Cell Cycle & Mitosis Meiosis

Context All living things (cells) come from other living things (cells) Cell division is necessary for: Reproduction Growth and development Tissue renewal Cell cycle – describes the life cycle of a cell

Somatic Cells vs. Gametes Somatic cells – body cells Contain entire genome within nucleus (or two copies of every chromosome) Gametes – sex cells (i.e. sperm or egg) Contain only half (or one copy (from mom or dad) of every chromosome)

Phases of the Cell Cycle M (mitotic) phase: When mitosis (cell division) occurs Interphase: G1 (Gap 1) Cell grows S (Synthesis aka DNA replication) Cell replicates DNA G2 (Gap 2) Cell grows and prepares for cell division

Chromatin  DNA + proteins (histones) in eukaryotic cells Chromosome  structures consisting of chromatin Sister chromatid  one half of a replicated chromosome

Centromere  point of connection between sister chromatids Kinetochore  protein complex found at centromere

Centrosome  organelle that organizes microtubules Centriole  animals cells only (function unknown)

Phases of Mitosis (literal cell division) 1st - G2 of interphase Nuclear envelope forms Centrosomes (& centrioles in animal cells) appear 2nd - Prophase Chromatin condenses into chromosomes Mitotic spindle appears 3rd - Prometaphase Nuclear envelope breaks up (fragments) Microtubules attach to centromeres at kinetochore

4th – Metaphase 5th – Anaphase Chromosomes meet @ middle (metaphase plate) Spindle fibers attached to each chromatid at kinetochore 5th – Anaphase Two sister chromatids pull apart at centromere and move towards opposite end of cell (towards centrosomes)

6th – Telophase and Cytokinesis Two daughter nuclei form from fragments of original nucleus Chromatin becomes less condensed Cytokinesis – division of cytoplasm and formation of two daughter cells Animal cells  involves cleavage furrow Plant cells  involves cell plate (formed by vesicles!)

Reminder: Binary Fission (prokaryotic cell division)

Cell Cycle Control Frequency of cell division varies with cell type RBCs every 24hr Mature nerve cells never divide Cell cycle checkpoints:

Cyclin-dependent kinases (Cdk) Kinase – an enzyme that activate or inactivate other proteins by phosphorylation Cyclin – protein who’s concentration fluctuates cyclically

Cancer Cancer cells derive from normal cells gone wrong Ex: Mutation in gene that regulates cell cycle checkpoint; now cell does not stop at that checkpoint but just keeps dividing. Cancer cells also Have no contact inhibition Aren’t anchorage dependent (thus metastasize) Express vascularization proteins (bring in blood vessels to feed tumor) And so much more.

Meiosis Process whereby gametes (sex cells) are produced for sexual reproduction purposes Eggs in female; sperm in male Sexual reproduction  egg meets sperm = fertilization Offspring have genetic variation Asexual reproduction  mitotic cell division in single-celled eukaryotes (e.g. amoeba) Clones (parent and offspring are identical)

Diploid vs. Haploid In humans, each somatic cell has 46 chromosomes (23 from mom & 23 from dad) or 23 homologous chromosomes (homologues) Somatic cells are diploid (full set of chromosomes) or 2n Gametes are haploid (half set) or n

Steps of Meiosis 2 rounds: Meiosis I (4n to 2n) Meiosis II (2n to n)

Meiosis I Interphase: Diploid cell’s chromosomes duplicate during interphase (2n  4n) Centrosomes replicate

Prophase I Chromosomes condense Homologous pairs match up and become physically connected at synaptonemal complex via process called synapsis CROSSING OVER  Genetic exchange of information between non-sister chromatids

Telophase I & Cytokinesis Metaphase I Homologous pairs line up on metaphase plate in tetrads Anaphase I Homologous pairs split up BUT sister chromatids stay together! Telophase I & Cytokinesis 2 daughter cells, both 2n, which go on to divide again

Meiosis II Prophase II Metaphase II Anaphase II Chromosomes meet at metaphase plate; sister chromatids are NOT identical (due to crossing over) Anaphase II Centromeres of each chromosome separate (sister chromatids pull apart) Telophase II and cytokinesis 4 haploid non-identical daughter cells are produced

End Result of Meiosis 2n  4n  2n  n Production of four haploid daughter cells all of which are genetically distinct from each other and the parent cell

Genetic variation Mutations in DNA Crossing over during prophase I of meiosis followed by (hopefully) random sexual reproduction Independent assortment of homologous pairs during metaphase I

Independent Assortment