Cell Cycle Flashcards
cell division is used for (3)
growth, repair, and reproduction
2 types of cell division
mitosis and meiosis
mitosis
- produces 2 genetically identical daughter cells (clones)
- preserves the diploid (2n) chromosome number
meiosis
- occurs in sexually reproducing organisms, results in cells that are haploid
- half the chromosome number of the parent cell
structure of a chromosome
highly coiled/condensed strand of DNA
replicated chromosome
consists of 2 sister chromatids (exact copies of each other)
centromere
specialized region that holds 2 chromatids together
kinetochore
disc-shaped protein on centromere that attaches chromatid to mitotic spindle during division
cell cycle
complex sequence of growth and division of cells
What is the frequency of cell division?
varies with cell type, but tightly regulated by complex mechanism involving kinases and allosteric interaction
- bone marrow cells always dividing in order to produce constant supply of blood cells
- liver cells arrested in G0 (stopped dividing) but can be induced to divide/regenerate when liver tissue is damaged
- human intestine cells normally divide 2 times a day to renew damaged tissue during digestion
- nerve cells do not divide at all
Frequency of cell division?
varies with cell type
2 factors that limits cell size and promote cell division
ratio of volume of cell to surface area + capacity of nucleus to control entire cell
How does ratio of cell volume to surface area affect cell division?
when cell grows: surface area of cell membrane increases as the square of the radius and volume increases as the cube of the radius
= volume increases at faster rate than cell membrane
ratio is major factor of cell division
How does the capacity of the nucleus affect cell division?
nucleus must be able to provide enough info and substances to meet cell’s needs
- metabolically active cells generally small
- larger cells have evolved to exist with multiple nuclei
5 major phases of the cell cycle
interphase (G1, S, G2), mitosis, cytokinesis
interphase
more than 90% of cell’s life, chromatin is threadlike and not condensed
1 nucleus, 1 centrosome or MTOC
G1 (Gap 1) phase
during interphase
period of intense growth and biochemical activity
S (synthesis) phase
during interphase
synthesis or replication of DNA
centrosome is duplicated
G2 (Gap 2) phase
cell continues to grow and complete preparation for cell division
centrosomes will seperate and move to opposite poles
mitosis
actual dividing of the nucleus
prophase, metaphase, anaphase, telophase
prophase (4)
1st phase of mitosis
- nuclear membrane begins to disintegrate
- chromosome strands condense (X)
- nucleolus disappears
- mitotic spindle begins to form, from each centrosome
metaphase (3)
2nd phase of mitosis
- chromosomes line up in single file on equator (metaphase plate)
- centrosomes positioned at opposite poles
- spindle fibers run from centrosomes to kinetochores in centromeres
anaphase
3rd phase of mitosis
- centromeres of each chromosome separate, spindle fibers pull apart sister chromosomes
telophase
4th phase of mitosis
- chromosomes cluster at opposite ends, and nuclear membrane reforms
- supercoiled chromosomes unravel and return to normal threadlike strands
- mitosis complete after 2 nucleoli form
cytokinesis
- dividing of cytoplasm
- begins during anaphase
- animal cells: cleavage furrow forms down middle as actin and myosin microfilaments pinch in cytoplasm
- plant cell: cell plate forms during telophase as vesicles from Golgi merge down middle, daughter plant cells do not seperate but new wall forms and middle lamella joins adjacent cells together
cancerous cells
no contact inhibition/anchorage dependence, divides uncontrollably
why they can migrate or metastasize to other regions
contact inhibition
or density-dependent inhibition
normal cells stop dividing and enter G0 as they get too crowded
anchorage dependence
normal cells must be anchored to surface to divide
eg petri dish (in vitro) or extracellular membrane (in vivo)
meiosis
- generates genetic diversity for natural selection + evolution
- produces gametes (ova/sperm), which have haploid/monoploid chromosome number (n), half the genetic material of parent cell
- nucleus divides twice
- every gamete is different
- sexual reprodution involves fusion of 2 haploid gametes and restores diploid chromosome number to offspring
meiosis I
- reduction division
- homologous chromosomes seperate
- each chromosome first pairs up precisely with homologue into synaptonemal copmlex by synapsis, forming tetrad/bivalent structure
- pairing process causes crossing-over
crossing-over
process by which nonsister chromatids exchange genetic material
highly organized mechanism to ensure greater variation among gametes
meiosis II
sister chromatids seperate into different cells
it is like mitosis
Prophase I
1st stage of meiosis I
- synapsis (pairing of homologues)
- crossing-over (exchange of homologous bits of chromosomes
- chiasmata (visible manifestation of crossover events)
- sets stage for separation of DNA
Metaphase I
2nd stage of meiosis I
- homologous pairs of chromosomes are lined up double file along metaphase plate
- spindle fibers from poles attached to centromeres of each pair of homologues
Anaphase I
3rd stage of meiosis I
separation of homologous chromosomes as they are pulled by spindle fibers and migrate to opposite poles
Telophase I
4th stage of meiosis I
homologous pairs continue to separate until they reach poles of cell
each pole has haploid number of chromosomes
Cytokinesis I
stage of meiosis I
same time as telophase I
Meiosis II
functionally the same as mitosis with same phases
chromosome number remains haploid and daughter cells same as parent cell
Meiosis + Genetic Variation (3)
3 types of genetic variation from meiosis and fertilization
1. independent assortment of chromosomes
2. crossing-over
3. random fertiliation of ovum by sperm
Independent Assortment of Chromosomes
- homologous pairs of chromosomes separate depending on random way they line up on metaphase palte
- each pair can line up in 2 ways
- gamete has 50% chance of receiving maternal chromosome, 50% chance of paternal chromosome
23 pairs of chromosomes in humans = 2^23 (8 mil) combinations
Crossover
produces recombinant chromosomes that combine genes inherited from both parents
human: average of 2 or 3 crossover events occur in each chromosome pair, recombinant chromosomes then line up on metaphase palte in random fashion (more possibilities of gametes)
random fertilization
1 human ovum/sperm = 8 mil chromosome combinations
8mil^2 recombinations
cell cycle control system
regulates rate of cell division with regular checkpoints
restriction point (R)
important checkpoint in the G1 phase
cell division most likely completed if it receives go-ahead
How is the timing of the cell cycle controlled?
initiated by growth factors and controlled by 2 molecules (cyclins and protein kinases)
cyclins
proteins which levels cyclically rise and fall in dividing cells (synthesized during S + G2 phase, broken down after M phase)
kinases
- ubiquitous class of proteins that activate other proteins by phosphorylating them
- kinases in cell cycle activated when they bind to cyclin (cyclin-dependent kinases/Cdks)
- Cdk binds to cyclin = cyclin-Cdk complex
MPF
- type of cyclin-Cdk complex
- triggers cell’s passage from G2 to M
- maturation promoting factor
- contributes to chromosome condensation and spindle formation
- during anaphase (after M phase), switches off by breaking down cyclin, Cdk persists in cell in inactive form until it becomes part of MPF again
