Chapter 13- Meiosis and Sexual Life Cycles-Modern Concepts of Bioscience PDF

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Meiosis and Sexual Life Cycles...

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Modern Concepts of Bioscience- Bio 1305 Chapter 13- Meiosis and Sexual Life Cycles Meiosis produces cells with half the chromosomes of the parent cell. It occurs only in specialized cells. Fertilization- unites a sperm and egg, re-establishing pair of homologous chromosomes, with both parental and maternal genes Humans have 46 chromosomes, only 1 pair of homologous chromosomes Meiosis I: homologs are separated Meiosis II: sister chromatids are separated Variation is demonstrated by the differences in appearance that offspring show from parents and siblings  Offspring acquire genes from parents by inheriting chromosomes  Genes are the units of heredity and are made up of segments of DNA  Genes are passed to the next generation via reproductive cells called gametes  Most DNA is packaged into chromosomes  Humans have 46 chromosomes in the nuclei of their somatic cells Fertilization and meiosis alternate in sexual life cycles Human chromosomes:  23 pairs  Karyotype  Homologs  Sex Chromosomes  Autosomes  Diploid cell (2n) DNA Replication → Each chromosome→ Two sister chromatids Technique: Pair of homologous chromosomes, Centromeres (one per sister chromatid) Sister chromatids, Metaphase chromosome Homologous Chromosomes  Sister chromatids of one duplicated chromosome  Centromere  Two nonsister chromatids in a homologous pair  Pair of homologous chromosomes Behaviour of Chromosomes Sets in the Human Life Cycle Meiosis reduces the number of chromosome sets from diploid to haploid  Meiosis takes place in two chromosomes Testcross Homozygous dominant or heterozygous  most important observation is that there was segregation in meiosis I, the splitting stage in the homologous alleles

Two characters transmitted to offspring as a package or independently? Mix because they are both independent 465 purple axial flowers 152 purple terminal flower 140 white axial flowers 53 white terminal flower (square to figure out) PpAa x PpAa Mendel’s laws of segregation and independent assortment reflect the rules of probability Probability of YYRR= ¼ (probability of YY) x ¼ (RR)= 1/16 Probability of YyRR= 1/2 (probability of Yy) x ¼ (RR)= ⅛ The addition rule states that the probability that any one of two or more ppyyRr ppYyrr Ppyyrr PPyyrr Ppyyrr Chance of at least two recessive traits=

Homozygous and Heterozygous (1)

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Heterozygous with a homozygous recessive (½)

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Monohybrid cross homozygous crossed with a heterozygous (¼)

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ppyRr ¼ (probability of pp) ¼ x ½ (yy) x ½ (Rr)= 1/16 ppYyrr= 1/16 Ppyyrr= 2/16 PPyyrr= 1/16 ppyyrr= 1/16

Chance of at least 2 recessive traits= 6/16 or ⅜ Meiosis 1)  Separates Homologous chromosomes Prophase I  In early prophase I, each chromosome pairs with its homolog and crossing over occurs  X-shaped regions called chiasmata are sites of crossovers Metaphase 1  Duplicate homologous pair Anaphase I  Pairs of homologous species Telophase I & Cytokinesis  Pairs Meiosis II is very similar to mitosis Prophase II  Nuclear membrane is broken down and structures or polymerizing Metaphase II  Sister chromatids are arranged at the metaphase plate  The kinesis Anaphase II  Split chromatids Telophase II & Cytokinesis  Chromosomes arrive at opposite poles  Nucleus form and the chromosomes begin decondensing  Cytokinesis separates the cytoplasm  End of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes  Each daughter cell is genetically distinct from the others and from the parent cells

Crossing over and synapsis during Prophase I  Interphase, the sister chromatids are held together by proteins called cohesins  Nonsister chromatids are broken at precisely corresponding positions  During synapsis, DNA breaks are repaired, joining DNA from one non sister chromatid to the corresponding segment of another non sister chromatid

A Comparison of Mitosis and Meiosis  Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell  Meiosis reduces the number of chromosomes sets from two diploid to one haploid, producing cells that differ genetically from each other and from the parent cell Genetic variation produced in sexual life cycles contributes to evolution  Mutations are the original source of genetic diversity  Mutations create different versions of genes called alleles  Reshuffling of alleles during sexual reproduction produces Origins of Genetic Variation Among Offspring  The behaviour of chromosomes during meiosis are fertilization is responsible for most of the cariatio that arises in each generation  Three mechanisms contribute to ensure variation: o Independent assortment of chromosomes o Crossing over o Random fertilization Independent Assortment of Chromosomes  2 equally probable arrangements of chromosomes at metaphase I...