The Cell Cycle Flashcards

Question

What happens during meiosis 1?

Answer 1

The homologous chromosomes line up on spindle and separate to opposite spindle poles.

Answer 2

The sister chromatids lines up on the spindle and separate to opposite spindle poles.

Answer 3

- Nondisjunction – failure of homologous chromosomes to separate from one another, either at meiotic division I or meiotic division II - Autosomes – usually fatal, exceptions are: a) Trisomy 21 (down’s syndrome) b) Trisomy 18 (Edward’s syndrome) c) Trisomy 13 (Patau syndrome) * No viable autosomal monosomies - Sex chromosomes: a) XO (Turner’s syndrome) b) XXX (Triple X syndrome) c) XXY (Klinefelter’s syndrome)

Answer 4

A sophisticated control system is used to ensure that the cell cycle is tightly regulated ensuring that: -each phase only occurs once per cell cycle -each phase is carried out in the correct order -phases must be non-overlapping To do this, the cell has a number of checkpoints which are used by the cell to monitor the progress of the cell cycle and decide whether it should go to the next phase or not, examples of checkpoints being: -Extracellular environment – is it favourable for the cell to enter the cell cycle? -DNA damage checkpoints – check the cell DNA is not damaged before DNA replication occurs and before the cell enters M phase -G2 checkpoint – check that the cell DNA is fully replicated before entry into M phase -Metaphase checkpoint - check that all chromosomes are aligned on the mitotic spindle before chromosome separation is initiated

Answer 5

They are regulated by negative pathways that arrest the cell cycle rather than through the removal of positive signals telling it to proceed through the cell cycle. It is controlled by cyclin dependant kinases.

Answer 6

It is a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependant kinase enzymes.

Answer 7

- Cdk must be complexed with cyclin in order to be activated. - Cyclin levels are cyclical (hence their name) during the cell cycle. - Once bound, the kinase ability of the Cdk is turned on and it is able to phosphorylate its target protein and allow the cell to proceed to the next stage of the cell cycle. - Cyclin degradation terminates Cdks activity.

Answer 8

-In lower eukaryotes, such as yeast, a single class of Cdk protein binds successively to different classes of cyclins in order to trigger the different cell cycle phases. This concept is illustrated in the diagram below. -However in higher eukaryotes, the cell cycle controls are more complex and different classes of Cdks act at each stage of the cell cycle. Below is a summary of the different classes of cyclins and Cdks corresponding to each of the cell cycle checkpoints.

Answer 9

LOOK AT CELL CYCLE NOTES TO SEE THE TABLE FOR THE DIFFERENT CYCLINS AND CDKS AND THEIR ROLES.

Answer 10

-Extracellular signals (mitogens) leads to the expression of cyclin D (a G1 cyclin) -Cyclin D binds to its specific Cdk molecule (Cdk4), activating Cdk4 kinase activity -Cyclin D-Cdk4 phosphorylates pRb -Phosphorylated pRB dissociates from the E2F protein -E2F is now active and can stimulate gene expression of cyclin E (G1/S cyclin) and cyclin A (S cyclin) -S phase entry can now proceed

Answer 11

The DNA is checked for damage and the extracellular enviroment is also checked.

Answer 12

- The G1 DNA damage checkpoint inhibits progression into S phase via the activation of p53. - Usually, cell levels of p53 are low because it is constantly being degraded by the proteasome. - Phosphorylated (active) p53 is not degraded - DNA damage results in decreased p53 degradation and this allows p53 to stimulate the expression of p21. - So basically, active p53 promotes transcription of genes that induce cell cycle arrest, in particular it binds to promotor region of p21 gene and stimulates p21 expression - p21 then binds to and inhibits G1/S-Cdk and S-Cdk complexes, leading to cell cycle arrest. - The cell arrest allows time to repair the damaged DNA. - If DNA repair not possible cycle will undergo apoptosis.

Answer 13

The G2 damage checkpoint relies on a different set of proteins to inactivate M-Cdk and block the cell from entering M phase

Answer 14

- Inhibitor of kinase 4 family (INK4): specifically inhibits G1 CDKs (e.g.: CDK 4) - CDK inhibitory protein/ kinase inhibitory protein (CIP/KIP) family: inhibit all other CDK-cyclin complexes (late G1, G2 & M) gradually sequestered by G1 CDKs thus allowing activation of later CDK.

Answer 15

- In a multicellular organism, an adult no longer requires cells to grow and divide to increase the overall number of cells, instead cell death must be balanced by cell proliferation to maintain cell numbers. - When the checkpoints regulating the cell cycle malfunction, cell escapes normal cell cycle checkpoint ----> uncontrolled progression through the cell cycle. - This is because there are no barriers to prevent the cell from uncontrolled entry into the progression through the cell cycle and this can result in cancer. - In human cancers, many genes that regulate cell cycle (e.g.: p53 and pRB) are often mutated.

Answer 16

- One such barrier is the retinoblastoma protein pRB. - pRB is an inhibitor of the cell cycle, it binds to E2F and its loss therefore results in deregulation of the cell cycle. - This is particularly noticeable in the immature cells of the retina, in which loss of pRB function (both copies of the gene no longer functioning), leads to a childhood cancer. - pRB is therefore known as a tumour suppressor gene.

Answer 17

``` DNA repair defects: -ataxia telangiectasia -Bloom's syndrome -Fanconi's anaemia -Li-Fraumeni syndrome -Lynch type II (HNPCC) -Xeroderma pigmentosum Chromosomal instability: -Down's syndrome -Klinefelter's syndrome ```

Answer 18

LOOK AT NOTES TO SEE THE TABLE AT THE END TO SEE THE DIFFERENT DNA DAMAGES.

The Cell Cycle Flashcards

(42 cards)