AP Biology: Gene Expression and Regulation (High School)

What three components make up a nucleotide?

Answer: A phosphate group, a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

What are DNA's base-pairing rules and how many hydrogen bonds form in each pair?

Answer: Adenine pairs with Thymine (2 H-bonds); Guanine pairs with Cytosine (3 H-bonds). Purines always pair with pyrimidines.

Name three structural differences between DNA and RNA.

Answer: DNA has deoxyribose, RNA has ribose; DNA uses thymine, RNA uses uracil; DNA is typically double-stranded, RNA is typically single-stranded.

Which enzymes unwind the DNA helix and relieve the resulting strain during replication?

Answer: Helicase unwinds the double helix at the replication fork; topoisomerase (DNA gyrase) relieves supercoiling/torsional strain ahead of the fork.

Why does DNA synthesis require an RNA primer?

Answer: DNA polymerase can only add nucleotides to an existing 3'-OH group, so primase first synthesizes a short RNA primer to provide that starting point.

Compare the leading and lagging strands during replication.

Answer: Leading strand is synthesized continuously 5'→3' toward the fork; lagging strand is made discontinuously as Okazaki fragments away from the fork, later joined by DNA ligase.

Why is DNA replication called 'semiconservative'?

Answer: Each resulting double helix contains one original (parental) strand and one newly synthesized strand.

What are the three stages of transcription and what happens in each?

Answer: Initiation: RNA polymerase binds the promoter; Elongation: RNA synthesized 5'→3' from the template strand; Termination: polymerase reaches a terminator and releases the transcript.

How does mRNA sequence relate to the DNA template and coding strands?

Answer: RNA polymerase reads the template strand 3'→5'; the resulting mRNA (5'→3') matches the coding (sense) strand sequence, except U replaces T.

What three modifications are made to pre-mRNA in eukaryotes?

Answer: Addition of a 5' cap (modified guanine nucleotide), addition of a 3' poly-A tail, and splicing out of introns.

What is RNA splicing and what complex carries it out?

Answer: Removal of non-coding introns and joining of exons from pre-mRNA; carried out by the spliceosome (small nuclear ribonucleoproteins, snRNPs).

What is alternative splicing and why is it important?

Answer: Different exon combinations from the same pre-mRNA are joined, allowing one gene to produce multiple distinct proteins, increasing proteome diversity.

Where does translation occur, and what three components are required?

Answer: At ribosomes in the cytoplasm (or rough ER); requires mRNA, tRNA, and ribosomes (rRNA plus protein).

What is the function of tRNA, and what is its anticodon?

Answer: tRNA carries a specific amino acid to the ribosome; its anticodon is a 3-nucleotide sequence that base-pairs with a complementary mRNA codon.

Name the three ribosomal sites and their functions.

Answer: A (aminoacyl) site accepts incoming charged tRNA; P (peptidyl) site holds tRNA with the growing polypeptide; E (exit) site releases uncharged tRNA.

What are the start and stop codons in translation?

Answer: AUG is the start codon (codes for methionine); UAA, UAG, and UGA are stop codons, recognized by release factors since no tRNA matches them.

What are the three stages of translation?

Answer: Initiation: ribosome assembles at the start codon; Elongation: amino acids added via peptide bonds as the ribosome moves codon by codon; Termination: stop codon reached, polypeptide released.

What is a codon?

Answer: A sequence of three mRNA nucleotides that specifies a particular amino acid or a stop signal during translation.

What does it mean that the genetic code is 'degenerate' (redundant)?

Answer: Most amino acids are encoded by more than one codon (synonymous codons), which can reduce the impact of certain point mutations.

What does it mean that the genetic code is 'universal'?

Answer: Nearly all organisms use the same codon-to-amino-acid assignments, reflecting shared evolutionary ancestry and enabling genetic engineering across species.

What is an operon?

Answer: A cluster of prokaryotic genes transcribed together from a single promoter, with a shared operator, allowing coordinated regulation.

How does the lac operon respond to lactose?

Answer: Without lactose, a repressor binds the operator and blocks transcription (off). With lactose, allolactose binds the repressor, releasing it from the operator so transcription proceeds (on) — an inducible operon.

How does the trp operon respond to tryptophan?

Answer: Normally 'on'; when tryptophan is abundant it binds the repressor, activating it to bind the operator and stop transcription — a repressible operon showing negative feedback.

What is the key difference between inducible and repressible operons?

Answer: Inducible operons (e.g., lac) are normally off and switched on by a molecule; repressible operons (e.g., trp) are normally on and switched off by a molecule, often an end product.