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