From Gene to Protein

Transcription and TranslationMechanisms of Regulation

DNA RNA ProteinTranscription Translation

LO 3.4 The student is able to describe representations and models illustrating how genetic information is

translated into polypeptides. [See SP 1.2]

• Initiation of Transcription•Transcription•Eukaryotic processing of RNA•Translation•Comparison of Eukaryotic and prokaryotic protein synthesis

To Make a Protein: Transcription and Translation

Initiation of Transcription• RNA polymerase can assemble mRNA in the 5’-3’

direction• Does not need a primer to initiate making mRNA

PromoterTATAAAA

5’5’3’

3’

TEMPLATE STRAND

CODING STRAND

Transcription Start Point

Direction of Transcription (downstream)

Synthesis of mRNA

• Transcription factors must bind to the promoter

• RNA-polymerase scans the template DNA molecule in the 3' to 5' direction until it recognizes transcription factors

Transcription• RNA-polymerase synthesizes a mRNA strand

complementary to the template strand (in the 5’-3’ direction just like DNA polymerase

Eukaryote mRNA processing

• 5’ GTP cap• 3’ poly A tail

• Splicing: Excision of introns• Small nuclear Ribonucleprotein

particles (snRNPs) assist in splicing

5’ UTR

Translation: Initiation, Elongation, Termination

• Initaition: mRNA, small subunit of the ribosome and initiator tRNA (methionine) form a complex

• Large subunit binds • Translation is initiated at the start codon (AUG)

5’ UTR

Elongation of the polypeptide1. tRNAs brings the

correct amino acid to the ribosome.

2. Energy from GTP hydrolysis is needed to form the peptide bond

3. The ribosome shifts one codon over and is ready for a new tRNA.

Termination• When a stop codon is reached a release factor

enters the A site• The bond between the polypeptide and tRNA is

broken• Protein, ribosome, tRNA and mRNA disassociate

How tRNAs decode the message:• tRNAs have an anticodon- complementary to

the mRNA codon. • tRNAs carry a specific amino acid.

Codon chart• The genetic code is highly

conserved• It is unambiguous: each

codon codes for only one amino acid

• It is redundant: many amino acids are coded for by more than one codon.

• Decode the following mRNA sequence

• 5’-AUGAAACGCGUUUAA-3’

Eukaryotic and Prokaryotic Protein Synthesis have Conserved Features

• RNA polymerase• Ribosomes and transfer RNAs decode mRNA into

a protein one codon at a time in the cytoplasm.• The codon AUG initiates protein synthesis • Codons are triplets of bases and are conserved.

Prokryotic Protein Synthesis• Because prokaryotes lack a nucleus

Transcription and Translation are coupled (can occur at the same time)

• No introns• This is more efficient

Eukaryotic Protein Synthesis

• The nuclear membrane separates transcription and translation

• Before mRNA leaves the nucleus it is modified by enzymes:

• Modifications includes– Addition of a 5’ GTP cap– Addition of a 3’ poly-A tail– Splicing: Excision (removal) of introns (interrupting

sequences)

Producing multiple proteins from the same mRNA

• Multiple Ribosomes can bind to the same mRNA

• Multiple mRNAs from the same gene.

• Bacteria are streamlined: – transcription and translation

are not separated by a nuclear membrane.

Summary of Eukaryotic

Transcription and Translation

Watch DNA learning center mRNA translation (advanced)

Translation by tRNA in the ribosome• Ribosomes essentially “hold everything together”

and catalys the formation of the peptide bond between the amino acids braught in by transfer RNA

• The transfer RNA do the decoding

Transcription and translation in prokaryotes and eukaryotes.

Coupled transcription and translation in bacteria

Transcription • Coding strand: on this figure is the “sense strand”• Template strand: on this figure is the “anti-sense

strand”• mRNA is complementary to the anti-sense strand,

and a “copy” of the sense strand, with Uracil instead of Thymine.