Chapter summary
DNA and gene expression, at a glance
Scan the key ideas, or hide the answers and try to recall them.
01
Read the DNA molecule
How do pairing and direction support information storage?
Key idea and reminders
DNA has two complementary, antiparallel nucleotide strands. The base sequence carries information; the covalent backbone and base pairing maintain the molecule.
- DNA uses deoxyribose and thymine.
- Antiparallel means opposite strand directions.
- Base sequence stores information.
Keep in mind: Phosphodiester bonds join the backbone within a strand. Hydrogen bonds link complementary bases between strands.
02
Three RNAs, three related jobs
Which RNA carries, matches and helps assemble?
Key idea and reminders
RNA usually has one nucleotide strand with ribose and uracil. mRNA carries codons, tRNA matches codons to amino acids, and rRNA forms a functional part of ribosomes.
- RNA contains ribose and uracil.
- mRNA has codons; tRNA has an anticodon.
- rRNA is structural and catalytic.
Keep in mind: tRNA and rRNA fold into functional structures, and rRNA has a catalytic role in the ribosome.
03
Copy each strand before division
Why does each daughter DNA contain an old strand?
Key idea and reminders
The two parental strands separate and each templates a complementary new strand. Each resulting DNA molecule contains one old strand and one new strand.
- Semi-conservative: one parental and one new strand per daughter molecule.
- DNA polymerase extends at the 3' end.
- Ligase seals backbone breaks between fragments.
Keep in mind: DNA polymerase synthesises both new strands 5' to 3'. Discontinuous fragment synthesis accommodates the antiparallel template arrangement.
04
Copy a gene into RNA
Which DNA strand is the template?
Key idea and reminders
RNA polymerase uses one DNA strand as a template, making a complementary RNA strand 5' to 3'. The DNA sequence remains in the chromosome.
- Transcription product: RNA, not protein.
- RNA polymerase uses one DNA template strand for a gene.
Keep in mind: mRNA is complementary to the template. It resembles the coding strand when both are written 5' to 3', with U replacing T.
05
Prepare a mature mRNA
What changes between transcription and translation?
Key idea and reminders
Pre-mRNA is processed by intron removal, exon joining, addition of a 5' cap and addition of a 3' poly-A tail before mature mRNA is used in translation.
- Splicing removes RNA introns and joins exons.
- A 5' cap and 3' poly-A tail support mature mRNA function.
Keep in mind: It is a sequence of adenine-containing RNA nucleotides at the 3' end of the RNA.
06
Read codons to build a polypeptide
How does the message become an amino-acid order?
Key idea and reminders
A ribosome reads mRNA codons 5' to 3'. Matching tRNAs deliver amino acids, which are joined in order until a stop codon ends translation.
- Codons are read on mRNA.
- tRNAs are adaptors and can be reused.
- The released polypeptide may need further folding or processing.
Keep in mind: Stop codons signal termination; no stop amino acid is incorporated into the chain.