Ch 11: Cell cycle and cell division Flashcards
Steps of DNA condensation
o 1) wrapping of DNA about 2 rimes around a protein complex called a NUCLEOSOME
• made up protein histones
H2A, H2B, H3, H4
• Each nucleosome has to copies
o 2) nucleosomes coil around each other to form the “30nM fiber” which requires a third histone H1
o 3) 30nM fiber is condensed further into chromatin
greatest amount of condensation occurs in
prophase
after DNA synthesis, two identical chromosomes are called
sister chromatids that are held together by cohesion protein
o Centromere forms where the two legs cross
humans have
46 chromosomes
how long does the eukaryotic cell cycle take
from 30 min to 24 hrs
G1 phase
- Takes 11 hours
- Cells prepare for DNA synthesis
- Cellular metabolism occurs to increase cell mass
S phase
- Synthesis
- Takes 8 hours
- DNA synthesis occurs
G2 phase
- 4 hrs
* cell prepares for the final M phase and cell division
M phase
- Mitosis
* 1 hr
G1 checkpoint
• Cell decides whether its DNA is fit to be replicated
• RB protein
Has a negative effect
Prevents cell from enetering S phase
Must be inactivated for cell to proceed into s phase
what happens if cell can’t continue in cell cycle
o Due to uv radiation
o RB protein stays active
o Cell cycle stays blocked in G1 until DNA damage induced by UV light has been removed
o Aka DNA repair
Cell can go thru a checkpoint only when what becomes active
when a cdk becomes active
o CDK adds a phosphate to a large molecule (amino acid of the surface of a protein)
o Cdk is an inactive kindase
• Only becomes active when it interacts with another protein called a cyclin
• THAT’S WHY IT’S CALLED A CYCLIN DEPENDENT KINASE
o When the cell can go thru the duplication, a cyclin interacts with a cdk and together they signal the cell to proceed thru the G1 checkpoint
• Begins when the active cdk/cyclin complex puts a phosphate onto the RB protein→inactivating RB and allowing DNA to move into S phase to duplicate DNA
Mitosis
• Cell divides
• M phase has two parts
o Mitosis: nuclear division
o Cytokinesis: actual cell division
Interphase
G1, G2, S
Prophase
- Chromosomes become maximally condensed
* Shaped like an X with centromere at crossing point
prometaphase
• Tubulin protein forms into microtubules which stretch across the nucleus to form 46 spindle fibers
Looks like an arc
• All 46 spindle fiber arcs meet on each side of the cell at a structure called a pole
• 46 chromatids (chromosome pairs) use their centromeres to attach to the spindle fibers
• nuclear membrane disappears
metaphase
• 46 chromatids align at the midcell
anaphase
- Cohesion protein is degraded
- Sister chromatids separate and each sister chromosome moves to a pole
- 46 sister chromosomes are clustered around two poles on either side of the cell
telophase
- Around each cluster of chromosomes, the nuclear membrane reforms
- Spindle fibers disappear
- Cell divides (cytokinesis) via a pinching process, which involves mainly actin and myosin
cytokinesis
• After nuclear division is complete, the cell divides, which requires actin/myosin to pinch off the middle of the cell turning it into two cells
Somatic vs gamete
• Majority of cells in body are somatic
o Divide by mitosis
o 46 chromosomes and 23 pairs (1 from mom and 1 from dad
o 2n chromosomes
o diploid
• minority of cells in the sex organs are called gametes
o sperm and egg cells
o 1n: one copy of each of the 23 chromosomes
o haploid cells
• arise from special diploid cells in the sex organs via meiosis
meiosis vs mitosis
• Mitosis requires 1 round of DNA replication and one cell division
o One cell becomes two cells
• Meiosis requires 1 round of DNA replication followed by two cell divisions aka M1 and M2
o 1 cell becomes four cells
meiosis 1
- Only 23 spindle fibers form and each pair of sister chromatids attaches side by side on a single spindle fiber
- During anaphase of meiosis 1, cohesion is NOT DEGRADED and 1m sister chromatids move together as a unit toward one pole and 1f sister chromatids move together as a unit to the other pole
- After meiosis I, there has been 1 round of DNA replication and cell division, so both new cells have 2n chromosomes
meiosis ii
- Second cell division
- No DNA replication
- Only 23 spindle fibers form and each pair of sister chromatids attaches to one spindle fiber
- During anaphase, cohesion IS DEGRADED and the two sister chromatids of each pair separate and move toward opposite poles
- When cell division occurs, each cell is now haploid and contains only a 1n complement of chromosomes, meaning that each cell has only copy of chromosome 1, one copy of chromosome 2, etc
meiosis generates genetic diversity in two ways
- No cellular mechanism distinguishes between the mother’s ves the father’s verion of a chromosome in terms of movement toward the poles during meiosis I
- During prophase I a region on the 1m chromatid can be exchanged with the identical region on the 1f chromatid in a process called chiasma which occurs by recombination
the replacement of cells that die
• To maintain cell number stais, cell division must only occur when a cell dies and must be replaced
• Goes thru stem cell division
o Asymmetric
o One of the daughter cells remains a stem cell while the other cell replaces the cell that died
cell death
apoptosis
