What base does guanine pair with

DNA has four nucleobases: adenine, thymine, guanine, and cytosine. They form base pairs. Adenine bonds with thymine, and guanine bonds with cytosine.

Nucleotides and Bases

Nucleotide Structure
Courtesy of the National Human Genome Research Institute

Nucleotides

A nucleotide is the basic structural unit and building block for DNA. These building blocks are hooked together to form a chain of DNA. A nucleotide is composed of 3 parts:

* five-sided sugar * phosphate group

* nitrogenous base (nitrogen containing)

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Guanine (G) is one of the four nucleotide bases in DNA, with the other three being adenine (A), cytosine (C) and thymine (T). Within a double-stranded DNA molecule, guanine bases on one strand pair with cytosine bases on the opposite strand. The sequence of the four nucleotide bases encodes DNA’s information.

Guanine is one of the building blocks of DNA. It's the G in the A, C, G, or T. Guanine in the double helix pairs with cytosine, so you will see CG pairs; one on one strand and one on the other. And the CG pairs happen to bind more tightly than the AT pairs, so long stretches of CG make stronger helixes than stretches of AT.

Jyoti G. Dayal, M.S.

Program Director

Division of Genomic Medicine

A base pair consists of two complementary DNA nucleotide bases that pair together to form a “rung of the DNA ladder.” DNA is made of two linked strands that wind around each other to resemble a twisted ladder — a shape known as a double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G) [GWA-NeeN] or thymine (T). The two strands are held together by hydrogen bonds between pairs of bases: adenine pairs with thymine, and cytosine pairs with guanine. 

One copy of the human genome consists of approximately 3 billion base pairs of DNA, which are distributed across 23 chromosomes. Human chromosomes range in size from about 50 million to 300 million base pairs. Because the bases exist as pairs, and the identity of one of the bases in the pair determines the other member of the pair, scientists do not have to report both bases of the pair — which is why DNA sequence is typically represented as single strings of letters. DNA sequencing involves determining the exact order of the base pairs across a DNA segment of interest or across an entire genome. A signature goal of the Human Genome Project was to generate the first high-quality sequence of the human. The effort was successful in generating a such a sequence for over 90% of the human genome, but it took nearly two more decades to sequence the remaining bits of the human genome — which were heavily enriched for highly repetitive and difficult-to-sequence stretches of DNA.

Sarah A. Bates, M.S.

Chief

Office of Communications