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Quizzes > Science

MCQs in Molecular Biology: Practice Core Concepts

Quick, free molecular biology quiz to check your understanding. Instant results.

Editorial: Review CompletedCreated By: Gabriella HurleyUpdated Aug 23, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art style DNA helix ribosome protein icons on dark blue background for free molecular biology MCQ quiz

This molecular biology MCQ quiz helps you check your grasp of DNA replication, transcription, translation, and gene regulation in a few minutes. Get quick feedback on each question and build confidence for exams; for more practice, try the molecular genetics quiz and the molecular basis of life quiz.

What is the basic building block of DNA?
Monosaccharide
Nucleotide
Fatty acid
Amino acid
DNA is composed of nucleotide monomers, each containing a phosphate group, a sugar, and a nitrogenous base. These nucleotides join via phosphodiester bonds to form the DNA backbone. Understanding nucleotides is fundamental to grasping DNA structure and replication.
Which sugar is found in RNA but not in DNA?
Glucose
Ribose
Fructose
Deoxyribose
RNA contains ribose sugar, which has a hydroxyl group at the 2' carbon, whereas DNA contains deoxyribose lacking that hydroxyl. This difference contributes to RNA's greater reactivity and shorter half-life. Identifying ribose is key to distinguishing RNA from DNA.
According to Chargaff's rules, what percentage of bases in a DNA molecule do adenine and thymine constitute if guanine makes up 30%?
30%
60%
20%
40%
Chargaff's rules state that A=T and G=C. If G is 30%, C is also 30%, totaling 60%. Thus A and T together must make up the remaining 40%. This principle underlies base pairing in the DNA double helix.
What is the primary function of ribosomes in the cell?
Protein synthesis
DNA replication
Lipid metabolism
Glycolysis
Ribosomes facilitate translation by reading mRNA codons and assembling amino acids into polypeptide chains. They consist of rRNA and proteins and operate in the cytoplasm or on the rough ER. Without ribosomes, protein synthesis cannot occur.
Which molecule carries genetic information from DNA to the ribosome?
rRNA
snRNA
tRNA
mRNA
Messenger RNA (mRNA) is transcribed from DNA and carries codon sequences to ribosomes for translation. Transfer RNA (tRNA) and ribosomal RNA (rRNA) serve different roles in translation. Recognizing mRNA's role is critical for understanding gene expression.
In which direction does DNA polymerase synthesize new DNA strands?
Both directions
Randomly
5' to 3'
3' to 5'
DNA polymerase adds nucleotides to the 3' hydroxyl end of the growing strand, synthesizing DNA in a 5' to 3' direction. This unidirectional synthesis is due to the enzyme's requirement for a free 3' OH group. The antiparallel template dictates leading and lagging strand synthesis.
Which base in RNA pairs with adenine during transcription?
Uracil
Guanine
Thymine
Cytosine
During RNA transcription, adenine in the DNA template pairs with uracil in the RNA transcript instead of thymine. Uracil is unique to RNA and pairs specifically with adenine via two hydrogen bonds. This substitution is a key feature of RNA structure.
What enzyme is responsible for adding DNA nucleotides during replication?
RNA polymerase
Ligase
Primase
DNA polymerase
DNA polymerase catalyzes the addition of deoxyribonucleotides to the growing DNA strand. Primase synthesizes RNA primers, and ligase joins Okazaki fragments. DNA polymerase's high fidelity is critical for accurate genome duplication.
What is the role of promoter regions in transcription?
Origin of replication
Signal for ribosome assembly
Binding site for RNA polymerase
Spliceosome recognition
Promoters are DNA sequences where RNA polymerase and transcription factors bind to initiate transcription. They determine transcription start sites and regulate gene expression levels. Mutations in promoters can disrupt transcription initiation.
Okazaki fragments are synthesized on which DNA strand during replication?
Neither strand
Lagging strand
Leading strand
Both strands
The lagging strand is synthesized discontinuously as Okazaki fragments because DNA polymerase can only synthesize in a 5' to 3' direction. RNA primers are laid down periodically, and fragments are later joined by ligase. This contrasts with continuous leading strand synthesis.
Which of the following modifications occurs at the 5? end of eukaryotic mRNA?
7-methylguanosine cap
Splicing
Deamination
Polyadenylation
Eukaryotic mRNAs receive a 7-methylguanosine cap at the 5? end immediately after transcription begins. This cap protects mRNA from degradation and assists in ribosome binding. Polyadenylation and splicing occur at later stages of mRNA processing.
What characterizes rho-independent termination in prokaryotes?
GC-rich hairpin followed by a U-rich tract
Methylation of terminator region
Cleavage by exonucleases
Rho factor binding upstream of terminator
Rho-independent termination relies on the formation of a GC-rich hairpin in the nascent RNA followed by a run of uracils, causing RNA polymerase to pause and dissociate. It does not require the rho protein. This mechanism is one of two main prokaryotic termination processes.
Which enzyme unwinds the DNA helix during replication?
Topoisomerase
Ligase
Helicase
Polymerase
