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6.4Gene Expression and Regulation

Translation — AP BIOLOGY Revision Notes & Topical Questions

5 min read 9 parts Lesson 5 of 9 1 quiz

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Lesson summary

6.4 Translation — Notes — AP Biology notes: Explain how the genetic code in mRNA is used to build a polypeptide and how genotype determines phenotype.Translation = converting the genetic code in mRNA into a polypeptide; it requires energy.Ribosomes — the site of translation, in t…

  • Start codon
  • Stop codon
  • Polyribosome
  • Release factor
  • Ribosomes — the site of translation, in the cytoplasm of prokaryotes and eukaryotes and on the rough ER of eukaryotes
  • Ribosome sites — A (aminoacyl), P (peptidyl) and E (exit); two tRNAs fit at a time

Section 1 of 9Translation and the ribosome

1
Translation = converting the genetic code in mRNA into a polypeptide; it requires energy.
2
Ribosomes — the site of translation, in the cytoplasm of prokaryotes and eukaryotes and on the rough ER of eukaryotes.
3
Ribosome sites — A (aminoacyl), P (peptidyl) and E (exit); two tRNAs fit at a time.
4
Polyribosomes — many ribosomes translate one mRNA at once.
5
Rough ER — proteins for export are made on it.
6
Prokaryotes — translation can begin while the mRNA is still being transcribed because there is no nuclear membrane; in eukaryotes the processes are separated by the nuclear envelope.

Section 2 of 9The genetic code

1
Codon = three mRNA nucleotides coding for one amino acid or a stop signal.
2
Count — 64 codons: 61 code for amino acids and 3 are stop codons (UAA, UAG, UGA).
3
Redundant (degenerate) — most amino acids have more than one codon, e.g. GAU and GAC both code for aspartic acid.
4
Non-overlapping — read in a fixed reading frame.
5
Start codon — AUG codes for methionine.
6
Near-universal — nearly all organisms use the same code, evidence of common ancestry that lets genes from one organism work in another.
7
Chart use — the genetic code chart is for mRNA codons, not DNA bases.

Section 3 of 9tRNA

tRNA
Amino acid end — carries a specific amino acid at its 3' end.
Anticodon — complementary to the mRNA codon.
Loading — aminoacyl-tRNA synthetases attach the correct amino acid using ATP.

Section 4 of 9Stages of translation

1
Initiation — the small ribosomal subunit binds mRNA and finds the start codon AUG, the initiator tRNA binds and the large subunit joins.
2
Elongation — a tRNA with a complementary anticodon enters the A site, rRNA in the large subunit catalyses a peptide bond,
  • and the ribosome translocates one codon along the mRNA 5' to 3'; this repeats to build the chain.
3
Termination — a stop codon enters the A site, a release factor binds, the polypeptide is released and the ribosome subunits separate.
4
After translation — polypeptides fold and may be modified (e.g. in the ER and Golgi) to become functional proteins.

Section 5 of 9Deducing an amino acid sequence

1
Convert the DNA sense strand to the template (antisense) strand by base pairing, or read the sense strand with T replaced by U.
2
Transcribe the template strand into mRNA codons, remembering U replaces T.
3
Find the first, second and third bases of each codon on the genetic code chart.
4
Read off the amino acid for each codon.
  • Genotype to phenotype — the DNA sequence sets the amino acid sequence, which sets protein shape and function.

Section 6 of 9Translation stages compared

StageEventSiteEnergyResult
InitiationSmall subunit binds mRNA; initiator tRNA at AUGP siteGTPComplete ribosome
ElongationtRNA enters A site; peptide bond formsA to P to EGTPChain grows, ribosome moves 5' to 3'
TerminationStop codon reached; release factorA siteGTPPolypeptide released

Section 7 of 9More from the specification

More from the specification
Pentose sugar units in dividing cells
  • Pentose sugar units in dividing cells nucleic acid chains in growing cells link repeating pentose sugar rings to nitrogenous bases via covalent linkages.
Enzyme
  • A protein that functions as a biological catalyst.
Lagging strand
  • Strand made in short fragments during replication.
Replication
  • Exact copying of chromosomes before division.
Transcription
  • Rewriting the DNA base code as mRNA.

Section 8 of 9Key terms

TermMeaning
Translationsynthesis of a polypeptide from mRNA codons
Codonthree-nucleotide unit of mRNA
Anticodonthree bases on tRNA complementary to a codon
Start codonAUG, coding for methionine
Stop codonUAA, UAG or UGA
Polyribosomeseveral ribosomes translating one mRNA
Release factorprotein that frees the polypeptide at a stop codon

Section 9 of 9Exam technique

1
Saying the start codon codes for a stop — AUG is methionine and UAA, UAG, UGA are stops.
2
Confusing codon and anticodon — the codon is on mRNA and the anticodon on tRNA.
3
Saying the ribosome moves 3' to 5' — it moves along the mRNA 5' to 3'.
4
Reading the genetic code chart with DNA bases — the chart is for mRNA codons.

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