BIOL 300 - Chapter 9

how many rounds of replication occur per cell cycle?

one

what happens when humans inherit too much DNA?

mutations like trisomy (down syndrome)

what is the pairing between replication and division?

if a cell replicates, it must also divide - can’t have one occur without the other

what happens when replication happens LESS than the amount of cell divisions?

there is a loss of genetic materials

what are the three checkpoints during DNA replication?

G1 checkpoint, G2 checkpoint, M checkpoint

what is the purpose of the G1 checkpoint?

looks for necessary nutrients, energy, growth factors, and if there’s damage to DNA

what is the purpose of the G2 checkpoint?

sees if cell is big enough after synthesis, and if DNA properly replicated

what is the purpose of the metaphase checkpoint?

looks to see if chromosomes are equally inherited

what happens if replication occurs MORE than cell division?

leads to endocycle

what is endocycle?

replication occurs without mitosis, leading to polyploidy and increased cell size

where is the endocycle necessary?

seen in liver, reproductive cell because they do a lot of activity

what are the characteristics of the prokaryotic cell division mechanism?

fast cell cycle (fast replication, less regulation), consists of external and intrinsic rate limiting factors

what are the external rate limiting factors for prokaryotes?

nutrient availability

what are the intrinsic rate limiting factors for prokaryotes?

presence of necessary nucleotides and proteins, cell size and membrane amount, even ratio of molecules nm

how does ecoli shrink its cell cycle down to 20min when it should take 60min?

first replication starts, then 20mins into the first cycle, 2nd replication starts, and etc

what is the process of bacterial cell division?

bacterial plasmid will start replicating left and right, and nucleases will get into the terminus to pull them apart and put them together. Cell forms septum to ensure one plasmid is on one side of the wall and vice versa. topoisomerase cuts everything apart into two identical plasmids, and septum finishes forming.

what is mutagenesis?

production of mutated strains of genes - each strain has 1 mutation that randomly occurs

what can mutagenesis be used for in studying bacterial cells?

can identify if the mutation affects cell division by using its phenotypes since bacteria are ss

what is the issue with studying mutagenesis in bacterial cells?

bacteria are monoploid, so any LOF mutations won’t survive and therefore can’t be studied

what is the solution to studying mutagenesis in bacterial cells?

temperature sensitive mutations - proteins lose structure at high temperatures

what are minicells?

bacterial cells with misplaced septum due to different genes

what is a multinucleated filament?

strands of bacterial DNA that didn’t completely separate

what are anucleate cells?

bacterial cells that didn't inherit DNA, or contain both copies of plasmid

what genes control the rod shape of bacteria?

MreB, PBP2, and RodA

what proteins control the location of the septum in bacterial cells?

minC, minD, minE, noc/slmA

what ratio of proteins determines the number and location of septa?

minE/minCD

what is the process of septum location and movement through proteins?

minD associates with membrane, minC then binds to MinD. MinE is at interphase between minD presence and absence and forms a ring, and the three proteins move around the cell due to lack of stability. Z ring assembly is highest at poles and lowest at midcell (means they’re more stable there & for longer)

what is the purpose of slmA/Noc proteins in bacterial septums?

prevents septation from occuring in space where chromosome is

what is the process (or “mentality”) of prokaryotic cell division?

divide until you run out of nutrients

what controls multicellular eukaryotic cell division?

dividing only when there are appropriate growth factor signals and all necessary checkpoints are good to go

which cells in our body can divide?

stem cells

what happens if stem cells are activated and told to divide when they’re not supposed to?

tumors start to form

what requirements do G0 cells have to meet before returning to G1?

must be able to respond to cell signals like growth factors, can’t be defective and has to pass checkpoint mechanisms

what is epigenetics?

study of changes in how genes are expressed wtihout altering base DNA

why can most cells not proliferate?

epigenetics - cells have controls and regulations to prevent proliferation, because that’s what cancer is

what steps have to be taken before cells can exit G0 and enter G1?

growth factor signaling > inc cyclin levels, checkpoints > CAK, CKI, phosphatase, CDK activation

what is the process of growth factor signalling in a cell?

ligand (growth factor) binds to receptor tyrosine kinase (receptor) on cell surface. the receptors dimerize and can phosphorylate each other in transphosphorylation. phosphorylation then recruits Ras GEF (primary effectors) which then recruits Ras GTP, a CA; this gene will go turn on other genes that are involved in nucleotide synthesis - transcription factors

what is a CA allele?

