Question

Phosphodiester linkage is formed between:
A. C-5 of one nucleotide and C-3 of the next
B. C-1 of one nucleotide and C-2 of the next
C. Purines of opposite strands
D. Pyrimidines of opposite strands

Answer 1

Hint: DNA and RNA are made up of nucleotides that are connected together in a chain through chemical bonds termed ester bonds that form in between the sugar base of one nucleotide and the phosphate group of the next nucleotide. At C-5 of nucleotide, a phosphate group attaches and a hydroxyl group at C-3 of another nucleotide has the ability to form phosphodiester bonds.

Complete Step by Step Answer:
DNA and RNA are considered polynucleotides, which are long chains of nucleotides linked together. A nucleotide is made up of a nitrogenous base (adenine, guanine, thymine, cytosine, or uracil), a pentose sugar, and a phosphate molecule. Nucleotides are connected together in a polynucleotide chain through phosphodiester bonds, wherein each phosphate group forms 2 ester bonds with its neighboring sugar molecules. The very first ester bond already appears to exist in between the phosphate group and the 5' carbon of a nucleotide's pentose sugar. The 2nd ester bond develops between the phosphate group and the hydroxyl group attached to the 3' carbon of the previous sugar molecule. Each ester bond formed eliminates one water molecule. Polymerases are a type of enzyme that catalyzes or accelerates the creation of phosphodiester bonds.
In a polynucleotide chain, phosphodiester bonds form an alternating pattern of sugar and phosphate byproducts known as the sugar-phosphate backbone. Phosphodiester bonds give a polynucleotide chain directionality. A free 5' phosphate group exists at one end of the polynucleotide chain and a free 3' hydroxyl group at the other end. These are known as the 5' and 3' ends, respectively. Nucleic acid directionality is required for DNA replication as well as RNA synthesis. Therefore, phosphodiester linkage is formed between C-5 of one nucleotide and C-3 of the next.
The correct option is A.

Note: Purines and pyrimidines are nitrogenous bases found in DNA. Purines possess a double ring structure, whereas pyrimidines have a single ring structure. Purines are adenine and guanine, whereas pyrimidines are thymine, uracil, and cytosine. A hydrogen bond connects a purine to a pyrimidine. Two hydrogen bonds connect adenine to thymine, whereas three hydrogen bonds connect guanine and cytosine.