Chapter 17 Protein Translation (PART 4) From Gene to Protein AP Biology Ms. Gaynor

mRNA  polypeptide (protein) Translation mRNA  polypeptide (protein)

tRNA A cell translates mRNA message into protein with help from transfer RNA (tRNA) Type of RNA ~80 nucleotides “t” shape Carries amino acids Matches codons to anticodons

tRNA Molecules of tRNA are not all identical Each carries a specific amino acid Each has an specific anticodon on the other end Aminoactyl tRNA Synthase (enzyme) puts tRNA together with it amino acid

Aminoactyl tRNA Synthase tRNA Assembly Aminoactyl tRNA Synthase ACTIVATED tRNA

Ribosomes Help bind tRNA anticodons with mRNA codons during translation Found on ROUGH Endoplasmic Reticulum RER (“bound”) or in cytoplasm (“free”) Anticodon (tRNA) = codon (mRNA)

tRNA (with anticodons) mRNA (with codons)

Ribosomes There are 2 ribosomal subunits Constructed of proteins and ribosomal RNA or rRNA

Binding Sites in a Ribosome The ribosome has three binding sites for tRNA The P site The A site The E site P site (Peptide Bond-tRNA binding site) E site (Exit site) mRNA binding site A site (Amino acid- tRNA binding site) Large subunit Small E P A

tRNA mRNA Growing polypeptide Amino acid Next amino acid to be added to polypeptide chain tRNA mRNA Codons 3 5 \

Building a Polypeptide Translation can be divided into 3 phases Initiation Elongation Termination

Initiation of Translation initiation stage brings together mRNA tRNA bearing 1st amino acid (“start”) called MET 2 subunits of a ribosome come together

Translation initiation complex Large ribosomal subunit Initiator tRNA mRNA mRNA binding site Small Translation initiation complex P site GDP GTP Start codon Met U A C G E 3 5 Figure 17.17

Elongation Amino acids are added one by one to the preceding amino acid Peptide bonds are formed Amino acid

Termination of Translation When the ribosome reaches a stop codon on the mRNA There are 3 stop codons UAA, UAG, and UGA Release factor (protein) binds to stop codon in A site (NOT tRNA)  polypeptide released

Termination of Translation Release factor Free polypeptide Stop codon (UAG, UAA, or UGA) 5 3

Phe tRNA Anticodon 5 3 Amino acids “EMPTY” tRNA tRNA with amino acid TRANSCRIPTION TRANSLATION DNA mRNA Ribosome Polypeptide Amino acids tRNA with amino acid attached tRNA Anticodon Trp Phe Gly A G C U Codons 5 3 “EMPTY” tRNA Figure 17.13

Polyribosomes http://www.sumanasinc.com/webcontent/animations/content/polyribosomes.html

Polypeptide  3D Protein Primary Structure Amino acids is a row (LINEAR) Secondary Structure Amino Acids BEND and FOLD towards each other Tertiary Structure Amino acids BIND to each other 3D shape made Quartnary structure (only some) 2 or more polypeptides BIND to each other

1 2 4 3

Completing and Targeting the Functional Protein Polypeptide chains undergo modifications after the translation process Proteins may be modified in ways that affect their three-dimensional shape

PROTEINS THAT LEAVE THE CELL… Proteins destined for the endomembrane system or for secretion (ROUGH ER) Must be transported into the ER Have signal peptides to which a signal-recognition particle (SRP) binds, enabling the translation ribosome to bind to the ER

The signal mechanism for targeting proteins to the ER http://www.sumanasinc.com/webcontent/animations/content/proteinsecretion_mb.html

Translation (Protein Synthesis) Animation http://www.wisc-online.com/objects/index_tj.asp?objID=AP1302 http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html# http://www.ucopenaccess.org/courses/APBiologyI/course%20files/multimedia/lesson13/lessonp.html

What happens to proteins? http://www.sumanasinc.com/webcontent/animations/content/lifecycleprotein.html

RNA plays multiple roles in the cell: a review Can H-bond to other nucleic acid molecules Can assume a specific 3-D shape Has functional groups that allow it to act as a catalyst

Types of RNA in a Eukaryotic Cell

What is a gene? REVIEW A gene region of DNA  final product either a polypeptide OR an RNA molecule Genes determines 1º structure 1º determines 2º and 3º structure

Key differences in gene expression in prokaryotes & eukaryotes Prokaryotic cells NO nucleus Allowing translation to begin while transcription is still in progress Eukaryotic cells The nuclear envelope separates transcription from translation Extensive RNA processing occurs in the nucleus Polyadenylation signal = termination signal on DNA