DNA Base Pairs — Overview & Structure

Base Pairs

The genetic code, or DNA sequence, is usually represented as a string of letters. There are only four available letters: A, T, C, and G. Each of these letters represents a base or a single rung along the genetic ladder. From just a small number of base pairs, we can make an endless number of proteins. How can we do that? By combining them in different ways! The sequence of bases itself is responsible for every aspect of life. But first, let’s discuss the bases themselves.

How Are the Bases Packed Inside DNA?

DNA is a double-stranded molecule, with rungs connecting the two strands. Each rung of the DNA ladder is actually a pair of bases that meet in the middle: a base pair. But each base is always only paired with one specific other base. 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.

Complementary Base Pairing

If you look at the diagram, you will see that it is impossible for a base to bond with any base other than its counterpart. This means that if you know the sequence on one strand of DNA, you automatically know the sequence on the other strand. This trait of DNA makes it easy for the cell (and researchers) to make changes to DNA. The phenomenon of having specific base pairs is known as complementary pairing.

The most important thing to remember is that A always pairs with T, and C always pairs with G. One way you can do this is to remember that G and C look similar.

The Four Bases

DNA has four nucleobases: adenine, thymine, guanine, and cytosine.

The nucleobases in a DNA strand have preferred partners to form hydrogen bonds with. Cytosine pairs with guanine, and adenine pairs with thymine. These are the base pairing rules that allow DNA replication and protein synthesis to happen.

A and T are connected by two hydrogen bonds, while C and G are connected by three hydrogen bonds. These bonds are shown in the picture above with the blue arrows.

$$\color{#8925ae}{\text{Vocabulary Review:}}$$

$\color{#8925ae}{\text{Monomer}}$ : Any molecule that can react with other molecules of the same or different makeup to form a chain, or polymer

$\color{#8925ae}{\text{Polymer}}$ : A chain of monomers made of a specific type of compound

$\color{#8925ae}{\text{Macromolecule}}$ : A polymer that has a large (macro) molecular mass. There are four main types in organisms: carbohydrates, proteins, lipids, and nucleic acids

$\color{#8925ae}{\text{Nucleotides}}$ : The monomers of nucleic acids. Made up of a nitrogen group, a phosphorus group, and a 5-carbon ring

$\color{#8925ae}{\text{Nucleic Acid}}$ : Complex polymer that stores genetic information in a cell in the form of a code. DNA and RNA

Complementary Pairing

DNA is made up of chains of nucleotides. Each nucleotide is made of a sugar, a phosphate group, and a nitrogenous base. There are four different nucleotides in the DNA molecule and each one is characterized by its nitrogenous base. The four nitrogenous bases are A, T, C, and G. They stand for adenine, thymine, cytosine, and guanine.

The four different bases pair together in a way known as complementary pairing. Adenine always pairs with thymine, and cytosine always pairs with guanine. The pairing nature of DNA is useful because it allows for easier replication. If you know one side of a DNA molecule, you can always recreate the other side. Each base has only one other base it can pair with.

The double helix structure of the DNA molecule places the four nitrogenous bases on the inside of the molecule. This protects them from any type of damage that could alter their order. Protecting the nitrogenous bases is extremely important because their order determines the proteins that they code for.

(Video) DNA Structure

by Teacher's Pet

This video offers a short explanation of what DNA's four nitrogenous bases are and what rules they use to pair together when forming DNA's characteristic double helix.

(Video) DNA: Complementary Base Pairing

by Study.com

On this episode of DNA & RNA, the defendant explains how the four nitrogenous bases of DNA (adenine, guanine, thymine, and cytosine) help the cell build proteins.