MCM 2-12 Cell Division Flashcards
what are the four phases of the cell cycle and brief description
G1 phase - Gap before DNA synthesis
S Phase - DNA synthesis phase
G2 Phase - Gap after DNA synthesis
M Phase - Mitosis
Describe G1
The gap before DNA synthesis. Cells in this phase have two copies of each chromosome (one copy of each homologue). It is the typical “resting” phase wherein general biosynthesis occurs. External signals (e.g. receptor activation-MAP kinase receptors) and internal state (e.g. cell size, DNA integrity) jointly influence whether a cell proceeds to division (S phase) or remain in G1 (G0 phase).
S Phase
The DNA synthesis phase. Cells in this phase have an amount of DNA that gradually increases from that found in G1. During this phase histones and other proteins needed exclusively for DNA replication are produced. DNA is replicated from multiple replication origins (bidirectionally). After this is accomplished, the replication licensing factors are removed from the origin recognition complex to prevent re-replication. Centrioles duplicate but remain as a single centrosome (for now).
afterwards, replication “licensing factors” are removed from original recognition complex to prevent re-replication
-centrioles duplicate
G2 phase
The gap after DNA synthesis. Cells in this phase have four copies of each chromosome (two copies of each homologue). Proteins required for mitosis and cytokinesis are continued to be produced. Addition of cohesins links sister chromatids together, and addition of condensins combined with phosphorylation of histone H1 allows for chromosomes to condense at the beginning of M phase.
M Phase
Mitosis. The onset of mitosis is controlled by kinase activity…the nuclear and cytoplasmic events are coordinated during the stages of mitosis largely through the actions of kinases and phosphatases that are active throughout the cytoplasm. Cells undergoing mitosis have four copies of each chromosome (two copies of each homologue) until cytokinesis. This phase is triggered by the activity of a cell-cycle regulatory kinase. Mitosis is further broken down into the following phases: Prophase Prometaphase metaphase anaphase telophase
what are the major checkpoints controlling progression from one stage to the next?
G1/S border (start)
G2/M border (enter M)
Metaphase/anaphase transition (exit M) - tension on all kinetochores activates APC (an E3 ubiquitin kinase) targeting the mitotic cyclin and other proteins for destruction.
-Securin destruction releases separase to separate sister chromatids.
-Destruction of mitotic cyclin turns off Motitoc CDK.
describe mechanism of cyclin dependent kinase (CDK) Regulation
CDK’s regulate cell cycle
complete CDK’s (cyclin-CDK complexes) are composed of catlytic CDK subunit and regulatory (cyclin) subunit
-changes in cyclin concentration change CDK activity- the cell cycle is dependant upon this activity
-in order for an active cyclin-CDK complex to be formed, cyclin subunit must be synthesized and CDK (catalytic subunit) must be phosphorylated at one of two sites (one is inhibitory, one is activating).
when catalytic subunit is phosphorylated- both sites receive phosphase, keeping the subunit inactive. A phosphorylase then removes the inhibitory phosphate, activating the cyclin CDK complex
what are the kinases that phosphorylate CDK?
two different kinases are involved in phosphorylating the CDK’s two sites
wee1 adds the inhibitory phosphate
cak (CDK activating kinase) adds the activating phosphate.
cdc25 removes the inhibitory phosphate to activate the complex
How are CDK’s turned off?
by ubiquination. An E3 ubiquitin ligase tags the cyclin with polyunbiquitin chain, causing its degredation by the proteosome.
a phsophatase removes the activating phosphate.
describe the DNA damage repair pathway
p53 is major protein.
in response to DNA damage, protein kinases are activated that phosphorylate P53, stabilizing/activating it
active p53 binds to regulatory regions of small inhibitory protein genes leading to the synthesis of the small inhibitory proteins that inhibit active CDK’s. This arrests the cell cycle in whichever phase it is in.
protein produced depends on phase of the cell cycole which the damage is detected.
During G1/S checkpoint, if damage is detected, p53 upregulated the gene p21, which leads to synthesis of the protein p21; it inhibits G1/S- and S-CDK’s
4 basic goals of cell division
approximate doubling in volume and content
exact duplication of genome
exact segregation of duplicated genomes
division into 2 new cells,
each toerh approximate equal volumes/content and 1 genome per copy
describe G1, S, G2, and M in terms of chromosome number
G1 - cell growing but dna not replicating, 2n
S - dna syntehsis. from 2 to 4n
G2 - 4n cells
mitosis - reduction of 4n into 2 2n cells
what phase is this?
typical resting phase
general biosynthesis occurs (growth/maintenance)
G1
what influences the movement from G1 to S
external - receptor mediated pathways (growth factors, hormones, MAP kinase)
Internal - look at internal state, must have accumulated enough “stuff”, enough nutrition, decides its big enough to divide
What events occur during S phase
Duplicating DNA, means you must also duplicate histones
- synthesizing nezymes required for DNA replication
- DNA replication occurs at multiple origines
- removal of replication licensing factors (via the phosphorylation of cdc6 by s-CDK)
centrioles duplicate, but do not separate until mitosis
why is S-phase unidirectional?
because you destroy the replication licensing factors (cdc6) that recongie origin. must wait until cell cycle ends to make more.
