Nucleotides are composed of a nitrogenous base, pentose sugar, and a phosphate group. There are 5 nitrogenous bases that are classified as either purines or pyrimidines based on their chemical structure. The purine bases are adenine (A) and guanine (G). Cytosine (C), thymine (T), and uracil (U) are categorized as pyrimidines.
Deoxyribonucleic acid (DNA) only has nucleotides that have 2’-deoxyribose as the sugar and A, G, C, T nitrogenous bases. Ribonucleic acid (RNA) is formed by nucleotides with ribose as the sugar and A, G, C, U nitrogenous bases. The phosphate group (PO43-) is unchanged between the nucleotides that form RNA and DNA.
The DNA double-helix is the helical structure formed by 2 DNA strands with complementary base pairing. In DNA adenine only binds to thymine and cytosine only binds to guanine. Although RNA is typically single-stranded, base pairing does occur. In RNA adenine binds to uracil and cytosine binds to guanine. The bond between guanine and cytosine is stronger with 3 hydrogen bonds than that between thymine (or uracil) and adenine where there are only 2 hydrogen bonds.
There are 3 prominent types of RNA with distinct functions in the cell: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Most RNA does not code for protein, only mRNA holds the code for the amino acid sequence of protein. tRNA and rRNA are both noncoding, but do play a role in translation. As its name implies tRNA transfers an amino acid to the newly forming polypeptide chain. rRNA is the RNA that forms the ribosome.
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