Which base pair is held together by three hydrogen bonds in double-stranded DNA?
Guanine and cytosine
✓
Adenine and thymine
Adenine and uracil
Guanine and thymine
Correct Answer
Guanine and cytosine
Guanine pairs with cytosine through three hydrogen bonds, which makes G-C pairs slightly more stable than A-T pairs. Adenine and thymine pair through only two hydrogen bonds. Adenine and uracil pair in RNA, not in double-stranded DNA. Guanine and thymine are not a standard complementary pair.
Category: DNA Structure
Question 2
Which two DNA bases are purines, with a double-ring structure?
Cytosine and thymine
Adenine and guanine
✓
Thymine and uracil
Adenine and cytosine
Correct Answer
Adenine and guanine
Adenine and guanine are purines, built from two fused rings. Cytosine and thymine are pyrimidines with a single ring. Thymine and uracil are both single-ring pyrimidines, and uracil is not found in DNA. Adenine and cytosine mixes one purine with one pyrimidine.
Category: DNA Structure
Question 3
How does the sugar in an RNA nucleotide differ from the sugar in a DNA nucleotide?
It has six carbons instead of five
It has no attachment site for phosphate
It has a hydroxyl group on its 2' carbon
✓
It has a nitrogen atom in its ring
Correct Answer
It has a hydroxyl group on its 2' carbon
Ribose, the sugar in RNA, carries a hydroxyl group on its 2' carbon, while deoxyribose has only a hydrogen there. Saying it has six carbons instead of five is wrong because both sugars have five carbons. Saying it has no attachment site for phosphate is wrong because RNA also has a sugar-phosphate backbone. Saying it has a nitrogen atom in its ring is wrong because nitrogen is found in the bases, not the sugar.
Category: DNA Structure
Question 4
In eukaryotic chromosomes, what structural unit forms when DNA wraps around a core of histone proteins?
Nucleoid
Centromere
Ribosome
Nucleosome
✓
Correct Answer
Nucleosome
A nucleosome is a length of DNA wound around a core of histones, and chains of them look like beads on a string. Nucleoid is the region of a prokaryotic cell that holds its chromosome. Centromere is the constricted region where sister chromatids are joined. Ribosome is a protein-making structure built from rRNA and protein, not DNA.
Category: DNA Structure
Question 5
After one round of replication, each new DNA double helix contains one original strand and one newly made strand. What is this model of replication called?
Semiconservative replication
✓
Conservative replication
Dispersive replication
Reciprocal replication
Correct Answer
Semiconservative replication
In semiconservative replication, the parental strands separate and each serves as a template, so every daughter helix keeps one old strand. Conservative replication would leave the original helix intact and make an entirely new one. Dispersive replication would scatter old and new pieces along both strands. Reciprocal replication is not a model of DNA copying.
Category: Replication and Repair
Question 6
Which enzyme unwinds and separates the two DNA strands at a replication fork?
Primase
Helicase
✓
DNA ligase
RNA polymerase
Correct Answer
Helicase
Helicase breaks the hydrogen bonds between base pairs, opening the helix into a Y-shaped replication fork. Primase makes the short RNA primers. DNA ligase seals gaps in the sugar-phosphate backbone. RNA polymerase carries out transcription, not the unwinding step of replication.
Category: Replication and Repair
Question 7
Why does DNA polymerase need a primer before it can begin making a new DNA strand?
It cannot bind to single-stranded DNA
It uses the primer as its energy source
It can add nucleotides only to an existing 3' end
✓
It can copy only RNA templates
Correct Answer
It can add nucleotides only to an existing 3' end
DNA polymerase cannot start a strand from nothing; it can only extend a chain that already has a free 3' end, which the short RNA primer supplies. Saying it cannot bind to single-stranded DNA is wrong because single-stranded DNA is exactly the template it reads. Saying it uses the primer as its energy source is wrong because the energy comes from the incoming nucleotides. Saying it can copy only RNA templates is wrong because it copies DNA templates.
Category: Replication and Repair
Question 8
Which enzyme seals the gaps between Okazaki fragments on the lagging strand?
Helicase
Primase
Telomerase
DNA ligase
✓
Correct Answer
DNA ligase
After the RNA primers are replaced with DNA, DNA ligase forms the final bonds in the sugar-phosphate backbone, joining the fragments into one continuous strand. Helicase unwinds the double helix ahead of the fork. Primase lays down RNA primers. Telomerase extends the ends of linear chromosomes.
Category: Replication and Repair
Question 9
Which enzyme uses its own built-in RNA template to lengthen the ends of linear eukaryotic chromosomes?
Telomerase
✓
Primase
RNA polymerase
Helicase
Correct Answer
Telomerase
Telomerase carries an RNA template that it uses to add repeated sequences to chromosome ends, making up for the shortening that happens because the lagging strand cannot be finished at the very tip. Primase makes primers but does not extend chromosome ends. RNA polymerase transcribes genes into RNA. Helicase unwinds DNA.
Category: Replication and Repair
Question 10
Which repair process cuts out thymine dimers, along with a few neighboring bases, and fills the gap by copying the undamaged strand?
Mismatch repair
Nucleotide excision repair
✓
Polymerase proofreading
Telomere extension
Correct Answer
Nucleotide excision repair
Nucleotide excision repair removes damaged bases such as ultraviolet-induced thymine dimers together with some flanking bases, and DNA polymerase then fills the gap using the intact strand as a template. Mismatch repair fixes incorrectly paired bases left over after replication. Polymerase proofreading corrects a wrong base as it is being added. Telomere extension lengthens chromosome ends and does not repair damage.
