Cell cycle 4.6

  1. Interphase

  • Cell growth & DNA synthesis

  • a cell spends the most time in this phase

  • Individual chromosomes are difficult to see

    • not condensed into easily viewed chromosome arms

  • look like disorganised balls of yarn called chromatin

  • nuclear membrane surrounds the chromatin

  • nucleolus is visible

G1 Phase

  • Cell grows in size

  • organelles are duplicated

  • molecular building blocks necessary for cell division are produced

  • synthesizes proteins and organelles needed for cellular activities

  • when conditions = not appropriate

    • cells may exit the cell cycle and enter a nondividing phase called the gap 0 (G0 phase)

    • resting phase during which cells are not actively preparing for cell division

G1 Checkpoint

- Ensure that the cell is ready for DNA synthesis

  • Ras

    • growth regulator protein

    • checks that cells are big enough to enter the next part of the cell cycle

    • as the cell grows, the amount of ras cyclin increases. When amount of ras cyclin reaches a certain level, it signals the cell to proceed to the next part of the cycle

  • p53

    • inspect the chromosomes’ DNA for damage

    • if there is DNA damage, p53 will stop the cell cycle until DNA repair enzymes can fix the damage

    • if DNA damage can’t be repaired, p53 may signal programmed cell death (apoptosis)

S phase

  • centrosomes replicated

  • the cell replicates its DNA

    • resulting in two identical sets of chromosomes

  • at the end of this phase, each chromosome consists of two sister chromatids (not yet condensed)

G2 phase

  • continues to grow and produce proteins needed for chromosome segregation (ex. microtubules)

G2 checkpoint

  • ensure that DNA has been fully and correctly replicated before the cell enters mitosis (M phase)

ATM/ Nibrin

  • S/ G2 checkpoint

  • protein complex

  • inspects to make sure that DNA was copied without mistakes

DNA Replication

  • cells must duplicate their DNA before mitosis so each new cell has an exact copy of the original

Process:

  1. DNA unwinds and unzips, separating the bases

  1. weak hydrogen bonds between nitrogenous bases break

  2. new nucleotides pair with original DNA (A pairs with T, G pairs with C)

Result:

  • two identical DNA molecules

    • each containing one old and one new strand

  1. Mitosis

  • shortest stage of the cell cycle

  • process of dividing the nucleus to create two identical daughter nuclei

  • key to genetic stability

    • ensuring each daughter cell has the same DNA

  • Phases:

Prophase → prometaphase → metaphase → anaphase → telophase

Prophase

  1. DNA from replication coils up into sister chromatids which are joined in the middle by a centromere

  • forms the X shaped chromosomes    

  1. cylindrical organelles called centrioles organize spindle fibres to form star shape called aster

  • asters help align and group chromosomes in cell division

  1. the asters and centrioles move to different poles

  • chromatin condenses into distinct chromosomes

    • each chromosome has two chromatids, two identical copies of the genetic information it holdsb

  • nucleolus disappears (late prophase, early prometaphase)

  • spindle fibres grow from centrosomes

  • centrosomes begin to move to opposite sides of the cell called the poles

centrosome

  • a section of the chromosome that the spindle fiber attaches to

  • critical for microtubule organization and cell structure

  • comprised of two centrioles

  • replicates to ensure each daughter cell receives a centrosome

  • eventually move to opposite poles and forms mitotic spindles

cell in mitosis → two centrosomes → one centrosome = two centrioles→ centrioles = made of microtubules

Nucleolus

  • area of the nucleus important for ribosome production

All the DNA in a cell constitutes the cells genome

  • the complete set of genetic material that contains instructions for growth, development, and repair

the chromatin further condenses into a single package called a chromatid

  • when chromatid duplicate they form an x called a chromosome

nucleosome

  • consists of DNA wrapped around a core of histone proteins

  • combine and twist to make the chromatin

Distribution of chromosomes during eukaryotic cell division

  • eukaryotic chromosomes consist of chromatin

    • complex of DNA and protein that condenses during cell division

  • in preparation for cell division

    • DNA is replicated and the chromosomes condense

  • each duplicated chromosome has two sister chromatids

    • separate during cell division

  • centromere = narrow “waist” of the duplicated chromosome

    • where the two chromatids are most closely attached

genome = different in prokaryote and eukaryote

eukaryote

  • linear DNA molecule

  • associated with histone proteins

  • no plasmids

  • two or more different chromosomes

Mitotic Spindle

  • structure made of microtubules that plays a crucial role in moving chromosomes during mitosis

  • aster (a radial array of short microtubules) extends from each centrosome

  • the spindle includes the centrosomes, spindle microtubules, asters

  • aster is not a part of the spindle microtubules

Prometaphase

  • nuclear membrane starts to disintegrate and disappeared

  • microscopic protein structures called spindle fibers grow towards the center and attach to the chromosomes at their centromeres

  • chromosomes start to move to the center of the cell

  • some spindle microtubules attach to the kinetochore (proteins at the centromere) of chromosomes and begin to move the chromosomes

  • Non-kinetochore microtubules interact with those from the opposite pole of the cell

    • contributing to the elongation of the cell

kinetochore (ropes pulling chromsomes)

  • protein structure located at centromere of chromosomes

  • pull and move chromosomes

  • help align chromosomes at metaphase plate

non-kinetochore microtubules (poles pushing the cell apart)

