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