You need to understand the structure of the molecule DNA before you read this post.
DNA stands for deoxyribonucleic acid and is the chemical that makes up the genetic information in all living organisms on earth. DNA is a double-stranded molecule in which each strand is made of a polymer of simple molecules called nucleotides. There are four nucleotides in DNA, with each nucleotide differing in the base present in the molecule. Adenine, Thymine, Cytosine and Thymine are the four bases found in the nucleotides in DNA. Every nucleotide in DNA contains the same sugar, deoxyribose and a phosphate group as shown in the diagram below.
But DNA is not the only nucleic acid found in cells. All living cells also contain a similar molecule RNA that serves a whole variety of different functions. It is not the main genetic material in any cell but is essential in allowing the information contained in a DNA molecule to be expressed as a protein (see post on protein synthesis to come)
RNA stands for Ribonucleic Acid and is also a polymer of nucleotides. But whereas DNA is always a double-stranded molecule, RNA is always single-stranded (although certain forms of RNA can fold back on themselves at points so they can appear double stranded). The sugar in every RNA nucleotide is ribose (as opposed to deoxyribose in DNA).
There are also four different bases found in RNA nucleotides. Three are identical to the bases found in DNA (Adenine, Cytosine and Guanine) but there is no Thymine in RNA. RNA can contain nucleotides with the similar base Uracil in its place.
So in summary:
- RNA is single stranded whereas DNA is double stranded
- RNA contains the sugar ribose in every nucleotide whereas DNA contains deoxyribose
- DNA contains 4 bases (ATCG) whereas RNA contains A,U,C and G (thymine is replaced by uracil)
I have been working today on making a video to explain DNA structure, chromosomes and cell division to post on YouTube. This has proved harder than I anticipated (not just because I look ridiculous and keep stuttering….) but I hope there may be something for you by lunchtime tomorrow….
So I will have to resort to the more old-fashioned medium of the blog. (The times they are a’changing)
Firstly DNA is from the family of molecules called Nucleic Acids. These are examples of biological polymers (macromolecules) and you should know that a polymer is a large molecule made of a chain of repeating subunits.
The monomers that make up a DNA molecule are called nucleotides. A single nucleotide is made up of a phosphate group attached to the sugar, deoxyribose which in turn is attached to a nitrogenous (nitrogen-containing) base.
Every nucleotide in DNA has the same phosphate group, the same sugar (deoxyribose) but there are four alternative bases in DNA nucleotides. You don’t need to worry about the structure of these four bases but you do need to know their names: Adenine, Guanine, Cytosine and Thymine.
Now the next idea is that a single DNA molecule is actually made up of two chains of nucleotides joined together. These two polynucleotide chains line up alongside one another and are held together by hydrogen bonds between the pairs of bases in the middle of the molecule. There are two antiparallel sugar-phosophate backbones on the outside and the pairs of bases in the middle. You can see that the bases always pair together in a predictable way. A pairs with T (joined by two hydrogen bonds) and C pairs with G (joined by three hydrogen bonds)
Can you see why the two strands that make up the DNA molecule are described as being antiparallel?
There are only two more things to appreciate about the structure of the molecule DNA:
Firstly it is appreciating how long the actual DNA molecule might be. The diagram above shows a structure 5 base pairs in length. The DNA molecules in the nuclei of your cells might be hundreds of millions of base pairs in length. If you look at the sum total in a single human nucleus there are DNA molecules 3 billion base pairs long – a molecule that if allowed to line up in a straight line would extend to around 2 metres in length.
And finally the fact about the structure of DNA that everyone remembers – it is a double helix. The two sugar-phosphate backbones do not run in a straight line as in the diagram above but coil around each other into the infamous double helix.