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
Ans. A
Alleles, or alternative forms of a gene, are always found at the same loci of homologous chromosomes:
Explanation
Homologous chromosomes are chromosome pairs, one from each parent, that are similar in length, gene position, and centromere location. Alleles are located at the same position, or locus, in each homologous chromosome.
Example
A gene responsible for eye color can have two alleles, one for black and the other for brown.
Function
Homologous chromosomes are important in the process of mitosis and meiosis, and allow for recombination and the production of new gene combinations. They are also used in repairing double-strand breaks of DNA.
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
Ans. A
One advantage of using embryonic stem cells over adult stem cells in the treatment of spinal cord injuries is that embryonic stem cells have a greater ability to differentiate into a wider variety of cell types. This pluripotency could potentially enable more effective regeneration and repair of damaged tissues in the spinal cord, compared to the more limited differentiation capacity of adult stem cells.
Many types of stem cells have been explored for the treatment of spinal cord injuries. Embryonic stem cells possess pluripotency, allowing them to form any cell type in the body, including nerve cells. Induced pluripotent stem cells, reprogrammed from adult cells, share a common potential.
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
Markscheme : D
The correct answer is:
D. The methylation patterns of both parents are erased after fertilization.
Explanation:
The graph shows that both maternal (black line) and paternal (gray line) DNA undergo demethylation after fertilization.
The paternal DNA loses its methylation faster than maternal DNA.
The maternal DNA undergoes gradual demethylation but also loses its methylation during early development.
Later, embryonic DNA (dashed line) undergoes re-methylation, which corresponds to tissue-specific modifications necessary for differentiation.
Thus, both parents’ DNA lose their methylation patterns after fertilization, supporting option D.
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
Markscheme : C
The correct answer is:
C. RNA polymerase binds to the promoter region.
Explanation:
A is incorrect: Only one strand of DNA (the antisense/template strand) is used as a template for mRNA synthesis. The other strand (the sense/coding strand) has the same sequence as the mRNA (except uracil replaces thymine).
B is incorrect: Nucleoside triphosphates (NTPs) lose two phosphates (not three) to become incorporated into the growing RNA chain.
C is correct: RNA polymerase recognizes and binds to the promoter region of DNA to initiate transcription. This is a key step in gene expression.
D is incorrect: The antisense/template strand, not the sense strand, is used to synthesize mRNA. The sense strand has the same sequence as the mRNA but is not directly used as a template.
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
Markscheme : C
The correct answer is:
C. RNA polymerase separates DNA strands.
Explanation:
A is incorrect: Polysomes (polyribosomes) are groups of ribosomes translating an mRNA simultaneously. This occurs during translation, not transcription.
B is incorrect: Nucleosomes are modified by processes like acetylation, methylation, or phosphorylation, but this is related to chromatin remodeling and gene regulation, not the core process of transcription.
C is correct: During transcription, RNA polymerase binds to DNA, unwinds the double helix, and separates the strands to allow RNA synthesis. This occurs at the promoter region and continues along the gene.
D is incorrect: Okazaki fragments are short DNA segments synthesized during DNA replication, specifically on the lagging strand, not during transcription.