Week 12 Flashcards
What are the three main kinds of Cdks?
- G1/S
- S
- M
How do cyclins interact with Cdks?
Cyclins associate with the kinase regions. They act as master regulators and control when the kinases are active. Cdk levels don’t change but cyclin levels do.
Function of G1-cyclin
Helps govern activity of G1/S-cyclin
Function of G1/S-cyclin
Helps trigger progression through Start, resulting in a commitment to cell-cycle entry.
Function of S-cyclin
Helps stimulate chromosome duplication
Function of M-cyclin
Helps stimulate entry into mitosis
How is G1-Cdk activated?
G1-CDK is activated by corresponding G1 cyclin, which is regulated by various external factors.
Role of mitogens
Mitogens can serve as external factors that stimulate entry into cell cycle
Proto-oncogenes and examples
Proto-oncogenes are genes that become oncogenes (cancer promoting) when they acquire mutations that make them constitutively active. Examples: Ras GTPase, MAP kinases, and Myc
What causes expression of Myc (a transcriptional factor)?
The MAP kinase cascade creates downstream effects that result in expression of Myc (a transcriptional factor)
What does Myc activate?
Myc activates the expression of the G1 cycle (regulates entry into the cell cycle)
Rb
Named after retinoblastoma, a type of cancer of the eye.
Rb is considered a tumor suppressor because loss of its activity leads to cancer.
E2F
Transcriptional regulator that turns on the expression of many genes, including the G1 cyclin and S-cyclin. Promotes its own expression.
Role of Rb
When there’s no signal, Rb is constitutively bound to E2F. When Rb is phosphorylated and inactivated, this allows E2F to be released from Rb.
If Rb is always active, what effect will this have?
Rb is a tumor suppressor gene. If the protein’s activity is inactivated, E2F will always be active and driving the cell cycle, leading to tumors sometimes
Role of CAK
Cdk-activating kinase (CAK) is always present and phosphorylates Cdk.
Why is CAK not sufficient for M-Cdk activation?
Wee1 kinase adds inhibitory phosphate to Cdk.
How is M-Cdk activated?
Removal of the inhibitory phosphate by Cdc25 phosphatase activates M-Cdk.
What does M-Cdk activate and inhibit?
M-Cdk activates Cdc25 phosphatase and inhibits Wee1 kinase (positive feedback!)
What initiates the metaphase to anaphase transition (degrades S- and M-cyclin)?
APC/C
APC/C
APC/C is an E3 ubiquitin ligase – ubiquitinates its substrates and targets them for degradation via the proteosome. Is partially activated by M-Cdk
Challenges of DNA replication in S phase
- Minimize mutational errors during replication
- Every nucleotide must be copied once and only once.
Origin recognition complex (ORC)
- DNA replication begins at the origins of replication.
- Many origins across the genome.
- Origins are bound with Origin Recognition Complex (ORC).
Role of helicases
To unwind DNA
What forms the PreRC, and what effect does this have?
Pair of Mcm helicases (enzymes that unwind DNA) are loaded onto origins to form a prereplicative complex (PreRC).
PreRC is ‘licensed’ and ready to ‘fire’ in the S phase.
How are Mcm helicases activated?
Via phosphorylation
How are origins inactivated and why?
Via phosphorylation so they cannot be licensed again.
Importance of phosphorylation of the preRC after replication
Phosphorylation of the preRC is critical because it prevents replication from happening more than once (undesirable)
At the end of S phase, what do the chromosomes look like?
At the end of S phase, each replicated chromosome consists of a pair of identical sister chromatids glued together along their length.
Cohesin
Cohesin is a protein complex, made in part of SMCs (Structural Maintenance of Chromosomes).
Role of cohesin complex
Cohesin complex needs to be removed for anaphase to proceed (i.e., separation of the sister chromatids) because it keeps sister chromatids together.
Role of condensin complex
The condensin complex helps package chromosomes during prophase
How are condensin complexes similar to cohesin complexes?
- Both have ring structures.
- The condensin complex (like cohesin) is composed of SMCs (structural maintenance of chromosomes)
How does the condensin complex help package chromosomes?
Forms an 8 ring oligomer that uses ATP hydrolysis to pull and loop DNA strands
Three types of microtubules make up the mitotic spindle
- Kinetochore
- Interpolar
- Astral
Kinetochore role
interact with the chromosomes. Help center chromosomes
Interpolar microtubule role
Interact in an antiparallel fashion (via kinesins and dyneins). Help position spindle poles at appropriate distance from each other
Astral microtubule role
Interact with the cell cortex. Help position spindle poles relative to the cell
Directionality of kinetochores
The minus end faces the centrosome and the plus end faces the DNA (chromosomes)
Directionality of interpolar microtubules
Interpolar microtubules are unique in that they can be in either orientation (plus or minus end can be switched)
How are chromosomes attached to the mitotic spindles?
Via the kinetochores. The kinetochore assembles at the centromere region of the chromosome. This is where the microtubules will come and bind.
How is bi-orientation in metaphase achieved?
By sensing tension. When tension is unstable, the kinetochore will let go of the microtubules. When stable, additional microtubules will be recruited.
What happens when there are incorrect attachments of the spindles to the chromosome in cases of low tension?
In case of low tension, when there are incorrect attachments of the spindles to the chromosome, Ndc80 is within reach of Aurora-B kinase. This phosphorylation event decreases affinity of Ndc80 for microtubules, thus weakening these attachments.
