Pre AP Biology -GEN 1.2 The Structure of DNA- MCQ Exam Style Questions -New Syllabus

Pre AP Biology -GEN 1.2 The Structure of DNA- MCQ Exam Style Questions – New Syllabus 2025-2026

Pre AP Biology -GEN 1.2 The Structure of DNA- MCQ Exam Style Questions – Pre AP Biology – per latest Pre AP Biology Syllabus.

Pre AP Biology – MCQ Exam Style Questions- All Topics

Question

Which of these statements best describes the human genome?

a. The protein-coding sequences occupy about $75\%$ of the genome.

b. About $45\%$ of the genome consists of transposable element sequences.

c. The genome sequence comprises approximately $30$ million base pairs.

d. Human cells have about $10\,500$ different protein-coding genes.

▶️ Answer/Explanation

Detailed solution

The correct option is b.
Protein-coding sequences (exons) actually make up less than $2\%$ of the human genome, not $75\%$.
Transposable elements, including LINEs and SINEs, account for roughly $44\%-45\%$ of the total sequence.
The human haploid genome is much larger than stated, containing about $3.2$ billion base pairs ($3.2 \times 10^9$).
Current biological estimates suggest humans have approximately $20\,000$ to $25\,000$ protein-coding genes.
Therefore, the statement regarding transposable elements is the most accurate description of the genome’s composition.

Question

Which of the following statements best describes the E. coli genome?

a. It has a much lower gene density than the human genome.

b. It contains longer genes than the human genome.

c. All the genes are transcribed from the same template strand of the DNA double helix.

d. About half the genes in its genome are grouped with other genes in operons.

▶️ Answer/Explanation

Detailed solution

The correct answer is d.

In E. coli, approximately $50\%$ of genes are organized into functional clusters called operons.
These operons allow for the coordinated expression of genes involved in the same metabolic pathway.
Unlike eukaryotes, prokaryotes like E. coli have a very high gene density with very little non-coding DNA.
Human genes are typically much longer than bacterial genes due to the presence of large introns.
Transcription in E. coli occurs on both strands of the DNA, depending on the orientation of the specific gene.
Therefore, the grouping into operons is the most distinguishing and accurate characteristic listed.

Question

Which of the following is the main reason that searching for ORFs is more useful for annotating bacterial protein-coding genes than it is for annotating eukaryote protein-coding genes?

a. Eukaryote protein-coding genes contain introns.
b. The density of protein-coding genes is much higher in eukaryote genomes.
c. In most bacteria, all the protein-coding genes are located on a single circular chromosome.
d. Bacterial protein-coding genes are much longer than eukaryotic protein-coding genes.

▶️ Answer/Explanation

Detailed solution

The correct answer is a. Eukaryote protein-coding genes contain introns.

In bacteria, protein-coding sequences are continuous Open Reading Frames ($\text{ORFs}$).
Eukaryotic genes are interrupted by non-coding sequences called introns.
These introns break a single polypeptide’s code into multiple small $\text{ORF}$ fragments.
Computer algorithms struggle to distinguish true small $\text{ORFs}$ from random sequences in eukaryotes.
In contrast, bacterial $\text{ORFs}$ are long and easy to identify from start to stop codon.
Therefore, simple $\text{ORF}$ scanning is highly effective for bacterial genome annotation.
Eukaryotic annotation requires more complex models to predict splice sites and exon-intron boundaries.

Pre AP Biology -GEN 1.2 The Structure of DNA- MCQ Exam Style Questions -New Syllabus