A, B and Z forms

Right handed DNA

1. ‘A’-DNA

• This is right handed DNA.
• In this type of DNA, the length of helix is 28.15 Å 
• There are 11 base pairs per helix. 
• The diameter of the helix (20.2) with the axial rise per base pair is about 2.36 Å. 

2. B’-DNA or Right handed DNA

• The structural model for this type of DNA was proposed by Watson and Crick. 
• It is a most common form of DNA. 
• The helix is right handed with the length of about 34 Å. 
• Each turn contains 10 pairs of mononucleotide units. 
• The rise of helix per base pair is 3.4 Å and the tilt of base pair is 6.3. 
• The diameter of DNA molecule is 20 A. 
• The repeating unit is a mononucleotide and the orientation of sugar molecule is not alternating. 
• Each successive base pair in the stack turns 36° 

3. ‘C’-DNA and ‘D’-DNA

• Both types of DNAS are right handed double helical structures, 
• similar to ‘B’-DNA with minor differences.
• In ‘C’ type of DNA, the size of helix is greater than found in A type of DNA but smaller than found in B-DNA. 
• It is about 31 Á. There are 9.33 base pairs per turn of the helix. 
• There is an axial rise of about 3.32 Ă and the tilt of base pairs is about 7.8. 
• However D-DNA has only 8 base pairs. 
• It lacks guanine.

4. ‘Z-DNA or Left handed DNA

• Rich and his co-workers have proposed a left handed double helical model for DNA molecule. 
• In this model zig-zag sugar phosphate back bones run in antiparallel organization 
• This is known as ‘Z’-DNA. In this model the sugar residues have alternating orientation , 
• Its one coil is 4.66 nm long having 12 nucleotide base pair.
• The distance between two base pair is 0.375nm .
• In this type of DNA the sugar residues show alternating orientation therefore the repeating unit is a dinucleotide.

MCQs on Right-Handed and Left-Handed DNA

1. What is the length of the helix in A-DNA?

a) 34 Å
b) 28.15 Å
c) 20.2 Å
d) 31 Å

2. How many base pairs are present per turn in A-DNA?

a) 10
b) 8
c) 11
d) 12

3. Which type of DNA structure was proposed by Watson and Crick?

a) A-DNA
b) B-DNA
c) Z-DNA
d) D-DNA

4. What is the diameter of B-DNA?

a) 31 Å
b) 28.15 Å
c) 20 Å
d) 25 Å

5. How many base pairs are present per turn in B-DNA?

a) 9.33
b) 10
c) 11
d) 12

6. What is the axial rise per base pair in B-DNA?

a) 2.36 Å
b) 3.32 Å
c) 3.4 Å
d) 3.75 Å

7. What is the tilt of base pairs in B-DNA?

a) 7.8°
b) 6.3°
c) 5.2°
d) 10.4°

8. Which DNA type lacks guanine?

a) A-DNA
b) B-DNA
c) C-DNA
d) D-DNA

9. What is the length of the helix in C-DNA?

a) 34 Å
b) 31 Å
c) 28.15 Å
d) 20.2 Å

10. How many base pairs are found in each turn of C-DNA?

a) 10
b) 11
c) 9.33
d) 12

11. Who proposed the Z-DNA model?

a) Watson and Crick
b) Franklin
c) Rich and co-workers
d) Chargaff

12. What is the handedness of Z-DNA?

a) Right-handed
b) Left-handed
c) Both right and left-handed
d) None of the above

13. How many base pairs are found per turn in Z-DNA?

a) 10
b) 11
c) 12
d) 9

14. What is the length of one coil in Z-DNA?

a) 3.4 nm
b) 4.66 nm
c) 2.36 nm
d) 5.0 nm

15. What is the distance between two base pairs in Z-DNA?

a) 0.34 nm
b) 0.36 nm
c) 0.375 nm
d) 0.40 nm

16. Which DNA type has a zig-zag sugar-phosphate backbone?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

17. What is the repeating unit in Z-DNA?

a) Mononucleotide
b) Dinucleotide
c) Trinucleotide
d) Polynucleotide

18. Which DNA type is most commonly found in living cells?

a) A-DNA
b) B-DNA
c) Z-DNA
d) D-DNA

19. Which form of DNA has alternating sugar orientation?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

20. What is the axial rise per base pair in C-DNA?

a) 2.36 Å
b) 3.32 Å
c) 3.4 Å
d) 3.75 Å

MCQs on Right-Handed and Left-Handed DNA

21. Which of the following DNA types has the highest base pairs per turn?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