Helicase separates the two strands of the DNA helix by breaking hydrogen bonds, creating replication forks. Topoisomerases relieve supercoiling ahead of helicase. Without helicase activity, replication cannot proceed.
How does tRNA recognize the correct amino acid codon during translation?
Through ribosomal RNA binding
By amino acid side chain interactions
By release factors
Via anticodon base pairing
tRNA molecules have anticodon loops that base-pair with complementary mRNA codons in the ribosome. This ensures the correct amino acid is added to the growing polypeptide. The anticodon - codon interaction is central to the fidelity of translation.
What type of mutation results from the insertion or deletion of nucleotide(s) not in multiples of three?
Missense mutation
Frameshift mutation
Nonsense mutation
Silent mutation
Insertions or deletions not divisible by three shift the reading frame, altering downstream codons and often leading to nonfunctional proteins. These are known as frameshift mutations. They typically have more severe effects than single-base substitutions.
Which enzyme extends telomeres by adding repetitive sequences at chromosome ends?
Telomere ligase
Telomerase
Topoisomerase
Telomerase primase
Telomerase is a ribonucleoprotein that carries its own RNA template to extend telomeric DNA repeats at chromosome ends. This activity counteracts shortening during cell division. Telomerase is active in germ cells and many cancer cells.
Which enzyme removes the RNA primers during DNA replication in prokaryotes?
DNA polymerase III
Primase
DNA polymerase I
RNase H
DNA polymerase I has 5??3? exonuclease activity that removes RNA primers and replaces them with DNA. DNA polymerase III synthesizes the bulk of new DNA on both strands. RNase H also degrades RNA in hybrids but DNA polymerase I is primary for primer removal.
What is the size difference between prokaryotic and eukaryotic ribosomes?
60S vs 70S
90S vs 100S
50S vs 60S
70S vs 80S
Prokaryotic ribosomes are 70S, composed of 50S and 30S subunits, while eukaryotic ribosomes are 80S made of 60S and 40S subunits. The Svedberg unit reflects sedimentation rate, not a sum of subunits. Differences are targets for antibiotics.
Which process removes introns from eukaryotic pre-mRNA?
Capping
Splicing by the spliceosome
Polyadenylation
Editing by deaminases
The spliceosome, a complex of snRNAs and proteins, recognizes splice sites and catalyzes intron removal from pre-mRNA. Exons are ligated to produce mature mRNA. This process is essential for eukaryotic gene expression.
In the lac operon, which molecule acts as the inducer to initiate transcription?
Lactose
Glucose
cAMP
Allolactose
Allolactose, an isomer of lactose, binds the lac repressor causing it to release from the operator, thereby permitting transcription. Lactose must be converted to allolactose for induction. cAMP - CRP complex modulates expression but is not the inducer.
What defines the reading frame of an mRNA during translation?
The 5' cap
The Shine-Dalgarno sequence
The start codon position
The poly-A tail
The reading frame is set by the first start codon (usually AUG) that defines how downstream nucleotides are grouped into codons. Shifts in frame occur if insertion or deletion disrupts this register. Proper frame ensures correct protein synthesis.
Which sequence in prokaryotic mRNA aligns with the 16S rRNA to initiate translation?
Kozak sequence
TATA box
Shine-Dalgarno sequence
Poly-A signal
The Shine-Dalgarno sequence upstream of the start codon base-pairs with the 3' end of the 16S rRNA, positioning the ribosome for initiation. It is a hallmark of prokaryotic translation initiation. Eukaryotes use the Kozak sequence instead.
What is the primary function of topoisomerases during DNA replication?
Seal nicks in the backbone
Add RNA primers
Relieve supercoiling ahead of the fork
Synthesize Okazaki fragments
Topoisomerases cut one or both DNA strands to relieve torsional strain generated by unwinding at replication forks, then reseal the strands. Without them, supercoiling would stall replication. Ligase and primase perform different tasks.
How does the CRISPR-Cas9 system achieve targeted DNA cleavage?
Cas9 cuts at random double-strand breaks
Guide DNA pairs with Cas9 to target RNA
Guide RNA directs Cas9 to complementary DNA
Cas9 recognizes PAM without RNA
In CRISPR-Cas9, a guide RNA (gRNA) base-pairs with target DNA upstream of a PAM sequence, directing Cas9 to introduce a double-strand break. The PAM motif (e.g., NGG) is essential for Cas9 binding and cleavage. This precision underlies genome editing applications.
What mechanism do riboswitches use to regulate gene expression?
DNA methylation blocks RNAP
Small RNAs degrade mRNA
Ligand-induced folding alters transcription or translation
Proteins bind promoters
Riboswitches are mRNA elements that change structure upon binding small metabolites, affecting transcription termination or ribosome access. This allows direct control of downstream gene expression without proteins. They are common in bacteria.
Which model describes circular DNA replication in most bacteria?
Displacement loop replication
Theta replication
Linear replication
Rolling-circle replication
Theta replication occurs on circular bacterial chromosomes, forming a structure resembling the Greek letter theta (?) as the replication fork moves bidirectionally. Rolling-circle is typical of some plasmids or phages. Theta is the main bacterial genome replication mode.
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Study Outcomes

