Chapter 14: The cell division cycle Flashcards
• Events that need to take place for an organism to grow
- cell mass must increase
- dna must replicate
- cell division must occur
• Phases of the cell cycle and events that happen in each phase (also what G0 phase is)
- Interphase – 90% cell in this
- G1 (first gap phase) – interval between end of mitosis and the beginning of DNA synthesis
- S (synthesis phase) – DNA synthesis occurs to make copies of each chromosome (production of sister chromatids)
- G2 (second gap phase) – Interval between DNA synthesis and the start of mitosis
- Mitotic Cell Division/M phase includes mitosis and cytokinesis
- G0 phase is a temporary exit from the cell cycle
- Some cells remain in G0 for days to years and may never enter the cell cycle (skeletal/cardiac and nerve cells)
• The cell cycle checkpoints and what purpose they serve
Start Checkpoint – occurs in late G1 where the cell determines if enough growth signals, nutrients & proteins are available to commit to cell division
• G2 /M Checkpoint – determines if all DNA was replicated properly and enough signals and proteins are available to enter mitosis
• Metaphase/Anaphase Checkpoint – occurs during metaphase – determines if all chromosomes are aligned at metaphase plate
• The function of cyclins and Cdks during the cell cycle and how they function
- each checkpoint controlled by regulatory proteins below:
CDKs: phosphorylate and activate proteins involved in various stages of the cell cycle
(proteins involved in nuclear envelope breakdown, chromosome condensation, spindle assembly)
•cdk present always, but activated only key times in the cell cycle & then quickly deactivated
• Only when the Cdks are bound by their cyclin can they be activated and able to phosphorylate targets
• Know specific Cdk and cyclin pairs specified and what they stimulate
- g1-cdk = cyclin d and cdk4,6 -> g1 phase exit and cyclin production
- g1- scdk = cyclin e and cdk2 -> stimulates entry to s phase
- s-cdk = cyclin a, cdk 2 -> s phase exit g2 phase entry
- m- cdk = cyclin b, cdk 1 n-> forms active mcdk phosphorylates targets that allow entry into m phase
• Understand the cyclic nature of cyclins throughout the cell cycle and how APC works (and why)
- As soon as cyclins have been produced in high enough number to push past a checkpoint, they are quickly targeted for destruction
- Cyclins that trigger entry into M phase are abruptly degraded partway through M phase due to anaphase-promoting complex (APC) which tags these cyclins with ubiquitin
- Proteins with a ubiquitin tag a re targeted for degradation by the proteasome and their levels drop quickly
• Know the three mechanisms that control Cdk activation and proteins involved in these processes
- cyclin binding & their phosphorylation state
- Removal inhibitory po4 by an activating phosphatase called Cdc25 = needed for activation
- Cdk inhibitor proteins, p27, which specifically binds to S-Cdk and prevents entry into S phase = more time in G1 phase
• Know the purpose of mitogens and how they initiate entry into the cell cycle
• mitogens: tell mammalian cells to divide
• W/o mitogens, cells are arrested in G1
• pRB normally binds and inhibits a transcription factor -> expression of many cell cycle genes for G1→S transition
• Mitogens stimulate activation-> G1 & G1 /S cyclins which phosphorylate and inactivate pRB causing its release from
the transcription factor
• Know how G1 phase is regulated and how p53 is involved in shutting down entry into S phase if DNA damage is detected
- to enter s phase dna has to be intact
• DNA damage -> high conc. & activation of a transcription factor p53
• p53 controls transcription of Cdk inhibitor p21 -> inactivation of G1 /S CDK complexes
• p53 defects cause over-proliferation of cells by allowing entry into S phase even when DNA is damaged; mutations
in p53 found in ≈50% of all human cancers
• The mechanism of S phase and how S-Cdk, ORC and Cdc6 are involved in stimulating DNA replication
- After G1, S phase: DNA replication carried out; accuracy is key and replication only x1
- Prep for replication starts in G1 phase where proteins, like ORC is recruited to OOR
- ORC stays bound during entire cell cycle, but during G1 it recruits Cdc6: functions to load DNA helicase onto DNA
- Once cell enters S phase, the newly made S-Cdk phosphorylates/ activates helicases = opening of dsDNA
- S-Cdk recruits rest of DNA replication proteins & prevents re-replication of DNA by phosphorylating Cdc6 -> degradation
• The basics of M phase and how M-Cdk is involved and regulated.
- M phase (mitosis & cytokinesis) 1 hour, but very dramatic stage in cell cycle, where duplicated chromosomes are segregated into daughter cells using cytoskeletal machinery.
- M-Cdk starts entire event, helping chromosomes prep for segregation and inducing formation of mitotic spindle
- M-Cdk begins accumulating during G2 and at end becomes activated by removal of inhibiting po4 by Cdc25
- Cdc25 is vital in determining whether entry into M phase happens.
- If errors in DNA replication, Cdc25 is inhibited -> cant remove inhibitory po4 from M-Cdk and cell wont enter M phase
- If DNA replication successful, activated M-Cdk stimulates + feedback loop, activates more Cdc25, activates more M-Cdk
• The function of cohesins and condensins
cohesins: dna copies are kept together by proteins
condensins: in prophase, the duplicated chromosomes, condensed by proteins called condensins, are phosphorylated and activated by M-Cdk.
• The phases of mitosis and events that happen during each phase. Know any Cdks or proteins specified in each stage
- prophase: chromatin condenses into chromosomes, nucleolus disassembles and disappears, mitotic spindle develops outside nucleus ( spindle mt, astral mt, centrosomes)
- prometaphase: -NE breaks down; caused by phosphorylation of NP & filament
proteins by M-Cyclin.
-Chromosomes form kinetochores on either side of centromere
-Kinetochore mt attach to kinetochores and start to move chromosomes to metaphase plate (middle of the cell) - metaphase: • Chromosomes arranged so their centromeres lie on metaphase plate in middle of cell
• Metaphase plate = future axis of cell division
• Kinetochore mt help align chromosomes on metaphase plate - anaphase: • To separate, cohesins degraded by separase, taken by securin
• APC, ubiquitinates/ targets securin for degradation
• Securin = inhibitory protein binds/inhibits protease called separase
• Active APC leads to degradation of securin, which frees separase to degrade cohesins allowing for chromatid
separation
• Kinetochore microtubules shorten to pull each chromatid (now a daughter chromosome) towards one side of the cell
• Polar microtubules from either end of the cell lengthen and push against each other to elongate the cell - telophase: • Once chromosomes are separated to either side of cell, kinetochore fibers (spindles)
are disassembled
• Nuclear envelope reforms around each set of chromosomes
• Nuclear membrane vesicles fuse & proteins originally phosphorylated are dephosphorylated
• Pores pump in nuclear protein, chromosomes expand, & transcription can begin
• Chromosomes become diffuse chromatin again
• Nucleoli reform
6 cytokinesis: cytoplasmic division
• The mechanism of the different microtubules that form the mitotic spindle and their function during cell division
-Spindle microtubules: polar and kinetochore
-Polar microtubules – extend from the two poles to the cell equator
-Kinetochore microtubules – attach to either side of chromosomes
Astral microtubules – anchor mitotic spindle to either side of the cell
• The function of APC, separins and separases during anaphase
- APC, ubiquitinates/ targets securin for degradation
- Securin = inhibitory protein binds/inhibits protease called separase
- separase to degrade cohesins allowing for chromatid separation
