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Nucleic Acids (AHL) - Translation

Grade 11IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Translation occurs in a 5′→3′5' \rightarrow 3' direction along the mRNA strand, involving the initiation, elongation, and termination stages.

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Ribosomes consist of a large and small subunit made of rRNArRNA and proteins. They contain three binding sites for tRNA: the AA (aminoacyl) site, the PP (peptidyl) site, and the EE (exit) site.

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tRNA activating enzymes (aminoacyl-tRNA synthetases) attach a specific amino acid to the 3′3' end of a tRNA molecule using energy from ATPATP hydrolysis. This process is called 'charging' the tRNA.

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Initiation involves the assembly of the mRNA, the small ribosomal subunit, and the initiator tRNA (tRNAMettRNA^{Met}) at the start codon (AUGAUG), followed by the attachment of the large ribosomal subunit.

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Elongation is a repeated cycle where a new aminoacyl-tRNA binds to the AA site, a peptide bond forms via peptidyl transferase, and the ribosome translocates the tRNA from the PP site to the EE site.

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Termination occurs when the ribosome reaches a stop codon (UAGUAG, UAAUAA, or UGAUGA). Instead of a tRNA, a release factor binds to the AA site, causing the disassembly of the translation complex.

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Free ribosomes in the cytoplasm synthesize proteins primarily for use within the cell, while bound ribosomes on the Rough Endoplasmic Reticulum (rERrER) synthesize proteins for secretion or for use in lysosomes.

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Polysomes (or polyribosomes) are structures where multiple ribosomes translate a single mRNA molecule simultaneously, significantly increasing the rate of protein synthesis.

📐Formulae

ATP+Amino Acid+tRNA→Aminoacyl−tRNA SynthetaseAminoacyl−tRNA+AMP+PPiATP + Amino\ Acid + tRNA \xrightarrow{Aminoacyl-tRNA\ Synthetase} Aminoacyl-tRNA + AMP + PP_i

Number of Nucleotides=(Number of Amino Acids×3)+3 (for stop codon)Number\ of\ Nucleotides = (Number\ of\ Amino\ Acids \times 3) + 3\ (for\ stop\ codon)

Directionality:5′→3′Directionality: 5' \rightarrow 3'

💡Examples

Problem 1:

A specific polypeptide contains 120120 amino acids. Calculate the minimum number of nitrogenous bases in the mRNA sequence required to code for this polypeptide, including the start and stop codons.

Solution:

120×3=360120 \times 3 = 360 bases for the amino acids, plus 33 bases for the stop codon = 363363 bases.

Explanation:

Each amino acid is coded by a triplet of bases called a codon. Therefore, 120120 amino acids require 360360 bases. The stop codon is also part of the mRNA sequence but does not code for an amino acid, adding 33 more bases.

Problem 2:

Identify the anticodon sequence on a tRNA molecule that would carry the amino acid Methionine (MetMet), given the start codon is 5′−AUG−3′5'-AUG-3'.

Solution:

3′−UAC−5′3'-UAC-5' (or 5′−CAU−3′5'-CAU-3')

Explanation:

The anticodon on the tRNA must be complementary and antiparallel to the mRNA codon. For the codon 5′−AUG−3′5'-AUG-3', the complementary bases are UU for AA, AA for UU, and CC for GG. Reversing the direction for antiparallel alignment gives 3′−UAC−5′3'-UAC-5'.

Problem 3:

Describe the role of ATPATP in the phosphorylation of tRNA molecules.

Solution:

Amino acid+ATP→Aminoacyl−AMP+PPi→Aminoacyl−tRNAAmino\ acid + ATP \rightarrow Aminoacyl-AMP + PP_i \rightarrow Aminoacyl-tRNA

Explanation:

Energy from ATPATP is used by aminoacyl-tRNA synthetase to link a specific amino acid to its corresponding tRNA. The 'charging' of the tRNA stores potential energy in the ester bond, which is later used to drive the formation of the peptide bond during translation.

Translation - Revision Notes & Key Diagrams | IB Grade 11 Biology