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How does DNA work?

DNA replication in actionMost people can tell you that DNA contains the "blueprint" for a living thing - but how does it work?

There are three main things to understand about what DNA does in the body:

  • Encoding: How is information stored in a DNA molecule?
  • Replication: How is DNA copied?
  • Expression: How is the information in the DNA used?

Encoding

A DNA molecule is made from smaller "building blocks" called nucleotides. Each nucleotide is made up of three parts: a sugar and a phosphate group, which form the structure of the DNA strand, and a base, which carries the information.

There are four different bases, usually referred to by their first letters: adenine (A), cytosine (C), guanine (G) and thymine (T). All of the information in DNA is stored in sequences of these four bases. Each sequence is a gene.

Replication

Every time a new cell is created, the DNA in the original cell has to be copied. This is possible because of the way DNA bases match up.

The DNA molecule is actually made of two "strings", which wind around each other in the famous double helix shape. These strings are connected together at the bases, and each base will only connect to one other: A and T always pair up, and G and C always pair up. You can think of them like a set of puzzle pieces that will only fit together with the right other piece, as in the diagram below.

DNA diagram 1

DNA diagram 2

This means that if you split a DNA strand down the middle, each half still has the full set of information: if one half goes AGCCTC, you know that the other side must go TCGGAG so that the bases match up.

This is exactly what happens when DNA is copied. An enzyme called helicase "unzips" the DNA into two strands, and a set of enzymes called DNA polymerase goes along these strands filling in the matching pairs.

Expression

But all this would be useless if nothing was done with the information in DNA.

The keys to making use of DNA are amino acids, the building blocks of protein. There are many different proteins in the body: proteins that form muscles; proteins that help with chemical reactions (enzymes); proteins that act as messengers (hormones) and many more.

The sequences of bases in DNA match up to the sequences of amino acids that make up these proteins. There are 20 different amino acids that can be used, so a single base isn't enough to represent each one. Instead, each set of three bases represents one amino acid. These sets are known as codons, and there are extra codons that indicate the start and end of the protein sequence.

The process starts like replication: the DNA is unzipped into separate strands. However, instead of creating two new strands, a new molecule called messenger RNA (mRNA) is created using one of the strands as a guideline. This is done with only part of the DNA at a time, so that an mRNA molecule contains the information for a single gene.

mRNA has to be created because most DNA always stays in the cell nucleus , and proteins are put together outside it. The mRNA can travel out of the nucleus to where it is needed.

In the same way that each base in DNA "fits together" with a particular other base, each codon "fits together" with a particular amino acid. This allows the right amino acids to be put together in the right order, creating the protein needed by the body.

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