  • extend from centrosome

  • overlap with microtubules (non-kinetochore) in the middle from the opposite pole

  • push poles apart to elongate the cell (anaphase)

  • help separate the poles

Metaphase

  • spindle fibers pull the chromosomes by their centromeres

  • chromosomes line up in the middle of the cell

    • metaphase plate

MAD1

  • protein

  • checks to be sure that the spindle fibers have attached properly

  • spindle fibers need to be attached correctly so chromosomes are equally distributed to the new cells

End of metaphase/ Beginning of anaphase

  • spindle fibers help to move the separated chromatids and pull them towards the mitotic poles, opposite sides of the cell

Anaphase

  • spindle fibers pull half of the chromatids to one pole and half to the other

  • cell membrane begins to pinch at the center

  • sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell

  • microtubules shorten by depolymerizing at their kinetochore ends

  • non-kinetochore microtubules from opposite poles overlap and push against each other

    • elongating the cell


Telophase

  • cell membrane constricts further

  • two nuclear membranes form around the separated groups

  • chromosomes begin stretching out into chromatin

  • actin + myosin filaments form a contractile ring

    • pulls the membrane in the middle of the two daughter cells together until the membrane fuses and creates two separate cells

  • area left around the “pinched” area of membrane = cleavage furrow

  1. Cytokinesis

  • cell membrane pinches in completely

  • mitosis is complete

  • two daughter cells exist

    • both are genetically identical with the same number of chromosomes

  • moment when cell divides into two separate daughter cells

Plant (cytokinesis)

  • no centrosomes/ centrioles

  • cell plate is formed

  • becomes new cell wall and cell membrane

Chromosomes decondense after they have been segregated into two nuclei

Cell cycle control system

  • sequential events of the cell cycle are directed by a distinct cell cycle control system

    • similar to a clock

  • regulated by both internal and external controls

  • has specific checkpoints where the cell cycle stops until a go-ahead signal is received

  • the activity of cyclins and Cdks (cyclin-dependent kinases) fluctuates (波動) during the cell cycle

    • trippers passage to the next stage

Cyclins (positive regulation, allowing moving forward in the cycle)

  • proteins who are “activators” for Cdks

G1 cyclins

  • prepare the cell for DNA replication

S cyclins

  • trigger DNA synthesis

M cyclins

  • drive the cell into mitosis

Cyclin-dependent kinases (Cdks)

  • enzymes that phosphorylate target proteins

  • when activated by cyclins

    • add phosphate groups to proteins that control cell cycle progression

  • cell can only move on when cdk activated

  • cyclins drive the cell cycle forward’ checkpoints stop if there is a problem

  • cyclins activate by default unless checkpoints stop

    • the proteins (ex. p53, ras) find something wrong → block cyclin-cdk activity until issue solved

  • Cyclin binds and activates CDK ✅

    • Cyclin levels rise → forms active cyclin–CDK complex

  • CDK phosphorylates target proteins ✅

    • These proteins then carry out the next phase functions (DNA replication, chromosome condensation, nuclear envelope breakdown, etc.)

  • Checkpoint proteins can block CDK activity if there’s a problem ✅

    • For example: p53 → activates p21 → inhibits CDK

  • This prevents cyclin–CDK from driving the cell forward

Those phosphorylated proteins initiate the events of the next phase

  • DNA replication

  • Chromosome condensation

  • Nuclear envelope breakdown

  • Entry into mitosis

cyclins+ cdks = default “go” signal

checkpoint proteins only act (activated) if something is wrong

if nothing happens

  • cdk stays active

  • triggers next stage by phosphorylating target proteins (adding phosphate groups)

if problem occured

  • inhibit cyclin activity until issue solved

Target proteins

  • Phosphorylated by CDKs

  • Purpose: carry out the next phase of the cell cycle

Checkpoint proteins

  • detect problems

  • purpose: stop cell cycle if there is a problem

cyclin, cdk = positive regulator

p53= negative regulator

Evidence for Cytoplasmic Signals

  • cell cycle appears to be driven by specific chemical signals present in the cytoplasm

  • some evidence for this hypothesis comes from experiments

    • cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei

Experiment 1

  • cell in S phase fused with cell in G1

  • G1 nucleus immediate entered S phase

    • DNA was synthesized

Experiment 2

  • cell in M phase fused with cell in G1

  • G1 nucleus immediately began mitosis

    • spindle formed

    • chromatin condensed

  • even though chromosomes had not been duplicated (skipped s phase)

  • = cytoplasms contain signals that can trigger cell progression

  • Cytoplasmic proteins and signals control progression through the cell cycle, independent of the nucleus.

  • The cytoplasm contains signals that force the nucleus to act

  • The nucleus just responds to those signals

If: mutations in proteins

Mutation in Ras protein

  • too little or too many organelles

    • disrupts cell functions

Mutation in P53 protein

  • cells avoid apoptosis

  • allowing cells with mutation to divide

  • might lead to cancer

Mutation in ATM/ Nibrin

  • increase genomic instability

    • cells with incomplete/ damaged replicated DNA to divide

  • increased risk of cancer

Mutation in Mad1

  • chromosomes not distributed equally and properly during anaphase

    • different number of chromosomes in each daughter cell