Gene expression

What is gene expression?

Using information in a gene to make a functional product, such as a protein.

What is the central dogma?

DNA → RNA → protein.

What is transcription?

Making RNA from a DNA template.

What is translation?

Using mRNA instructions to build a chain of amino acids (a protein).

DNA vs. RNA: sugar

DNA has deoxyribose; RNA has ribose with an extra OH group.

DNA vs. RNA: bases

DNA uses A, T, C, G; RNA uses A, U, C, G.

DNA vs. RNA: typical strand structure

DNA is usually double-stranded; RNA is usually single-stranded.

What does mRNA do?

Carries genetic instructions to the ribosome.

What does rRNA do?

Forms part of the ribosome and helps make peptide bonds.

What does tRNA do?

Brings specific amino acids to the ribosome.

What is a codon?

A 3-base sequence in mRNA that specifies an amino acid or stop signal.

What is a tRNA anticodon?

A 3-base sequence that pairs with an mRNA codon.

What tRNA anticodon pairs with 5′-AUG-3′?

3′-UAC-5′.

How many codons and amino acids are in the standard genetic code?

64 codons and 20 common amino acids.

What does unambiguous genetic code mean?

Each codon specifies only one amino acid or stop signal.

What does redundant genetic code mean?

Multiple codons can specify the same amino acid.

What is the wobble idea?

The third codon position often varies without changing the amino acid.

Where does translation begin on mRNA?

At the AUG start codon, not necessarily at the first mRNA base.

What does AUG specify?

The start of translation and the amino acid methionine.

Transcription initiation step 1

RNA polymerase binds the DNA promoter.

Transcription initiation step 2

RNA polymerase opens a short part of the DNA helix.

Transcription initiation step 3

RNA polymerase begins adding complementary RNA nucleotides.

Transcription elongation: which DNA strand and direction are read?

The template DNA strand is read 3′→5′.

Transcription elongation: which direction is RNA built?

RNA is made 5′→3′ using complementary bases (A-U, T-A, G-C, C-G).

Transcription termination step 1

RNA polymerase reaches a DNA termination signal and stops.

Transcription termination step 2

RNA is released, RNA polymerase leaves, and DNA closes back up.

Does transcription need a primer?

No; RNA polymerase starts without a primer.

RNA processing step 1: 5′ cap

A cap is added to the 5′ end; it helps export and ribosome recognition.

RNA processing step 2: Poly-A tail

Many adenines are added to the 3′ end to protect mRNA.

RNA processing step 3: RNA splicing

Introns are removed and exons are joined.

What is pre-mRNA?

The RNA transcript before eukaryotic processing is complete.

What is mature mRNA?

Processed RNA with a 5′ cap, poly-A tail, and spliced exons.

Where do transcription and RNA processing occur in eukaryotes?

Mainly in the nucleus.

Where does translation occur in eukaryotes?

On ribosomes in the cytoplasm or on rough ER.

What do the ribosome A, P, and E sites do?

A: new tRNA arrives; P: holds growing chain; E: empty tRNA exits.

Translation initiation step 1

Small ribosomal subunit, mRNA, and first tRNA come together at AUG.

Translation initiation step 2

The first tRNA starts in the P site; the large subunit joins.

Translation elongation step 1: Codon recognition

A new tRNA enters the A site and pairs its anticodon with the codon.

Translation elongation step 2: Peptide bond

The growing amino acid chain transfers from the P-site tRNA to the amino acid at A.

Translation elongation step 3: Translocation

Ribosome moves one codon; A-site tRNA goes to P and empty P-site tRNA goes to E.

Translation termination step 1

A stop codon reaches the A site; no tRNA matches it.

Translation termination step 2

A release factor binds, the polypeptide is released, and ribosome subunits separate.

Which direction does a ribosome read mRNA?

5′→3′, one codon at a time.

Is tRNA built during translation?

No; existing tRNAs bring amino acids, and the ribosome builds a polypeptide.

How is transcription termination different from translation termination?

Transcription uses a DNA termination signal; translation uses an mRNA stop codon and release factor.

What is the DNA coding strand during transcription?

The non-template strand; RNA matches its bases except U replaces T.

What happens to DNA behind RNA polymerase?

The two DNA strands come back together as the RNA leaves.

Why is ribosomal RNA (rRNA) called a ribozyme?

It acts as an enzyme that catalyzes peptide bond formation.

During translation, what moves P→A versus A→P→E?

The growing amino acid chain transfers P→A; tRNAs move A→P→E.

What starts transcription versus translation?

Transcription starts at a DNA promoter; translation starts at an AUG codon.