Gene Expression and Stem Cells Revision

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PROTO-ONCOGENES

encode proteins that STIMULATE cell division

mutation = oncogene

makes too much active protein all the time

cell divides unchecked - can lead to cancer

mutation is DOMINANT

TUMOUR - SUPPRESSOR GENES

encode protein that INHIBIT cell division

mutation = stop protein made so cell can pass through checkpoints

mutation is recessive - both copies of gene must mutate

OESTROGEN

hormone that can affect transcription

binds to a transcription factor called receptor complex

complex moves from cytoplasm - nucleus

it binds to specific DNA site near start of gene

complex either acts as an ACTIVATOR - helping RNA polymerase. or acts as a (REPRESSOR - blocking RNA polymerase this depends on type of cell)

level of oestrogen affects rate of transcription of genes

RNA polymerase attaches to DNA double helix at start of gene

hydrogen bonds broken so DNA uncoils

one strand is then used as a template to make mRNA copy

RNA lines up free RNA nucleotides along template. specific base pairing - mRNA is complementary copy of DNA

nucleotides paired with specific bases on DNA - joined together forming mRNA

RNA polymerase moves along - separating DNA and assembling mRNA

hydrogen bonds reform on DNA once RNA polymerase has passed - strand recoils into double helix

when RNA polymerase reaches stop codon - stops making mRNA and detached from DNA

mRNA is then edited

EDITING mRNA

eukaryotic DNA contain section that dont code for amino acids - INTRONS

bits needed - EXONS

during transcription both are copied to mRNA from DNA - this forms pre-mRNA

introns removed from pre-mRNA - SPLICING

exons joined together forming mRNA

mRNA then leaves nucleus onto ribosome in cytoplasm

mRNA attaches to ribosome and tRNA carries amino acids to ribosome

tRNA with complementary anticodon to first codon on mRNA attaches to mRNA by specific base pairing

2nd tRNA molecule attaches to mRNA in the same way

2 amino acids attached to tRNA are joined by peptide bond. 1st tRNA molecule moves away leaving amino acid behind

3rd tRNA molecule binds to next codon behind the 2nd. its amino acid binds to the first 2 and the 2nd tRNA moves off

process continues producing a polypeptide chain until it reaches a stop codon on mRNA

polypeptide chain moves away from ribosome.

STEM CELLS

TOTIPOTENT STEM CELLS = can develop into a complete organism. found in embryos up to the 32 cells stage

PLURIPOTENT STEM CELLS = can differentiate into nearly all cells. found in blastocysts (5 day old embryo - 150 cells) - embyronic stem cells

MULTIPOTENT STEM CELLS = can differentiate into related family cells. found in some tissues throughout life. - adult stem cells - used in body to replace and repair damaged tissue

DELETION AND INSERTION MUTATION

cause frame shift

may not change the actual sequence of bases.

all amino acids after the mutation are shifted left or right

so protein is completely wrong and non functional

SUBSTITUTION MUTATION

NONSENSE MUTATION = base change resulting in the formation of a STOP CODON - may result in shorter polypeptide chain thus changing primary structure.

MIS-SENSE MUTATION = base change resulting in different amino acid coded for

SILENT MUTATION = base change dpesnt result in different amino acid - may be more than one triplet coding for the same amino acid - DNA code is DEGENERATE

cancer and oestrogen

transcription

editing mRNA and mutations

translation

stem cells