topic 9

What does diploid mean?

Having two copies of each chromosome — one from the mother (via the egg) and one from the father (via the sperm). Human somatic (body) cells are diploid: 22 pairs of autosomes + 1 pair of sex chromosomes (XX = female, XY = male) = 46 chromosomes. Each pair is a set of homologous (matching) chromosomes.

What does haploid mean?

Having only one copy of each chromosome (a single set). In humans, only the gametes (sperm and eggs) are haploid, with 23 chromosomes.

What are the general requirements for cell division?

1. Duplication of the chromosomes (DNA replication).

2. Separation of each full set of chromosomes to opposite sides of the cell.

3. Division of the cell into two daughter cells, each with a complete copy of the genome.

4. All of this must be regulated so it happens at the right time and only when needed.

What is G1 phase?

The first growth phase. The cell has received a signal to divide. The external and internal environment is monitored to check all is good to go, and the cellular contents (but NOT the chromosomes/DNA) are duplicated — including ER membranes, Golgi, ribosomes, mitochondria, vesicles, cytoskeletal protein monomers, the centrosome and centrioles — so there is enough for two cells.

What is S phase?

Synthesis phase. DNA replication occurs — all 46 chromosomes are copied so each daughter cell will receive a full complement of genetic material. Each chromosome becomes two sister chromatids.

What is G2 phase?

The second growth phase. The cell synthesises proteins required for mitosis and cytokinesis. The duplicated chromosomes are double-checked for errors and most errors are repaired. Cell size is checked again to confirm it has enough contents to divide into two functional cells.

What is M phase?

Mitosis (and cytokinesis). The duplicated chromosomes (sister chromatids) are separated to opposite poles, and the cell divides into two daughter cells, each with a copy of the genome.

What is G0 phase?

The resting stage. The cell is not cycling (cell cycle arrest) — it has exited the cell cycle and is quiescent or senescent. The cell is just doing its job/tissue function and can differentiate to perform that function. Cells can enter G0 from G1.

What is interphase?

The portion of the cell cycle before mitosis, consisting of G1, S, and G2. During interphase the cell contents have been built up and the DNA has been copied and errors repaired. At the end of G2: the nucleolus and nuclear envelope are still distinct, the chromosomes have been duplicated but are still thread-like (chromatin), and the centrioles have been copied.

How long is the cell cycle and its phases?

The full cell cycle is approximately 24 hours. DNA replication (S phase) takes about 10-12 hours of this.

What does a copied (duplicated) chromosome consist of?

Two sister chromatids joined at the centromere. The distinctive X shape is only visible for a short time (in metaphase). Sister chromatids are identical copies, and when separated they become the individual chromosomes of the two new cells.

What is the order of mitosis stages? (mnemonic)

I Pat Monkeys At The Circus = Interphase, Prophase, Metaphase, Anaphase, Telophase, Cytokinesis. (Note: prometaphase falls between prophase and metaphase.)

What happens during prophase?

Step 1 of mitosis. The chromosomes begin to condense and the nuclear envelope/membrane breaks down (becomes not apparent). The two centrosomes (each with two centrioles, duplicated earlier) move to opposite sides/poles of the cell. The mitotic spindle, made of microtubules, begins to form.

What happens during prometaphase?

Step 2 of mitosis. The chromosomes continue to condense. The condensing chromosomes begin to attach to the spindle at their centromeres via the kinetochores. Each centrosome forms a spindle pole.

What is the kinetochore?

A protein complex that assembles on the DNA wrapped around histones at the centromere. It is the site where the spindle microtubules attach to the chromosome. The kinetochore moves along the microtubule to pull one sister chromatid away from its pair toward the pole.

What happens during metaphase?

Step 3 of mitosis. The chromosomes are fully condensed (now clearly seen as sister chromatids) and lined up on the metaphase plate — a line midway between the two centrosomes/poles. All chromosomes are attached to spindles and lined up ready to be separated.

What happens during anaphase?

Step 4 of mitosis. The kinetochores retract along the spindle, pulling the sister chromatids apart from their centromeres and toward opposite poles. The separated sister chromatids are now individual daughter chromosomes, so each new cell will have a full copy of the chromosomes.

What happens during telophase?

