Genetic variation in fishes

As for all sexually producing organisms, ones that had passed on, present ones and the ones that are to come are all different i.e including fishes. The uniqueness of each individual within a specie is the consequence of two factors:
1) Deoxyribose nucleic acid (DNA)
2) Sexual reproduction
These two factors produce and maintain the genetic diversity within a specie.
DNA consist of chains of nucleotide monomers and each nucleotide contains a deoxyribose sugar, a nitrogenous base and a phosphate group. The base may be a purine base (Adenine,A, and Guanine,G) or a pyrimidine base (Cytosine,C, and Thymine,T) in which A can only pair with T and C with G. A complete DNA molecule consists of two polynucleotide chains, or strands, wrapped around each other in the form of a double helix. This double strand contain the template(non-coding) strand and non-template(coding) strand. The non-template strand is transcribed  to RNA (i.e copying the template strand) with the help of RNA polymerase so as to be able to express a gene. The single nucleotide strand formed, which contain ribose sugar and uracil(U) in place of thymine(T), is then translated to chains of amino acids which codes for a particular protein. There are 4^3=64 possible combinations of the four bases into a triplet code, and it is these 64 triplet codons which define the 20amino acids. The ‘words’ of the code consist of three bases using the symbol U for uracil, C for cytosine, A for adenine and G for guanine. Two or more codons may code for the same protein while it may be only one for some others like tryphtophan and methionine. AUG (methionine) signals start of a protein synthesis though it may later be removed in some gene expression. Three codons (UAA, UAG and UGA) do not encode amino acids but act as signals for protein synthesis to stop and are  called termination codons or stop codons.
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