Notes
DNA replication
Replication is semiconservative: each new DNA molecule has one old strand and one new strand.
- Helicase unwinds the helix; primase lays an RNA primer; DNA polymerase adds nucleotides only to the $3'$ end, so it builds $5' \to 3'$.
- The leading strand is continuous; the lagging strand is made in Okazaki fragments joined by ligase.
From gene to protein
- Transcription (nucleus): RNA polymerase reads the template strand and builds mRNA $5' \to 3'$. In eukaryotes, the pre-mRNA gets a $5'$ cap, a poly-A tail, and introns spliced out, leaving exons. Alternative splicing lets one gene make several proteins.
- Translation (ribosome): mRNA codons are read in threes; tRNA anticodons pair with them and bring amino acids. Translation starts at AUG and ends at a stop codon.
- The genetic code is nearly universal, which is evidence of common ancestry.
Regulating genes
- Prokaryotes: operons. The lac operon is inducible (on when lactose is present, because lactose inactivates the repressor). The trp operon is repressible (off when tryptophan is plentiful).
- Eukaryotes: transcription factors, enhancers, DNA methylation (usually silences genes), and histone acetylation (usually opens chromatin to turn genes on). Different cell types express different genes, which drives cell differentiation.
Mutations and biotechnology
- Point mutations: silent (same amino acid), missense (different amino acid), nonsense (early stop codon). Frameshifts from insertions or deletions change every codon downstream.
- Tools: PCR copies DNA; gel electrophoresis separates DNA by size (smaller fragments travel farther toward the positive end); bacterial transformation inserts a plasmid; sequencing reads the bases.
Topics in this unit
Each topic has its own short notes and practice.
Practice
The two old strands separate and each serves as a template.
On the lagging strand, synthesis must restart as the fork opens.
Complementary RNA bases (U instead of T), built antiparallel.
Prokaryotic genes generally lack introns, and their mRNA isn't processed this way.
Without lactose, the active repressor keeps the operon off.
Codons are read in threes, so a deletion scrambles everything downstream.
DNA is negative and moves toward the positive end; small pieces move through the gel fastest.
Gene regulation differs by cell type.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A mutation changes one DNA base in a gene, and the codon on the mRNA changes from UAU (tyrosine) to UAA.
(a) Identify the type of mutation and predict its effect on the protein.
Show the answer and scoring guide
UAA is a stop codon, so this is a nonsense mutation. Translation ends early, producing a shortened protein that likely doesn't work.
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(b) A different mutation changes UAU to UAC, which also codes for tyrosine. Explain why this has no effect.
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The code is redundant: more than one codon codes for the same amino acid, so the protein's sequence doesn't change (a silent mutation).
Check off the points you earned:
(c) Explain how methylation of the gene's promoter could reduce production of this protein even without a mutation.
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Methylation makes chromatin tightly packed and blocks transcription factors and RNA polymerase, so the gene is transcribed less.
Check off the points you earned: