Transcription and Translation

What is a gene?

A region of DNA that contains information for building a protein or other RNA product.

What is gene expression?

The process of using information in a gene to make a functional product. Has two stages: transcription then translation

What are the three components of a nucleotide?

 A nitrogenous base, a 5-carbon sugar, and a phosphate group.

What distinguishes an intron from an exon?

Exons are coding sequences kept in mature mRNA. Introns are non-coding sequences spliced out during RNA processing.

mRNA sequence: 5'-AGU-AGA-3'. What amino acid sequence is produced?

Ser-Arg. AGU codes for serine; AGA codes for arginine.

The template strand at the transcription start site reads 3'-G…. What is the first base of the RNA transcript?

RNA polymerase reads the template 3'→5' and builds complementary, so G on the template gives C in the RNA.

A poly-A mRNA is injected into a cell. Will it be translated?

 No — it lacks a ribosome binding site and AUG start codon. Translation cannot initiate without those signals.

A DNA coding strand reads TTA and mutates to TAA. What type of mutation is this?

A nonsense mutation. The mRNA codon changes from UUA (Leucine) to UAA (stop codon), prematurely ending the polypeptide.

How does the terminator in transcription parallel the stop codon in translation?

Both signal "end the process." The terminator stops RNA polymerase; the stop codon signals the ribosome to release the finished polypeptide.

How are the promoter and the AUG start codon functionally related?

Both signal "start the process." The promoter is where RNA polymerase binds to begin transcription; AUG is where the ribosome begins translation.

DNA, RNA, and polypeptides are all polymers. What are the monomers for each?

DNA and RNA are made of nucleotides. Polypeptides are made of amino acids.

How does alternative splicing allow one gene to produce multiple proteins?

Different combinations of exons can be included in mature mRNA from the same pre-mRNA, producing polypeptides with different amino acid sequences.

What is the relationship between a codon and an anticodon?

 They are complementary base-pair partners. The anticodon on tRNA base-pairs with the mRNA codon, delivering the correct amino acid to the ribosome

A drug blocks all tRNA from entering the ribosome. What happens?

Translation elongation halts — no amino acids can be added to the growing chain. Transcription is unaffected.

A single base is deleted from a coding sequence. Why is this potentially more damaging than a point mutation?

It causes a frameshift — every codon downstream is re-read in a shifted frame, changing many amino acids. A point mutation only affects one codon.

Why does the genetic code allow 64 codons but only 20 amino acids?

4 bases in groups of 3 gives 4³ = 64 combinations, exceeding the 20 amino acids. Most amino acids are encoded by multiple codons — called codon degeneracy.

In eukaryotes, why can't translation begin immediately after transcription?

 Pre-mRNA must be processed first (5' cap, 3' tail, intron splicing) and exported from the nucleus before ribosomes in the cytoplasm can translate it.

A sense mutation changes a codon but the same amino acid is still produced. Why?

Codon degeneracy — most amino acids have multiple codons, so the mutation lands on a synonym codon for the same amino acid.