Lecture 11: Cell Cycle, DNA and mutation

Cell cycle controlThe cell cycle needs to be regulated so that cells know when they can divide and when not to divide. Certain eventsmust happen for a dividing cell to progress in order to preventchromosomes becoming unstable and casueing cancer or premature ageing: DNA has to be undamaged DNA replication must have been completed Future DNA replication must be delayed until copies are safely deposited in daughter cells Cells use checkpoints to regulate the cell cycle. The cell clock receives signals from outside the cell ( tyrosine kinase receptors, G protein coupled receptors, TGF-beta receptors, integrins and nutrient status) and processes/integrates them to decide if the cell should start replication or retreat to a non-proliferating state.Checkpoints:G1: Once the cell has crossed the restriction point it can't go back. The G1/S checkpoint ensures that DNA is not damaged.S: DNA is synthesized G2: G2/M checkpoint ensures all the DNA has replicatedM: Chromosome are drawn apart. The spindle checkpoint ensures that chromosomes are attached to spindles before segregation.ControlThe cell cycle is controlled by phosphorylation: Cyclins regulate CDKs (serine/threonine kinases) that control the cell cycle Cyclins and CDKs must be bound together to be active, the initial cyclin-CDK complex is inactive but becomes active after a series of phosphorylation and dephosphorlation steps. This complex is called mitosis-promoting factor(MIF) and phosphorylates target proteins.

MutationsThey occur all the time, most are repaired but those that aren't result in mutations. Single-base substitutions are most likely to occur when DNA is being copied (S phase for eukaryotes). In human for every cell generation there is ~150 mutations which is relatively low compared to the number of base pairs. Males contribute more mutations than females as:~24 mitotic divisions occur from the fertilised egg that starts a female's embryonic development.~400 mitotic divisions have occured in a 30 year old mand since the fertilized egg that formed himSingle Base Substituition (point mutation)A single base is replaced by another base TRANSITION: one purine (A or G) or pyrimidine (C or T) is replaced the other. Caused by Oxidative deamination or tautomerisation TRANSVERSION: a purine is replaced by a pyrimidine or vice-versa. Caused by ionising radiation and Alkylating agents. MISSENSE: a new nucleotide alters the codon so it changes the amino acid NONSENSE: the changed nucleotide results in a stop codon (TAA, TAG or TGA). This prematurely stops translation of the mRNA transcribed from the mutant gene. The earlier the mutation the more likely the protein will be unable to function. SILENT MUTATION: as most amino acids are encoded by several codons some mutations do not change the product and can only be detected by gene sequencing. Splice site mutationRemoval of intron sequences as pre-mRNA is being processed to mRNA has to be very precise as a mutation resulting in one or more introns remaining in the mature mRNA can change the protein product. Nucleotide siganls at each intron/exon boundary guide the enzymatic machinery so if a mutation alters one of these signal then the intron is not removed and remains as part of the mature mRNA.Insertions and deletionsOne or several thousand base pair may be added or removed from a DNA sequence, collectivly they are called INDELS. They can have devistating effects as they cause a frameshift. Frameshift mutations can create new stop codons. Indels of 3 base pairs (trinucleotide repeat) or multiples may be less serious as the reading frame is preserved.