Question

Read the following four statements (A - D). (A) In transcription, adenosine pairs with uracil. (B) Regulation of lac operon by repressor is referred to as positive regulation. (C) The human genome has approximately 50,000 genes. (D) Haemophilia is a sex-linked recessive disease. How many of the above statements are right?

• A. Two
• B. Three
• C. Four
• D. One.

Answer 1

The Correct Option is A

Transcription is the process of transferring of information stored in DNA to wRNA through the synthesis of RNA over the template DNA. As RNA is a polymer of ribonucleotide and is made up of pentose ribose sugar, phosphoric acid and nitrogenous bases (A, U, G and C), so during base pairing ribonucleoside triphosphate come to lie opposite to the nitrogen bases of DNA template and form complementary pair; uracil opposite adenine, adenine opposite thymine, cytosine opposite guanine and guanine opposite cytosine. Haemophilia is a sex-linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny, hence shows criss-cross inheritance pattern. It is also known as bleeder's disease as the patient continues to bleed from a minor cut as he does not possess the natural phenomenon of blood clotting due to absence of antihaemophiliac globulin or factor VIII and plasma thromboplastin factor IX. It is genetically, due to recessive sex-linked gene h carried by X chromosome. A female becomes haemophiliac only when both its X chromosomes carry the gene (XhXh) whereas in case of males, a single gene for the defect is able to express itself (XhY) due to presence of a recessive sex-linked gene h carried by X chromosome. The repressor of the operon is synthesized (all-the- time - constitutively) from the / gene. The repressor protein binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon. In the presence of an inducer, such as lactose or allolactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase to access the promoter and transcription proceeds. Hence, regulation of lac operon by repressor is referred to as negative regulation. Human genome is said to have approximately 3 x 109 bp and approximately 20,000 - 25,000 genes.