Download Cell Cycle

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
– 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 singlecelled 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 nonsister 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