Biochem: Protein Synthesis

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Transcription

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is carried out by RNA polymerase

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Translation

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is performed on ribosomes

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Replication

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is carried out by DNA polymerase

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Reverse transcripts

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copies RNA into DNA

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mRNA

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Messenger RNA: Encodes the amino acid sequence of a polypeptide

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tRNA

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Transfer RNA: Brings amino acids to ribosomes during translation

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rRNA

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Ribosomal RNA: With ribosomal proteins, makes up the ribosomes, the organelles that translate the mRNA

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Transcription is?

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the synthesis of an RNA molecule called mRNA having a complementary base sequence to the DNA molecule

Only parts of the DNA are copied

Selective

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RNA polymerase I

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located in the nucleolus and is concerned with the biosynthesis of rRNA

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RNA pol II

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found in the chromatin and nucleoplasm

mRNA synthesis

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RNA pol III

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found in the chromatin and nucleoplasm

in tRNA synthesis and some rRNA

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Stage 1 — Activation of Amino Acids

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Occurs in the cytosol

Each of the 20 aa is covalently linked to a specific tRNA at the expense of ATP

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Stage 2 - Initiation of Polypeptide chain

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mRNA binds to the ribosome followed by an initiating aa' attached to its tRNA — this is known as the initiation complex

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Stage 3 - Elongation

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Polypeptide chains is lengthened by covalent attachment of successive aa units placed in position by the tRNA to which it is attached.

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Stage 4 - Termination and Release

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Completion is signaled by a termination codon in the mRNA.

The polypeptide chain is then released from the ribosome, aided by proteins called release factors

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Stage 5 - Folding and Processing

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Attachment of prosthetic groups

folded into it's 3D shape and processed to remove initiating aa

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Codon?

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a group of 3 consecutive bases in mRNA which specifies an amino acid or a signal for termination of translation

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Anticodon

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group of 3 consecutive bases (3 nucleotides) in tRNA complementary to a codon on mRNA.

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Wobble Hypothesis

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allows some tRNA's to recognize more than one codon

There is relaxed base pairing in the 3rd position of the codon — the third base is said to `wobble

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What is the difference between replication and transcription??

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Replication is the whole DNA molecules

Transcription is only part of the DNA

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How is transcription selective??

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it is turned on by specific regulatory sequences indicating the beginning & ending of these sequenes

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What creates mRNA strands

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RNA polymerase

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What does Uracil replace?

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Thymine

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Structure of tRNA:

Amnio acid arm

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carries the specific Aa esterified by its carboxyl group to the terminal adenine residue on the 3' end

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Structure of tRNA:

Anticodon arm

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Contains the anticodon: the specific triplet of nucleotide complementary to the bases in the codon on mRNA

Anticodon is characteristic of each tRNA molecule

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What are ribosomes

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they are multi component structures that contain several enzymatic activities needed for protein synthesis

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Initiating Aa what are they for?

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a specific Aa will initiate the protein synthesis

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What is the initiator Aa??

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Methionine (Met)

It enters the ribosome attatched to tRNA

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What enzyme attatches Met to the tRNA?

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tRNA synthetase

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Post Translational Modifications

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1. Amino & Carboxyl terminal modifications

2. Attatchment of CHO side chain

3. Addition of prosthetic groups

4. Formation of S-S bridges