  1. Understand DNA and RNA Structures -

    Explain the chemical building blocks and configurations of DNA and RNA to answer questions about genetic material.

  2. Analyze DNA Replication Mechanisms -

    Break down the sequential steps and key enzymes involved in DNA replication to solve replication-based MCQs.

  3. Apply Transcription and Translation Principles -

    Trace the flow of genetic information from DNA to RNA to protein to accurately tackle protein synthesis questions.

  4. Differentiate Prokaryotic and Eukaryotic Pathways -

    Contrast the molecular processes of transcription, translation, and replication in prokaryotes versus eukaryotes to select correct quiz answers.

  5. Identify Key Molecular Enzymes -

    Recall the functions of polymerases, ligases, ribosomes, and other essential factors to navigate questions about molecular machinery.

  6. Evaluate Your Molecular Biology Knowledge -

    Use quiz feedback to assess your strengths and pinpoint areas for further study in molecular biology concepts.

Cheat Sheet

  1. DNA Double Helix & Base Pairing -

    The DNA molecule consists of two antiparallel strands forming a right-handed helix stabilized by hydrogen bonds, with A - T pairs forming two bonds and G - C pairs forming three. Use the mnemonic "GC is the strong clutch" to remember that guanine and cytosine share three hydrogen bonds. This structural principle underpins accurate DNA replication and PCR primer design.

  2. Central Dogma & Information Flow -

    The central dogma describes the unidirectional flow of genetic information: DNA is transcribed into RNA, which is then translated into protein. A handy phrase, "Do RNA Proteins," can help recall DNA → RNA → Protein. Understanding this pathway is vital for molecular biology MCQ quiz questions on gene expression and viral exceptions.

  3. DNA Replication Mechanism -

    Replication begins at origins of replication and proceeds bidirectionally, with DNA polymerase synthesizing the leading strand continuously and the lagging strand discontinuously via Okazaki fragments. Remember "Okaza-key puzzle" to visualize how the lagging strand pieces fit together. Key enzymes like DNA Pol III in prokaryotes and DNA Pol δ in eukaryotes ensure high fidelity.

  4. Transcription & RNA Processing -

    In eukaryotes, RNA polymerase II transcribes pre-mRNA, which undergoes 5' capping, splicing to remove introns, and 3' polyadenylation before nuclear export. The mnemonic "CAP, SPLICE, TAIL" helps recall each processing step. Mastery of promoter elements like the TATA box and enhancers is crucial for quiz items on transcription regulation.

  5. Translation & the Genetic Code -

    Translation decodes mRNA codons into amino acids at ribosomal A, P, and E sites, guided by tRNA anticodon pairing; wobble at the third codon position allows codon degeneracy. For example, G-U pairing in the wobble position exemplifies flexibility in the code. Familiarity with start (AUG) and stop codons (UAA, UAG, UGA) anchors your understanding of protein synthesis completion.

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