Question
What is consistently located at the same loci on homologous chromosomes?
A. Alleles with the same function
B. Alleles with identical base sequences
C. Genes with identical base pairs
D. Genes with the same mutations
▶️Answer/Explanation
Answer: A. Alleles with the same function
Explanation:
- Homologous chromosomes have genes at the same loci but may carry different alleles of those genes.
- These alleles may have different base sequences but usually code for proteins with the same function (even if one allele is mutated).
- So, while alleles can differ, they generally perform the same biological function at the same loci on homologous chromosomes.
Why other options are incorrect:
B. Alleles with identical base sequences: Alleles can have slight sequence variations.
C. Genes with identical base pairs: Alleles can differ in base pairs due to mutations.
D. Genes with the same mutations: Not all homologous chromosomes carry the same mutation.
Question
What would be an advantage of using embryonic stem cells over adult stem cells in the potential treatment of spinal cord injuries in humans?
A. More cell types can be obtained.
B. Possibilities of rejection are lower.
C. The risk of stem cells forming malignant tumors is lower.
D. Unlimited numbers of cells can be extracted from the umbilical cord.
▶️Answer/Explanation
Answer: A. More cell types can be obtained.
Explanation:
Embryonic stem cells (ESCs) are pluripotent, meaning they can develop into almost any cell type. Adult stem cells are multipotent, usually limited to forming certain tissue types.
Let’s analyze the options:
A. Correct – ESCs can become more cell types than adult stem cells, which is especially useful in complex injuries like those in the spinal cord.
B. Incorrect – ESCs are more likely to be rejected by the immune system compared to adult stem cells from the patient.
C. Incorrect – ESCs have a higher risk of forming tumors due to their rapid division and pluripotency.
D. Incorrect – Embryonic stem cells are not extracted from the umbilical cord; that refers to cord blood stem cells, which are distinct.
Very soon after fertilization, parental epigenetic methylation is reversed in the DNA.
Later, tissue-specific epigenetic modifications are made to the embryonic DNA. The graph follows the degree of methylation from different sources during embryonic development.
According to the graph, what are the changes in DNA methylation during embryonic development?
A. Only the paternal DNA becomes demethylated.
B. The maternal DNA becomes demethylated first.
C. The methylation patterns of the parents’ DNA are erased before fertilization.
D. The methylation patterns of both parents are erased after fertilization.
Answer/Explanation
Answer: D. The methylation patterns of both parents are erased after fertilization.
Explanation:
Methylation silences genes. After fertilization, both maternal and paternal DNA undergo demethylation, wiping previous epigenetic marks. Later, new tissue-specific methylation is added during development.
A. Incorrect – Both maternal and paternal DNA undergo demethylation, not just paternal.
B. Incorrect – The paternal genome demethylates slightly earlier than maternal, not the other way around.
C. Incorrect – Methylation is not erased before fertilization, it’s erased after.
D. Correct – After fertilization, both parental methylation patterns are erased, followed by reprogramming.
What is a feature of transcription?
A. Both strands of a DNA molecule act as a template for mRNA.
B. Nucleoside triphosphates become nucleotides by losing three phosphates.
C. RNA polymerase binds to the promoter region.
D. The sense strand acts as a template for mRNA.
Answer/Explanation
Answer: C. RNA polymerase binds to the promoter region.
Explanation:
What happens in transcription?
Transcription is the process where RNA polymerase reads the DNA template and makes mRNA. Only one DNA strand (the template or antisense strand) is used.
A. Incorrect – Only one strand is used as a template, not both.
B. Incorrect – Nucleoside triphosphates lose two phosphates (not three) to become nucleotides in RNA synthesis.
C. Correct – RNA polymerase binds to the promoter region, a key step to initiate transcription.
D. Incorrect – The antisense strand is the template. The sense strand has the same sequence as mRNA (except T → U).
What happens during transcription in eukaryotes?
A. Polysomes move.
B. Nucleosomes are phosphorylated.
C. RNA polymerase separates DNA strands.
D. Okazaki fragments are produced.
Answer/Explanation
Answer: C. RNA polymerase separates DNA strands.
Explanation:
What is transcription in eukaryotes?
Transcription is the process where RNA polymerase reads a DNA template strand and builds a complementary mRNA molecule. It occurs in the nucleus of eukaryotic cells and involves unwinding DNA, pairing RNA nucleotides with DNA bases, and synthesizing a pre-mRNA transcript.
Evaluating Each Option:
A. Incorrect – Polysomes are clusters of ribosomes translating a single mRNA strand. This happens during translation, not transcription.
B. Incorrect – While histone modification (including phosphorylation) can affect gene expression, phosphorylation of nucleosomes is not a direct step in transcription. It’s part of epigenetic regulation, not transcription itself.
C. Correct – This is a key step in transcription. RNA polymerase binds to the promoter region, unwinds the DNA, and separates the strands to read the template (antisense) strand for mRNA synthesis.
D. Incorrect – Okazaki fragments are formed during DNA replication on the lagging strand, not during transcription.