Step 5 of mitosis. The chromosomes have reached the poles and begin to decondense (become more diffuse/unwind). Two new nuclear membranes and nucleoli begin to form. The spindle fibres depolymerise and disappear. The cytoplasm begins to divide along the cleavage furrow.

What happens during cytokinesis?

Step 6. The plasma membrane invaginates (bends inward) along the same plane as the metaphase plate — the cleavage furrow — until the cytoplasm is divided in two. The parent cell becomes two daughter cells with identical genetic information. The genomic DNA returns to the thread-like chromatin form. The cellular contents and cytoplasm are split between the two cells.

How does cytokinesis differ in plant cells?

In plant cells, because of the rigid cell wall, a cleavage furrow does NOT form. Instead, a new plasma membrane (cell plate) is assembled between the two new nuclei.

What is the end product of mitosis?

Two genetically identical daughter cells, each diploid with a complete copy of the genome (46 chromosomes in humans).

What is the role of centrioles in mitosis?

The centrioles (within the centrosomes) were copied during interphase. They organise microtubules, including those of the mitotic spindle. During prophase they move to opposite poles of the cell.

Why is the cell cycle highly regulated?

• To control the timing of events

• to turn each process on/off at the right time

• to initiate each event in the correct order

• to ensure each event is triggered only once per cell cycle

• to ensure events occur in a linear, irreversible direction;

• to provide redundancy/back-ups so the cycle works even with some malfunctioning parts

• and to be adaptable to different cell types and environmental conditions.

What are the three classic cell cycle checkpoints? (study checklist)

G1 checkpoint, G2 checkpoint, and M (metaphase) checkpoint. Failure to pass a checkpoint can result in cell death.

What does the G1 checkpoint check?

Whether the cell is an adequate size to divide into two, has enough nutrients and cellular contents, and is receiving the correct growth signals from other cells (growth factors binding receptors to activate proliferation).

What does the G2 checkpoint check?

Whether the chromosomes have been copied correctly (DNA replication complete and error-free) and whether the cell is an adequate size.

What does the M (metaphase) checkpoint check?

Whether the chromosomes are correctly attached to the mitotic spindle before separation proceeds.

What are the four important cell cycle checkpoints? (lecture diagram)

1. G1 checkpoint (cell size, nutrients, DNA damage, growth factors) — Cyclin D with CDK4/CDK6. 2. Intra-S-phase checkpoint (DNA damage, DNA replication) — Cyclin A with CDK2. 3. G2/M checkpoint (cell size, DNA replication) — Cyclin A with CDK1. 4. Spindle assembly checkpoint (chromosome attachment to spindle) — Cyclin B with CDK1.

What are cyclins?

Regulatory proteins that build up in concentration (the cell makes more of them) during a particular stage of the cell cycle. Different cyclins increase during different stages. Once a checkpoint is passed, the cyclin detaches from its CDK and is degraded (broken down).

What are cyclin-dependent kinases (CDKs)?

Enzymes (kinases) that are present at a constant concentration all the time but are mostly inactive, waiting to be activated by binding a cyclin. Kinases activate or inactivate other proteins by phosphorylation (transfer of an inorganic phosphate group).

How do cyclins and CDKs work together to control the cell cycle?

A specific cyclin builds up in concentration during its stage. When there is enough cyclin, it binds to the CDK to form a complex (a "promoting factor"). Binding of the cyclin activates the kinase, which then phosphorylates (activates) proteins that move the cell into the next phase. Once the threshold of cyclin-CDK activation is reached, the cell moves past the "restriction point." After passing the checkpoint, the cyclin detaches and is degraded, the CDK becomes inactive again.

What is the restriction point?

The point the cell moves past once the threshold of cyclin-CDK activation is reached, committing it to progress through the cell cycle.

What other signals/proteins regulate the cell cycle (besides cyclins and CDKs)?

Presence of growth factors (e.g. platelet-derived growth factor, PDGF — cultured fibroblasts proliferate with PDGF but fail to divide without it); enough nutrients; and the density of cells in tissues (cell contact can prevent proliferation to avoid overcrowding — contact inhibition).

What happens when cell cycle control goes wrong?

Cancer. Cancer happens when cells continue to proliferate (grow and divide) when they shouldn't. Progression: a cell with a mutation → hyperplasia → dysplasia → in situ cancer → invasive cancer.