Chapter summary
DNA replication and gene expression, at a glance
Scan the key ideas, or hide the answers and try to recall them.
01
DNA and the three RNA roles
How does nucleic-acid structure store and use information?
Key idea and reminders
A nucleotide sequence stores information; complementary base pairing enables copying and recognition.
- DNA: deoxyribose and thymine; RNA: ribose and uracil.
- DNA strands are antiparallel and complementary.
- mRNA carries codons; tRNA adapts; rRNA helps catalyse.
Keep in mind: Complementarity concerns base identity; the backbones are antiparallel.
02
Copy both DNA strands
Why is one new strand made in fragments?
Key idea and reminders
DNA polymerase extends a 3-prime end, so antiparallel templates require continuous and discontinuous synthesis.
- Each daughter helix retains one old strand.
- New DNA always extends 5-prime to 3-prime.
- Lagging fragments need primers and ligase.
Keep in mind: Helicase separates strands. Ligase seals nicks between DNA fragments.
03
The end-replication problem
Why can the last RNA primer leave an unfillable gap?
Key idea and reminders
After a terminal primer is removed, no upstream 3-prime end may remain to fill its gap, so a linear chromosome end can shorten.
- Internal gaps can use an upstream 3-prime end.
- A terminal primer may lack that replacement route.
- Telomeres buffer and protect linear ends.
Keep in mind: The end-replication problem follows from primer removal and polymerase directionality, not helicase excising a gene.
04
Transcribe the template strand
Which DNA strand determines the RNA sequence?
Key idea and reminders
RNA polymerase reads one DNA template strand and makes a complementary RNA molecule 5-prime to 3-prime.
- Read template 3-prime to 5-prime.
- Make RNA 5-prime to 3-prime.
- RNA matches coding DNA except U replaces T.
Keep in mind: It is complementary to the template strand and matches the coding strand apart from U/T.
05
Process a eukaryotic RNA message
Why can one gene produce more than one polypeptide?
Key idea and reminders
Eukaryotic pre-mRNA is capped, polyadenylated and spliced; alternative exon combinations can produce different mature messages.
- Introns removed; exons retained.
- 5-prime cap and 3-prime poly-A tail aid stability and use.
- Alternative splicing changes RNA combinations, not DNA.
Keep in mind: Splicing acts on RNA, removing introns from a transcript while the DNA remains.
06
Translate codons into a polypeptide
How does a triplet of RNA bases specify an amino acid?
Key idea and reminders
A ribosome reads mRNA codons 5-prime to 3-prime; matching charged tRNAs deliver amino acids for peptide-bond formation.
- Codon is on mRNA; anticodon is on tRNA.
- Ribosomes catalyse peptide-bond formation.
- Stop codons recruit release, not an extra amino acid.
Keep in mind: Termination uses a release factor; there is no stop amino acid.