what are the steps of mitosis as visualized in the light microscope?
what do microtubules and actin filaments do during mitosis?
do they have distinct or redundant functions in cell division?
microtubules: form as spindles during promteaphase; attach to the chromosome, anchor it to mitotic spindle, interdigitate with each other from opposite poles
actin: drives cytokinesis, when G1 daughter cells form
in what important way does meiosis differ from mitosis?
Mitosis: process of cell division, from 4N parent to 2 2N daughter cells that are diploid; somatic cells
Meiosis: germ cells; 2N to 1N haploid cells = reductive division; goes through 2 rounds, Meiosis I and II
during what phase of meiosis does recombination btwn maternal & paternal chromosomes occur?
what are the 2 major ways by which genetic recombination occurs during meiosis?
1) chromosomal crossing-over (infinite recombination possibilities)
2) independent assortment (limited, 223 recombination possibilities)
what are phases of cell cycle?
S phase: DNA synthesis
M phase: mitosis, cell division
G1 & G2: "gap" or "growth" phases, cell syntheizing things that aren't necessary for DNA synthesis (G1) and synthesizing things needed for mitosis (G2)
what is DNA content in each phase of somatic cell cycle?
S phase: between 2N and 4N
M phase: 4N
what does human somatic cell alternate beween?
interphase and mitosis
what is mitosis?
process of cell division
go from one 4N G2 cell to 2 2N G1 daughter cells
G2 > prophase, prometaphase, metaphase, anaphase, telophase > G1 + G1
what happens to centrosomes / microtubule organizing centers during S phase?
1) centrosomes (2) move to the poles
2) nuclear membrane breaks down
3) chromosomes condense, become visible
4) each pair of sister chromatids are held together by the centromere
1) spindle fibers (microtubules) form, attach to the kinetochore
have 3 kinds of microtubules:
1) astrac MT: position at mitotic spindle
2) kinetochore MT: attach to chromosomes
3) polar MT: interdigitate w/ MTs from opposite pole
1) chromosomes align in center of mitotic spingle
2) karyotiping done at this phase
1) sister chromatids separate and move to opposite poles
2) cell elongates in preparation for telophase
1) reformation of nuclear membrane
2) chromosomes decondense (reversal of prophase)
3) spindle, MT, disappear
when is cytokinesis?
what drives it?
6th, after telophase
actin filaments polymerization/depolymerization drives cytokinesis
result: separation of daughter cells
what is karyotyping?
number and appearance of chromosomes
process of reductive cell division for germ cells
each germ cell = 1N, haploid DNA content
describe chromosome content change over meiosis
start w/ 4N DNA content cell
Meiosis I: 4N to 2N
Meiosis II: 2N to 1N
DNA content in germ cell vs. somatic cell?
germ cell: 1N DNA content, are haploid
somatic cell: 2N DNA content, are diploid
what happens in Meiosis I?
pairing and segregation of homologues (paternal or maternal), aka homologous chromosomes separate
go from 4N to 2N DNA content by lining up separate homologues, separting them into poles
result: each daughter gets 1 materal or paternal copy via sister chromatid
prophase I characteristics?
DNA is condensed
spindle forms at chiasmata, which holds maternal and paternal paris together
homologous chromosomes align and exchange segments
metaphase I characteristics?
tetrads of sister chormatids align in middle
microtubules form, attach to kinetochore
anaphase I characteristics?
pairs of homologous chromosomes split up
sister chromatids remain attached
holds paternal and maternal chromatid pairs together in meiosis I
when does spindle form in meiosis I?
once chiasmata is in place
what happens in meiosis II?
pairing and segregation of sister chromatids - sister chromatids line up, separate, like in mitosis
what happens to DNA conent during Meiosis II?
start w/ 2N, 23 chromosomes, end up w/ 1N DNA content, each of which has single sister chromatid
why go through 2 rounds of meiosis?
to cause genetic reassortment
what are 2 ways to accomplishment genetic reassortment?
1) independent assortment: random distribution where 1 maternal chomosome and 1 paternal chromosome / 2nd maternal or paternal chromosome coudl randomly go to a pole; cerates 223 or 8 millino possibilities
2) crossing over via localized recombination between maternal and paternal chromosomes at the chiasma - can occur limitless number of times in both directions, generates infinite amount of diversity!
what is chiasma's function?
1) physical association: holds together maternal/paternal chromosomes effectively, so at metaphse I of meiosis I they can separate
2) facilitates chromosomal crossing over, recombination: localized recombination btwn maternal & paternal chromosomes at chiasma, can occur limitless # of times in each direction, creates infinite amount of diversity
why does meiosis occur by DNA syntehsis, followed by Meiosis I and II?
b/c during meiosis I, chromosomal crossing over occurs - homoogous chromosomes exchange genetic materal
this recombination results in limitless genetic diversity
explains why meiosis doesn't occur by simple homologues' separation - would not get crossing-over and exchange of genetic material if just had homologues' separation
what is synaptonemal complex?
highly ordered, well-defined cellular structure of 2 lateral elements and a central element
it is what facilitates recombination btwn paternal and maternal homologues
underlies the chiasma
what do chiasma do?
when are they present?
chiasma = remnants of synaptonemal complex
are sites of crossing-over that persist into metaphase of Meiosis I
physically hold homologues together until segregation occurs, like centromeres for sister chromatids
how do male sex chromosomes have crossing over?
during Meiosis I
small region of homology between X and Y chromosomes, allows their pairing
crossing-over only in this small region
allows physically keeping the X and Y chromosomes together until segregation is ready to occur
what is non-disjunction?
why does it occur?
what is result?
when homologues fail to separate in Meiosis I
occurs if chiasma aren't properly disrupted in Meiosis I
homologous chromosomes stay associated
Meiosis II then occurs normally
resulting germ cells have N+1 (3) or N-1 (1) chromosomes, when should each have 2 chromosomes
3N gametes can continue to fertilization, resulting embryo has extra chromosome, results in a trisomy
when can nondisjunction occur?
most common in Meiosis I, as sister chromatid separation is more tightly controlled process
can occur if sister chromatids fail to separate during Meiosis II