RNA

Cells have a second type of nucleic acid - RNA (Ribonucleic Acid) which can also carry genetic information. Unlike DNA, which is located primarily in the nucleus, RNA can also be found in the cell's cytoplasm. Like DNA, RNA is also built from purine and pyrimidine nucleotides (Uracil taking the place of Thymine), but forms a single helix (unlike the DNA's double helix). The messenger RNA (mRNA) carries genetic information from the DNA to the ribosomes - the intra-cellular constructs where it is translated into a protein. mRNA is synthesized in the nucleus based on a single DNA strand, using the RNA polymerase enzyme. mRNA is transcribed from a DNA strand only in locations called open reading frames. When such transcription occurs, the two DNA strands are split apart, and one of them serves as a mold for the generated mRNA molecule, which is complementary to this strand, and therefore, a replica of the other one. Consecutive triplets of mRNA bases, called codons, each determine a certain amino acid (see below). In eukaryotes, the mRNA is formed of coding and non-coding regions. Coding regions are the regions used to carry real genetic information. Non coding regions do not carry such information (see below).

  

Figure: coding and non-coding sections of DNA

The coding regions are called exons, since they are able to leave the nucleus and reach the Ribosome. The non-coding regions are called introns and never leave the nucleus.

  

Figure: exons and introns

After being synthesized from a DNA strand, the exons of the RNA molecule are merged together, exiting the nucleus, heading for a ribosome. The transfer RNA (tRNA) is a small RNA molecule serving as an adapter between mRNA and amino acids. The molecule is composed of two parts. On one part the tRNA holds an anticodon. The anticodon is a sequence of three RNA bases. On the other side, the tRNA holds an amino acid. The many-to-one mapping from anticodons to amino acids defined by tRNA molecules is the universal genetic code (see below).