Question

BIOCHEM

PLEASE ONLY ANSWER IF YOU TRULY UNDERSTAND IT!! I posted this many times, and people have been giving wrong answers. Also ONLY answer if you can answer it correctly and FULLY. Also a brief explanation helps! Thank you!

2) The double helix of B-form DNA is stabilized most significantly by:

A) nonspecific base-stacking interactions between two adjacent bases in the same strand

B) hydrogen bonding between the phosphate groups of two side-by-side strands

C) covalent bonds between the 3’-end of one stand and the 5’-end of the other

D) hydrogen bonds between the ribose of each strand

3) The DNA oligonucleotide abbreviated 5’-pATCGAC-3’ (the p indicated a phosphate on the 5’-end):

A) has an A at its 3’ end

B) has 6 phosphate groups

C) has a phosphate on its 3’ end

D) violates Chargaff’s rules

4) Which of the following is a palindromic sequence?

A) GGATCC

B) GTATCC

C)GAATCC

D) CCTTCC CCTAGG CATAGG CTTAGG GGAAGG

6) What is the principal effect of UV light on DNA?

A) melting of B-form helices

B) production of T-dimers

C) deamination of cytosine residues

D) oxidation of guanosine residues

7) Chargaff’s rules state that in typical DNA:

A) A = G

B) A = C

C) A + T = G + C

D) A + G = T + C

8) In the Watson-Crick structure of DNA, the:

A) nucleotides are arranged in the A-form

B) 2’-hydroxyl groups in ribose sometimes participate in hydrogen bonding

C) two strands are antiparallel

D) purine content must be the same in both strands

9) The recognition site of some restriction enzymes are listed below. Which will not produce a “sticky end”?

A) GA↓ATTC

B) CCC↓GGG

C) C↓TCGCG

D) all will produce “sticky ends”

10) Proteins that cut DNA to unwind it are called:

A) helicases

B) topoisomerases

C) ligases

D) endonucleases

Answer 1

2) The double helix of B-form DNA is stabilized most significantly by:

A) nonspecific base-stacking interactions between two adjacent bases in the same strand

Correct: base stacking (non-specific because bases stacked one upon another depend on the DNA sequence only and can be anything) facilitates H-bonding between oppositely placed bases along tha antiparallel strands.   

B) hydrogen bonding between the phosphate groups of two side-by-side strands

Incorrect: No H-bond formed between phosphate groups of two stands.

C) covalent bonds between the 3’-end of one stand and the 5’-end of the other

Incorrect: such bonding does not occur

D) hydrogen bonds between the ribose of each strand

Incorrect: DNA has deoxyribose sugar. Sugar residues in DNA does not form H-bond to stabilize double helix.

3) The DNA oligonucleotide abbreviated 5’-pATCGAC-3’ (the p indicated a phosphate on the 5’-end):

A) has an A at its 3’ end

B) has 6 phosphate groups

Correct: Two subsequent bases along the DNA strand have one phosphate group in between them. So, there are 5 phosphate groups in between subsequent 6 bases. The 6^th phosphate group is at the 5’end of the A residue.   

C) has a phosphate on its 3’ end

D) violates Chargaff’s rules

4) Which of the following is a palindromic sequence?

A) GGATCC

Correct option: A palindromic sequence reads the same in 5’-3’ and 3’-5’ direction.

5’-GGATCC-3’ (one strand) // 3’-GGATCC-5’ (antiparallel strand)

B) GTATCC

C)GAATCC

D) CCTTCC CCTAGG CATAGG CTTAGG GGAAGG

6) What is the principal effect of UV light on DNA?

A) melting of B-form helices

B) production of T-dimers

Correct option: UV light forms thymine dimer.

C) deamination of cytosine residues

D) oxidation of guanosine residues

7) Chargaff’s rules state that in typical DNA:

Correct option: D) A + G = T + C

Chargaff’s rule: Total number purine bases (A+G) equals total number of pyrimidine bases (T+C) in a double stranded DNA

8) In the Watson-Crick structure of DNA, the:

A) nucleotides are arranged in the A-form

Incorrect: B-form DNA

B) 2’-hydroxyl groups in ribose sometimes participate in hydrogen bonding

Incorrect: DNA has ribose sugar

C) two strands are antiparallel

Correct: Both strands are antiparallel

D) purine content must be the same in both strands

Incorrect: Not necessary

9) The recognition site of some restriction enzymes are listed below. Which will not produce a “sticky end”?

A) GA↓ATTC

B) CCC↓GGG

Correct option: the resultant DNA fragments have no flanking bases at the restricted end, thus produce blunt ends. Or, sticky ends (ends with a base overhang) is not produced.

C) C↓TCGCG

D) all will produce “sticky ends”

10) Proteins that cut DNA to unwind it are called:

A) helicases

Incorrect: helicase generally breaks interstrand H-bonds to unwind two strands.

B) topoisomerases

Correct : Topoisomerase I and II results single and double strand breaks, respectively, to unwind DNA.

These enzymes can also mediate DNA supercoiling.

C) ligases

Incorrect: Join DNA stretches together

D) endonucleases

Incorrect: hydrolyse DNA but not to unwind them