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I know that DNA is universal meaning that all organisms have the same 4 base pairs and also that the same codons code for the same amino acid in all organisms. What I am wondering is that say humans and bacteria and rats all have the same gene which codes for the same metabolic enzyme. Will the amino acid sequence of that protein enzyme also be the same in all three of those organisms? And that specific gene segment will also have the exact same base pair sequence in all three organisms too correct?

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  • $\begingroup$ Welcome to Biology.SE. What do you mean by "same gene", if not "same base pair sequence"? $\endgroup$ – Remi.b Jan 8 '18 at 6:19
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Before I go into answering your question, I just want to correct some mistake you have. Although DNA nucleotides (bases) are (almost) universal, the genetic code is not - there are organisms that use a slightly different genetic code. See here for example. Moreover, even the mitochondria in humans (and vertebrates in general) uses a slightly different code.
Now, to answer your question, it all comes down to evolution. Humans, rats and bacteria all have a common ancestor somewhere in our evolutionary past. Let's assume that this ancestor had some crucial and universally-useful gene. Since then, human+rat diversified from bacteria and then from each other, but all 3 species kept this gene, since its function was absolutely necessary. However, mutations were acquired along the way in each lineage. These mutations can be random and may not even have an effect on the amino acid results (synonymous), or they can be a result of natural selection and have an effect on the amino acid sequence (non-synonymous). In many cases, the different life styles and selective pressures acting on each species lead to rather different nucleotide and amino acid sequences. So to sum up, you can expect DNA, RNA and protein sequences to be different, although more similar between human and rat than bacteria.
In fact, the concept of the "same gene in different organisms" is somewhat wrong. As I explained, this is not the same gene, but you may say that it is homologous between the species, and more specifically orthologous.

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  • $\begingroup$ Alright, but say that 2 organisms do have the exact same gene for some protein enzyme. I know there can be mutations and all and this is unlikely, but say that happens. Will those two widely different organisms have the same DNA sequence for that gene and hence the same amino acid sequence, resulting in the same protein? Or can the exact same protein enzyme be coded from different DNA sequences in different organisms? I'm thinking that in order to be the exact same protein, the gene sequence MUST be the exact same as well but I'm not sure. $\endgroup$ – Dequavis Jan 8 '18 at 12:22
  • $\begingroup$ Just to make sure I understand: you're asking something like "Suppose I see that two organisms produce the exact same protein (same AA sequence), does that mean that the respective DNA sequence is the same?" Is that correct? If so, then the answer is of course not! Take a look at the genetic code table. How many ways are there to code Proline? Glycine? There are numerous way to code the same protein. On top of that, there is splicing, RNA editing, protein processing etc that could be going on and different between the organisms. $\endgroup$ – soungalo Jan 8 '18 at 12:39

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