All three share the common feature in that they are made up of multiple nucleotides connected to form a chain (a polynucleotide). Their main differences lie in their varying functions accompanied with their corresponding structural differences. Since mRNA and tRNA variants of RNA, lets first look at the structural differences between DNA and RNA. DNA contains the four base types Adenine(A), Cytosine(C), Guanine(C) and Thymine(T). RNA molecules contain the same bases with the exception of Thymine which is replaced by Uracil(U). Furthermore, RNA is much shorter and single stranded whereas DNA is long and double stranded. The individual nucleotides of DNA and RNA are also slightly different. All nucleotides are made up of a sugar a base and a phosphate group. In DNA the sugar used is called deoxyribose whereas in RNA the sugar is ribose (hence DNA and RNA).The important structural difference between the two types of RNA is that mRNA takes on the shape of a line whereas tRNA has a clover-like shape. However, here their main differences lie in their function. Both play a vital role in protein synthesis. mRNA is used as a vessel to carry genetic information, in the form of bases, encoded in the DNA of the cell to the Rough Endoplasmatic Reticulum (RER) where it is to be translated into protein. tRNA plays a role in this process by carrying amino acids (the building blocks that make up a protein) and aligning them in the right order to create the right protein. So looking back at tRNA's structure, it has an amino acid binding site on one end to enable it to carry amino acids. On the other side there are three unpaired bases. These will bind to their complementary opposite bases on the mRNA strand we mentioned earlier. So, depending on the sequence of bases on the mRNA, a specific protein will be built. Now, looking at the picture as a whole, we can trace the sequence of bases on the mRNA back to the sequence of bases on the DNA as the mRNA's bases are the complementary opposites of the bases on the relevant section of the DNA.
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