Chapter 5: Cell Division Flashcards
cell division
nuclear division (karyokinesis) followed by cytokinesis
- diploid cells= two copies of every chromosome, forming pairs (homologous chromosome)
- Humans have 46 chromosomes, 23 homologous pair, a total of 92 chromatids (depends on stage)
microtubule organizing centers (MTOCs)
aka centrosomes
- pair of these lay outside of nucleus
- in animal cells, each MTOC contains pair of centrioles.
* ** plants do have MTOCs called centrosomes, but they aren’t composed of centrioles
Mitosis
prophase, metaphasem anaphase, telophase, cytokinesis
- occurs in somatic cells
- occurs in asexual reproduction such as in plants and singled celled organisms
prophase
- nucleus diassembles: nucleolus disappear, chromatin condenses into chromosomes, and nuclear envelope breaks down
- mitotic spindle is formed and microtubules (composed of tubulin) begin connection to kinetochores
metaphase
- chromosomes line up single file at center, each chromatid is complete with a centromere and a kinetochore, once separated, it is a chromosome ***to keep track of total=count the centromeres
- centrosomes at opposite ends of cell *** (note: once separated that’s the end of metaphase, so to be precise the chromosomes # doubles at anaphase.
- karyotyping performed here
anaphase
- microtubules shorten, each chromosome is pulled apart into two chromatids (once separated it is a chromosome; chromosome # doubles)
- pulls the chromosome to opposite poles (disjuction)
- at end of this phase, each pole has a complete set of chromosomes, same as original cell before replication
telophase
- nuclear division, nuclear envelope develops, chromosomes=> chromatin, nucleoli reappear
cytokinesis
- actually begins during the later stages of mitosis (most sources indicate it begins toward the end of anaphase)
- division of cytoplasm to from 2 cells
cleavage furrow
actin and myosin microfilaments shorten, pull plasma membrane into center (animal cells)
cell plate
- vesicles from the golgi bodies migrate and fuse to form cell plate, out growth and merge with plasma membrane separating the two new cells (plants).
- cells don’t actually separate from each other, middle lamella cements adjacent cells together
interphase
begins after mitosis and cytokinesis are complete, and consists of G1, S, G2
- cell cycle= M, G1, S, G2 phases
- G1=cell increases in size, and the G1 checkpoint ensures everything is ready for DNA synthesis
- S phase= second molecule of DNA replicated from the first, provides sister chromatids–DNA synthesis
- G2=rapid cell growth, preparation for of genetic material for cellular division
- more time spent in interphase than mitosis (>90%). Growth occurs in ALL 3 PHASES, not just the G’s
- There are checkpoints in these cycles to make sure things are going as planned
- —End of G1(restriction point** most important one)=cell growth assessed and favorable conditions checked. If fails, cell enter G0 (checks extracellular environment)
* ** nerve and muscle cells remain here, rarely divide after maturing!!
