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Well-Structured Explanation
Introduction
This question tests conceptual clarity on transcription, gene regulation (lac operon), the approximate number of genes in the human genome, and the inheritance pattern of a genetic disorder (haemophilia). We have four statements (A–D), and we need to determine how many of these statements are correct.
Analysis of Each Statement
Statement (A): In transcription, adenosine pairs with uracil
• During transcription, the DNA strand is used as a template to form the complementary RNA strand.
• In RNA, the base uracil (U) replaces thymine (T). Consequently, on the mRNA strand, uracil pairs with adenine (A) in the DNA template.
• Therefore, the statement that adenosine (in the RNA) pairs with uracil (from the template perspective) is a correct representation of the base-pairing rule for transcription.
Hence, Statement (A) is correct.
Statement (B): Regulation of lac operon by repressor is referred to as positive regulation
• The lac operon in E. coli is generally regulated by a repressor protein that binds to the operator region and blocks transcription.
• When lactose is absent, the repressor binds to the operator, preventing RNA polymerase from transcribing the structural genes. This is known as negative regulation because binding of the repressor stops transcription.
• In positive regulation, an activator (rather than a repressor) aids RNA polymerase in binding to the promoter and enhances transcription.
Hence, Statement (B) is incorrect because repressor-based regulation is classified under negative regulation.
Statement (C): The human genome has approximately 50,000 genes
• The Human Genome Project has revealed that humans have around 20,000 to 25,000 genes.
• Therefore, stating about 50,000 genes is an overestimation.
Hence, Statement (C) is incorrect.
Statement (D): Haemophilia is a sex-linked recessive disease
• Haemophilia (commonly Haemophilia A and B) is indeed a sex-linked recessive disorder, typically associated with the X chromosome.
• Males (XY) are more frequently affected while females (XX) are carriers unless they inherit two affected X chromosomes.
Hence, Statement (D) is correct.
Conclusion
From the analysis, we see that Statements (A) and (D) are correct, while (B) and (C) are incorrect. Therefore, the number of correct statements is two.
