Knowledge Organiser: Protein Synthesis (OCR A Higher)
Key Terms
- Transcription: DNA to mRNA, in the nucleus, by RNA polymerase
- Translation: mRNA to polypeptide, at the ribosome
- Codon: 3-base sequence on mRNA coding for one amino acid
- Anticodon: 3-base sequence on tRNA, complementary to a codon
- Peptide bond: bond linking neighbouring amino acids
- Frameshift: change in the grouping of bases into codons, caused by a deletion or insertion
Key Processes
- Transcription: RNA polymerase uses one strand of DNA as a template to build a complementary mRNA strand; base pairing: A with U, T with A, G with C, C with G
- mRNA carries the genetic code out of the nucleus through nuclear pores to the ribosome in the cytoplasm
- Translation: the ribosome reads mRNA codons; tRNA molecules bring matching amino acids (anticodon pairs with codon)
- Amino acids are joined by peptide bonds to form a polypeptide chain
- A stop codon signals the end of translation
- One amino acid is added per codon (three bases)
Sickle Cell Facts
- DNA template triplet: CTC to CAC (T replaced by A)
- mRNA codon: GAG to GUG
- Amino acid: glutamic acid replaced by valine
- Haemoglobin molecules stick together when oxygen is low
- Red blood cells become sickle-shaped
- They carry oxygen poorly and block capillaries
Must-Know Facts
- There are 64 possible codons (4 × 4 × 4) but only 20 amino acids, so several codons can code for the same amino acid
- The genetic code is almost the same in all living organisms
- Substitution mutation: only one codon is changed; it may be silent if the new codon codes for the same amino acid
- Deletion or insertion mutation: causes a frameshift, so every codon after the mutation is altered, usually giving a protein that does not work
- A substitution is generally less damaging than a deletion or insertion because only one amino acid may change instead of the rest of the protein
Common Mistakes
- Confusing transcription and translation: Transcription copies DNA into mRNA in the nucleus; translation uses mRNA to build a protein at the ribosome.
- Saying DNA leaves the nucleus: DNA stays in the nucleus. It is mRNA that carries the code out through the nuclear pores.
- Mixing up codons and anticodons: A codon is on mRNA; an anticodon is on tRNA. They are complementary and pair during translation.
- Saying all mutations change the protein: Several codons can code for the same amino acid, so a substitution may be silent.
Memory Aid
- DNA is the pattern for transcription
- MRNA leaves the nucleus through nuclear pores
- Polypeptide is assembled at the ribosome with the help of tRNA
- The mRNA codon is the lock (held in place at the ribosome)
- The tRNA anticodon is the key (the tRNA moves in, matches, and delivers its amino acid)
- The key is complementary to the lock. If the codon is GCU, the anticodon is CGA.
- Substitution changes at most one codon
- Deletion or insertion destroys the reading of codons (frameshift, so all later codons are wrong)