- —end of G2=checks for sufficient mitosis promoting factor (MPF) levels to proceed… check if DNA is duplicated properly
- —-M checkpoint (metaphase checkpoint) during mitosis that triggers start of G1??… make sure chromosomes are attached .. make sure mitotic spindle is grabbing the kinetochore properly
meiosis 1
meiosis 1 is reduction division
- homologous chromosomes pair at plate, migrate to opposite poles (no separation of sister chromatids)
- occurs in germ cells (egg, sperm, pollen)
- fertilization/syngamy=fusion of two haploid gametes=diploid zygote
- ends with 4 haploid daughter cells, with half the number of chromosomes (one chromosomes from each homologous pair … get either moms or dads)
prophase 1
- nucleus disassembles: nucleolus disappears and nuclear envelope breaks down, chromatin condenses, spindle develps. 2. MT’s begin attaching kinetochores
- crossing over meansn
genetic recombination
—synapsis= homologous chromosomes pair up. these pairs are referred to as tetrads (group of 4 chromatids) or bivalents
— chiasmata: region where crossing over occur of non-sister chromatids
—-synaptonemal complex: protein structure that temporarily forms between homolgous chromosomes: gives rise to the tetrad w/ chiasmata and crossing over
metaphase 1
- homologous pairs are spread across metaphase plate
- microtubules attached to kinetochores of one member of each homolgous pair
- microtubules from other site attach to 2nd member of pair
anaphase 1
- homologues within tetrads uncouple and pulled to opposite sides (disjuction)
telophase 1
- nuclear membrane develops
- each pole forms a new nucleus that has half number of chromosomes (from homolgous pair to each chromosome =chr 2 sister chromatids)
- chromosome reduction phase to haploid
- — interphase may occurs in between here
meiosis 2
chromosomes spread across metaphase plate and sister chromatids separate and migrate to opposite poles. It is similar to mitosis
prophase 2
nuclear envelope disappears and spindle develops etc, no chiasmata and no crossing over
metaphase 2
chromosomes align on plate like in mitosis but now with half number of chromosomes (no extra copy)
anaphase 2
each chromosome is pulled into 2 separate chromatids and migrate to opposite poles of cell
telophase 2
nuclear envelope reappears and cytokinesis occurs=> 4 haploid cells (each chromosome=1 chromatid)
zygote))
forms after fertilization
- singled celled and then divides by mitosis to produce a multicellular organism
- Note that one copy of each chromosome in the zygote originates from one parent, and the second copy from the other parent
- Thus, a pair of homolgous chromosomes in the diploid zygote represents both maternal and paternal heritage
alternation of generations: plant life cycle
- meiosis in sporangia produces spores (haploid)
- spores undergo mitosis to become multicellular (gametophyte) which is haploid (n) since spores are already haploid
- the gametes fuse and produce a diploid cell (zygote 2N) that grows by mitosis to become sporophyte. Cells in sporophyte (sporangia) undergoes meiosis to produce haploid spores which germinate and repeat life cycle
* ** alternation of generations refers to the diploid and haploid stages
3 components of genetic variation
genetic recombination during meiosis and sexual reproduction originates from three events:
- crossing over during prophase 1 of non-sister of homologous chromosomes
- independent assortment of homologues during metaphase 1 (which chromosomes goes into which cell)
- random joining of gametes aka germ cells (which sperm fertilizes which egg–genetic composition of gamete affects this)
regulation of cell cycle
functional limitations
- surface to volume ration (S/V)
- as volume gets much larger when cell grows (x^3) vs SA (x^2)
- when S/V is large (such as 4:8) … exchange becomes easier
- when S/V is small ( such as 4:512).. exchange is hard, leads to cell death or cell division to increase
- genome - volume-ratio (G/V)
- genome size remains constant; as cell grows, only volume increase.
- G/V will be small and this exceed the ability of its genome to produce sufficient amounts of regulatory activities
- some large cells (parmecium, human skeletal muscle) are multinucleated to deal with this
A.checkpoints
cell specific regulations
1.G1 checkpoints
- aka restriction point, the most important one
- at the end of G1 phase, if the cell is not ready to divide it may arrest here (G0 phase- nerve and muscle cells remain here, rarely divide after maturing) and never proceed or wait until it is ready
- G2 checkpoint
- end of G2 phase, evaluates accuracy of DNA replication and signal whether to begin mitosis
- M checkpoint
- during metaphase, ensures microtubules are properly attached to all kinetochores.. prevents anaphase if not
B. cyclin-dependent kinases (Cdk’s)
- Cdk enzyme activates proteins that regulate cell cycle by phosphorylation; Cdk’s are activated by protein cyclin
C. growth facotrs
plasma membrane has receptors for growth factors that stimulate cell for division (such as damaged cell)
D. density-dependent inhibition
cell stop dividing when surrounding cell density reaches maximum
E. anchorage dependence
most cells only divide when attached to an external surface such as neighboring cells or side of culture dis
cancer
defy all of the 5 conditions above (called transformed cells)
