Gene Gene ExpressionExpression

From gene to proteinFrom gene to proteinTranscription and TranslationTranscription and Translation

• Prokaryotes – mRNA translated immediately• Eukaryotes – pre-mRNA processed before leaving the

nucleus as mRNA

Transcription

STEP 1 – InitiationOccurs at a promoter

STEP 2 – ElongationmRNA transcript lengthens

STEP 3 – TerminationmRNA transcript released

RNA processingalteration of pre-mRNA

RNA processingRNA splicing

• Introns – intervening sequences – noncoding segments on pre-mRNA– May regulate gene activity– Enable genes to give rise to two or more

different polypeptides– Facilitate evolution through exon shuffling

• Exons – expressed sequences on pre-mRNA

• Signal for splicing is a short sequence at the ends of introns

• small nuclear ribonucleoproteins (snRNPs) with snRNA recognize splice sites

• snRNPs + proteins spliceosome– release introns– join together exons

that flank introns

Translation Overview• mRNA moves through

ribosome• codons are translated

into amino acids• tRNA molecules:

anticodon and amino acid ends

• amino acids added to a growing polypeptide chain

• rRNA molecules + proteins ribosomes

tRNA structure

Ribosome structure

Building a polypeptide

Step 1 - Initiation

Step 2Elongation

Step 3 – Termination

Given the following sequence on a template DNA strand 3’ TAT AAA CCG TAC GGA TAG ACA CCG AAA ATC CGG GCA 5’• What is the sequence on the non-template strand? 5’ ATA TTT GGC ATG CCT ATC TGT GGC TTT TAG GCC CGT 3’• What is the mRNA sequence transcribed? 5’ AUA UUU GGC AUG CCU AUC UGU GGC UUU UAG GCC CGU 3’

• What is the STOP codon? UAG• What is the anticodon attached to the tRNA that

corresponds to the STOP codon?There is no tRNA that corresponds to the STOP codon. Release factors take their place.

• What is the amino acid sequence in the polypeptide product?Met – Pro – Ile – Cys – Gly - Phe