Post on 11-Jan-2016
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Cell Cycle & MitosisMeiosis
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– 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)
• 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
Diploid vs. Haploid
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
• 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– 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