BIOL 152 1st Edition Lecture 6 Outline of Last Lecture Origins and Molecular Basis of Genetics Outline of Current Lecture The Cell Cycle Mitosis two daughter cells Meiosis four genetically different cells going from diploid to haploid Cellular organization of genetic material Genome Chromosomes o 46 in human somatic cells 23 pair o 23 in gametes o Chromatin DNA protein o Sister chromatids o Centromeres These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Cell division occurs within a cycle of cell activity Interphase o o o G1 S G2 The Process of Mitosis The stages of mitosis Interphase Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis The Mitotic Spindle Forms between 2 centrosomes Microtubules stretch outward from centrosome o Hollow tubes of protein Kinetochore protein in centromere where microtubules attach The Mitotic Division of an Animal Cell G2 of Interphase Centrosomes replicate Nuclear envelope still intact Chromosomes duplicated S phase but not condensed Prophase Chromatin condenses forms sister chromatids Mitotic spindle forms Centrosomes move to opposite ends of cell Kinetochores develop Prometaphase Nuclear envelope fragments Microtubules extend towards kinetochores Non attached microtubules interact with opposite microtubules Metaphase Chromosomes converge on metaphase plate Each chromatid pair has kinetochore attached to microtubules cell cycle check point Enzyme unravels DNA strands at centromeres Anaphase Sister chromatids separate Chromosomes move via motor protiens o 2 hypotheses for how motor proteins move 1 Pacman type movement where protein gnaws on microtubules to 2 Reeling in when proteins are pulled in by microtubules Cell elongates as non attached microtubules lengthen Telophase Nuclear envelope reforms Chromosomes become less condensed Cytokinesis In animals cleavage furrow pinches cell o Actin and myosin filaments contract In plants cell plate is laid down and cell wall forms Meiosis and Sexual Life Cycles Sexual versus Asexual Reproduction Sexual requires two mates of different sex asexual is an organism able to reproduce by itself fission or cell replication In human somatic cells there are 46 chromosomes 23 homologous pairs Autosomes and Sex chromosomes o XX for females and XY for males Karyotype process of staining chromosomes to view the number and genetic factors of the autosome and sex chromosomes Overview of Meiosis Haploid versus Diploid Prophase I Nuclear envelope breaks down centrosomes move to poles spindle microtubules form and chromosomes begin to condense Homologous pair chromosomes pair up Synapsis forms where pairs of chromosomes attach themselves firmly Crossing over occurs site of occurrence is known as chiasmata Metaphase I Homologous pairs line up on metaphase plate one chromosome from each pair facing each pole Kinetochore microtubules attached to each homologous pair Anaphase I Homologous chromosomes composed of two sister chromatids move to opposite poles Telophase I and Cytokinesis Each half of cell is haploid but each chromosome still composed of two sister chromatids Cytokinesis division of the cytoplasm occurs simultaneously In some species the chromosomes de condense and the nuclear envelope reforms No replication occurs between Meiosis I and II Prophase II Spindle apparatus forms Metaphase II Because of crossover the two sister chromatids are not genetically identical Kinetochores from sister chromatids are attached to microtubules and they line up on the metaphase plate Anaphase II Centromeres of each chromosome come apart and sister chromatids separate Telophase II and Cytokinesis Nuclei form Chromosomes decondense All four haploid daughter cells are distinct from each other and their parent cells Comparison of Meiosis and Mitosis Chromosome number Genetic variation Three Mechanisms for Genetic Variation 1 Independent Assortment random orientation of homologous pairs on the metaphase plate 2 23 possibilities 2 Crossing over DNA molecules of two non sister chromatids are broken at the same place and rejoined to each other s DNA 3 Random Fertilization Heritable variation is what makes evolution possible