Question

Explain how DNA is a directional molecule in a chemical sense.

Short answer

DNA is a directional molecule due to its chemical structure. Each nucleotide in DNA has a 5' end, where phosphate is attached, and a 3' end, where a hydroxyl group is attached. The phosphate of one nucleotide is joined to the 3' carbon of the next nucleotide making the DNA strand have a directionality from 5' to 3'. Additionally, the two DNA strands in a double helix run anti-parallel to each other, meaning one strand runs 3' to 5', while the other runs 5' to 3'.

Step-by-step solution

Understanding DNA Structure

DNA or deoxyribonucleic acid is a long polymer made up of deoxyribonucleotides. Each nucleotide is composed of a phosphate group, a 5-carbon sugar (deoxyribose), and a nitrogenous base (adenine, guanine, cytosine, thymine). These nucleotides are connected in a chain by phosphodiester bonds between the phosphate group of one and the sugar of the next, forming a sugar-phosphate backbone.

Identifying the Directionality

The structure of DNA evidences directionality due to the asymmetry of each nucleotide's sugar component. The sugar molecule has a 5' end (where the fifth carbon atom is bound to a phosphate group) and a 3' end (where the third carbon atom is bound to a hydroxyl group). As a result, when nucleotides are connected into a strand, the strand possesses a 5' end (with a free phosphate group) and a 3' end (with a free hydroxyl group). So, DNA directionality is always referred to as being from 5' to 3'.

Elaborating on the Antiparallel Structure

Moreover, in the DNA double helix, two DNA strands run in opposite directions which are referred to as antiparallel. This means that whilst one strand might run in the 5' to 3' direction (5' to 3' strand), the other runs in the 3' to 5' direction (3' to 5' strand). The bases in the center of the helix adhere to base pairing rules: adenine pairs with thymine (A-T), cytosine pairs with guanine (C-G). The antiparallel and complementary nature of DNA assures its accurate replication and transcription.