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6.2.36 · Nucleic Acids and Protein Synthesis

Constructing Polypeptides — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 10 of 13 Unit quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

6.2.3 Constructing Polypeptides — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the process of translation, including the roles of mRNA, tRNA, ribosomes and ATP, and explain how a polypeptide is assembled.

  • Free ribosomes
  • Bound ribosomes
  • tRNA structure
  • Switching off
  • Modification
  • Quaternary structure

Part 1 of 8Where translation takes place

Translation
  • Translation assembly of a polypeptide at a ribosome using the sequence of codons in mRNA, in the cytoplasm after the mRNA leaves the nucleus.
Ribosomes
  • Ribosomes made of rRNA and protein, with a large and a small subunit.
  • Eukaryotic cytoplasmic ribosomes are 80S and prokaryotic ribosomes are smaller, 70S.
Free ribosomes
  • Free ribosomes they float in the cytoplasm and make proteins for use in the cell, such as glycolysis enzymes.
Bound ribosomes
  • Bound ribosomes they sit on the rough endoplasmic reticulum and make proteins for secretion or membranes.

Part 2 of 8Activating tRNA

Activating tRNA
tRNA structure
  • tRNA structure a triplet of unpaired bases (the anticodon) at one end and an amino acid binding site at the other.
Types
  • Types there is at least one type of tRNA for each amino acid, so at least 20 types, each with a specific anticodon and binding site.
Loading
  • Loading an enzyme attaches the correct amino acid to its tRNA using energy from ATP.
Accuracy
  • Accuracy the enzyme recognises both the amino acid and the matching tRNA, so the genetic code is applied correctly.

Part 3 of 8Steps of translation

1
Initiation: the small ribosomal subunit binds to the mRNA near its 5′ end and finds the start codon AUG.
2
A tRNA with the anticodon UAC carrying methionine pairs with the start codon, and the large subunit joins.
3
A second tRNA, whose anticodon is complementary to the next codon, enters the second tRNA site, so two tRNAs lie side by side.
4
A peptide bond forms between the two amino acids by a condensation reaction catalysed by the ribosome, using energy from ATP.
5
The ribosome moves along the mRNA by exactly one codon (translocation); the first tRNA, now free of its amino acid, leaves to be reloaded and a new tRNA enters the vacant site.
6
These steps repeat, lengthening the polypeptide one amino acid at a time.
7
Termination: at a stop codon no tRNA matches, the polypeptide is released and the ribosome subunits separate.

Part 4 of 8Polysomes and efficiency

Polysome
  • Polysome several ribosomes translate the same mRNA at once, each at a different stage.
Benefit
  • Benefit many copies of the polypeptide come from one mRNA, so a cell makes large quantities of protein quickly from few mRNA copies.
Switching off
  • Switching off mRNA is eventually broken down by enzymes, so protein synthesis can be stopped.

Part 5 of 8After translation

After translation
Folding
  • Folding the polypeptide folds into secondary and tertiary structure as R groups interact.
Modification
  • Modification in eukaryotes, proteins entering the Golgi apparatus may be processed, for example by adding carbohydrate to form glycoproteins or by cutting the chain.
Quaternary structure
  • Quaternary structure several polypeptides may combine, as in haemoglobin, which has four chains.
Secreted proteins
  • Secreted proteins ribosomes on the rough ER feed the polypeptide into the lumen, vesicles carry it to the Golgi,
    • and secretory vesicles release it by exocytosis,
    • as in a pancreatic acinar cell.

Part 6 of 8Transcription and translation compared

FeatureTranscriptionTranslation
Location (eukaryote)NucleusRibosome in cytoplasm
TemplateDNAmRNA
ProductmRNAPolypeptide
Key enzyme or structureRNA polymeraseRibosome

Part 7 of 8Energy and antibiotics

Energy and antibiotics
ATP use
  • ATP attaches amino acids to tRNA and is used in forming peptide bonds.
Cells making much protein
  • Cells making much protein they have many mitochondria and a prominent nucleolus where rRNA is made.
Ribosome functions
  • Ribosome functions it holds mRNA and tRNA close together, has two tRNA sites so two amino acids are adjacent, catalyses the peptide bond,
    • and moves codon by codon to a stop codon.
Antibiotics
  • Antibiotics some, such as tetracycline, bind the smaller 70S bacterial ribosome and stop translation,
  • which kills bacteria but leaves human 80S ribosomes unharmed.

Part 8 of 8More from the specification

More from the specification
Transcription of structural genes
  • RNA polymerase transcribes specific protein-coding genes into messenger RNA sequences for ribosomal translation into polypeptides.
Gene Structure and Coding
  • A gene is a sequence of nucleotides along a DNA molecule strand that codes for the specific sequence of amino acids in a polypeptide chain.
Transcription and Translation Compartmentalisation
  • Transcription occurs within the nucleus producing messenger RNA molecules,
  • whereas translation occurs in the cytoplasm at ribosomes where transfer RNA delivers amino acids.
Gene Mutations
  • A mutation is a spontaneous change in the nucleotide sequence of DNA,
  • which can alter triplet codes and result in altered or non-functional protein molecules.
Gene Structure and Coding
  • A gene is a sequence of nucleotides along a DNA molecule strand that codes for the specific sequence of amino acids in a polypeptide chain.
Transcription and Translation Compartmentalisation
  • Transcription occurs within the nucleus producing messenger RNA molecules,
  • whereas translation occurs in the cytoplasm at ribosomes where transfer RNA delivers amino acids.
Gene Mutations
  • A mutation is a spontaneous change in the nucleotide sequence of DNA,
  • which can alter triplet codes and result in altered or non-functional protein molecules.
Mutation
  • A random change in genetic material.
Nucleotide
  • The unit of DNA made of a phosphate, a deoxyribose sugar and a base.
Insertion
  • Addition of one or more nucleotides into a DNA sequence.
Intron
  • A non-coding section of a gene removed from the mRNA.
Universal
  • Describes a code that is the same in nearly all organisms.
Hydrogen bond
  • The bond between paired bases that holds the two strands together.
Template strand
  • The DNA strand read to make mRNA.

Key terms

TermMeaning
RibosomeA structure of rRNA and protein with two subunits, where translation takes place.
Peptide bondCovalent bond between the carboxyl group of one amino acid and the amino group of the next, formed by condensation.
TranslocationMovement of the ribosome along the mRNA by one codon.
PolysomeA group of ribosomes translating the same mRNA at the same time.
InitiationStart of translation, when the small subunit and first tRNA bind at the start codon.
TerminationEnd of translation when a stop codon is reached and the polypeptide is released.
Condensation reactionA reaction in which two molecules join with the loss of water.

Avoid these mistakes

1
Saying the ribosome makes mRNA — ribosomes translate mRNA and RNA polymerase makes it.
2
Forgetting that peptide bond formation is a condensation reaction needing energy from ATP.
3
Thinking tRNA is used up — it leaves the ribosome and is recharged with another amino acid.
4
Saying the ribosome moves one base at a time — it moves one codon (three bases).
5
Confusing eukaryotic 80S ribosomes with prokaryotic 70S ribosomes.

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