Gene Expression and Regulation — Free AP Biology Review Games.
This unit covers DNA replication, transcription, translation and gene regulation — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers DNA replication, transcription, translation and gene regulation — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 DNA Replication
DNA replication is semiconservative, meaning each new double helix contains one original strand and one newly synthesized strand. Students must know the roles of key enzymes (helicase, primase, DNA polymerase III, DNA polymerase I, ligase) and the directionality constraint (synthesis only 5'→3'). The leading strand is synthesized continuously; the lagging strand is synthesized in Okazaki fragments.
Key Points
- Helicase unwinds the double helix at the replication fork; primase lays down an RNA primer to provide a free 3'-OH for DNA polymerase
- DNA polymerase III adds nucleotides 5'→3', reading the template 3'→5'; it cannot initiate a new strand de novo
- DNA polymerase I replaces RNA primers with DNA; ligase seals the nicks between Okazaki fragments
- A mutation in the template strand is passed to one daughter cell; errors not caught by proofreading become heritable mutations
A cell is replicating DNA and a researcher adds a drug that inhibits primase. Which process is most directly disrupted and why?
Without primase, no RNA primers can be synthesized. Because DNA polymerase III requires a free 3'-OH group to begin adding nucleotides, it cannot start synthesis on either the leading or lagging strand. Replication halts entirely, not just on the lagging strand, because even the leading strand requires one primer at the origin.
2 Transcription
Transcription is the synthesis of a pre-mRNA from a DNA template by RNA polymerase, occurring in the nucleus in eukaryotes. Students must know the three stages (initiation, elongation, termination), the role of the promoter and transcription factors, and eukaryotic pre-mRNA processing (5' cap, poly-A tail, splicing out introns). The template strand is read 3'→5', producing an mRNA 5'→3'.
Key Points
- RNA polymerase binds to the promoter (e.g., TATA box) with the help of transcription factors; no primer is needed
- The non-template (coding) strand has the same sequence as the mRNA, except T is replaced by U
- Pre-mRNA processing: 5' methylguanosine cap protects from degradation; poly-A tail added to 3' end; introns removed by spliceosomes
- Alternative splicing of exons allows one gene to code for multiple proteins — a key concept for gene regulation questions
A mutation changes the TATA box sequence in the promoter of a gene. Predict the most likely effect on transcription.
The TATA box is a core promoter element where the transcription initiation complex assembles. If it is mutated, transcription factors cannot bind properly, and RNA polymerase is not recruited to the correct start site. The result is a significant reduction or complete elimination of transcription of that gene.
3 Translation
Translation is the synthesis of a polypeptide from an mRNA sequence by ribosomes, occurring in the cytoplasm. Students must understand the codon–anticodon interaction, the three ribosomal sites (A, P, E), and the sequential steps of initiation, elongation, and termination. The genetic code is degenerate (multiple codons per amino acid) and nearly universal.
Key Points
- Ribosomes read mRNA codons 5'→3'; tRNA anticodons are antiparallel and complementary to the codon
- Initiation: small ribosomal subunit binds mRNA at the start codon (AUG, Met); large subunit joins forming P and A sites
- Elongation: aminoacyl-tRNA enters A site, peptide bond forms (peptidyl transferase activity of rRNA), ribosome translocates 5'→3'
- Stop codons (UAA, UAG, UGA) recruit release factors, not tRNAs, causing the polypeptide to be released
A point mutation changes codon 47 in an mRNA from GAA (Glu) to GAG. What is the most likely effect on the protein?
Both GAA and GAG code for glutamic acid — this is a synonymous (silent) mutation due to the degeneracy of the genetic code. The amino acid sequence of the protein is unchanged, so protein function is most likely unaffected. This example tests whether students can use a codon table and understand degeneracy rather than assuming all mutations alter protein function.
4 Gene Regulation
Gene expression is regulated at multiple levels: transcriptional, post-transcriptional, translational, and post-translational. For the AP exam, students must know prokaryotic operon models (lac and trp operons) and eukaryotic regulation via transcription factors, enhancers, and epigenetic mechanisms (DNA methylation, histone modification). The core principle is that cells regulate which genes are expressed to respond to environmental signals.
Key Points
- Lac operon (inducible): in the absence of lactose, a repressor blocks transcription; lactose (as allolactose) binds the repressor, causing it to detach and allowing transcription
- Trp operon (repressible): when tryptophan is abundant, it acts as a corepressor binding the repressor protein, which then blocks transcription
- Eukaryotic enhancers are DNA sequences that, when bound by activator proteins, increase transcription even from a distance via DNA looping
- Epigenetic regulation: methylation of cytosine residues silences genes; acetylation of histones loosens chromatin and increases transcription — neither changes the DNA sequence
E. coli is grown in a medium with both glucose and lactose present. Predict whether the lac operon genes will be highly expressed, and justify your answer.
Even though lactose is present (which inactivates the repressor), the lac operon will NOT be highly expressed because glucose is also present. When glucose is available, cAMP levels are low, so the CAP activator protein cannot bind the promoter to stimulate transcription. This demonstrates that the lac operon requires both the absence of glucose (for CAP activation) and the presence of lactose (to relieve repression) to be maximally transcribed — a classic two-factor regulation question on the AP exam.
Questions, answered.
What is Gene Expression and Regulation?
Gene Expression and Regulation is Unit 6 of AP Biology, covering DNA replication, transcription, translation and gene regulation.
How to study for AP Biology Unit 6?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.