G2 events
continued syntehsis of proteins required for mitosis and cytokinesis
addition of cohesins links sister chromatids
once condensins and histones are phosphorylation, enters into Mitosis prophase
Mitosis - Prophase
defined by chromosomes condensing and 2 centrosomes separating.
KEY - still has nuclear membrane. This defines prophase.
- chromosomes condense
- spindle forms (duplicated chromosomes separate)
- kinetochores (proteins) assemble on the centromeric portion of DNA
what markes transition from Mitosis prophase to prometaphase?
breakdown of nuclear envelope by the phorphorylation of lamins which is a function of CDK (cyclin dependant kinase)
what triggers mitosis?
activity of a cell-cycle regulatory kinase
Prometaphase
after nuclear envelope breakdown
-allows microtubules radiating from centrosomes to be captured by the kinetochores, forcing chromosome pair to middle of cell
metaphase
defining feature - all chromosomes attatched to both sides and lined up on metaphase plate.
Once lined up, kinetochore cascade gets turned off (if cascade on, wont proceed).
describe the balanced forces during metaphase
tension on kinetochore fibers pull inward, balanced by outward forces of overlapping polar microtubules being pulled in opposite directions by kinesin
metaphase/anaphase checkpoint
tension on kinetochores activates the Anaphase Promoting Complex (APC) that triggers destruction of coheins (which keep sister chromtids togetner).
Chromatids separate and move towards opposite poles (anaphase)
anaphase can be broken down into two types of movement
- A - microtubules get shorter, the kinetochores chew the microtubules, puling the chromatids towards the poles
- B - kinesins continue to actively push the poles apart on the bipolar microtubule arrays, furhter separating and elongating the cell.
telophase
finish dividing cell by forming ring of actin filaments (non-muscle myosin 2)
-spindle disassembles, ring assembles via signals sent at middle of spindle.
chromosomes decondense via phosphatase activity reversing effects of mitotic kinase on the chromosomes (causing condensation)
prepares for cytokinesis
cytokinesis
contractile ring of antiparellel microfilaments associated with cell membrane and non-muscle myosin II. Similar to belt junction, constricts the cell
orientation of mitosis apparatus determines position of contractile ring
what does cell lose during mitosis and cytokinesis?
cells lose adhesion and round up
cells that fail to complete cytokinesis…
cells that fail to complete mitosis…
binucleate
polyploid - multiple copies of genome in one nucleus
Describe CDK’s
what do they do?
2 subunit kinase. the cyclin dependant kinase subunit (catalytic) and the cyclin subunit (Regulatory)
regulate the cell cycle
how are CDK’s regulated?
cyclin must be synthesiszed and bind to kinase
catalytic subunit gets phosphorylated by two kinases
- one phosphate is removed (the inhibitory) one by an activating protein phosphatase
- active cyclin CDK complex created
what happens if you are lacking wee1?
wee1 = inhibitory kinase, without wee1, the activating phosphate is still added and its never stopping when it should be stopped.
Cells divide too soon, before they are large enough, therefor they are “wee” or small.
how do we get rapid CDK activation?
there is positive feedback, once an active M-CDK is formed, it stimualtes more phosphatase to remove the inhibitory phosphates from other M-CDK’s
2 ways that APC triggers anaphase
- by turning off mitotic CDK
- promotes chromosome separation by chewing away at Securin. When securin destroyed, it releases Separase. Separase chews up cohesin that was previously holding the sister chromatids together, allows the sistesr to separate.
each checkpoint has a different..
combo of kinase subunits and regulatory subunits that determines if you proceed through the cell cycle
what will mess up the cell cycle regulation?
DNA damage
damage in G1 can block movement to S phase
Damage to incompletely replicated DNA blocks movement from S to G2
damaged DNA in G2 = blocks mitosis
all the DNA damage checkpoints involve
p53 which is made and degraded all the time via ubiquitination
when DNA is damaged, p53 gets phosphorylated and stabalized, turning it into a transcription factor
it goes into the nucleus, turns on a set of genes (one being p21) that encode an inhibitory mRNA which binds a cyclin and prevents it from becoming active (can be g1 cyclin, m cyclin, etc..) stops the cell cycle process
when DNA is fixed, p53 degraded, p21 levels drop, can move on in cell cycle
Prophase
Chromosome condensation begins in parallel with the phosphorylation of histone H1 and the addition of condensin. The centrosome with its duplicated centrioles splits into two separate centrosomes, each containing a single pair of centrioles and the mitotic apparatus forms from growth of microtubules between these duplicated, separated centrosomes.. Overlapping microtubules push each other apart via kinesin motors → centrosomes moving away from each other. Nuclear envelope still intact. Kinetochores form from centromere DNA + associated proteins.
