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Meiosis and Sexual Reproduction
Chapter 10
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Impacts, Issues Video Why Sex
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Why Sex In nature, the main function of sex is to perpetuate one’s genes Perpetuation of life includes reproduction and development
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Why Sex Asexual reproduction is easier and faster
Single parent produces offspring All offspring are genetically identical to one another and to parent Sexual reproduction can be an alternative adaption in changing environments chromosomes are duplicated in germ cells Involves Meiosis Gamete production Fertilization Produces genetic variation among offspring
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Cost of Sexual Reproduction
Specialized cells and structures must be formed Special courtship, and parental behaviors can be costly Nurturing developing offspring, either in egg or body, requires resources from mother
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Homologous Chromosomes Carry Different Alleles
Cell has two of each chromosome One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles
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Homologous Chromosomes
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Sexual Reproduction Shuffles Alleles
Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits This variation in traits is the basis for evolutionary change
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Gametes are sex cells (sperm, eggs) Arise from germ cells
Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells ovaries anther testes ovary
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Gamete Formation Reproductive organs
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Chromosome Number Sum total of chromosomes in a cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number
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Human Karyotype 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY)
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Meiosis: Two Divisions
Two consecutive nuclear divisions Meiosis I Meiosis II DNA is not duplicated between divisions Four haploid nuclei form
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Meiosis I Each homologue in the cell pairs with its partner,
then the partners separate
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Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated)
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Prophase I Each duplicated chromosome pairs with homologue Homologues swap segments Each chromosome becomes attached to spindle
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Metaphase I Random alignment of homologous chromosomes Chromosomes are pushed and pulled into the middle of cell The spindle is fully formed
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Anaphase I Homologous chromosomes segregate The sister chromatids remain attached
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Telophase I The chromosomes arrive at opposite poles Usually followed by cytoplasmic division
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Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes
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Metaphase II Duplicated chromosomes line up at the spindle equator, midway between the poles
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Anaphase II Sister chromatids separate to become independent chromosomes
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Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes Four haploid cells
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Anaphase II Telophase II Prophase II Metaphase II Meiosis II
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Mitosis & Meiosis Compared
Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces 2 diploid cells Produces clones Meiosis Function Sexual reproduction Occurs in germ cells Produces 4 haploid cells Produces variable offspring
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Prophase vs. Prophase I Prophase (Mitosis)
Homologous pairs do not interact with each other Prophase I (Meiosis) Homologous pairs become zippered together and crossing over occurs
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Anaphase, Anaphase I, and Anaphase II
Anaphase I (Meiosis) Homologous chromosomes separate from each other Anaphase/Anaphase II (Mitosis/Meiosis) Sister chromatids of a chromosome separate from each other
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each chromosome duplicated during interphase
germ cell germ cell each chromosome duplicated during interphase n MEIOSIS I separation of homologues MEIOSIS II separation of sister chromatids gametes gametes 2n diploid number restored at fertilization zygote
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Factors Contributing to Variation among Offspring
Crossing over during prophase I Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization
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Crossing Over Crossing over
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Crossing Over Each chromosome becomes zippered to its homologue
All four chromatids are closely aligned Nonsister chromosomes exchange segments Crossing-Over
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Effect of Crossing Over
After crossing over, each chromosome contains both maternal and paternal segments Creates new allele combinations in offspring
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CROSSOVER FREQUENCY Proportional to the distance that separates genes
Crossing over will disrupt linkage between A and B more often than C and D A B C D 33
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Animal Life Cycle Random alignment
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Random Alignment During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes Initial contacts between microtubules and chromosomes are random Either the maternal or paternal member of a homologous pair can end up at either pole The chromosomes in a gamete are a mix of chromosomes from the two parents
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combinations possible
1 2 3 combinations possible Alignment at metaphase I or or or
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Possible Chromosome Combinations
As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2n (n is number of chromosome types)
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Fertilization Male and female gametes unite and nuclei fuse
Fusion of two haploid nuclei produces diploid nucleus in the zygote Which two gametes unite is random Adds to variation among offspring 38
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Animal Egg Formation Egg formation 39
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Oocytes Arrested in Meiosis I
Girl is born with primary oocytes already in ovaries Each oocyte has entered meiosis I and stopped Meiosis resumes, one oocyte at a time, with the first menstrual cycle 40
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Animal Sperm Formation
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Spermatogenesis Spermatogonium (2n) divides by mitosis to form primary spermatocyte (2n) Meiosis produces haploid spermatids Spermatids mature to become sperm 42
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Fertilization Fertilization 43
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Fertilization Sperm penetrates to egg cytoplasm Secondary oocyte undergoes meiosis II; forms mature egg Egg nucleus and sperm nucleus fuse to form diploid zygote
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Life Cycle of a Leopard Frog
Leopard frog life cycle
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midsectional views top view side view
Gamete Formation Fertilization Cleavage Gastrulation midsectional views top view side view Organ Formation Growth, Tissue Specialization
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Experimental Evidence of Localized Differences
Cytoplasmic localization 47
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Cleavage begins within 24 hours of fertilization blastula forms Cell secretions produce a fluid-filled cavity in center of ball of cells called the blastocoel blastoceol blastula
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Gastrulation - Day 15 Primitive streak forms along one axis of the inner cell mass Cells migrate inward here to produce a 3-layered embryo Endoderm digestive tract respiratory tract Mesoderm muscle, bone, circulatory Ectoderm skin, nervous tissue,
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Stages of Human Development
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