What triggers sister chromatid separation in anaphase?
The APC/C
What cleaves cohesin?
A protease (enzymes that cut other proteins) called separase cleaves cohesin.
How and when is separase activated?
Separase is activated only in the metaphase to anaphase transition. It is kept inactive by securin which is constitutively bound to separase.
Activation and activity of APC/C
The APC/C complex will target securin for degradation in the proteasase.
APC/C complex is activated by Cdc20 (at least in part)
Quality check in progression to anaphase
Spindle assembly checkpoint blocks progression to anaphase (chromosome separation) until all chromosomes are bi-oriented Mad2 inhibits APC/C function and prevents the cell from progressing to anaphase.
What does Mad2 protein bind to?
Mad2 protein binds to unattached kinetochores
What causes chromosome separation?
Shortening of the kinetochore microtubules (through depolymerization) (Anaphase A), and moving apart of the spindle poles (Anaphase B) results in chromosome separation
How do the kinetochore microtubules generate a pulling force?
The kinetochore microtubules will start to depolymerize at the plus end (one of the pulling forces)
The poles will be pulled toward the periphery of the cell.
Meiosis
Meiosis reshuffles DNA through recombination to generate 4 genetically distinct haploid cells (super important because genes are combined in novel and unique ways. Evolution can act on these and select for the ‘fittest’ combinations).
Meiosis in females vs males
In females, 3 of the 4 haploid cells becomes a polar body and only one cell becomes an oocyte. In males, all 4 haploid cells become sperm.
Is meiosis I or II more similar to mitosis?
Meiosis II is similar to mitosis.
What holds homologous chromosomes together?
Homologous chromosomes are held together by the synaptonemal complex to allow for pairing and crossover
Key features that distinguish meiosis I from mitosis
- Both sister chromatids attached to the same spindle pole
- Crossovers physically link homologs together
- Cohesion is not removed from regions near the centromere.
When are meiosis I and II completed in an oocyte?
Although oocytes are made during development, meiosis I isn’t complete until ovulation! And meiosis II not until fertilization
What causes Down syndrome?
Down syndrome (inheritance of an extra copy of chromosome 21) is due to nondisjunction.
Tight junction
Seals gap between epithelial cells
Adherens junction
Connects actin filament bundle in one cell with that in the next cell
Desmosome
Connects intermediate filaments in one cell to those in the next cell
Gap junction
Allows the passage of small water-soluble molecules from cell to cell. Allow for communication between cells.
Hemidesmosome
Anchors intermediate filaments in cell to ECM
Actin-linked cell-matrix junction
Anchors actin filaments in cell to ECM
Which junctions link cytoskeleton between cells?
Adherens and desmosomes
Which junctions link cytoskeleton to the ECM?
Actin-linked cell-matrix junction and hemidesmosomes.
Which junctions connect to actin?
Adherens and actin-inked cell-matrix junction
Which junctions connect to intermediate filaments?
Desmosomes and hemidesmosomes
Cadherins
transmembrane adhesion proteins in cell-to-cell junctions.
Integrins
transmembrane adhesion proteins in cell-to-matrix junctions
The cadherin superfamily of proteins is found in which type of organisms?
Found in all multicellular animals, but not in fungi, plants, bacteria, or arhaea.
homophilic binding
The same protein interacts in both cells. This is the type of binding cadherins use.
Cadherin and development
The epidermal, neural plate, and mesoderm cells of an early embryo, when mixed in a random manner, resort themselves into the three groups. Three different cadherins were later shown to play a critical role in this process of cell sorting.
How do differences in cadherin expression play a role in tissue organization?
- Cells expressing N-cadherin will only interact with other cells expressing N-cadherin (and same for E-cadherin)
- This is how cells are able to congregate and form clumps.
- With high and low levels of expression of E-cadherin, an inside and outside clump forms (but all one clump)
Cancer and cadherin
By mutating cadherin, cancer cells can break away from epithelial cell layer.
Cadherin in the absence of calcium
In absence of calcium, cadherin molecules become ”floppy” and thus lose their ability for homophilic interactions. (on the extracellular side of the membrane)/
What type of tissues are desmosomes commonly found in?
Heart muscle (because desmosomes impart mechanical strength) and epidermis.
How do tight junctions assist with maintaining certain aspects of polarity?
Tight junctions also form a “fence” between the apical and basal plasma membrane compartments (in other words, help to maintain certain aspects of cell polarity)
In the example with glucose transporters, there is an asymmetrical localization of Na+ driven glucose transporters and passive glucose transporters.
Claudin and occludin in tight junctions
Claudins are important for assembly and the structure of the “seal”. Occludins are primary determinants of permeability.
Examples of how gap junctions couple cell electrically
- Heart and smooth muscle contractions synchronized via gap junctions
- Neurons are also coupled via gap junctions
What are gap junctions permeable to?
permeable to ions and small molecules (e.g., amino acids, nucleotides, very small peptides)
What regulates the opening and closing of connexins that compose gap junctions?
They can be regulated by pH or Ca2+
Structure of integrins
Heterodimers of alpha and beta subunits.
Characteristics of integrins
- Bind to matrix components in Ca and Mg dependent manner
- Dynamic (i.e. switch between being active and inactive) at the actin-linked cell matrix junctions. Important in migrating or crawling cells.
- Can be activated from the outside OR the inside of the cell