Prometaphase
Nuclear envelope breaks down. Microtubules can attach to kinetochores on each chromosome. Kinesins push opposing microtubules apart while dyneins pull astrals (microtubules that don’t attach to kinetochores), acting to pull the centrosomes apart.
Metaphase
Microtubules have attached to chromosomes bipolarly. Once they have all reached equal tension, they align on the metaphase plate. Astrals from opposing centrosomes overlap and kinesins push them apart.
Anaphase
Once equal tension is reached, the Anaphase Promoting Complex (APC) triggers separation of chromatids. Anaphase A consists of chromosome-to-pole movement - kinetochore microtubules shorten, pulling chromatids apart. Anaphase B consists of pole-pole separation. Polar microtubules (astrals) push apart, by kinesin motors, and the centrosomes are pulled apart, by dynein motors, from each other.
Telophase
daughter chromosomes arrive at separate ends of the cell. Nuclear envelope begins reforming around each nucleus. Actin contractile ring begins forming.
Cytokinesis
Contractile ring undergoes microfilament sliding and constricts the cell, eventually pinching into two. The orientation of cytokinesis is is controlled by the spindle orientation during meiosis. Typically occurs immediately after mitosis.
During cell division, the forces that lead to cytokinesis are generated by ____.
a non muscle myosin ATPase.
Although microfilaments must assemble into the contractile ring, the force for cytokinesis comes from contraction of that ring by a myosin.
Small G-proteins of the ras family are important for signal transduction that is initiated by _______________
receptor protein kinases
Hydrophobic signaling molecules such as steroid hormones stimulate cells by binding to ___.
receptor proteins that bind to DNA
During the progression through each cell cycle, the molecule whose concentration/amount is MOST CAREFULLY controlled is
nuclear genomic DNA
The one essential function of all cell cycles is to accurately and precisely double the amount of DNA and then accurately and precisely segregate one copy to each daughter cell. The actual amount of cyclin subunit synthesized in each cell cycle is NOT precisely regulated.
A normal diploid cell that completes mitosis but fails to complete cytokinesis becomes a single ____ cell.
binucleate
The term tetraploid refers to DNA copy number per nucleus NOT per cell.
Cytoplasmic dynein, a minus-end directed microtubule motor protein, has been localized to kinetochores of mitotic chromosomes in mammalian cells. During which of the following stages of mitosis could there be NO dynein-dependent forces exerted on the chromosomes?
prophase
Small intracellular second messengers are important in many signal transduction pathways. Which of the following receptor types do NOT act through a change in the concentration of a small intracellular second messenger?
A. Ligand-gated ion channel
B. Gs-linked trimeric G protein receptor
C. Gq-linked trimeric G protein receptor
D. Gi-linked trimeric G protein receptor
E. Steroid hormone receptor
E. Steroid hormone receptor
Pathways regulated by small G-proteins of the ras family become activated when the G-proteins are stimulated to hydrolyze GTP by a GAP. TF
False - Hydrolysis of GTP inactivates small G-proteins.
A reduction in receptor affinity should result in a reduction in cellular response, even at a saturating concentration of ligand. TF
False
Re-formation of the nuclear membrane around chromosomes requires dephosphorylation of ____________ and occurs during _________ phase of mitosis.
lamins, telophase
Cells taken from three different tissues of the same organism are found to all express the same steroid hormone receptor. When each cell type is treated with the same concentration of hormone, under the same cell culture conditions, three very different patterns of gene expression are observed. This is best explained by assuming that each cell type has ___________.
D. a different pattern of co-activator gene expression
What mechanism prevents DNA from being replicated more than once per cell cycle?
replication origin complexes formed during G1 bind to DNA and are primed by passage through the G1/S checkpoint.
these complexes are modified by the replication fork and cannot reform until the next G1 phase
what mitotic events occur before the nuclear envelope breaks down?
chromosome condensation and spindle elongation
Why is microtubule assembly important for anaphase B movement in most cells?
during anaphase B movement, spindles elongate beyond the original length of the MT overlap in the central spindle. In order for MT sliding to continue, MT must grow longer.
what determines the site of cytokinesis?
signals from the metaphase plate determine the plane of cytokinesis
how does Kinesin contribute to anaphase chromosome movement?
kinesin molecules PUSH on overlapping microtubules of the central spindle, contributing to forces that cause spindle elongation. As spindle poles separate, they pull chromatids apart.
what is a correct description of an inhibitory mechanism that normally degrades cdks?
translocation of kinase subunit out of the nucleus
