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Transcript
Chapter 2 – Chromosomes and Sexual Reproduction Basic Cell Types - Prokaryotic • “before nucleus” • Unicellular • Simple structure – No internal membranes • Eubacteria – “true bacteria” • Archaea – “ancient bacteria” – More closely related to eukaryotes Basic Cell Types - Eukaryotic • “true nucleus” • Unicellular or mutlicellular • Large, complex – Have internal membranes Genetic Material • Prokaryotes – Single, circular chromosome • May have small, accessory plasmids – Eubacteria • DNA within cytoplasm (nucleoid region) – Archaea • DNA may have associated protein (histones), but different from eukaryotic chromosomes • Eukaryotes – Multiple, linear chromosomes – DNA with associated histone proteins • Chromatin • Chromatin arranged in specific complex to form chromosomes – Allows for packaging within a small nucleus Genetic Material - Viruses • Protein coat surrounding nucleic acid (DNA or RNA) • Not classified as living organism – Dependent on host cell for reproduction – Evolved after cells – Closely related to host • Similar genes Cell Reproduction - Prokaryotic • Binary fission • Origin of replication – Initiation site of DNA replication • 2 DNA molecules move to opposite ends of cells – Proteins bind near replication origins and anchor new DNA molecules to plasma membrane Cell Reproduction - Prokaryotic • New cell wall forms to produce 2 cells • Identical to each other, and parent cell • Asexual reproduction Cell Reproduction - Eukaryotic • Chromosomes – Each species has a characteristic number – Diploid cell/organism • Has 2 copies of each chromosome • Homologous chromosomes – Save same genes at same locus – May have different alleles – Haploid cell/organism • One set/one copy of each chromosome Chromosome Structure • 3 components of a functional chromosome – Centromere • Attachment point for microtubules • Kinetochore – protein complex attached – Telomere • Ends of linear chromosomes • stabilize – Origins of replication • 2 identical copies of DNA = sister chromatids – Held together by common centromere Chromosome Classification • Classified by location of centromere • “p” arm and “q” arm • Humans do not have telocentric chromosomes Cell Cycle • Interphase – Extended period of growth • Mitotic phase – Mitosis – Cytokinesis • Key areas are regulated at checkpoints Interphase • G1 • S • G2 Mitosis • Nuclear Division • 5 stages – Prophase – Prometaphase Mitosis • 5 stages con’t – Metaphase – Anaphase Mitosis • 5 stages cont – Telophase Movement of Microtubules Cytokinesis • Division of cytoplasm – Animal cells – cleavage furrow – Plant cells – cell plate • Usually occurs simultaneously with telophase • End result of mitosis/cytokinesis is 2 identical cells – Asexual reproduction Sexual Reproduction and Genetic Variation • Meiosis – Creates gametes/sex cells/egg and sperm – Chromosome number is reduced by half • Diploid to haploid • Fertilization – Fusion of egg and sperm to restore diploid condition • Forms diploid, single cell - zygote – Sexual reproduction • 1 or 2 parents Meiosis • One DNA replication followed by two rounds of division • Meiosis I – Reduces number of chromosomes • Diploid to haploid • Meiosis II – Separates sister chromatids Meiosis I • Prophase Sub-Stages of Prophase I Meiosis I • Metaphase • Anaphase Meiosis I • Telophase • Interkinesis – Nuclear envelopes are reformed and spindle breaks down • Some cells skip to metaphase II – Cell may or may not split into two cells Meiosis II • Prophase • Metaphase Meiosis II • Anaphase • Telophase Consequences of Meiosis • End result is 4 haploid cells from one diploid cell – Each is genetically different • Crossing over – Exchange between homologous chromosomes – Sister chromatids are not identical • Random assortment – Shuffles maternal and paternal chromosomes in different combinations – Metaphase I Crossing Over and Random Assortment Separation during Mitosis • Cohesin holds sister chromatids together – Established during S – Broken down during anaphase by separase • Separase is inactive during Interphase and early mitosis Separation during Meiosis • Cohesin aids in formation of synaptonemal complex • Anaphase I – Cohesin broken down by separase – Centromeric cohesin is protected by shugoshin • Keeps sister chromatids together • Metaphase II – Separase breaks down shugoshin • Allows separation of sister chromatids Meiosis in Animals Alternation of Generations in Plants