AP Biology: Gene Expression and Regulation — Practice Questions & Answers
How genetic information stored in DNA is copied, decoded into proteins, and selectively turned on or off to produce specialized cells, plus the biotechnology tools used to study and manipulate genes.
981 practice questions available for this unit — here are 10 with full answers and explanations.
Which of the following correctly pairs a nitrogenous base found in DNA with its complementary base?
Adenine with cytosine
Guanine with cytosine
Adenine with guanine
Thymine with cytosine
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WhyIn DNA, adenine pairs with thymine (A-T) via two hydrogen bonds, and guanine pairs with cytosine (G-C) via three hydrogen bonds. Uracil replaces thymine only in RNA.
2Multiple choice · Easy
Which sugar is found in the backbone of an RNA molecule but NOT in DNA?
Deoxyribose
Glucose
Ribose
Fructose
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WhyRNA contains ribose, which has a hydroxyl (-OH) group on its 2' carbon. DNA contains deoxyribose, which lacks that oxygen at the 2' position, making DNA more chemically stable.
3Multiple choice · Easy
DNA replication is described as semiconservative. This means that each new DNA molecule consists of:
Two newly synthesized strands
One original strand and one new strand
Two original strands
Fragments of both old and new strands randomly mixed
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WhyIn semiconservative replication, the two original strands separate and each serves as a template. The result is two daughter molecules, each containing one original (parental) strand and one newly synthesized strand. This was demonstrated by the Meselson-Stahl experiment.
4Multiple choice · Easy
In what direction does DNA polymerase synthesize a new DNA strand?
3' -> 5'
5' -> 3'
Both directions simultaneously
Either direction, depending on the template
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WhyDNA polymerase adds nucleotides only to the 3' hydroxyl end of a growing strand, so synthesis always proceeds in the 5' -> 3' direction. The template is read in the antiparallel 3' -> 5' direction.
5Multiple choice · Medium
During DNA replication, the lagging strand is synthesized discontinuously as Okazaki fragments. Why must the lagging strand be made this way?
DNA polymerase can only add nucleotides in the 5' -> 3' direction, opposite to fork movement on that strand
The lagging strand template has more guanine and cytosine bases
Helicase only unwinds one strand at a time
RNA primers cannot bind to the leading strand
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WhyBecause DNA polymerase can only synthesize 5' -> 3' and the two template strands are antiparallel, the lagging strand template runs in a direction that forces synthesis away from the replication fork. The polymerase therefore works in short pieces (Okazaki fragments) that are later joined by DNA ligase.
6Multiple choice · Medium
A eukaryotic pre-mRNA undergoes three major processing steps before leaving the nucleus. Which step involves the removal of non-coding sequences?
Addition of the 5' cap
Addition of the poly-A tail
Splicing to remove introns
Phosphorylation of the ribose backbone
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WhyIntrons are non-coding intervening sequences removed during splicing by the spliceosome, which joins the remaining exons together. The 5' cap and poly-A tail are added separately and protect the mRNA and aid in export and translation.
7Multiple choice · Medium
A segment of the DNA template strand reads 3'-TACGGCATT-5'. What is the sequence of the mRNA transcribed from it?
5'-AUGCCGUAA-3'
5'-ATGCCGTAA-3'
5'-UACGGCAUU-3'
5'-AAUGCCGUA-3'
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WhyRNA polymerase reads the template 3' -> 5' and builds mRNA 5' -> 3' using complementary base pairing, with uracil replacing thymine. Template 3'-TAC GGC ATT-5' yields mRNA 5'-AUG CCG UAA-3'.
8Multiple choice · Medium
What is the role of tRNA during translation?
It catalyzes the formation of peptide bonds in the ribosome
It carries a specific amino acid and base-pairs its anticodon with an mRNA codon
It unwinds the mRNA so ribosomes can read it
It adds the poly-A tail to the mRNA before translation
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WhyEach tRNA carries a specific amino acid and has an anticodon that base-pairs with the complementary mRNA codon in the ribosome. This matching ensures the correct amino acid is added to the growing polypeptide in the order specified by the mRNA.
9Multiple choice · Medium
In the lac operon of E. coli, what happens to transcription of the lac genes when lactose is present but glucose is absent?
Transcription is blocked because the repressor stays bound
Transcription occurs at maximal levels
Transcription is permanently shut off by CAP
The genes are deleted from the chromosome
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WhyLactose (via allolactose) binds the repressor and removes it from the operator, allowing transcription. When glucose is low, cAMP is high, so the cAMP-CAP complex binds the promoter and strongly activates transcription. Both conditions together produce maximal expression.
10Multiple choice · Medium
The trp operon is a repressible operon. When tryptophan levels in the cell are HIGH, transcription of the trp genes is:
Turned off because tryptophan acts as a corepressor
Turned on because tryptophan inactivates the repressor
Unaffected, since the trp operon is constitutive
Increased to store extra tryptophan
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WhyThe trp operon controls genes for making tryptophan. When tryptophan is abundant, it acts as a corepressor, binding the repressor and enabling it to block the operator. This turns off transcription, so the cell does not waste energy making an amino acid it already has.
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DNA (deoxyribonucleic acid): The double-stranded molecule that stores hereditary information; its sequence of nucleotides encodes the instructions for building RNA and proteins.
RNA (ribonucleic acid): A typically single-stranded nucleic acid that uses ribose sugar and uracil; it carries and helps express genetic information in roles such as mRNA, tRNA, and rRNA.
Nucleotide: The repeating building block of nucleic acids, made of a five-carbon sugar, a phosphate group, and one nitrogenous base.
Nitrogenous base: The information-carrying component of a nucleotide; the purines adenine and guanine and the pyrimidines cytosine, thymine, and uracil.
Purine: A nitrogenous base with a double-ring structure, namely adenine and guanine.
Pyrimidine: A nitrogenous base with a single-ring structure, namely cytosine, thymine, and uracil.
Phosphodiester bond: The covalent linkage joining the phosphate of one nucleotide to the sugar of the next, forming the sugar-phosphate backbone of a nucleic acid strand.
Sugar-phosphate backbone: The alternating chain of sugars and phosphate groups that forms the structural outer framework of a nucleic acid strand.