Category: Replication and Repair
Question 11
During transcription, RNA polymerase builds an RNA molecule that is complementary to which strand of DNA?
The nontemplate strand
The leading strand
The template strand
✓
The lagging strand
Correct Answer
The template strand
RNA polymerase reads the template strand and pairs incoming RNA nucleotides with it, so the RNA is complementary to that strand. The nontemplate strand has nearly the same sequence as the RNA, with T in place of U. The leading strand and the lagging strand are terms from DNA replication, not transcription.
Category: Transcription and Processing
Question 12
What is the DNA sequence, usually just upstream of a gene, where RNA polymerase and related proteins bind to begin transcription?
Terminator
Exon
Codon
Promoter
✓
Correct Answer
Promoter
The promoter is the binding site for the transcription machinery, and its sequence helps set how often a gene is transcribed. Terminator is a sequence at the end of a gene that signals transcription to stop. Exon is a coding segment within the gene. Codon is a three-nucleotide unit of mRNA read during translation.
Category: Transcription and Processing
Question 13
Which modification is added to the 3' end of a eukaryotic pre-mRNA?
A poly-A tail
✓
A modified guanine cap
An RNA primer
A histone coat
Correct Answer
A poly-A tail
An enzyme adds a long string of adenine nucleotides, the poly-A tail, to the 3' end, which protects the transcript and helps signal its export from the nucleus. A modified guanine cap is added to the 5' end, not the 3' end. An RNA primer is used in DNA replication. A histone coat is not part of mRNA processing; histones package DNA.
Category: Transcription and Processing
Question 14
How many different codons are possible in the genetic code?
20
64
✓
61
16
Correct Answer
64
Each codon has three positions and each position can hold one of four bases, so there are 4 x 4 x 4 = 64 codons. The number 20 is the count of common amino acids, not codons. The number 61 is the count of codons that specify amino acids, leaving out the three stop codons. The number 16 is what two-base codons would give.
Category: Translation
Question 15
Which molecule acts as an adaptor, pairing with an mRNA codon and delivering the matching amino acid to the ribosome?
Messenger RNA
Ribosomal RNA
Transfer RNA
✓
Small nuclear RNA
Correct Answer
Transfer RNA
Each transfer RNA carries a specific amino acid and has an anticodon that base pairs with a codon, linking the language of nucleotides to the language of amino acids. Messenger RNA carries the coded message itself. Ribosomal RNA forms the structure and catalytic core of the ribosome. Small nuclear RNA works in splicing inside the nucleus.
Category: Translation
Question 16
During elongation, at which ribosomal site does each incoming charged tRNA first bind?
P site
E site
S site
A site
✓
Correct Answer
A site
A charged tRNA carrying a new amino acid enters the A site, where its anticodon pairs with the exposed codon. The P site holds the tRNA attached to the growing polypeptide chain. The E site is where an empty tRNA exits. The S site is not one of the three tRNA binding sites of the ribosome.
Category: Translation
Question 17
Why is the genetic code described as degenerate?
Many amino acids are specified by more than one codon
✓
Each codon can specify more than one amino acid
Codons overlap so that one base belongs to two codons
Different species read codons in different ways
Correct Answer
Many amino acids are specified by more than one codon
Degenerate means redundant: 61 codons code for only 20 amino acids, so most amino acids have two or more codons. Saying each codon can specify more than one amino acid is wrong because each codon is unambiguous. Saying codons overlap is wrong because the code is read in non-overlapping triplets. Saying different species read codons in different ways is wrong because the code is nearly universal.
Category: Translation
Question 18
In the lac operon of E. coli, what happens when lactose is absent from the cell?
An activator binds the promoter and boosts transcription
A repressor binds the operator and blocks transcription
✓
The lactose genes are transcribed at a high rate
RNA polymerase binds the operator in place of the promoter
Correct Answer
A repressor binds the operator and blocks transcription
With no lactose present, the repressor protein sits on the operator and keeps RNA polymerase from transcribing the genes, so the cell does not waste energy making enzymes it does not need. An activator boosting transcription does not describe this situation. The genes are transcribed at a high rate only when lactose is present and the repressor has let go of the operator. RNA polymerase binds the promoter, not the operator.
Category: Gene Regulation
Question 19
What does alternative RNA splicing allow a eukaryotic cell to do?
Make extra copies of a gene
Translate mRNA inside the nucleus
Make different proteins from one gene
✓
Repair errors in the DNA
Correct Answer
Make different proteins from one gene
By keeping or removing different combinations of segments from the same pre-mRNA, alternative splicing produces different mature mRNAs and therefore different proteins from a single gene. Making extra copies of a gene is gene duplication, a separate process. Translating mRNA inside the nucleus does not happen in eukaryotes. Repairing errors in the DNA is the job of repair enzymes, not splicing.
Category: Gene Regulation
Question 20
What happens during regulation at the epigenetic level in eukaryotes?
A 5' cap and poly-A tail are added to the mRNA
A finished protein is chemically modified
Ribosomes change how quickly they translate mRNA
DNA is loosened from nucleosomes so transcription factors can reach it
✓
Correct Answer
DNA is loosened from nucleosomes so transcription factors can reach it
Epigenetic regulation controls how tightly DNA is packed; uncoiling it from nucleosomes makes a gene accessible to transcription factors. Adding a 5' cap and poly-A tail is post-transcriptional processing. Chemically modifying a finished protein is post-translational control. Changing how quickly ribosomes translate mRNA is translational control.
Category: Gene Regulation