Human Genome Molecular Biology Practice Quiz (26 questions)

1. Which relationship among DNA bases is expressed by Chargaff’s rules?

A. A equals T and G equals C, making the ratio $(A+G)/(T+C)=1$

B. The overall GC-to-AT ratio must equal one in every DNA molecule

C. A equals C and G equals T, making the ratio $(A+C)/(G+T)=1$

D. A equals G and T equals C, making the ratio $(A+T)/(G+C)=1$

Answer: (A)

Explanation

Chargaff’s rules describe complementary pairing in which adenine matches thymine and guanine matches cytosine, yielding $(A+G)/(T+C)=1$. The GC-to-AT ratio need not equal one, so that interpretation confuses base pairing with overall base composition.

2. Which publication did Watson and Crick publish in 1953?

A. “Molecular Structure of Nucleic Acids” in Science

B. “Photographic Evidence for DNA Helices” in Biochemistry

C. “A Structure for Deoxyribose Nucleic Acid” in Nature

D. “The Genetic Material of the Cell” in Nature

Answer: (C)

Explanation

Watson and Crick published “A Structure for Deoxyribose Nucleic Acid” in Nature in 1953. Photograph 51 was produced by Rosalind Franklin, so it was not the title of their publication.

3. What three components make up a nucleotide?

A. A fatty acid, a nitrogenous base, and a phosphate group

B. A pentose sugar, a nitrogenous base, and a phosphate group

C. A pentose sugar, an amino acid, and a phosphate group

D. A pentose sugar, a nitrogenous base, and an amino group

Answer: (B)

Explanation

A nucleotide consists of a pentose sugar, a nitrogenous base, and a phosphate group. A nucleoside has the sugar and base but lacks the phosphate group.

4. Which structural difference distinguishes DNA from RNA?

A. DNA contains deoxyribose and thymine, whereas RNA contains ribose and uracil

B. DNA contains ribose and thymine, whereas RNA contains deoxyribose and uracil

C. DNA contains deoxyribose and uracil, whereas RNA contains ribose and thymine

D. DNA contains ribose and uracil, whereas RNA contains deoxyribose and thymine

Answer: (A)

Explanation

DNA generally contains deoxyribose and thymine, while RNA contains ribose and uracil instead of thymine. The other choices reverse or incorrectly combine the characteristic sugars and bases of the two nucleic acids.

5. A DNA sample is maintained under common physiological conditions. Which helical form is most consistent with those conditions?

A. B-DNA, a right-handed helix with about 10 base pairs per turn

B. B-DNA, a left-handed helix with about 11 base pairs per turn

C. Z-DNA, a left-handed helix with about 12 base pairs per turn

D. A-DNA, a right-handed helix with about 11 base pairs per turn

Answer: (A)

Explanation

B-DNA is the common physiological form and is a right-handed helix with approximately 10 base pairs per turn. A-DNA is associated with dehydrated conditions or RNA–DNA hybrids, while Z-DNA is left-handed.

6. What distance corresponds to one complete turn of the B-DNA double helix?

A. Approximately 3.4 nm

B. Approximately 2.0 nm

C. Approximately 0.34 nm

D. Approximately 2.3 nm

Answer: (A)

Explanation

One turn of B-DNA measures about 3.4 nm and contains 10 base pairs. The value 0.34 nm refers to the separation between adjacent nucleotides on the same strand, not the length of a full turn.

7. When was the Human Genome Project launched to sequence the entire human genome?

A. Late 2022

B. Early 1995

C. Late 1988

D. Mid-2001

Answer: (C)

Explanation

The Human Genome Project began in late 1988 with the goal of sequencing the entire human genome. The 2001 date refers to publication of first draft sequences, not the project’s launch.

8. What major achievement did Celera Genomics and the International Human Genome Sequencing Consortium announce in 2001?

A. They published first drafts covering about 92% of the human genome.

B. They launched the project to sequence the entire human genome.

C. They identified approximately 200 million previously unsequenced base pairs.

D. They completed a sequence containing 3.055 billion base pairs.

Answer: (A)

Explanation

In 2001, the two groups published first drafts covering approximately 92% of the human genome. The 3.055-billion-base-pair sequence and the additional 200 million base pairs were associated with the 2022 T2T-CHM13 result.

9. What did the T2T-CHM13 Consortium accomplish in 2022?

A. It produced the first complete human genome sequence, containing 3.055 billion base pairs.

B. It sequenced approximately 200 million base pairs without determining the full genome.

C. It launched the international effort to sequence the human genome.

D. It published the first human genome draft, covering approximately 92% of the sequence.

Answer: (A)

Explanation

The T2T-CHM13 Consortium produced the first complete human genome sequence in 2022, with 3.055 billion base pairs and approximately 200 million additional base pairs of DNA. The 92% draft was published in 2001 by earlier sequencing efforts.

