week 4:From Nucleic Acids to Proteins: The Genetic Code and Translation

These flashcards cover key terms and concepts related to the genetic code and translation as discussed in the lecture notes.

Primary Structure

The linear sequence of amino acids in a protein, which is determined by the corresponding gene.

Secondary Structure

The local folding of a protein's polypeptide chain into structures such as alpha helices and beta sheets.

Tertiary Structure

The overall three-dimensional shape of a protein formed by the folding and interactions of secondary structures.

Quaternary Structure

The structure formed when two or more polypeptide chains assemble into a functional protein.

Initiation

The codon (AUG) that signals the start of protein synthesis, coding for methionine.

3 main stages:

1) mRNA binds the small subunit of the ribosome

2) Initiator tRNA binds to the mRNA (anticodon-codon binding)

3) The large ribosomal subunit joins the complex

Termination Codon

Codons (UAA, UAG, UGA) that signal the termination of protein synthesis, with no corresponding tRNA.

Essential Amino Acids

9 Amino acids that cannot be synthesized by the body and must be obtained from the diet.

Non-Essential Amino Acids

11 Amino acids that can be synthesized by the body and are not required in the diet.

Aminoacyl-tRNA Synthetases

Enzymes that attach the appropriate amino acid to its corresponding tRNA during protein synthesis.

Elongation Factors

Proteins that facilitate the elongation phase of translation by assisting in the binding of tRNA and the formation of peptide bonds.

Release Factors (RFs)

Proteins that recognize stop codons and promote the release of the newly synthesized polypeptide chain from the ribosome.

List four major functions of proteins.

Enzymatic, structural, regulatory, and transport.

What is the general structure of an amino acid?

An amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R group attached to a central carbon.

how are peptide bonds formed

condensation reaction of OH from carboxyl group on one amino acids and H from Amine group on the other

What are protein domains?

Discrete functional units within a protein that can fold independently, usually associated with specific biochemical functions.

how are the 64 possible codons split up

3 stop codons and 61 sense codons

What is the start codon and what does it code for?

AUG; it codes for methionine

What conserved sequence is found at the 3′ end of all tRNAs?

CCA.

Which enzyme attaches amino acids to tRNA?

Aminoacyl-tRNA synthetase.

The stages of translation

tRNA charging,include initiation, elongation, and termination.

Why must the small and large ribosomal subunits be separate at the start of translation?

Because mRNA can only bind to the small ribosomal subunit.

What is the 43S pre-initiation complex composed of and what is it

The 40S ribosomal subunit, initiator Met-tRNAᵢᴹᵉᵗ, and eukaryotic initiation factors (eIFs).

A eukaryotic translation initiation assembly (40S + Met-tRNAᵢᴹᵉᵗ + eIFs) that binds the 5′ cap and scans for AUG.

What does the 43S complex do once bound to mRNA?

Scans 5′→3′ for an AUG in a Kozak sequence, enabling start codon recognition.

What happens after AUG recognition by the 43S complex?

Codon–anticodon pairing → eIF release → 60S binding → 80S initiation complex.

Outline the complete process of translation elongation in eukaryotes.

Elongation consists of

(1) charged tRNA to the A site by eEF1A–GTP forms codon–anticodon pairing,

(2) peptide bond formation between amino acids catalysed by 28S rRNA in the large subunit, and

(3) ribosome translocation in the 5′→3′ direction mediated by eEF2 with GTP hydrolysis.

How is a charged tRNA accurately delivered to the ribosomal A site during elongation?

A charged tRNA binds eEF1A–GTP and is delivered to the A site, where correct codon–anticodon pairing triggers GTP hydrolysis and release of eEF1A as GDP.

Describe how peptide bond formation occurs during translation elongation.

he amino acid in the A site forms a peptide bond with the growing polypeptide in the P site, releasing the amino acid from the P-site tRNA; this reaction occurs in the large ribosomal subunit and is catalysed by 28S rRNA, which acts as a ribozyme.

Explain ribosomal translocation during elongation and the fate of tRNAs.

eEF2–GTP drives movement of the ribosome one codon in the 5′→3′ direction; the tRNA carrying the polypeptide shifts from A to P, the empty tRNA moves from P to E and exits the ribosome to be recharged.

How is elongation maintained efficiently along the mRNA?

The polypeptide remains attached to the P-site tRNA after each cycle, elongation repeats until a stop codon is reached, and helicase activity in the small subunit unwinds mRNA secondary structures.

What triggers translation termination and how is the stop codon recognised?

Termination begins when a stop codon enters the A site, which lacks a corresponding tRNA and is recognised by the release factor eRF1.

Describe the molecular events that complete translation termination.

eRF3 uses GTP hydrolysis to cleave the tRNA–polypeptide bond, releasing the protein, followed by release of tRNA and mRNA and dissociation of the ribosomal subunits.