 22 Meiosis II Compared to Mitosis 23 Life Cycle Basics: Plants 24 Meiosis & Sexual Reprod Haploid multicellular “individuals” alternate with Haploid diploid multicellular “individuals” The haploid individual: The 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 In haploid individuals haploid Life Cycle Basics: Animals In familiar animals: 25 Meiosis & Sexual Reprod “Individuals” are diploid; produce haploid Individuals” gametes 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 Production Spermatogenesis All four cells become sperm ­ Production of eggs Production Oogenesis Only one of four nuclei get cytoplasm Becomes the egg or ovum Others wither away as polar bodies 26 Meiosis & Sexual Reprod 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 The generation generation Undergoes mitosis Results in multicellular embryo that gradually takes Results on features determined when zygote was formed on 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 Has formed formed The Human Life Cycle 27 Gametogenesis in Mammals 28 29 Meiosis & Sexual Reprod 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 Ending Slide Chapter 10 Meiosis & Sexual R ...
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