3. Which of the following represents correctly paired nitrogen bases? (a) Adenine- Cytosine; (b) Cytosine-Thymine; (c) Guanine-Thymine; (d) Guanine-Cytosine. ​

The option that correctly represents paired nitrogen bases is D ( Guanine-Cytosine)

Explanation:

Deoxyribonucleic acid( DNA) is found in chromosomes which are located in the nucleus of cells. One of its main functions is to store the organism's hereditary traits and direct the day to day metabolic activities of each cell in the organism.

The DNA molecule consists of two strands that curve spirally to form a double helix structure which consists of:

--> phosphate,

--> A sugar called deoxyribose, and

--> Four nitrogenous bases—adenine, guanine, cytosine, and thymine

The four different NITROGEN BASES pair together in a way known as complementary pairing. ADENINE always pairs with THYMINE, and CYTOSINE always pairs with GUANINE.This is because of their molecular structures. Each base contains one or two nitrogen rings, which makes it long or short. It also has a sticky end of binding sites that connects with exactly one other base.

--> Adenine is long and has two binding sites.

--> Thymine is short and has two binding sites.

--> Guanine is long and has three binding sites.

--> Cytosine is short and has three binding sites.

From the above analysis, GUANINE can only bind to CYTOSINE.