DNA in different bacteria will have different sequences, because they will generally have different genetic content – this means they will make different proteins, as genes are the code for proteins. DNA in Salmonella, for example, has a similar core genome, for usual everyday Salmonella functions, but also has what is called a cloud genome. These are genes which are only present in some of the bacteria. These differences are usually from rogue bits of DNA which end up inside the bacteria and code for things such as antibiotic resistance. The bacteria with these ‘genetic islands’ are usually the ones which become dominant and cause a lot of illness.
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