Overview
Molecular biology covers how information in DNA becomes functional molecules, and how cells control that process. It is also where most experimental questions come from. Techniques such as PCR, gel electrophoresis, and reporter assays show up in scenarios about genetics, physiology, and development.
Learn the central processes well enough to predict what a specific mutation or inhibitor does. For example, if a promoter is deleted, is the protein still made? If a stop codon appears early, how long is the protein?
Core concepts
2.1DNA replication
Replication is semiconservative. DNA polymerases add nucleotides only to a 3′-OH, so synthesis runs 5′→3′. The leading strand is made continuously, and the lagging strand is made as Okazaki fragments that are joined by ligase. Primase lays down RNA primers, helicase unwinds, and topoisomerases relieve supercoiling.
Proofreading (3′→5′ exonuclease activity) and mismatch repair keep error rates low. Telomerase extends chromosome ends in cells that express it.
Key terms: semiconservative · Okazaki fragment · primase · proofreading · telomerase
2.2Transcription and RNA processing
RNA polymerase reads the template strand 3′→5′ and makes RNA 5′→3′. The RNA sequence matches the coding (non-template) strand, with U in place of T.
In eukaryotes, pre-mRNA gets a 5′ cap and a poly-A tail, and introns are removed by the spliceosome. Alternative splicing lets one gene produce several proteins. Bacteria transcribe and translate at the same time, and their mRNAs are often polycistronic.
Key terms: template vs. coding strand · promoter · splicing · alternative splicing · polycistronic
2.3Translation and the genetic code
Ribosomes read mRNA codons 5′→3′. tRNAs carry amino acids, and aminoacyl-tRNA synthetases attach the correct amino acid to each tRNA. That attachment step is where the code is actually enforced.
Mutation types: silent (same amino acid), missense (different amino acid), nonsense (early stop codon), and frameshift (insertion or deletion that is not a multiple of three). The effect of a missense mutation depends on where it is and how different the new amino acid is.
Key terms: codon · reading frame · aminoacyl-tRNA synthetase · nonsense mutation · frameshift
2.4Gene regulation
In the lac operon, the repressor blocks transcription unless lactose (as allolactose) is present. CAP–cAMP boosts transcription when glucose is low. Be able to predict expression for mutants such as lacI⁻, lacOᶜ, and lacIˢ in both haploid cells and partial diploids. That means knowing whether each element acts in cis or in trans.
Eukaryotic regulation works at many levels: chromatin (acetylation generally opens it, and DNA methylation at promoters generally silences it), transcription factors and enhancers, RNA processing, microRNAs, translation, and protein degradation.
Key terms: operon · cis vs. trans · enhancer · chromatin remodeling · microRNA
2.5Molecular techniques
PCR amplifies a region between two primers. Gel electrophoresis separates DNA by size, and smaller fragments move farther. Restriction enzymes cut at specific sequences. Western blots detect proteins with antibodies. qPCR and RNA-seq measure transcript levels. Reporter genes (GFP, luciferase) show when and where a promoter is active.
In experiment questions, always ask what the control is and what each lane or sample shows.
Key terms: PCR · gel electrophoresis · Western blot · reporter gene · CRISPR-Cas9
Practice
Review the concepts above, then complete the practice set. Missed a question? Read the explanation and try a similar problem.
Common mistakes
| Mistake | Instead |
|---|---|
| Writing the mRNA as the complement of the coding strand. | mRNA matches the coding strand (with U for T). It is complementary to the template strand. |
| Assuming every point mutation changes the protein. | Check the codon table. Many third-position changes are silent. |
| Treating lacOᶜ as acting in trans. | An operator mutation affects only the genes on the same DNA molecule. That is the definition of acting in cis. |
Recommended resources
- OpenStax Biology 2e, Chapters 14–17 Textbook (free)
- Scitable by Nature Education Website
- MIT OpenCourseWare 7.01SC Fundamentals of Biology University lectures
Further reading
- Campbell Biology, chapters on gene expression and regulation.
- Alberts et al., Molecular Biology of the Cell, chapters on DNA replication and control of gene expression.