LEC41: Mitosis and Meiosis

Question 1

what are the steps of mitosis as visualized in the light microscope?

Answer 1

1) prophase
2) prometaphase
3) metaphase
4) anaphase
5) telophase
6) cytokinesis

Question 2

what do microtubules and actin filaments do during mitosis?

do they have distinct or redundant functions in cell division?

Answer 2

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

Question 3

in what important way does meiosis differ from mitosis?

Answer 3

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

Question 4

during what phase of meiosis does recombination btwn maternal & paternal chromosomes occur?

Answer 4

Prophase I

Question 5

what are the 2 major ways by which genetic recombination occurs during meiosis?

Answer 5

1) chromosomal crossing-over (infinite recombination possibilities)
2) independent assortment (limited, 2²³ recombination possibilities)

Question 6

what are phases of cell cycle?

Answer 6

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)

Question 7

what is DNA content in each phase of somatic cell cycle?

Answer 7

G1: 2N

S phase: between 2N and 4N

G2: 4N

M phase: 4N

Question 8

what does human somatic cell alternate beween?

Answer 8

interphase and mitosis

Question 9

what is mitosis?

Answer 9

process of cell division

go from one 4N G2 cell to 2 2N G1 daughter cells

G2 > prophase, prometaphase, metaphase, anaphase, telophase > G1 + G1

Question 10

what happens to centrosomes / microtubule organizing centers during S phase?

Answer 10

they double

Question 11

prophase steps?

Answer 11

first

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

Question 12

prometaphase steps?

Answer 12

2nd

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

Question 13

metaphse steps?

Answer 13

3rd

1) chromosomes align in center of mitotic spingle
2) karyotiping done at this phase

Question 14

anaphase steps?

Answer 14

4th

1) sister chromatids separate and move to opposite poles
2) cell elongates in preparation for telophase

Question 15

telphase steps?

Answer 15

5th

1) reformation of nuclear membrane
2) chromosomes decondense (reversal of prophase)
3) spindle, MT, disappear

Question 16

when is cytokinesis?

what drives it?

result?

Answer 16

6th, after telophase

actin filaments polymerization/depolymerization** **drives cytokinesis

result: separation of daughter cells

Question 17

what is karyotyping?

Answer 17

number and appearance of chromosomes

Question 18

meiosis?

produces?

Answer 18

process of reductive cell division for germ cells

each germ cell = 1N, haploid DNA content

Question 19

describe chromosome content change over meiosis

Answer 19

start w/ 4N DNA content cell

Meiosis I: 4N to 2N

Meiosis II: 2N to 1N

Question 20

DNA content in germ cell vs. somatic cell?

Answer 20

germ cell: 1N DNA content, are haploid

somatic cell: 2N DNA content, are diploid

Question 21

what happens in Meiosis I?

Answer 21

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

Question 22

prophase I characteristics?

Answer 22

DNA is condensed

spindle forms at chiasmata, which holds maternal and paternal paris together

homologous chromosomes align and exchange segments

Question 23

metaphase I characteristics?

Answer 23

tetrads of sister chormatids align in middle

microtubules form, attach to kinetochore

Question 24

anaphase I characteristics?

Answer 24

pairs of homologous chromosomes split up

sister chromatids remain attached

Question 25

chiasmata function?

Answer 25

holds paternal and maternal chromatid pairs together in meiosis I

Question 26

when does spindle form in meiosis I?

Answer 26

once chiasmata is in place

Question 27

what happens in meiosis II?

Answer 27

pairing and segregation of sister chromatids - sister chromatids line up, separate, like in mitosis

Question 28

what happens to DNA conent during Meiosis II?

Answer 28

start w/ 2N, 23 chromosomes, end up w/ 1N DNA content, each of which has single sister chromatid

Question 29

why go through 2 rounds of meiosis?

Answer 29

to cause genetic reassortment

Question 30

what are 2 ways to accomplishment genetic reassortment?

Answer 30

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!

Question 31

what is chiasma’s function?

Answer 31

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

Question 32

why does meiosis occur by DNA syntehsis, followed by Meiosis I and II?

Answer 32

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

Question 33

what is synaptonemal complex?

Answer 33

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

Question 34

what do chiasma do?

when are they present?

Answer 34

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

Question 35

how do male sex chromosomes have crossing over?

Answer 35

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

Question 36

what is non-disjunction?

why does it occur?

what is result?

Answer 36

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

Question 37

when can nondisjunction occur?

Answer 37

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