DNA is the abbreviation for deoxyribonucleic acid. A long word for an extremely long molecule; there is about two meters of DNA in each human cell. There is so much to say about this fascinating molecule, but here we will focus on the basic structure and function.
Firstly, where is DNA found? In eukaryotic cells (including plant and animal cells) DNA is stored as chromatin in the cell nucleus, whereas in prokaryotic cells (bacteria, for example) it is stored as a circular loop in the cytoplasm. DNA can be divided into sections called genes. Each gene codes for a protein that will have a specific function. To find out more about chromatin, read this article.
One basic thing to remember about the structure of DNA is that it has a double helix structure. That means that there are two strands which make up the classic twisted shape that is often seen in images such as this first diagram. The two strands are anti-parallel. When we look more closely at what DNA consists of it will become clearer what anti-parallel means. Before that, let’s take a look at nucleotides.
Nucleotides, sometimes called mononucleotides, are the very basic building blocks of the two DNA strands. We could call DNA a biological polymer because it is a repeating unit made up of monomers (the nucleotides), and each strand of the double helix is a polynucleotide chain.
There are three parts to a nucleotide:
- A phosphate group.
- A pentose sugar (in a DNA nucleotide this is called deoxyribose).
- A nitrogen-containing base (these are often just called bases).
There are four possible bases that could be in a DNA nucleotide: adenine (A), thymine (T), guanine (G) or cytosine (C). It is the order of these bases which determines the genome of the organism. The bases are what hold the two polynucleotide strands together because they are attracted to each other with hydrogen bonds (these are the type of bonds found between water molecules). However, each base can only bind to its complementary base – T binds to A with two hydrogen bonds, and G binds to C with three hydrogen bonds. There is never pairing of A with G, for example. This is called complementary base pairing.
The pentose sugar and the phosphate group make up the sugar-phosphate backbone of each polynucleotide strand. A pentose sugar gets its name from having five carbon molecules and is drawn as a pentagon. The carbon atoms in the pentose sugar can be numbered from 1′ to 5′ (one prime to five prime) as shown in the nucleotide diagram above.
The 5′ carbon is not in the pentagon itself as there is an oxygen atom at the top point of the pentagon. The phosphate group joins the pentose sugars with a phosphodiester bond between the 5′ carbon of one pentose sugar and the 3′ carbon of another pentose sugar. The sugar-phosphate backbones run in the opposite direction on each strand which is what makes them anti-parallel (this is easier to see in the diagram). This also is what gives the strands their “5′ to 3′” or “3′ to 5′” direction. It is important to know the directionality of the strands in genetic research.
Being familiar with the structure of DNA is key to understanding processes such as DNA replication and transcription, and to understanding lab techniques used in genetic research. Make sure you know these facts and you are off to a good start in molecular biology.
- DNA is a double helix of two anti-parallel polynucleotide strands.
- The polynucleotide strands are made up of single nucleotides (containing a pentose sugar, a phosphate group and a base) joined with phosphodiester bonds.
- The bases hold the strands together with hydrogen bonds between complementary bases – A binds to T and G binds to C.