RNA and the Genetic Code

Key Concepts

Organisms must store and pass on genetic information to future generations and express it to carry out life processes.
Genetic information is encoded in DNA and RNA with nitrogenous bases, while proteins are composed of amino acids.
The genetic code translates genetic information into proteins through transcription (DNA to RNA) and translation (RNA to protein).
A gene is a unit of DNA encoding a specific protein or RNA molecule.

Transcription: DNA ➡️ RNA
- Synthesis direction: 5′ to 3′.
- Complementary and anti-parallel to the DNA template strand.
Translation: RNA ➡️ Protein
- Ribosomes translate mRNA in the 5′ to 3′ direction.
- Synthesizes protein from amino (N-terminus) to carboxyl terminus (C-terminus).

mRNA (Messenger RNA)
- Carries genetic information from DNA to ribosome for protein synthesis.
- Synthesized in the nucleus, transported to cytoplasm.
- Stability allows gene expression regulation.
- Eukaryotic mRNA is monocistronic; prokaryotic mRNA can be polycistronic.
tRNA (Transfer RNA)
- Brings correct amino acid to the ribosome during protein synthesis.
- Contains an anticodon complementary to mRNA's codon.
- Catalyzes peptide bond formation between amino acids.
rRNA (Ribosomal RNA)
- Structural and functional component of ribosomes.
- Involved in catalyzing peptide bonds and positioning mRNA/tRNA.

Basic unit: Three-letter word (codon) translating into an amino acid.
Genetic code consists of 64 codons.
- 61 codons for 20 amino acids.
- 3 codons signal translation termination.
Codons read in 5′ to 3′ direction, unambiguous (one codon per amino acid).
Start Codon: AUG (methionine)
Stop Codons: UAA, UAG, UGA

Degenerate Code: Multiple codons can specify same amino acid.
- Example: Six codons specify serine.
Wobble Position: Third base in codon can vary without altering amino acid (silent mutation).

Point Mutations
- Silent Mutation: No effect on protein (e.g., GTA to GTT, still valine).
- Missense Mutation: Substitutes one amino acid for another (e.g., CCC to ACC).
- Nonsense Mutation: Codon becomes a premature stop codon (e.g., TAC to TAG).
Frame Shift Mutations
- Addition or deletion of nucleotides shifts reading frame.
- Results in altered amino acid sequence or premature protein truncation.

Occurs in the nucleus.
Enzymes (helicases and topoisomerases) unwind DNA.
RNA polymerase transcribes DNA template into hnRNA (heterogeneous nuclear RNA).
hnRNA undergoes post-transcriptional modifications:
- Splicing: Removal of introns, ligation of exons by spliceosome.
- 5′ Cap: 7-methylguanylate triphosphate cap added to protect mRNA.
- 3′ Poly-A Tail: Polyadenylation to stabilize mRNA and assist ribosome binding.

mRNA exits nucleus, binds to ribosome in cytoplasm.
Initiation: Small ribosomal subunit binds mRNA, large subunit joins after initiator tRNA (methionine) binds to start codon.
Elongation: Ribosome moves along mRNA, adding amino acids to polypeptide chain.
- A site: Holds incoming tRNA-amino acid complex.
- P site: Holds growing polypeptide chain.
- E site: Where inactive tRNA pauses before exiting ribosome.
Termination: Ribosome reaches stop codon, release factors terminate translation, releasing polypeptide.

Proteins undergo additional modifications:
- Phosphorylation: Adds phosphate groups.
- Carboxylation: Adds carboxylic acid groups.
- Glycosylation: Adds oligosaccharides.

Operons: Cluster of genes with a single mRNA transcription.
- Inducible Systems: Repressor binds to operator, blocking RNA polymerase until induced (e.g., Lac operon).
- Repressible Systems: Transcription constant until repressor and corepressor complex inhibits transcription (e.g., Trp operon).

Transcription Factors: Bind specific DNA sequences to regulate expression.
- Promoters: Near start site (within 20 base pairs).
- Enhancers: Distant from start site (more than 25 base pairs).