Genetics: Cell Cycle, Mitosis, and Meiosis

Prophase II

Chromatin condenses, nuclear envelope degenerates, spindle fibers form

Describe M phase in general

Mitosis or Meiosis

Gene conversion

occurs when the sequence of one allele of a gene is changed, and actually becomes the sequence of other allele

What two things must the cell check during G1?

-Phosphorylation of certain proteins ( the retinoblastoma (RB) protein) -the cell makes sure it has stockpiled all the enzymes it needs to replicate DNA before it goes into S phase

What happens at the end of metaphase? (pertaining to the spindle fiber, chromatin, and nuclear membrane)

-Spindle fibers are bound to the kinetochore (at the centromere) of each chromatid (binding occured in late prophase) The cell undergoes a spindle-assembly checkpoint to be sure every chromatid has been bound by a spindle fiber -chromosomes are in their most condensed configuration and aligned at the equatorial plane -nuclear envelope still disintegrated

Late prophase (prometaphase)

-centrosomes have migrated to the poles -chromsomes are condensed -nuclear membrane is gone -spindle fibers are reaching in and binding to the chromosomes

Early prophase

-chromosomes have just started to condense -centrosomes move towards the opposite poles of the dividing cells

Anaphase (spindle fiber, chromatin, nuclear membrane)

-visible spindle fibers have bound the chromosomes by connecting to the kinetochores -nuclear membrane disintegrated -sister chromatids are pulled apart toward opposite poles and now each chromatid becomes a chromosome

If the two pieces that are cut out came from two different homologs (right side—one red, one blue), you get the expected __________ ratio of alleles in the gametes

50%/50%

If the two pieces that are cut out came from the same homolog (left side—both blue), you get a_____________ ratio of alleles in the gametes

75%/25%

In order to package as much cytoplasm and organelles into the egg as possible what happens?

All the cytoplasm and organelles migrate into one daughter cell

So when do the sister chromatids separate and what must happen to shugoshin (and when) for this to occur?

Anaphase II and shugoshin must be degraded during Metaphase II

Prometaphase I

Centrosomes have migrated to poles Chromosomes are obviously condensed Nuclear membrane has disappeared Spindle fibers are beginning to bind to chromosomes Homologous recombination occurs

Early Prophase I

Centrosomes, beginning to move toward opposite poles of the dividing cell Chromosomes are beginning to condense

Late metaphase I

Chromosomes are aligned at the equator Spindle fibers are binding the chromosomes

What is cohesin and how long does it stick around?

Cohesin holds the sister chromatids together at the centromere until the enzyme separase degrades the cohesin (end of metaphase—beginning of anaphase)

The sperm has donated only its _________________; so the egg must provide _______________________________________.

DNA and centrosome; all the cytoplasm and organelles that are available to be divided among the newly created cells

Interphase before Meiosis

DNA has already been replicated and each chromosome has been doubled to form two sister chromatids

another way meiosis increases genetic diversity

Different recombinant chromosomes 1, for example, contain different relative amounts of the grandfather's chrom 1 and grandmother's chrom 1

Holliday junction

Your DNA repair proteins recognize these bubbles and cut the mismatched bases out of one of the DNA strands, then use the remaining strand as a template to synthesize new DNA

What is apoptosis?

If the cell fails to correct the problems detected at any of its checkpoints, the cell commits suicide, by turning on genes that induce apoptosis, or programmed cell death

What is the G0 phase and why would it occur?

In G0, the cell stops replicating and dividing. A cell goes into G0 if it senses a lack of growth factors, nutrients or other molecules that are necessary for the cell cycle to continue

How do cells in your body become specialized?

In each cell type, a different subset of genes is turned on. The set of genes that gets turned on determines the set of proteins the cell possesses.

Diakinesis

chromatin condenses further, the two chromosomes in the bivalent separate farther apart, the two chromosomes now remain joined only at the ends

What are the 5 stages of Prophase and Prometaphase I?

Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis

Early-mid telophase I

Nuclear membrane has not yet reformed Chromatin is still somewhat condensed Cells only beginning to cleave apart, new cell membrane is not yet visible

Early Mid-Telophase

Nuclear membrane hasn't reformed yet Chromatin is a little condensed Cells are just beginning to cleave; new cell membrane not visible yet

Interkinesis

Nuclear membrane reforms, spindle fibers degenerate, chromatin decondenses Several proteins that are necessary for chromatin condensation in Prophase II get phosphorylated and assembled into complexes, including some of the cohesins (there are several versions of cohesion) and condensin II

Telophase II

Nuclear membrane reforms, spindle fibers disappear, chromatin decondenses

*know the chart from the recitation problem set*

Number of chromsomes/DNA molecules in each phase

How do we get rid of cohesin?

Once separase has degraded cohesin, this allows the sister chromatids to segregate apart during anaphase

So two sister chromatids held together is how many chromosomes and how many DNA molecules?

One chromosome, two DNA molecules

What happens to the cells that get the cytoplasm and organelles and the ones that don't?

One will get everything and become the egg; others will become polar bodies and die off

Differences between Meiosis and Mitosis

Only gametes undergo meiosis; somatic cells undergo mitosis. Meiosis = 4 haploid daughter cells (one set of chromosomes per cell) Mitosis = 2 diploid daughter cells (two sets per cell) Recombination occurs between homologous chromosomes during meiosis, which results in the child inheriting some chromosomes that contain a combination of gene alleles that came down from the grandfather and grandmother

What are the 5 stages of Mitosis?

