cell cycle Flashcards
cell replication:
cells replicate to fix damage and keep proper function and reduce mutations. this stops premature ageing.
single cell vs multi cell organisms:
unicellular- complete new organism.
multicellular- require long and complex series of divisions to form functioning organism.
insufficient content in cells- shrink, smaller surface area, wont survive easily and wont get through all stages of cycle.
cycle:
G0: resting phase, not going through the cycle.
G1: cell grows, cells get bigger with more molecules.
R: start/restriction point. cells decide whether to continue. chemicals allow cells to continue.
S: synthesis, cells have most energy.
G2: cell prepares to divide.
end of cycle is mitosis (cytokinesis).
24 hours in typical human cell. 1 hour mitosis. 23 hour for interphase.
most cells are in G0. cells are senescent or differentiated. can remain in this state for long periods.
neurons cant leave G0 as they are too specialised, once damaged they cant repair.
features of cell cycle:
clock or timer
continuous processes. organelle/protein synthesis.
stepwise processes. once per cell cycle.
purpose of cycle:
first 3 phases (interphase). duplicate cell content (DNA and other cellular macromolecules). transcribes genes, synthesises proteins.
mitosis- divide the genetic material into 2 exact halves.
cytokinesis- divide the rest of the cell contents into 2 cells (except nucleus).
checkpoints:
the cycle is controlled at key checkpoints which monitor the status of the cell- cell size, adequate supply on nutrients, DNA quantity and quality, favourable environment.
checkpoints can be regulated by signals from other cells, growth factors and other extracellular signal molecules.
checkpoints of the cell cycle:
metaphase- exit M phase if all chromosomes are attached to spindles. stop if chromosomes are misaligned.
R point- enter S phase if environment is favourable. stop if damaged DNA.
S- stop if unreplicated or damaged DNA.
G2- enter M phase if environment is favourable and all DNA is replicated. stop if unreplicated or damaged DNA.
types of cell division:
mitosis- occurs in somatic cells (body). growth and repair. asexual reproduction (yields identical cells).
meiosis- occurs in specific reproductive cells. sexual reproduction (yields different cells).
mitosis:
production of 2 genetically identical daughter cells which are also genetically identical to the parent cell.
5 phases- prophase, prometaphase, metaphase, anaphase, telophase.
prophase:
genetic material condenses to prevent chromosomes tangling- easier to separate. chromosomes become visible.
centrosomes replicate and move to opposite sides of nucleus. driven by motor proteins and ATP.
mitotic spindle assembles between the 2 centrosomes. used to separate the 2 sister chromatids.
nucleolus disappears.
prometaphase:
the nuclear membrane breaks down, releasing the centrosomes (normally associated with nuclear membrane). allows centrosomes to interact with chromosomes to build the mitotic spindle.
chromosomes further condense.
a kinetochore (point where the sister chromatids are attached) forms at the centromere.
contains 2 regions- the inner kinetochore tightly associated with the centromere DNA, the outer kinetochore interacts with microtubules.
microtubules attach at the kinetochore and the chromosomes attach to the mitotic spindle.
metaphase:
chromosomes align on an axis (the metaphase plate). the spindle consists of microtubules, one attached to each chromosome.
chromosomes act independently and align randomly.
anaphase:
each centromere splits. results in 2 free chromatids (called sister chromatids). each chromatid moves towards a pole.
cell changes shape becoming elongated due to microtubules not associated with the kinetochore.
telophase:
formation of nucleolus and nuclear membrane. short and thick chromosomes begin to elongate to form long and thin chromatin.
formation of cleavage furrow (a shallow groove in the cell near the old metaphase plate). position determined by the spindle.
cytokinesis:
pinching of cytoplasm due to contraction of ring of actin and myosin II filaments. location of ring determined by position of mitotic spindle. cleavage occurs in plane perpendicular to the spindle.
meiosis:
meiosis means to make smaller. duplicated cells have a reduced chromosome number. diploid germ cells (2N) undergo DNA replication followed by 2 rounds of division (meiosis I and meiosis II).
results in haploid cells (N) called gametes. essential for sexual reproduction.
each gamete contains a complete set of chromosomes. half of the genetic content of the original cell.
genetic diversity:
meiosis enables genetic diversity.
important as it facilitates evolution. lack of diversity promotes genetic disease. a healthy population has a large gene pool.
genetic diversity arises by recombination: independent assortment of chromosomes crossing over (genetic exchange).
meiosis:
meiosis I: prophase 1 metaphase 1 anaphase 1 telophase.
meiosis II: prophase 2 metaphase 2 anaphase 2 telophase.
meiosis I:
each starting diploid cells contains 2 copies of each chromosome. 1 from father and 1 from mother.
first step is duplication of chromosomes, which proceeds meiosis in interphase.
interphase (G1, S, G2) is the same in cells undergoing mitosis and meiosis.
prophase 1:
virtually identical to prophase in mitosis. development of spindle, breakdown of membrane, centrosomes move towards poles.
chromosomes partially separate in late prophase. areas where crossing over occurred remain attached and are called chiasmata.
metaphase 1:
critical difference between mitosis and meiosis.
the orientation is random, there is 50-50 chance for the daughter cells to get either the mothers or fathers homologue for each chromosome.
crossing over can also occur during this phase.
anaphase 1:
kinetochore spindle fibres contract.
pulls homologous pairs away from each other toward cell poles.
chromosomes, each with 2 chromatids, move to separate poles. each daughter cell is haploid. centromeres do not separate so each chromosome has 2 chromatids.
telophase 1:
a cleavage furrow forms followed by cytokinesis.
nuclear envelope usually is not reformed and chromosomes do not disappear.
each daughter cell has a single set of chromosomes, half the total number in the original cell. original cell was diploid. daughter cells are haploid.
meiosis II:
meiosis II is similar to mitosis.
chromatids of each chromosome are no longer identical because of recombination. meiosis II separates the chromatids producing 2 daughter cells each with haploid chromosomes.
each chromosome has only 1 chromatid.
prophase II:
a new set of spindle fibres form.
chromosomes begin to move toward the equator of the cell.
metaphase II:
the chromosomes in the 2 cells align with the metaphase plate.
anaphase II:
centromeres split and the kinetochore.
spindle fibres shorten. draws chromosomes toward each pole of the cell.
telophase II:
cleavage furrow develops.
cytokinesis follows.
nuclear envelope forms.
chromosomes begin to fade. becomes granular chromatin characteristic of interphase.
when meiosis II is complete produces 4 haploid (n) daughter cells compared to diploid (2n) original cell.
cell cycle during spermatogenesis:
maturation from puberty to old age.
each primary spermatocyte forms 2 secondary spermatocytes.
each secondary spermatocyte forms 2 spermatozoa. 4 in total.
spermatids mature. cytoplasm lost, nucleus condenses, acrosome forms.
sertoli cell nourishes developing sperm. consumes residual cytoplasm.