22. What is the structural difference between B-DNA and Z-DNA?

a) B-DNA is left-handed, while Z-DNA is right-handed
b) B-DNA is right-handed, while Z-DNA is left-handed
c) Both are right-handed
d) Both are left-handed

23. Which DNA type has the smallest number of base pairs per turn?

a) A-DNA
b) B-DNA
c) C-DNA
d) D-DNA

24. How many base pairs are present per turn in D-DNA?

a) 8
b) 9.33
c) 10
d) 11

25. Which of the following statements about A-DNA is correct?

a) It is the most stable form of DNA
b) It is a left-handed DNA
c) It has 11 base pairs per turn
d) It has a zigzag backbone

26. What is the orientation of sugar residues in Z-DNA?

a) Alternating orientation
b) Parallel orientation
c) Antiparallel orientation
d) Random orientation

27. In which DNA type is the sugar-phosphate backbone present in a zig-zag pattern?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

28. Which DNA type has an axial rise per base pair of 3.75 Å?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

29. The presence of alternating purine and pyrimidine sequences is a characteristic of which DNA type?

a) A-DNA
b) B-DNA
c) Z-DNA
d) D-DNA

30. Which of the following is NOT a right-handed DNA?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

31. How does the structure of Z-DNA differ from B-DNA?

a) Z-DNA is shorter and wider
b) Z-DNA is a single-stranded molecule
c) Z-DNA is left-handed and has a zigzag backbone
d) Z-DNA does not contain nitrogenous bases

32. What is the tilt of base pairs in C-DNA?

a) 7.8°
b) 6.3°
c) 5.2°
d) 8.5°

33. Which DNA type lacks guanine?

a) A-DNA
b) B-DNA
c) D-DNA
d) Z-DNA

34. What is the repeating unit in B-DNA?

a) Mononucleotide
b) Dinucleotide
c) Trinucleotide
d) Polynucleotide

35. In which type of DNA is the sugar residue orientation NOT alternating?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

36. What is the length of a single helical turn in Z-DNA?

a) 3.4 nm
b) 4.66 nm
c) 2.36 nm
d) 5.0 nm

37. How many nucleotide pairs are there in a complete turn of Z-DNA?

a) 10
b) 11
c) 12
d) 9

38. Which of the following DNA types has the highest tilt of base pairs?

a) A-DNA
b) B-DNA
c) C-DNA
d) D-DNA

39. What is the function of the major and minor grooves in B-DNA?

a) They provide sites for protein and enzyme binding
b) They stabilize the DNA structure
c) They help in DNA replication only
d) They prevent DNA unwinding

40. Which DNA type is formed under dehydrated conditions?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

41. The distance between two successive base pairs in B-DNA is:

a) 2.36 Å
b) 3.4 Å
c) 3.75 Å
d) 4.66 Å

42. The phosphate group in DNA contributes to its:

a) Basic nature
b) Acidic nature
c) Neutral nature
d) None of the above

43. What makes Z-DNA unique compared to other DNA types?

a) It is right-handed
b) It contains only purines
c) It has a left-handed helix with a zigzag backbone
d) It has only 8 base pairs per turn

44. Which DNA type is commonly found in transcriptionally active regions?

a) A-DNA
b) B-DNA
c) Z-DNA
d) C-DNA

45. What is the difference in base pair tilt between A-DNA and B-DNA?

a) A-DNA has a higher tilt than B-DNA
b) A-DNA and B-DNA have the same tilt
c) B-DNA has a higher tilt than A-DNA
d) Neither A-DNA nor B-DNA has a tilt

46. Which of the following statements is incorrect regarding C-DNA?

a) It is a right-handed DNA
b) It has 9.33 base pairs per turn
c) It lacks cytosine
d) Its axial rise per base pair is 3.32 Å

47. Which DNA type has the highest helical twist per base pair?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

48. Which DNA form has the widest helical diameter?

a) A-DNA
b) B-DNA
c) C-DNA
d) Z-DNA

49. In which type of DNA are base pairs nearly perpendicular to the helix axis?

a) A-DNA
b) B-DNA
c) Z-DNA
d) D-DNA

50. What is the main reason why Z-DNA forms in certain genomic regions?

a) Due to high GC content
b) Due to high AT content
c) Due to protein binding and supercoiling
d) Due to lack of phosphate groups