10. Which description correctly identifies the structure of a nucleosome?

A. A looped DNA domain attached to a scaffold of histone H1 and other structural proteins.

B. About 55 base pairs of linker DNA wrapped around histone H1 to connect neighboring nucleosomes.

C. A chromosome-end loop containing tandem repeats of the sequence 5′-TTAGGG.

D. About 146 base pairs of DNA wrapped around a histone octamer containing two copies each of H2A, H2B, H3, and H4.

Answer: (D)

Explanation

A nucleosome consists of approximately 146 base pairs of DNA wrapped around a histone octamer with two copies each of H2A, H2B, H3, and H4. Linker DNA connects neighboring nucleosomes, while histone H1 stabilizes that linker region.

11. Which sequence correctly represents the progression of DNA compaction from the least condensed level to the most condensed level?

A. Tertiary looped domains, primary nucleosomes, secondary solenoid, quaternary metaphase chromosome

B. Primary nucleosomes, secondary solenoid, tertiary looped domains, quaternary metaphase chromosome

C. Secondary solenoid, primary nucleosomes, quaternary metaphase chromosome, tertiary looped domains

D. Primary nucleosomes, tertiary looped domains, secondary solenoid, quaternary metaphase chromosome

Answer: (B)

Explanation

DNA compaction proceeds from primary nucleosomes to a secondary solenoid, then to tertiary looped domains, and finally to a quaternary metaphase chromosome. The other sequences place one or more compaction levels out of order.

12. A genomic region is loosely packed and readily accessible for transcription; which chromatin state best describes it?

A. Linker chromatin, because it connects adjacent nucleosomes through histone H1

B. Heterochromatin, because it is tightly packed and structurally associated

C. A metaphase chromosome, because it provides the highest level of compaction

D. Euchromatin, because it is generally open and transcriptionally active

Answer: (D)

Explanation

Euchromatin is loosely packed, accessible, and transcriptionally active, matching the described genomic region. Heterochromatin is more tightly packed and generally transcriptionally inactive.

13. What is the primary structural role of a telomere at the end of a chromosome?

A. It stabilizes the chromosome end by forming a loop and contains tandem 5′-TTAGGG repeats.

B. It condenses chromatin into tertiary looped domains during chromosome formation.

C. It attaches spindle microtubules through kinetochores to align chromosomes during division.

D. It connects neighboring nucleosomes through approximately 55 base pairs of linker DNA.

Answer: (A)

Explanation

Telomeres stabilize chromosome ends by forming a loop and contain approximately 1000 tandem repeats of 5′-TTAGGG. Spindle attachment and chromosome segregation are functions associated with centromeres, not telomeres.

14. How does a centromere contribute to chromosome behavior during cell division?

A. It provides the region where kinetochores attach spindle microtubules for alignment and equal segregation.

B. It links adjacent nucleosomes by stabilizing approximately 55 base pairs of linker DNA.

C. It maintains transcriptional activity by keeping nearby chromatin loosely packed and accessible.

D. It protects chromosome ends by forming a loop enriched in tandem 5′-TTAGGG repeats.

Answer: (A)

Explanation

A centromere is the chromosome region where kinetochores attach spindle microtubules, enabling alignment and equal segregation during cell division. Telomeres, rather than centromeres, stabilize chromosome ends through terminal loops and repeats.

15. What does the human genome comprise?

A. A single functional DNA sequence that produces one protein

B. The complete DNA-based genetic material of an individual or species

C. The protein-coding portion of DNA found in an individual

D. The chromosome regions responsible for regulating transcription

Answer: (B)

Explanation

The human genome is the complete set of DNA-encoded genetic material and contains approximately 3.055 billion base pairs. A gene is one functional DNA unit within the genome, while the genome also includes non-coding DNA.

16. Which comparison correctly describes the proportions of coding and repeated DNA in the human genome?

A. Coding DNA represents about 54%, while repeated sequences represent about 1–5%.

B. Coding DNA represents about 1–5%, while repeated sequences represent about 54%.

C. Coding DNA and repeated sequences each represent about 25%.

D. Coding DNA represents about 80%, while repeated sequences represent about 15%.

Answer: (B)

Explanation

Coding DNA accounts for approximately 1–5% of the human genome, whereas repeated sequences account for approximately 54%. The proportions are therefore highly unequal, with repeated DNA forming the larger fraction.

17. Which feature distinguishes a gene from a simple DNA segment?

A. A gene is found only within non-coding chromosome regions.

B. A gene encodes a functional RNA or protein product.

C. A gene consists entirely of repeated DNA units.

D. A gene must produce exactly one protein form.

Answer: (B)

Explanation

A gene is a DNA sequence that encodes a functional product, either RNA or protein, and alternative splicing can allow it to produce multiple protein isoforms. Producing multiple isoforms is therefore compatible with gene function.

