Question

Within a DNA molecule, which nitrogen base pairs with cytosine (C)?

Answer 1

The nitrogen base Guanine (G) pairs with cytosine (C) in the case of DNA molecules. Guanine binds with Cytosine with the help of $3$ hydrogen bonds. Guanine is a purine base and cytosine is a pyrimidine base. Adenine (A) binds with Thymine (T) in the case of DNA and with Uracil (U) in the case of RNA. Adenine binds to Thymine with $2$ hydrogen bonds.

Complete answer:
According to the structure proposed by James Watson and Francis Crick, DNA is a double-stranded helical structure made up of a $2$ polynucleotide chain and sugar-phosphate backbone. The two chains are anti-parallel to each other. There are $4$ types of nitrogenous bases found in DNA and based on their chemical structure they are classified into two groups - Purine bases – Adenine and Guanine and Pyrimidine bases – Thymine and Cytosine. The purine bases contain $2$ heterocyclic rings whereas the pyrimidine bases contain only one heterocyclic ring making purine rings greater in structure than the pyrimidine ring. Thus to maintain an approximately uniform distance between the strands of DNA, a purine base is paired with a pyrimidine base. The nitrogenous base Thymine can also be termed $5 -$methyl uracil.

Note:
According to the observation of Erwin Chargaff, for a double-stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and are equal $1$. The pitch of the helix is found to be $3.4$ nm. The plane of one base pair stacking over the other in the double-helical structure provides stability to the helix apart from the hydrogen bond.