why do cells divide
growth, repair, reproduction |
mitosis |
produces 2 genetically identical daughter cells (called clones) |
preserves diploid (2n) chromosome number |
passes a complete genome from parent to child |
genome |
whole of its hereditary information encoded in its DNA, includes both the genes and the non-coding sequences of the DNA |
meiosis |
in sexually reproducing organisms, results in haploid cells (have half the chromosome # of the parent)(n) |
structure of a replicated chromosome
replicated chromosome consists of two sister chromatis where one is an exact copy of the other.
centromere is a specialized region that holds the two chromatids together
kinetochore is a disc-shaped protein on the centromere that attaches the chromatid to the mitotic spindle during cell division
cell cycle basics
bone marrow cells |
always dividing to produce constant supply of red and white blood cells |
liver cells |
arrested in G0 (have stopped dividing) can be induced to divide about/regenerate when liver tissue is damaged |
human intestine cells |
divide ab twice a day to renew tissue destroyed during digestion |
specialized cell ex (nerve cells) |
do not divide at all |
process is regulated in any case by a complex mechanism involving kinases and allosteric interactions |
ratio of volume of cell to SA and capacity of nucleus to control the entire cell |
limit cell size and promote cell division |
ratio of cell volume to sa
as cell grows, sa increases as the square of the radius and volume increases as the cube of the radius |
volume inside cell grows at faster rate than cell membrane |
determines when cell divides |
capacity of nucleus
nucleus must be able to provide enough info to produce adequate quantities of all substances to meet the cells needs |
bc of this metabolically active cells are usually small |
can be large active cells like paramecium |
-has two nuclei that each control diff cell functions |
human skeletal muscle cells |
giant multinucleate cells |
fungus slime molds |
consist of one giant cell that has thousands of nuclei |
cell division and cancerous cells
contact inhibition//density dependent inhibition |
normal cells grow and divide until they become too crusaded then they stop and enter G0 |
anchorage dependence (ANIMALS) |
to divide, cell must be attached or anchored to some surface |
can be Petri dish (in vitro) or extracellular membrane (in vivo) |
cancer cells show none of these two things |
divide uncontrollably, and do not have to be anchored to any membrane |
^is why cancer cells can migrate or metastasize to other regions of body |
regulation and timing of the cell cycle
cell cycle control system |
regulates the rate at which cells divide |
check points act as stop signals that halt cell unless overridden by go signals |
checkpoints in G1, G2, and M |
G1 is most important, if receive go ahead, cell will most likely complete cycle |
if it doesn't, it will go to G0 and become a non dividing cell |
timing of cell cycle is initiated by growth factors and controlled by 2 molecules |
cyclins and protein kinases |
cyclins get name bc levels cyclicly rise and fall in dividing cells |
synthesized during every S and G2 phase |
broken down after M phase |
kinases are and ubiquitous class of proteins that activate other proteins by phosphorylating them |
only activated when bound to a cyclin |
named cyclin dependent kinases (cdk) |
when cdk binds to a cyclin, cyclin cdk compels is formed |
ex of this is mpf which triggers cells passage from G2 to mitosis |
maturation (mitosis) promoting factor |
contributes to molecular events required fro chromosome condensation and spindle formation during prophases |
after M phase, during anaphase, mpf switches off by initiating process that leads to the breakdown of cyclin |
cdk persists in cell in inactive form until becomes part of mpf again |
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interphase
G1 |
intense growth and biochemical activity |
s |
synthesis/replication of DNA |
G2 |
cell continues to grow and complete preparations for cell division |
more than 90% of cells life is in interphase |
in interphase, chromatin is threadlike, not condensed |
centrosome consisting of two centrioles may be seen in the cytoplasm of ANIMAL cell |
centrosome is duplicated during s phase |
G2 - M transition |
two centrosomes separate from one another and move to opposite poles |
plant cells lack centrosomes but have microtubule organizing centers (MTOCs) |
these have the same function |
mitosis
consists of the actual dividing of the nucleus
metaphase
equatorial plate=metaphase plate
centrosomes at opp poles of cell
spindle fibers run from centrosome to kinetochores in the centromeres
telophase
supercoiled chromosomes uncoil back to chromatin
telophase
supercoiled chromosomes uncoil back to chromatin
cytokinesis
dividing of the cytoplasm
begins during anaphase
animal cells: cleavage furrow froms down middle of cell as actin and myosin microfilaments pinch in the cytoplasm
plant cells: cell plate forms during telophase as vesicles from golgi coalesce down middle of cell, daughter plant cell DO NOT separate
new cell wall forms and sticky middle lamella cements adjacent cells together
cyclin vs cdk
activity of cdk rises and falls depedning on changes in concen of cyclin
peaks of mpf activity correspond to rise in cyclin concentration
cyclin leves rise during S and G2 phases and then fall abrubtly during the M phase
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meiosis
generates genetic diversity that is the raw material for natural selection and evolution |
produces gametes (ova and sperm) |
have haploid or monoploid chromosomes (n) |
half genetic material of parent cell |
nucleus divides twice |
each gamete differs genetically from every other gamete |
sexual reproduction involves fusion of two haploid gametes and restores diploid chromosome # to offspring |
meiosis I reduction division |
homologous chromosomes separate |
each chromosome pairs up w homologue in synaptonemal complex by process called synapsis |
forms structure called tetrad (set of 4) or bivalent (in pairs) |
by aligning/binding crossing over is likely |
^process by which non sister chromatids exchange genetic material |
results in recombination of genetic material |
ensures greater variation among gametes |
meiosis II like mitosis |
sister chromatids separate into diff cells |
prophase I
-synapsis, pairing of homologues occurs
crossing over, exchange of homologous bits of chromosomes
-chiasmata, visible manifestations of the crossover events are visible
-sets stage for separation (segregation of DNA)
metaphase 1
spindle fibers from poles of the cell are attached to the centromeres of each pair of homologues
telophase 1 / cytokinesis 1
in telophase: each pole has haploid # of chromosomes
cytokinesis occurs simultaneously w telophase 1
in some species interphase occurs bet meiosis 1 and 2, in other none
NO chromosome replication in bet meiosis 1 and 2
meiosis 2
same as mitosis
chromosome # remains haploid
meiosis and genetic variation
3 types of genetic variation occur from meiosis and fertilization |
independent assortment of chromosomes, crossing over, random fertilization of an ovum by a sperm |
independent assortment of chromosomes
homologous pairs separate depending on the random way they line up on the metaphase plate during metaphase 1 |
each pair of chromosomes can line up in two possible orientations |
50% chance receive maternal chrom |
50% chance receive paternal chrom |
possible # of combinations of chromosomes is 2^23 |
bc 23 pairs of chromosomes in humans |
crossover
produces recombinant chromosomes that combine genes inherited from both parents |
may be 2 to 3 crossover events in humans |
metaphase 2 recombinant chromosomes line up on metaphase plate in random fashion |
^increases possible types of gametes even more |
random fertilization
human ovum and sperm represent 8 million possible chromosome combinations respectively |
when one sperm fertilizes one ovum |
8 million x 8 million recombinations can occur |
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