Mitosis & Meiosis

What is the goal of mitosis?

production of two identical diploid daughter cells from a diploid parent cell

What is the cell cycle driven by?

cyclin dependent kinases (Cdk)

What is the function of Cdk?

promoting cell cycle progression by phosphorylating specific substrates

What is the order of mitotic events?

prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis, interphase

What occurs during prophase?

1- chromosomes condense

2- centrosomes migrate to poles

3- spindle starts to form outside of the nucleus

What occurs during pro-metaphase?

1- nuclear envelope breaks down

2- chromosomes attach to spindle

What occurs during metaphase?

1- sister chromatids become linked to opposite spindle poles

2- chromosomes align along equitorial plane

What occurs during anaphase?

1- sister chromatids separate

2- daughter chromosomes are pulled to opposite ends of cell

What occurs during telophase?

1- chromosomes arrive at poles

2- nuclear membrane reforms

3- chromosomes decondense

What is cytokinesis?

division of the cytoplasm, accomplished by a contractile ring made of actin and myosin

What plane does the contractile ring form in?

the same as the metaphase plate

What occurs during interphase?

G1, S, G2

What is a centrosome?

microtubule organizing center that contains a pair of centrioles

What is the centrosome cycle?

process required so each daughter cell inherits one centrosome

What are the steps of the centrosome cycle?

each cell has one centrosome (with 2 centrioles) → the centrosome replicates → centrosomes separate to poles at prophase, and nucleate spindle

How are sister chromatids held together?

cohesin, which is deposited during DNA replication

What does condensin do?

condenses DNA during prophase by promoting coiling

When does nuclear envelope breakdown occur?

pro-metaphase

How is nuclear envelope breakdown prompted?

Cdk-mediated phosphorylation of lamins that act as internal support for nuclear envelope

When does the nuclear envelope reform?

telophase

How do chromosomes attach to spindles?

the chromosome contains centromeric heterochromatin (highly condensed chromatin near center of structure), and a kinetochore protein attaches to the heterochromatin on each chromatid. the kinetochore complex has strands that attach to the kinetochore microtubules

What is the term for proper spindle alignment?

amphitelic/bipolar attachment

What are some terms for unstable spindle attachments?

syntelic, merotelic, and monotelic attachments

What is the benefit of nuclear envelope breakdown?

it allows for spindle assembly by the centrosomes

What types of microtubules are there?

astral, kinetochore, and overlap/polar microtubules

What is the function of kinetochore microtubules?

to link the spindle pole to the chromosomes via a kinetochore

Where are kinetochores produced?

at the centromere

What is the function of overlap microtubules?

give the spindle its symmetrical, bipolar shape and help with pole separation

What is the function of astral microtubules?

radiate out from the centrosome; act in pole separation and spindle positioning

What does the metaphase spindle look like?

a bit like a football

What is dynamic instability?

microtubules are in a state of dynamic instability, it describes a system that alternated between growth and decay rapidly

What occurs during chromosome separation?

1- kinetochore microtubules shorten

2- spindles poles separate with help of overlap and astral microtubules

How do sister chromatids separate during anaphase?

the cohesins near the centromeric heterochromatin are dissolved with help from the enzyme separase.

What is the anaphase promoting complex?

complex of proteins that acts to initiate anaphase

How does the APC promote anaphase?

the APC is activated by Cdk, a spindle checkpoint occurs to ensure chromosome alignment is correct, and APC activates separase by cleaving securin

What is the function of securin?

to bind to separase and keep it inactive until it is cleaved by APC and separase can activate

When are cohesin molecules along the arms of the sister chromatids removed?

during prophase

When does cytokinesis start?

during anaphase and is completed by the end of telophase

What is the goal of meiosis?

produce haploid gametes from a diploid parental cell

What is a gamete?

haploid cell specialized for fusion

What is the major benefit of meiosis?

offspring inherit a new combination of genes which is helped by independent assortment and crossing over

What are the steps of meiosis?

meiosis I (chromosome segregation), cell division, meiosis II (chromosome segregation), and cell division to result in 4 haploid gametes

What happens during meiosis I?

1- duplicated homologs pair

2- at least 1 cross-over per pair

3- homologs align on spindle

4- homologs segregate to opposite poles

What happens during meiosis II?

1- spindle re-forms

2- chromosomes align

3- chromatids segregate to opposite poles

What are the steps of meiosis I?

prophase I, metaphase I, anaphase I, and telophase I

What happens during prophase I?

pairing of homologous chromosomes, recombination between pairs, leptotene, zygotene, pachytene, diplotene, and diakenesis

What is leptotene?

when duplicated, paired sister chromatids condense

What is zygotene?

where a structure called the synaptonemal complex is formed between the 2 sets of sister chromatids

What is pachytene?

recombination occurs between non-sister chromatids, usually 2-3 per chromosome at recombination nodules

What is homologous recombination?

genetic exchange between 2 homologous sequences

1- helices break and rejoin with opposite partner

2- site of exchange can be anywhere

3- heteroduplex DNA forms at site of exchange

4- very precise process

What is diplotene?

synaptonemal complex is disassembled; cross-overs are maintained and chiasma can be observed

What are chiasma/chiasmata?

where non-sister chromatids remain linked following recombination

What is diakenesis?

transition to metaphase I, chromosomes recondense

What happens during metaphase I?

bivalents (complex of 4 chromatids) are held together by chiasma and align at the equatorial plane

What happens during anaphase I?

chiasma are resolved and cohesins at chromatid arms are dissolved leaving sister chromatids attached at centromere. homologs segregate to opposite poles

What is non-disjunction?

failure of chromosomes to separate and segregate properly at anaphase I or II

what happens during telophase I?

chromosomes dissociate from the spindle, nuclear membranes reform around their daughter nuclei, chromosomes may de-condense a little

Does anything happen between meiosis I and II?

Yes, brief interphase

What happens at prophase II?

the nuclear envelope breaks down and chromosomes attach to newly formed spindle

What occurs during metaphase II?

chromosomes align on the epuatorial plane

What occurs at anaphase II?

attachments between sister chromatids are dissolved, one chromatid segregates to each pole

What occurs at telophase II?

the chromosomes de-condense and the nuclear membrane reforms

What happens after telophase II?

cytokinesis leaving 4 haploid cells

What is independent assortment?

chromosomes randomly segregate to gametes which results in new assortments of maternal and paternal chromosomes

What is recombination?

exchange of sequences between 2 DNA molecules, makes new combinations of maternal and paternal alleles along a chromosome

Where does meiotic recombination occur?

between non-sister chromatids from homologous chromosomes during prophase I

What are the steps of meiotic recombination?

1- formation of dsDNA break mediated by endonuclease

2- break is resected to make 3’ overhangs

3- one 3’ end invades one of the homologous chromatids

4- DNA synthesis and pairing between strands makes a Holliday junction

5- resolvase cuts the joined molecules to make 2 recombined chromosomes