Credit Sprint / Study guides / Biology Unit 6

Credit Sprint's Guide to Biology Unit 6: Gene Expression and Regulation

DNA is copied before division and read to make RNA and proteins. Cells turn genes on and off so different cells do different jobs, mutations change the code, and biotechnology lets scientists read and edit it.

What you need to know

• DNA replication is semiconservative: each new double helix keeps one old strand. Helicase unwinds, primase adds a primer, DNA polymerase builds 5′→3′, ligase joins Okazaki fragments on the lagging strand.
• Transcription (nucleus): RNA polymerase reads the template strand and builds mRNA 5′→3′.
• Eukaryotic mRNA is processed: a 5′ cap, a poly-A tail and splicing out of introns. Alternative splicing lets one gene make several proteins.
• Translation (ribosome): tRNA anticodons pair with mRNA codons, bringing amino acids. It starts at AUG and stops at a stop codon.
• The genetic code is redundant (several codons for most amino acids) and nearly universal, evidence of common ancestry.
• Prokaryotes regulate genes with operons. The lac operon is inducible (on when lactose is present); the trp operon is repressible (off when tryptophan is high).
• Eukaryotes regulate with transcription factors, enhancers, DNA methylation and histone acetylation. All cells carry the same DNA but express different genes.
• Point mutations: silent (same amino acid), missense (different amino acid), nonsense (early stop codon). Frameshifts (insertions or deletions not in threes) change every codon after them.
• Biotechnology: gel electrophoresis separates DNA by size (small pieces travel farther), PCR copies DNA, and bacterial transformation inserts genes with plasmids.
• Mutations are random; selection acts on the variation they produce.

Equations and rules

Exam traps

Key terms

• Semiconservative replication: Each new DNA molecule has one old strand and one new strand.
• DNA polymerase: Builds new DNA 5′→3′ and proofreads.
• Okazaki fragments: Short pieces of the lagging strand, later joined by ligase.
• Transcription: Making mRNA from a DNA template. RNA polymerase does the work.
• Intron: A non-coding section removed from pre-mRNA.
• Codon: Three mRNA bases that code for one amino acid or a stop.
• Anticodon: Three tRNA bases that pair with an mRNA codon.
• Operon: A cluster of bacterial genes under one promoter and operator.
• Transcription factor: A protein that helps turn a gene on (or off) by binding DNA.
• Frameshift mutation: An insertion or deletion that shifts the reading frame.
• Nonsense mutation: A point mutation that creates an early stop codon.
• PCR: Polymerase chain reaction: makes millions of copies of a DNA segment.
• Gel electrophoresis: Separates DNA fragments by size. Small fragments move farthest.

Practice questions

Five of the 25 Unit 6 questions in Credit Sprint. Every choice is explained in the app.