Question

What is not relevant to DNA double helix?

• A. Distance between adjacent nucleotides $3.4\AA$
• B. Back bone has alternate ribose sugar and phosphate
• C. Diameter $20\AA$
• D. One turn every $34\AA$

Answer 1

The correct option is B Back bone has alternate ribose sugar and phosphate

One turn every 34 Å
According to Watson and Crick’s model of double helical DNA -
One turn of DNA helix has $10$ nucleotides or $10$ base pairs
Length of one complete turn of the DNA double helix is $34Å=3.4nm$

Diameter $20\AA$
The DNA double helix has a diameter of $20Å$, i.e, the space between the two sugar phosphate chains of DNA is $20Å$ or 2 nm.

Distance between adjacent nucleotides
Length between two adjacent base pairs =(Length of one complete turn/Number of base pairs in one turn) $=\left(3.4/10\right)nm=0.34nm$ or$3.4Å$

Ribose sugar and phosphate
DNA is a double stranded helical molecule in which each strand is composed of repeating units of deoxyribonucleotides.
Each deoxyribonucleotide is composed of a deoxyribose sugar with a phosphate attached to its $5^{\prime }$ carbon and a nitrogenous base attached to its $1^{\prime }$ carbon.
The adjacent nucleotides are connected by phosphodiester bonds between the $3^{\prime }$-OH group of one nucleotide and the $5^{\prime }$ phosphate group of the next nucleotide.
The deoxyribose sugar and phosphodiester bonds form the backbone of each DNA strand and the nitrogenous bases are projected inwards.
Ribose sugar and phosphates form the backbone of ribonucleic acid or RNA.