The Genetic Code, Translation and Gene Regulation: Quick Revision

A brief note on The Genetic Code, Translation and Gene Regulation. Read it before a test. Try each question at the end before you open its answer.

The genetic code

• 64 codons: 61 code for amino acids, and 3 are stop codons (UAA, UAG, UGA). AUG codes for methionine and is the start codon.
• The code is triplet, degenerate (up to six codons for one amino acid), unambiguous, read without gaps and nearly universal.
• Point mutation: silent, missense or nonsense. Sickle-cell anaemia: GAG to GUG, so Glu is replaced by Val.
• Frameshift: an insertion or a deletion of 1 or 2 bases. Of 3 bases, the frame is kept.

Translation

• tRNA: anticodon loop at one end, amino acid acceptor end at the other. Clover leaf on paper, inverted L in the cell. There is no tRNA for a stop codon.
• Charging: ATP joins an amino acid to its tRNA.
• Ribosome: small and large subunits, rRNA and about 80 proteins. The 23S rRNA is a ribozyme in bacteria.
• Steps: initiation at AUG with the initiator tRNA, elongation one codon at a time (5′ to 3′ on the mRNA), termination at a stop codon with a release factor.
• Amino acids: nucleotides divided by 3, minus 1 for the stop codon.

Regulation, the genome and fingerprints

• lac operon: i, p, o, z, y, a. The repressor binds the operator. Lactose, the inducer, takes it off, and the operon is on. z: beta-galactosidase, y: permease, a: transacetylase.
• HGP: 1990 to 2003. Methods: ESTs and sequence annotation. Vectors: BAC and YAC. Method of Sanger.
• DNA fingerprinting: Jeffreys, VNTR. Restriction enzymes, gel electrophoresis, blotting, labelled probe, autoradiography. Used in paternity tests and forensic work.

A worked example

An mRNA has a coding region of 303 nucleotides, from AUG to the end of the stop codon. How many amino acids?

1. $303 \div 3 = 101$ codons.
2. One is the stop codon, so $100$ amino acids.

Practice with new numbers

A program to try

The program counts the codons of a three-base code.

Mistakes to avoid

• Counting the stop codon as an amino acid.
• Saying that the tRNA carries the codon. It carries the anticodon.
• Saying that glucose is the inducer. Lactose is.
• Saying that a deletion of 3 bases spoils the frame.

Check yourself

1. Which codon is the start codon?

2. What is the result of a nonsense mutation?

3. What does the repressor do, and what removes it?

4. Name two steps of DNA fingerprinting.

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 5, Molecular Basis of Inheritance.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapters 15 and 16, Genes and Proteins, Gene Expression.