constitually active allele (also a hypermorph) - if a mutation occurs, the cell will constantly keep dividing

which protein does Ras-GTP lead to the synthesis of?

cyclins

what are cyclins?

proteins that regulate the progression of the cell cycle by binding to and activating cyclin-dependent kinases (CDKs)

what is a dominant negative allele?

allele that is a LOF allele - can’t bind to growth factor

how is Ras deactivated?

binding to GDP

how is Ras activated?

binding to GTP

is Ras’ default form active or inactive?

inactive - once GTP binds to activate it, Ras will eventually turn GTP into GDP as a safety mechanism to prevent constitual activity.

what happens if a Ras gene mutates into a CA allele?

you can’t stop cell division - tumors form

what do small GTPases act as?

molecular switches

what do Ran GTPases do?

facilitate transport of proteins to/from nucleus

what do Rab GTPases do?

regulate vesicle trafficking

what do Ras GTPases do?

control cell differentiation and growth signaling

what is a common secondary function of GTPases?

bind to and hydrolize GTP

what is GTP used for?

used as a switch to activate/deactivate GTPase enzymes

what is a GAP?

GTPase activating protein - speeds up the reaction of GTPase transforming GTP into GDP, so enzymes stay active for shorter periods.

what are the checkpoints looking for when scanning the DNA?

defects like massive chromosomal breaks, or changes in gene architecture

what are the cell cycle checkpoint genes?

Rb and p53 - transcription factor genes (also act as tumor suppressors!)

what type of mutation is present if Rb or p53 are mutated?

considered LOF tumor suppressor mutations - mutations in both genes are usually seen together

what are tumor suppressors?

genes that function to inhibit the cell cycle, to prevent uncontrolled cell division

what is an oncogene?

mutant allele of a gene that activates cell cycle without signals - considered hypermorphic, GOF alleles of cell cycle activators like CA Ras

what is the purpose of the p53 gene?

relays to CDK that DNA damage is present; if there’s an excess of p53, proteins designed to detect DNA damage something is wrong

what would happen if a LOF mutation occured in p53?

cannot signal to CDK and other proteins that DNA is damaged, leading to synthesis and division of a cell with damaged DNA (not good - leads to tumors)

what is the p53 gene referred to as?

guardian of the genome, because it detects if DNA is too damaged and whether a cell should undergo apoptosis

what is the signalling pathway for cyclin and CDK?

when cyclin and CDK bind, they' work with kinases like CAK and CKI to activate. p53 can promote the activity of p21, which inhibits active CDK-cyclin interactions. DNA damage can also promote the activity of Chk1 and Chk2, which inhibit proteins that promote cyclin-CDK activity.

What is Rb?

gene that prevents retinoblastoma - first tumor suppressor gene identified, and mutations in this gene are responsible for most retinoblastoma cases

what is the process of the Rb-e2f checkpoint, starting with growth factors?

Growth factors activate the Ras/Raf pathway, leading to Cyclin D–CDK complex formation, which phosphorylates and inactivates Rb, releasing E2F to trigger S-phase gene expression and cell cycle progression. If DNA damage or stress is detected, p53 (via p21) or Chk1/Chk2 (via Cdc25 inhibition) can block this process to stop the cycle.

what happens after Rb is activated by upstream genes/proteins?

binds to e2f factors that turn on genes needed to pass G1 and enter S phase, so it controls the restriction point - basically stops replication

what happens when Rb is phosphorylated by a cyclin-CDK complex after binding to e2f?

it releases e2f to permit cell cycle progression

what happens with homozygosity in Rb LOF mutations?

embryonic lethality

what happens with hereditary heterozygosity in Rb LOF mutations?

develops in youth in both eyes - there’s one mutated allele and one wild-type allele, so if the wild-type version is damaged the mutated allele will be expressed

what happens with nonhereditary heterozygosity in Rb LOF mutations?

born with no RB mutation, but the mutation develops in older age in usually one eye

for eukaryotic cell cycle regulation, what two mechanisms have to converge?

receive growth factor signaling to promote cyclin levels and activation of CDK, and checkpoints have to be released through CAK, CKI, and phosphatase