Prophase, Prometaphase, Metaphase, Anaphase, Telophase *cytokinesis occurs during telophase

Describe S in general

Replicate DNA

End of Interphase

Spindle fiber not organized, chromatin is decondensed, nuclear membrane is intact,

What happens at the end of interphase? (pertaining to the spindle fiber, chromatin, and nuclear membrane)

Spindle fibers are not organized Chromatin is decondensed Nuclear membrane is intact

What happens at the end of prophase? (pertaining to the spindle fiber, chromatin, and nuclear membrane)

Spindle fibers grow out of the centrosomes toward the centromeres of the chromosomes Chromatin condenses into visible chromosomes and migrates toward the equatorial plate (center of the dividing cell); because the DNA was replicated in S phase, each chromosome now has two sister chromatids and by late prophase, you can see them Nuclear envelope disintegrates

Describe G1 in general

Stockpile nucleotides, ATP, enzymes to replicate DNA

Describe G2 in general

Synthesize microtubules and other proteins needed for cell division, stockpile ATP and enzymes for mitosis/meiosis

Metaphase I

The bivalents are aligned in their tetrads along the equatorial plane The spindle fibers connect to the chromosomes by the kinetochores at their centromeres Cell checks to be sure every chromatid is bound by a spindle fiber

What three things must the cell check during G2?

The cell must accumulate a certain amount of mitosis promoting factor (MPF) and activate it through dephosphorylation to enter the M phase Cell must check for DNA damage: damage prevents MPF from activating and halts the cycle Cell checks the length of telomeres

Metaphase II

The chromosomes align at the equatorial plane, and the spindle fibers connect to the chromosomes (at the kinetochore at the centromere) Note that half the chromatids are recombinant chromatids, and half are nonrecombinant chromatids

Anaphase I

The enzyme separase degrades the cohesin that holds the two homologs together (but leaves the cohesin that is holding the sister chromatids together at the centromere intact) The two chromosomes in each bivalent migrate apart from each other toward opposite poles of the dividing cell

Reduction Division And Equational Division

The first cell division is sometimes referred to as the reduction division, because it reduces the number of chromosomes in the cell by half The second cell division is sometimes referred to as the equational division, because there is no change in the number of chromosomes per cell

How come in Anaphase I separase can degrade the cohesin between the two homologous chromosomes but not the cohesin holding the two sister chromatids at the centromere?

The protein shugoshin protects cohesin from separase at the centromeres.

Telophase

The sister chromatids (now called chromosomes) have been pulled completely toward the two poles during anaphase—no more movement is necessary Chromatin decondenses Nuclear envelope reforms Spindle fibers degenerate to some degree New cell membrane material is synthesized so the two daughter cells are now separate functional cells

Anaphase II

The sister chromatids migrate apart from each other toward opposite poles of the dividing cell

Why are telomeres important?

They shorten every cell cycle and the length is indicative of age.

Telophase I

chromosomes have migrated completely to opposite poles Cytokinesis occurs, aka cytoplasmic streaming—cytoplasm and organelles migrate into the two daughter cells

How can a child can inherit a combination of features from all four grandparents

When a recombinant chromosome is passed down, it contains a mixture of material from the child's grandmother and the child's grandfather

heteroduplex DNA

When recombination is over, for part of the chromosome, a strand from one homolog is bound to a strand from the other homolog—this

How are the sister chromatids pulled apart?

With a moter protein called dynein; dynein sits on two spindle fibers and pulls on two spindle fibers that are connected to chromosomes

Pachytene

chromatin condenses, synaptonemal complex develops between homologous chromosomes (i.e. the homologous chromosomes come into contact with each other at one or more places in each p and q arm) *RECOMBINATION OCCURS* Non-sister homologous chromatids (one from each member of the homologous chromosome pair) physically cross over at the points of contact and exchange material

Zygotene

chromatin condenses; homologous chromosomes pair up and engage in synapsis to produce bivalents and tetrads

Cytokinesis I

aka cytoplasmic streaming—cytoplasm and organelles migrate into the two daughter cells

Meiosis resumes___________, but only a __________ of primary oocytes resume meiosis each month, and they stop in ______________

at puberty; handful; metaphase II

number of chromosomes = number of _________

centromeres

Leptotene

chromatin condenses

Meiosis finishes after

fertilization occurs

tetrads

four chromosomes

Product of meiosis

four haploid daughter cells If you draw the chromosomes as they would appear if they recondensed, you can see how half your gametes contain a nonrecombinant chromosome from that pair, and the other half contain a recombinant chromosome

Immediately after fertilization, the zygote/embryo undergoes several rounds of cleavage and cell division with

no replication of cytoplasm and organelles

When authors use a prophase I/prometaphase I scheme, prometaphase I corresponds to the

pachytene, diplotene and diakinesis stages

In women, the ________________ begin meiosis from _____________ gestation to shortly after birth, but they stop at __________.

primary oocytes; 7 months ; diplotene

Each chromatid is a _______ molecule of DNA

separate

Middle of Anaphase I

spindle fibers draw chromosomes apart

What does the cell check during the M phase?

the cell checks to be sure every chromosome is properly aligned at the equatorial plane, and that every centromere has been contacted by a spindle fiber => if the chromosomes/spindles have not aligned properly, this inhibits activation of the MPF and keeps the cell in mitosis => or the cell may just enter apoptosis

Diplotene

the two chromosomes in each bivalent begin to separate, but remain joined at the chiasma—i.e. the places where the crossing over took place

After S phase of Interphase, each chromosome contains _____ sister chromatids.

two

bivalent

two chromosomes