Cell Bio 5: Cell Division and Cell Cycle

Three functions of chromosomes

Stop DNA getting tangled, provide strength and stability, allow DNA to be moved around easily by the cytoskeleton

What type of proteins is DNA bundled around

Histone proteins

DNA condenses from what to form chromosomes

Chromatin

How many centrosomes are there in a cell that is ready to divide by mitosis

(At least) 2

What is the mitotic spindle apparatus

A microtubule-based structure with a MTOC on either side of the cell

What are the three types of microtubules associated with the MTOC

Astral, polar, and kinetochore microtubules

What type of microtubules connect the two poles by interacting with each other at the centre of the microtubule spindle apparatus?

Polar mts

What type of microtubules radiate from the MTOC?

Astral mts

What type of microtubules attach directly to the chromosome at the centromere?

Kinetochore mts

What is the difference between the kinetochore and centromere?

Centromeres are the chromosomal regions on which the kinetochores (large protein complexes) assemble

Which two motor proteins are involved in positioning the chromosomes during mitosis

Kinesin and dynein

Name two ways in which mitosis can be observed

Polarisation light microscopy and fluorescence microscopy

Prophase

chromatin condenses into chromosomes

Prometaphase

nuclear membrane breaks down, the mitotic spindle invades the space where the chromosomes are and begin attaching to the chromosomes

Metaphase

chromosomes line up along the metaphase plate (equator) and quality control checkpoint

Anaphase

chromosomes break at centromeres and sister chromatids move to opposite ends of the cell. This separation occurs through the digestion of a protein called cohesin. The enzyme is called separase.

Telophase

reformation of nuclear membranes and two new daughter nuclei form, nucleoli reappear, chromosomes turn back into chromatin

How do the microtubules pulling apart the sister chromatids appear to shrink even though they are not capable of contracting?

Scientists believe there are motor proteins attached to the kinetochores which depolymerise the microtubules as they walk. Disassembly at + end.

What is the composition of the ring that cleaves the cell into two cells

Myosin II and actin

What is the difference between mitosis and cell division?

Mitosis refers to nuclear division only whereas cell division refers to nuclear division AND cytoplasm division (cytokinesis)

How long does it take for one total cell cycle

Around 24 hours

How long does a typical S phase last

10-12 hours

How long does mitosis typically take

30-60 mins

What happens in the G1 and G2 phase of the cell cycle

Growth

What happens in the S phase of the cell cycle

Copying of DNA

What happens in the M phase of the cell cycle

Mitosis (PPMAT) and cytokinesis

What is interphase?

Everything apart from M phase in the cell cycle

How can flow cytometry be used to measure DNA content in dividing cells?

A stream of individual cells suspended in liquid are passed through a flow cytometer. A laser is shone onto each cell and fluorescence measured. The DNA has been stained blue prior to cells going in the machine. The machine scans thousands of cells per second and creates a histogram (x axis is amount of fluorescence, y axis is number of cells).

What are Fucci cells?

Fluorescent, ubiquitination-based cell cycle indicator cells. They change colour depending on what stage of the cell cycle they are in (red in G1 phase and green at onset of S phase).

How do cell divisions in the early embryo differ from those in healthy adult tissue?

There is no G phase so the cells halve in size each division

What is the G0 phase?

When cells exit reversibly from the cell cycle and are not actively dividing (for example most neurons in the adult brain are in the G0 phase)

What are the three cell cycle ‘checkpoints’?

G1: ‘am I big enough?‘, G2: ‘how’s my DNA?‘, metaphase-anaphase transition: are all chromosomes present and aligned correctly?

What happens if the conditions for the cell cycle checkpoints are not met?

Cells can delay cell cycle progression or undergo apoptosis (cell suicide).

What does MPF stand for?

M-phase promoting factor, something in the cytoplasm (not spindle or DNA) that promotes mitosis!

What are cyclins?

Proteins that fluctuate in level during the cell cycle and control its different stages. They are regulatory subunits that activate CDKs.

How does the expression of cyclin B fluctuate in the cell cycle?

It increases at a steady rate from G1 to mid M-phase and then drops off during M phase

What are CDKs?

Cyclin-dependant kinases

How do the expression of CDKs fluctuate in the cell cycle?

Stay constant

How are CDKs activated and deactivated?

They are activated by cyclin binding and inactivated by cyclin destruction

How do the different cyclin-CDK pairs control each phase of the cell cycle?

CDK activation upon phase start, followed by later inactivation. CDK levels are stable with activity depending on cyclin concentration/binding. All cyclins are destroyed at the end of the cell cycle.

Cyclins and CDKs pair to regulate cell cycle progression and trigger mitosis through what process?

Protein phosphorylation