Question

18. The base pairs are held together primarily by a. Covalent bonds b. Hydrogen bonds c. lonic bonds d. Gyrases 19. How many hydrogen bonds between Adenine and Thymine? a. b. 2 c. 3 d. 4 20. How many hydrogen bonds between Cytosine and Guanine? b. 2 d. 4 21. Which ones are weaker? a. Hydrogen bonds b. Covalent bonds 22. The nature of the double helix is that it is: a. Parallel arranged b. Perpendicularly arranged c. Antiparallel arranged d. None of the above 23. One side of the helix goes from the 5' end to the 3', the other side goes from the a. 5 'end to the 3' b. 3' end to the 5'

Answer 1

18) A base pair of DNA molecule consists of two complementary nucleotides. As the two nucleotides are complementary to eachother , hence they are held together by Hydrogen bonds. Adenine bonds with Thymine and Guanine bonds with Cytosine.

Therefore option b) hydrogen bonds is the correct answer.

19) Adenine and Thymine are held together by two hydrogen bonds. One hydrogen bond is formed by the attachment of one Hydrogen atom of Adenine with an electronegative atom (Oxygen) of Thymine. Second hydrogen bond is formed by attachment of one hydrogen atom from Thymine with an electronegative atom ( Nitrogen) of Adenine.

Hence option b) 2 is the correct answer.

20) Cytosine and Guanine are held together by three hydrogen bonds. One hydrogen bond is formed by attachment of one hydrogen atom of cytosine with oxygen atom of Gaunine. Second hydrogen bond is formed by attachment of one hydrogen atom of guanine with Nitrogen atom of cytosine. Third hydrogen bond is formed by attachment of one hydrogen atom of guanine with oxygen atom of cytosine.

Hence option c) 3 is the correct answer.

21) The energy required to break a covalent bond is much higher than the energy required to break a hydrogen bond. The energy required to break a bond is directly proportional to the strength of a bond. Hence hydrogen bonds are much weaker compared to covalent bonds , as less energy is required to break a hydrogen bond.

Therefore option a) hydrogen bonds is the correct answer.

22) Double helix refers to the shape of a DNA molecule. In a double helix structure of DNA molecule , two strands wind against eachother and run in antiparallel directions.

Hence option c) antiparallel arranged is the correct answer.

23) Double helix structure of DNA molecule consists of two antiparallel strands. The two strands are arranged in such a way that , if one strand of helix runs in 5' to 3' direction , the other strand runs in 3' to 5' direction. Which means two strands are arranged in antiparallel direction with respect to eachother.

Hence option b) 3' to 5' is the correct answer.