gene expression + transcription

transcription

dna → rna

translation

rna → protein

mRNA

edited version; carries code from DNA that specifies amino acid

tRNA

carries a specific amino acid to ribosome base on its anticodon to mRNA

rRNA

60% of ribosome; site of protein synthesis

template strand

the dna strand that runs from 3’ to 5’

codons

sets of triples in mRNA; codes for amino acids in polypeptide chain

which rna is complementary to the template strand

mRNA

redudndancy

1+ codons code for each of 22 amino acids

purpose of transcription

create rna molecule

overview of transcription

rna polymerases use a template strand to direct the formation of bases in a new rna molecule

rna polymerase

separates dna strands and transcribes mRNA

promoter

start of gene

terminator

end of gene

steps of transcription

1. initiation

2. elongation

3. termination

initiation

1. TATA box = DNA sequence (TATAAAA) in promoter region upstream from transcription start site

2. transcription factors + rna polymerase = transcription initiation complex

3. transcription factors must recongize TATA box before rna polymerase can bind to dna promter

elongation

1. dna polyermase adds rna nucleotides to the 3’ end of the growing chain (a-u, g-c)

2. rna polymerase untwists dna, then rewinds it after mRNA is made

termination

1. rna polymerase transcribes a terminator sequence (prok) or a polyadenylation signal sequence (euk), then mRNA and polyermase detach

2. called pre-mRNA (euk)

post transcription

5’ and a 3’ poly-A tail are added so it can

1. export from nucleus

2. protect mRNA from enzyme degradation

3. attack mRNA to ribosomes in cytoplasm

rna splicing

pre-mRNA has introns (noncoding sequences) and exons (codes for amino acids)

splicing = introns cut out, exons joined together