The Genetic Code and Translation

What is the central dogma of molecular biology?

Replication → Transcription → Translation

How do protein synthesis processes differ between prokaryotes and eukaryotes?

In prokaryotes, transcription and translation are coupled; in eukaryotes, they are compartmentalized.

What is a codon?

A sequence of three nucleotides on mRNA that codes for one amino acid.

What is the start codon and what amino acid does it code for?

AUG; it codes for methionine (Met).

What are the stop codons?

UAA, UAG, and UGA

Why is the genetic code considered redundant or degenerate?

Multiple codons can code for the same amino acid.

What does the wobble hypothesis explain?

The flexibility in base pairing at the third codon position.

How many codons exist in total and how many code for amino acids?

64 total codons; 61 code for amino acids and 3 are stop codons.

What direction are codons read during translation?

5′ to 3′

What is the adaptor molecule that links codons to amino acids?

tRNA (transfer RNA)

What enzyme attaches an amino acid to its corresponding tRNA?

Aminoacyl-tRNA synthetase

What are the two steps in charging tRNA?

1. Activation (amino acid + ATP) 2. Transfer (amino acid attached to tRNA)

How many different tRNAs exist compared to codons?

About 45 tRNAs for 61 codons

What is meant by the “universal genetic code”?

Nearly all organisms use the same codon-to-amino acid assignments.

What evidence supports the endosymbiotic theory?

Mitochondria and chloroplasts have bacterial-like ribosomes and polycistronic genes.

What is the structure of a ribosome?

Two subunits: a small and a large subunit made of rRNA and proteins.

What are the three binding sites of a ribosome?

A site (Aminoacyl), P site (Peptidyl), and E site (Exit)

What is the Shine-Dalgarno sequence?

A ribosome-binding site in prokaryotic mRNA upstream of AUG.

What is the Kozak sequence?

A eukaryotic consensus sequence near the start codon.

What are initiation factors?

Proteins that help assemble the translation initiation complex.

What provides energy for translation?

GTP hydrolysis

What is a polysome (polyribosome)?

A cluster of ribosomes translating a single mRNA simultaneously.

What recognizes the stop codon during translation?

Release factors (RFs)

In what direction is the polypeptide chain synthesized?

From N-terminal to C-terminal

What are post-translational modifications?

Chemical changes after translation that modify protein structure and function.

What antibiotics target bacterial ribosomes and how?

Tetracyclines, aminoglycosides, erythromycin, clindamycin

What is the difference between polycistronic and monocistronic mRNA?

Prokaryotic mRNAs are polycistronic; eukaryotic mRNAs are monocistronic.

How do initiator tRNAs differ in prokaryotes and eukaryotes?

Prokaryotes use fMet-tRNA; eukaryotes use Met-tRNA.

What is the adaptor hypothesis by Crick and Hoagland?

tRNAs act as adaptors that match codons with their corresponding amino acids.

How does degeneracy contribute to mutation tolerance?

It allows some mutations (especially in the third codon position) to be silent.