Question

Which statement is not true with regard to Z-DNA?

1. Each turn of the two polypeptide chains contains 12 base pairs.
2. Distance between two subsequent base pairs is 3.7 A.
3. The distance between axis and sugar phosphate is 10 Å.
4. Alternate deoxyribose sugar units in the polynucleotide chain have inverse orientation,

• A. 3 only
• B. All
• C. None of these
• D. 4 only

Answer 1

Answer:

A. 3 only

• Z-DNA, a left-handed double helix, differs significantly from the more common B-DNA.

• Structure of Z-DNA:

  • It has 12 base pairs per helical turn, arranged into two distinct grooves: a narrow and deep minor groove and a wide and shallow major groove.

  • The distance between successive base pairs in Z-DNA is approximately 7.4 Å.

  • The sugar-phosphate backbone forms a zigzag pattern, hence the name 'Z-DNA'.

  • Alternate deoxyribose sugar units in the polynucleotide chain exhibit an inverse orientation, specifically a gauche conformation at the C4'-C5' bond.

  • The distance from the helical axis to the sugar-phosphate backbone is approximately 10 Å.

• Comparison with B-DNA:

  • B-DNA has 10.5 base pairs per turn, with a distance of 3.4 Å between successive base pairs.

  • The sugar-phosphate backbone in B-DNA is more uniform, and the sugar rings are in a staggered conformation.

  • The major groove in B-DNA is wider and more accessible than in Z-DNA, while the minor groove is narrower and deeper.

• Biological Significance:

  • Z-DNA is thought to play a role in gene regulation, recombination, and DNA replication.

  • Its formation is often induced by specific DNA sequences, such as alternating purine-pyrimidine sequences (e.g., GCGCGC).

  • Certain proteins, like the Simian virus 40 large tumor antigen (SV40 T-ag), can bind to and stabilize Z-DNA