What is the structure of DNA?

DNA stands for deoxyribonucleic acid. And what is DNA? Well, it’s basically a carrier of genetic information. DNA is found in all cells, and it contains the information needed for cells to live, to grow and to divide. The basic structure of DNA is as follows. DNA is a ‘polymer’. And what is a polymer, I hear you ask?! A polymer is a big long chain of smaller molecules, called ‘monomers’, joined together. In the case of DNA, these monomers are called ‘nucleotides’. A nucleotide is made of three things: a deoxyribose sugar, a phosphate group and a nitrogenous base. Nitrogenous just means ‘containing nitrogen’. There are 4 different bases – Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). The sequence of these bases in DNA forms a code which is read. This is essentially your ‘genetic information’. So, those three things – a sugar, a phosphate and a base, make up one ‘monomer’. To make the polymer that is DNA, these little monomers need first to be joined together to form a long chain. The bases don’t join with each other though, only the sugar part and the phosphate part. The sugar of one monomer is joined to the phosphate of the next by a ‘phosphodiester bond’. This bond is formed by a condensation reaction – that means a reaction where a molecule of water is removed and two things join together. This happens over and over until lots of sugars and phosphates are joined. This forms what is called the ‘sugar-phosphate backbone’ of DNA. But DNA doesn’t just consist of one polymer strand. It’s actually double-stranded. There are two of these long strands of nucleotides. These two polynucleotide chains run in opposite directions – they are ‘anti-parallel’. The end of each strand is given a name. One end is the ‘5 prime’ end, with a free phosphate group, and the other is the ‘3 prime’ end, with a free hydroxyl group. One strand goes from 5 prime to 3 prime and the other from 3 prime to 5 prime. These two strands are joined together. How? This time, the sugars and phosphates aren’t involved, but the bases. Bases of one strand are joined to bases of the other strand by hydrogen bonds. Individually, hydrogen bonds are fairly weak, so they can be easily broken when the strands need to be separated for DNA replication, but there are so many of them that the DNA molecule is very stable. Not all of the bases can pair with each other though. A can only pair with T, and C can only pair with G, due to their structures. Between A and T, there are two hydrogen bonds and between C and G there are three.
So, we’ve got two strands of DNA, joined together by hydrogen bonds between the bases. Next, I’ll describe the overall structure of a DNA molecule. DNA exists in a helix shape, which basically just means a spiral. DNA is found inside all cells, in the nucleus. But, just think about how complex humans are. DNA carries the genetic information coding for absolutely everything about the body, so, as you can imagine, there’s quite a lot of it! In order to fit it all inside a tiny nucleus, DNA has to be packaged very tightly. The helices are wrapped around proteins called ‘histones’, to form more condensed structures called ‘nucleosomes’. Then there is more and more packing until the DNA molecules form thread-like structures called ‘chromosomes’. Inside every cell, with the exception of gametes, there are 23 homologous pairs of these chromosomes.