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Every living organism needs to carry with it instructions to build a new version of itself, and to make products to keep it alive. These instructions are nucleic acids. All organisms with more than one cell use a nucleic acid called deoxyribonucleic acid (DNA), and less complicated ones, such as viruses, use ribonucleic acid (RNA.) Each of these nucleic acids is a string of many individual molecules, and when the life form reads these molecules in sequence, it identifies what product to make.
Of the two nucleic acids, RNA is the least complicated. It exists as a single strand. DNA, on the other hand, pairs up with another DNA strand, so it is present in cells as a double-stranded structure in a spiral shape.
Each nucleic acid is a string, that is made up of many building blocks, one after the other, called nucleotides. These nucleotides stick together through chemical forces on each end of the block. Only four different nucleotides make up DNA. These are adenine (A), guanine (G), thymine (T) and cytosine (C.) RNA also has only four nucleotide blocks, but instead of thymine (T), it has uracil (U.)
A living thing holds lots of nucleic acid instructions in its cells. Each cell can read the string of instructions and make the relevant products. As every organism needs to make lots of different products, the nucleic acid string contains many little sections of instructions. These sections are called genes, and the cell generally reads each gene as the instructions for one particular product.
It is the sequence of nucleotides that matters with nucleic acids, and the complicated instructions do not need more than four nucleotides. The human genome, for example, holds 3.2 billion nucleotides in each strand. Smaller organisms tend to have shorter nucleic acids, such as the bacterium Haemophilus influenzae at 1.8 million bases per strand.
An analogy is the fact that the English language has 26 letters, but English-speakers can put all of these letters together, in different word combinations, and have complex conversations. A very simple example is when someone says "pots". The same letters in reverse mean something completely different; "Stop." So, in another example, when the cell reads a gene with a sequence that begins CCTTGGAA...., it will make a different cell product to one that begins AATTGGAA...... even though the sequences are similar. Nucleic acid sequences in genes must be relatively accurate, as otherwise the organism may not be able to build the correct product.
Fundamentally, nucleic acids function as the computer that organizes the cell. They also provide the instructions that the cell needs to replicate itself. Without the nucleic acid, a cell, or an organism, cannot build another version of itself. Only those lifeforms that can replicate themselves can survive into the next generation. This is why nucleic acids are present in every life form on earth.