Gene Expression
Central Dogma
Information flows from: DNA RNA Protein
Exception: reverse transcriptase (retroviruses)
RNA DNA RNA Protein
3 kinds of RNA
Messenger RNA (mRNA) Carries info for making protein
Transfer RNA (tRNA) Bonds to 1 kind of amino acid and carries it to
ribosome
Ribosomal RNA (rRNA) Catalytic; protein synthesis
RNA
Synthesized from DNA template - transcription
Nucleotides differ from DNA nucleotides in that: Ribose instead of deoxyribose Uracil instead of thymine Single stranded
“The Code”: Important terminology
Codon – sequence of 3 consecutive mRNA bases – specifies 1 amino acid
Anticodon – 3 consecutive bases on tRNA which are complementary to codon on mRNA
Transcription: Copying DNA’s code to RNA
RNA synthesis catalyzed by RNA polymerases
DNA serves as a template – only 1 side of DNA strand
5’3’ direction; no primer needed
Initiation: RNA polymerase binds to promoter (not
transcribed) – upstream from gene Unwinds DNA, begins transcription
Elongation: Additional RNA nucleotides are added to 3’
end of growing RNA molecule Double helix rewinds
Termination: RNA polymerase recognizes termination
sequence RNA polymerase separates from DNA
template and RNA transcript
Post-transcription modification
Eukaryotic cells produce pre-mRNA from transcription This will be modified before leaving
the nucleus
http://207.207.4.198/pub/flash/26/transmenu_s.swf
mRNA made by transcription…
Non-coding leader sequence at 5’ end Recognition sites to bind to ribosome
Start codon – AUG Coding sequence Stop codon – UAA, UGA, UAG
Non-coding trailing sequence at 3’ end
Translation Overview
Conversion of triplet code into polypeptide
Takes place at ribosome in cytoplasm
Involves all 3 types of RNA
Ribosome Made of 2 subunits –
large and small 4 binding sites
mRNA binds to small subunit
Large subunit has 3 sites:
A site – tRNA delivering next amino acid
P site – hold growing polypeptide
E site - exit
tRNA Binds to specific
amino acid at 3’ end
Anticodon – complementary to codon on mRNA
Structure held by H-bonding
Translation:
Initiation: Initiation factors (proteins) attach to
small subunit mRNA binds to small subunit around AUG tRNA with UAC anticodon binds to start
codon Large subunit binds and initiation factors
released (initiator tRNA at P site)
Elongation: Next tRNA binds
to codon at A site Peptide bond
forms between 2 amino acids (released from P site tRNA and attaches to A site tRNA)
Translocation – ribosome moves down 1 codon
Termination: stop codon in A site – no
complementary tRNA Release factor binds to A site –
freeing polypeptide
http://207.207.4.198/pub/flash/26/transmenu_s.swf