Animal cells use much of their DNA to carry the information necessary for them to function efficiently, but some of their other DNA is not like this, it is simply junk that has been copied faithfully each time ancestral cells divided.

Simplified Animal Cell Structure

Animal cells are contained within a flexible membrane and have a single nucleus that contains a number of things including enormous lengths of DNA. Surrounding the nucleus there is the cytoplasm (the ‘stuff’ of the cell), and running throughout the cytoplasm there is a network of membranes which can have small structures called ribosomes attached. (These ribosomes are also found throughout the cytoplasm, along with much larger structures known as mitochondria.)

Making Proteins in an Animal Cell

• The DNA in the nucleus holds the code for every protein that the animal under consideration can make – at any stage of its life cycle.

• A bit of this code is needed whenever the cell needs to access the information for making a particular protein, and this copied code is transported to a ribosome where that protein is made.

• Proteins have very complex structure and are used in many ways within the cell. Perhaps the most important are the enzymes, since it is these that control the chemistry of the cell.

• (Mitochondria are concerned with the conversion of the ‘fuel’ obtained from food into the energy required for the cell to do work. Their DNA, RNA and ribosomes are involved in their own functioning.)

Important DNA Sequences in Related Animals

• DNA that codes for proteins or is involved in some way with efficient functioning is essential for the animal’s well being.

• All of the cell’s DNA needs to be copied every time a cell divides, and although this process is remarkably efficient, mistakes are sometimes made.

• If the mistake is with the copying of an important bit of DNA then that animal will not thrive – not being able to make a crucial protein can be the difference between life and death for the animal.

Junk DNA Sequences in Related Animals

Much of the DNA within the nucleus of a cell does not seem to carry any important information – it has been copied along with the important DNA and has accumulated as ‘junk’ over the millennia. Mistake made when copying this ‘junk DNA’ do not harm the offspring in any way, so these mistakes will always be passed on to all descendents and will cause them no harm.

Closely related animals (individuals, species, or groups) will have very similar ‘junk DNA’, while distantly related animals will have accumulated more random copying errors (and therefore differences) over time.