Chapter 14 – From DNA to Protein: Gene Expression

What did Archibald Garrod discover about genes?

Genes determine enzymes; mutations in genes can cause metabolic disorders like alkaptonuria.

What was Garrod’s “one gene–one enzyme” hypothesis?

Each gene produces one specific enzyme; mutant alleles produce inactive or missing enzymes.

What is gene expression?

The process of using genetic information to produce proteins.

What are the two main steps of gene expression?

Transcription (DNA → RNA) and translation (RNA → protein).

State the central dogma of molecular biology.

DNA → RNA → Protein; DNA can also replicate (DNA → DNA).

Name the three key types of RNA and their functions.

mRNA: carries coding info for proteins (~5% RNA); rRNA: forms ribosomes and catalyzes peptide bonds (~80%); tRNA: carries amino acids to ribosome (~15%).

How do RNA viruses replicate?

RNA viruses replicate like this:

1. +RNA viruses: RNA acts as mRNA → makes proteins → copies RNA.

2. −RNA viruses: RNA makes a complementary mRNA → makes proteins → copies RNA.

3. Retroviruses: RNA → DNA → integrates into host → makes RNA and proteins.

What is telomerase?

Telomerase is an enzyme that adds repetitive DNA sequences to the ends of chromosomes (telomeres).

• Telomeres protect chromosomes from losing important DNA during replication.

Which DNA strand is used as the template in transcription?

The template strand; the coding strand has the same sequence as RNA (T → U).

What are the requirements for transcription?

DNA template, ribonucleoside triphosphates (ATP, GTP, CTP, UTP), RNA polymerase, proper chemical environment.

What are the main steps of transcription?

• Initiation – RNA polymerase binds to the promoter region of DNA and unwinds the DNA.

• Elongation – RNA polymerase reads the template (antisense) strand and synthesizes pre-mRNA in the 5′ → 3′ direction.

• Termination – RNA polymerase reaches a termination signal, stops, and releases the newly made RNA.

• Processing (in eukaryotes) – pre-mRNA is modified: 5′ cap added, poly-A tail added, introns spliced out, producing mature mRNA.

What are introns and exons?

Introns: noncoding sequences spliced out; Exons: coding sequences forming mature mRNA.

What are the main steps of pre-mRNA processing?

5′ cap addition (ribosome binding/protection), 3′ poly-A tail (stability/export), splicing (introns removed, exons joined).

What is a codon?

A sequence of three mRNA nucleotides specifying one amino acid.

How many codons exist, and why is the code redundant?

64 codons (4³), 20 amino acids; most amino acids have multiple codons (degenerate), but code is unambiguous.

What are start and stop codons?

Start: AUG (methionine), signals translation initiation. Stop: UAA, UAG, UGA, terminate translation.

How does the DNA template relate to codons?

Template strand is complementary/antiparallel to mRNA codons; coding strand matches mRNA (T → U).

What is translation?

The process of converting mRNA codons into a polypeptide chain.

What are the roles of tRNA in translation?

Carry amino acids, pair anticodons with mRNA codons, interact with ribosome A, P, E sites.

What is wobble base pairing?

Flexibility at the third codon position reduces number of tRNAs needed without ambiguity.

Describe ribosome structure.

Large and small subunits (rRNA + proteins); rRNA catalyzes peptide bonds; A site = incoming tRNA, P site = growing peptide, E site = exit tRNA.

Steps of translation?

Initiation: small subunit + methionine tRNA + mRNA assemble; Elongation: peptide bonds form, ribosome moves 5′→3′; Termination: stop codon → release factor, polypeptide released.

What is a polysome?

Multiple ribosomes translating the same mRNA simultaneously.

What are template strands and non-template strands?

Template Strand

• The strand of DNA that RNA polymerase reads during transcription

• RNA is made complementary to this strand

• Also called the antisense strand

Non-template Strand

• The other DNA strand, not read by RNA polymerase

• Its sequence is almost identical to the mRNA (except mRNA has U instead of T)

Also called the coding or sense strand