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