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Quiz: Foundations of Molecular Biology

Test your understanding of core molecular biology concepts including DNA, RNA, proteins, the central dogma, mutations, and epigenetics with these review questions.

1. What are the three components of a nucleotide?

  1. Amino acid, phosphate group, ribose sugar
  2. Nitrogenous base, five-carbon sugar, phosphate group
  3. Purine, pyrimidine, hydrogen bond
  4. Codon, anticodon, peptide bond
Show Answer

The correct answer is B. A nucleotide consists of a nitrogenous base (purine or pyrimidine), a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group. Option A incorrectly includes amino acids, which are protein building blocks, not nucleotide components. Options C and D list related terms but not the structural components of a nucleotide.

Concept Tested: Nucleotides

2. In complementary base pairing, which base pairs with cytosine and how many hydrogen bonds are formed?

  1. Thymine, with two hydrogen bonds
  2. Adenine, with three hydrogen bonds
  3. Guanine, with three hydrogen bonds
  4. Uracil, with two hydrogen bonds
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The correct answer is C. Guanine (G) pairs with Cytosine (C) through three hydrogen bonds, while Adenine (A) pairs with Thymine (T) through two hydrogen bonds. Option A incorrectly pairs cytosine with thymine. Option B gives the wrong pairing partner. Option D describes an RNA base but with the wrong pairing partner and bond count.

Concept Tested: Complementary Base Pairing

3. What does the term "central dogma" describe in molecular biology?

  1. The principle that all organisms share identical genetic codes
  2. The rule that mutations always cause disease
  3. The directional flow of genetic information from DNA to RNA to protein
  4. The process by which proteins fold into their three-dimensional shape
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The correct answer is C. The central dogma, coined by Francis Crick in 1958, describes the standard directional flow of genetic information: DNA is transcribed into RNA, which is translated into protein. Crick later noted he misunderstood the word "dogma" — he intended it as a central principle, not an unquestionable belief. Options A, B, and D describe other biological concepts unrelated to information flow.

Concept Tested: Central Dogma

4. How many possible codons exist, and how many encode amino acids?

  1. 64 total codons; 61 encode amino acids
  2. 61 total codons; 58 encode amino acids
  3. 64 total codons; 64 encode amino acids
  4. 20 total codons; 20 encode amino acids
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The correct answer is A. With four possible bases at each of three positions in a codon, there are \(4^3 = 64\) possible codons. Of these, 61 encode the 20 standard amino acids (making the code degenerate), and 3 are stop codons (UAA, UAG, UGA) that signal the end of translation. Option D confuses the number of amino acids with the number of codons.

Concept Tested: Codons and Genetic Code

5. Which type of mutation involves a change at a single base pair and is the most common form of genetic variation in human populations?

  1. Structural variant
  2. Copy number variation
  3. Insertion or deletion (indel)
  4. Single nucleotide polymorphism (SNP)
Show Answer

The correct answer is D. Single nucleotide polymorphisms (SNPs) are changes at a single base pair and are the most common type of genetic variation, occurring roughly once every 1,000 base pairs. Structural variants (A) involve alterations of 50 bp or more. Copy number variations (B) involve differences in copy number of genomic regions. Indels (C) are the second most common variant type after SNPs.

Concept Tested: Single Nucleotide Polymorphism

6. What distinguishes a frameshift mutation from other types of insertions or deletions?

  1. It only affects non-coding regions of the genome
  2. It involves an indel whose length is not a multiple of three, altering the downstream reading frame
  3. It always results in a longer protein
  4. It only occurs in mitochondrial DNA
Show Answer

The correct answer is B. A frameshift mutation occurs when an insertion or deletion of nucleotides is not a multiple of three, which shifts the reading frame for all downstream codons. This typically produces a completely different amino acid sequence downstream and often introduces a premature stop codon, resulting in a nonfunctional protein. Option A is incorrect because frameshifts affect coding regions. Option C is wrong because frameshifts usually truncate proteins.

Concept Tested: Insertion and Deletion

7. Which epigenetic mechanism involves adding a methyl group to cytosine at CpG dinucleotides and is generally associated with gene silencing?

  1. Histone acetylation
  2. DNA methylation
  3. Phosphorylation
  4. Alternative splicing
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The correct answer is B. DNA methylation adds a methyl group (\(\ce{-CH3}\)) to the 5-carbon position of cytosine at CpG dinucleotides. Methylation of CpG islands in promoter regions is generally associated with gene silencing. Histone acetylation (A) is associated with active transcription, not silencing. Phosphorylation (C) is a histone modification but not specific to CpG sites. Alternative splicing (D) is a post-transcriptional regulatory mechanism, not an epigenetic mark.

Concept Tested: DNA Methylation

8. Which of the following is an exception to the central dogma where information flows from RNA back to DNA?

  1. Prion propagation
  2. RNA replication by RNA-dependent RNA polymerase
  3. Reverse transcription by retroviruses such as HIV
  4. Non-coding RNA gene silencing
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The correct answer is C. Reverse transcription, used by retroviruses like HIV, synthesizes DNA from an RNA template using the enzyme reverse transcriptase. This violates the standard DNA-to-RNA directionality. Prion propagation (A) represents protein-to-protein information flow. RNA replication (B) involves RNA-to-RNA copying. Non-coding RNA function (D) involves RNA acting as the final product without translation to protein.

Concept Tested: Central Dogma Exceptions

9. What is an open reading frame (ORF)?

  1. A region of DNA that contains only introns
  2. A stretch of DNA beginning with a start codon and ending with a stop codon, with no internal stop codons
  3. A non-coding regulatory element upstream of a gene
  4. A segment of RNA that has been fully processed and exported from the nucleus
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The correct answer is B. An open reading frame (ORF) is a stretch of DNA that begins with a start codon (ATG, encoding methionine) and ends with a stop codon, with no internal stop codons interrupting the sequence. ORF finding is one of the earliest bioinformatics tasks, identifying potential protein-coding regions in newly sequenced genomes. Option A describes intronic regions. Option C describes promoter elements. Option D describes mature mRNA.

Concept Tested: Open Reading Frame

10. Which level of protein structure describes the overall three-dimensional fold of a single polypeptide chain?

  1. Primary structure
  2. Quaternary structure
  3. Secondary structure
  4. Tertiary structure
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The correct answer is D. Tertiary structure describes the overall 3D fold of a single polypeptide chain, determined by hydrophobic interactions, disulfide bonds, ionic interactions, and van der Waals forces. Primary structure (A) is the linear amino acid sequence. Secondary structure (C) refers to local folding patterns like alpha helices and beta sheets. Quaternary structure (B) describes the arrangement of multiple polypeptide subunits into a complex.

Concept Tested: Protein Structure