HIV i-Base

Appendix 2: Supplementary information about genetics

DNA as a recipe book – for making new HIV

Sometimes it helps to think of DNA as a recipe book.

  • There are only four letters used in this book, (A, T, C and G: the four BASES)
  • There are only 20 different words (the 20 common AMINO ACIDS). Each word only has three letters. The place that each of these words is printed is also called a CODON.
  • Each sentence (a PROTEIN) is made up of chains of many 3-lettered words (AMINO ACIDS).
  • Each recipe – a chapter – (a GENE) is made up of several thousand sentences.
  • Each book (GENOME) is made up of many chapters (GENES)

The HIV genome has nine short chapters, using about 3,000 words (AMINO ACIDS/CODONS) and about 9,200 letters (BASES).

The human genome contains 23 large chapters (CHROMOSOMES), many thousands of sentences, around one billion words and three billion letters. If you read one word every second, 24 hours a day, it would take over 30 years to read the human genome.

In humans, only 10% of the 3 billion bases are thought to be important and active. So some changes in DNA may not make any difference. This means that a large percentage of human DNA is like advertisments or blank pages.

We know what some parts of the DNA chain relate to – ie one part will determine the colour of your eyes. Other sections of DNA have been linked to more critical functions including risk or protection from a range of hereditary health complications. So, many things about you are determined by the order of the four bases and 20 amino acids in your DNA.

Nucleotides (bases)

There are only four base chemicals that make up DNA:

A = adenine T = thymine

C = cytosine G = guanine

The order of these bases determine the structure and function of all life. To make it easier, in DNA these molecules only have two pairs of bonds:

A always binds to T and C always binds to G.

Amino acids

Each group of three bases will be one of 20 different amino acids.

Amino acids are the chemical building blocks to make proteins and almost everything in the body is either made of proteins or needs proteins to make it.

See Appendix 3: List of amino acids and their abbreviations.

You do not need to learn these or know about the differences for each amino acid. It is important to understand that each letter stands for a different amino acid.

For example:

The three bases ATG is the code for the amino acid called methionine (M)

The three bases GTA or GTG are two of the codes for the amino acid called valine (V)

Most amino acids can be made up from different combinations of letters.

See Appendix 4: DNA codes for amino acids.

For example:

Valine can be made from four different base combinations: GTT, GTA, GTG and GTC

  • Strings of amino acids make up different proteins.
  • Strings of proteins make up different genes.
  • Strings of genes (in humans) are called chromosomes.

In this example, notice that only one letter needs to change to get from methionine (M) to valine (V). This gives an indication that the mutation is a simple change.

>> Appendix 3: List of amino acids and their abbreviations.

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