The Basics: DNA

Everybody has heard of DNA but do you know how it works? How the information in your genes is accessed or encoded? Cue picture of the double helix….

DNA Structure

In order to understand DNA it is important to understand some elements of the structure. DNA has two strands and each stand is made up of nucleotides. A nucleotide is made up of three parts, a sugar, a phosphate group and a base. The sugar and phosphate group form the so-called backbone of each strand, but it is the base that is of the most interest. The bases encode all the information and through pairing with their complementary base they hold the two separate strands of DNA together.

The four bases of DNA

There are four bases adenine (A), cytosine (C), guanine (G) and thymine (T). When discussing DNA these bases are often referred to by their first letter only, therefore they will often be written as just A, C, G or T. The bases selectively pair with their complementary base. There are two factors that play a role in the pairing, whether the base is made up of two molecular rings (a purine) or one ring (pyrimidine) and the number of hydrogen bonds that are form between the bases. A base that has one molecular ring pairs with a base that has two rings. T and C have only one ring whereas A and G have two. However T always pairs with A, and C with G. These pairing are based on the number of hydrogen bonds that form, with T and A having two hydrogen bonds C and G having three.

As a side note, the bases can mis-pair, there are processes designed to prevent it but it can still happen and they are referred to as a mutations.

The way these bases pair means that one of DNA strand contains the bases in one order and the other contains the complementary bases. This leads to a confusing naming system for the two strands. The strand that is used to make copies from is called the template strand while the other strand is called the coding strand.

DNA code

When discussing the DNA code, it is the long length of repeating bases that is being referred to. Initially you may think that four bases are not very much to work with, especially when you see the complexity of everything around you, but the DNA works on sequences of bases, not one base alone.

Coding Sections

The genes! The word everyone has heard of, but does it surprise you that this is also very complicated?

Entire books could and have been written on the subject and I will endeavour to write a couple of The Basics Series covering aspects of gene expression in an accessible but more detailed way. But for now I will try to condense it down into its simplest terms.

Genes only make up a small amount of the DNA within the genome of a cell but have a dramatic effect on their functions. The gene in transcribed from the template strand into RNA. RNA is a molecule similar to DNA with a backbone and bases. The bases of RNA pair one at a time with those on the template strand thereby building up a strand of RNA that complements the DNA.

There are many stages of processing involved but the RNA can either function as a strand of RNA or be used to synthesise proteins. Proteins have many varied use in the cell, with a vast number of functions depending on them, from breaking down food, to signalling between your cells they all depend on proteins. Proteins are made from amino acids linked together in a string. There are 20 different amino acids and the RNA codes for them in triplets. A cellular organelle, the ribosome reads the RNA in groups of three bases, codons, and then attaches the correct amino acid to the growing protein. There is a particular RNA codon that signals where the ribosome should start making the protein from and other that tell it when to stop, leaving the protein to fold and perform its function.

Non-coding sections

Now everyone has heard of genes but this is the part that is not a gene, perhaps you may have watched a science-fiction film or show that refers to it as junk DNA, well it is very far from junk! This is the region that is involved in regulation. There are many complex proteins and molecules that interact with non-coding regions and they can exert many different effects over the expression of genes, including; promoting their activation, suppression or silencing them completely. This allows for the quantity and types proteins being expressed to adapt to different conditions.

Summary

DNA is remarkable in its simplicity, only 4 different bases encode all the information that allows an organism, live, breath and reproduce. However, that is where the simplicity ends. In order to create proteins that the organism requires to survive, the DNA must be transcribed into RNA then processed before being used to make proteins. To make the proteins the RNA bases are read in groups of three which correspond to one of twenty amino acids. The amino acids are joined together in order to make the proteins. The parts of the DNA that are not used to make RNA are used to control which regions are transcribed into DNA. This level of control ensures the proteins the cells need are made.

Further Reading

Britannica – DNA

Outlining the location of DNA in the cell and structure.

Genetics Home Reference – What is DNA?

Outlining the location of DNA in the cell and structure.

New Scientist – DNA

Covering DNA structure and DNA testing.

Coming soon in The Basics range

DNA replication

How does DNA reproduce so any cells produced by cell division have faithful copies of the original.

From DNA to proteins

An overview of the transcription and translation processes. How do the base triplets turn into all the complex proteins that the body needs to survive.

RNA

A fascinating multipurpose molecule that is synthesised from DNA and has a multitude of roles within the cell, including containing the blueprints for protein synthesis.