18. During gene expression, which statement correctly distinguishes exons, introns, promoters, and enhancers?

A. Exons remain in mature RNA, introns are removed, and regulatory elements can act upstream or at distant sites.

B. Promoters remain in translated RNA, while silencers become protein-coding sequences after transcription.

C. Exons are removed, introns remain, and promoters act at distant sites after translation.

D. Introns encode mature RNA, exons regulate transcription, and enhancers are removed during splicing.

Answer: (A)

Explanation

Exons remain in mature RNA, whereas introns are removed during RNA processing; promoters act upstream and enhancers or silencers can regulate transcription from distant sites. The other descriptions assign processing or regulatory roles to the wrong elements.

19. A DNA test detects a change in the number of repeated units at one locus rather than a change in nucleotide identity. What type of variant is this?

A. A short tandem repeat variant

B. A mitochondrial inheritance variant

C. A coding-sequence substitution

D. A single-nucleotide variant

Answer: (A)

Explanation

A short tandem repeat variant changes the number of repeated DNA units at a locus. A single-nucleotide variant instead changes the identity of one nucleotide.

20. Which sequence qualifies as a microsatellite?

A. A tandem repeat with units longer than those typical of microsatellites

B. A tandem repeat with units of 1–6 base pairs and variable repeat number

C. A mobile element of about 6–7 kilobases encoding reverse transcriptase

D. A non-autonomous element shorter than 500 base pairs requiring LINE machinery

Answer: (B)

Explanation

Microsatellites are short tandem repeats whose repeat units contain 1–6 base pairs, and their repeat number is highly variable. Longer repeat units characterize minisatellites rather than microsatellites.

21. Which statement correctly compares LINEs and SINEs?

A. LINEs are autonomous elements that encode reverse transcriptase, whereas SINEs depend on LINE machinery.

B. LINEs are short non-autonomous elements, whereas SINEs encode their own reverse transcriptase.

C. LINEs and SINEs are both autonomous elements that independently mobilize through translation.

D. LINEs are mitochondrial repeats, whereas SINEs are protein-coding nuclear genes.

Answer: (A)

Explanation

LINEs are approximately 6–7 kb autonomous retrotransposons that encode reverse transcriptase, while SINEs are shorter than 500 bp and rely on LINE machinery. SINEs therefore lack the autonomous machinery characteristic of LINEs.

22. Which inheritance pattern and molecular feature are characteristic of mitochondrial DNA?

A. It is histone-bound and transmitted through both parents.

B. It has a lower mutation rate than nuclear DNA and follows Mendelian inheritance.

C. It is linear and inherited biparentally through nuclear chromosomes.

D. It is circular and generally inherited maternally.

Answer: (D)

Explanation

Mitochondrial DNA is circular, lacks histones and a DNA repair system, and is maternally inherited. Nuclear DNA is instead linear, chromosomal, and biparentally inherited.

23. What is the primary role of messenger RNA during protein synthesis?

A. It carries amino acids directly to the ribosome for peptide assembly.

B. It guides the degradation of complementary RNA molecules.

C. It carries a DNA-derived coding message that serves as a translation template.

D. It combines with proteins to form the structural framework of the ribosome.

Answer: (C)

Explanation

Messenger RNA is transcribed from DNA and serves as the template for protein synthesis during translation. Transfer RNA carries amino acids, while ribosomal RNA forms ribosomal machinery.

24. Which RNA molecule delivers amino acids to ribosomes during translation?

A. Transfer RNA

B. Ribosomal RNA

C. Messenger RNA

D. Small interfering RNA

Answer: (A)

Explanation

Transfer RNA carries amino acids to ribosomes during translation and represents approximately 15% of total cellular RNA. Messenger RNA carries the coding message, whereas ribosomal RNA contributes to ribosome formation.

25. What is the principal structural role of ribosomal RNA?

A. It provides the temporary coding template copied from DNA.

B. It silences transposable elements through complementary targeting.

C. It carries amino acids to the sites of peptide synthesis.

D. It combines with proteins to form ribosomes.

Answer: (D)

Explanation

Ribosomal RNA combines with proteins to form ribosomes and represents approximately 80% of total cellular RNA. Amino acid transport is performed by transfer RNA, while transposable-element silencing is associated with piRNAs.

26. Which comparison correctly distinguishes DNA from RNA?

A. DNA and RNA have the same sugar and bases but differ mainly in the proteins they produce.

B. DNA is usually single-stranded with ribose and uracil, whereas RNA is usually double-stranded with deoxyribose and thymine.

C. DNA performs messenger, transfer, and ribosomal functions, whereas RNA stores hereditary information in chromosomes.

D. DNA is usually double-stranded with deoxyribose and thymine, whereas RNA is usually single-stranded with ribose and uracil.

Answer: (D)

Explanation

DNA is usually double-stranded, contains deoxyribose and thymine, and stores hereditary information, while RNA is usually single-stranded, contains ribose and uracil, and performs several functional roles. The alternative comparisons reverse or erase these chemical and biological distinctions.