Located at various places in the chromosomes of every species are the codes for the various types of RNA molecules. These include the tRNA molecules that "transfer" (carry) a specific amino acid for use in protein assembly, the ribosomal RNA (rRNA) molecules that are major constituents of ribosomes, where the tRNA-borne amino acids are concatenated, and the "small nuclear" snRNA molecules that participate in a variety of functions such as splicing mRNA and attending to telomeres (chromosome ends). These genes code for RNA that is itself directly functional, depending on its nucleotide sequence and autoannealing conformation.
RNA transcription is achieved by enzymes that move along a specific one of the two DNA strands at a given location (the two strands locally separate for this to happen); the enzyme "reads" the sequence incorporating (ribo)nucleotides one at a time into a chain by complementary base pairing to each nucleotide on the DNA template strand. The resulting RNA molecule is identical in sequence to that of the other "coding" DNA strand, except that ribonucleotides that are chained together to make an RNA molecule have a slightly different sugar than DNA and U replaces the T of DNA. Different polymerase (chain making) enzymes are used to transcribe different types of RNA; the control of which polymerase is used is encoded in the flanking sequence on the chromosome that represents a physical binding site (known as a promoter; see below) for the enzyme.
DNA is the "permanent" codebook. Once the RNA is transcribed, it leaves the DNA and proceeds to go about its business, while the DNA template—the "master" code—remains intact and can reanneal into a stable double-stranded state. When a cell divides, for example, the whole intact DNA molecule is copied and passed to
A. Lysine tRNAs,Various Species
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