Nucleic Acids: DNA & RNA Structure

You’ll be able to

Identify the three parts of a nucleotide and how nucleotides link into a strand
Apply complementary base pairing to determine a partner strand
Compare the structures of DNA and RNA and relate antiparallel strands to structure

The monomer: a nucleotide

Nucleic acids store and transmit genetic information, and their monomer is the nucleotide. Every nucleotide has three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. The bases come in two shapes — the double-ringed purines (adenine, guanine) and the single-ringed pyrimidines (cytosine, thymine, and in RNA, uracil). It is the sequence of these bases that spells out genetic information.

Linking nucleotides into a strand

Nucleotides join by dehydration synthesis into a chain: the phosphate of one links to the sugar of the next, building a sugar-phosphate backbone with the bases sticking out to the side. This bond runs between the 5′ carbon of one sugar and the 3′ carbon of the next, giving every strand a direction — a 5′ end and a 3′ end. That directionality matters: the two strands of DNA run in opposite directions, or antiparallel (one 5′→3′, the other 3′→5′).

Complementary base pairing (DNA)

A–T · G–C

Adenine pairs with thymine (2 hydrogen bonds); guanine pairs with cytosine (3 hydrogen bonds). In RNA, uracil replaces thymine, so A pairs with U.

The double helix and its RNA cousin

DNA is a double helix: two antiparallel strands twisted together, their bases paired in the middle by hydrogen bonds — a purine always pairs with a pyrimidine (A with T, G with C), which keeps the helix a uniform width. Because the strands are complementary, each one carries the information to rebuild the other, which is the key to copying DNA. RNA differs in three ways: it is usually single-stranded, its sugar is ribose (with an extra –OH) instead of deoxyribose, and it uses the base uracil (U) in place of thymine.

Worked example

A DNA strand reads 5′–A G G T C A–3′. Write the sequence of its complementary partner strand, in the correct orientation.

1.Apply base pairing to each base: A→T, G→C, G→C, T→A, C→G, A→T, giving T C C A G T.
2.The partner strand is antiparallel, so it runs in the opposite direction (3′→5′ beneath the original).
3.Written conventionally in the 5′→3′ direction, reverse that sequence: T G A C C T.

Answer: The complementary strand is 3′–T C C A G T–5′, or written 5′→3′: 5′–T G A C C T–3′.

Checkpoint

In a sample of double-stranded DNA, 30% of the bases are adenine (A). What percentage are guanine (G)?

Watch out

Uracil belongs to RNA only; thymine belongs to DNA only. If a strand contains U, it is RNA — and remember RNA is normally single-stranded, so it has no complementary partner strand of its own.

Checkpoint

Which statement correctly describes a difference between DNA and RNA?

On the exam

Two facts win most nucleic-acid questions: complementary pairing (A–T/U, G–C) lets you fill in any partner strand, and antiparallel orientation means you must flip direction when you write it out. State both explicitly on free-response answers.

Answer the 2 checkpoints as you read.