Hobbies And Interests
mRNA
Messenger RNA, or mRNA, is similar to DNA. However, while DNA is in the shape of a twisted ladder, or double helix, RNA is a single-chain molecule. It resembles half a DNA ladder; it is not twisted. Messenger RNA carries coding information contained in the DNA. The mRNA chain is composed of a series of molecules called nucleotides. The nucleotides in mRNA are adenine, guanine, cytosine and uracil. These nucleotides are ordered in sequences composed of three nucleotides each, each sequence being called a codon. Four main regions define an mRNA strand: a 5' cap and untranslated region, which is the front of the strand; a coding sequence, which carries the coding information from the DNA; a 3' untranslated region; and a poly-A tail, a long sequence of adenine nucleotides that allows the mRNA strand to leave the nucleus (in cells with a nucleus) and also protects the molecule from premature breakdown.
tRNA
Like mRNA, transfer RNA (tRNA) is a single-chain molecule. Unlike mRNA, tRNA is twisted around itself. It consists of two functional areas: an anticodon loop and a 3' end. The tRNA molecule is often pictured as a cross with its 3' end at the top and its anticodon loop at the bottom. The leftmost arm of the cross is called the D-loop; the rightmost arm is called the T-psi-C loop. A tRNA anticodon consists of three bases complementary to a three-base codon on the mRNA chain. Like mRNA, these three bases can be made up of adenine, guanine, cytosine or uracil. The 3' end of a tRNA molecule bonds to an amino acid in the cell's cytoplasm. Each type of tRNA can only bond to one type of amino acid. However, some tRNA anticodons will bond with more than one mRNA codon sequence.
rRNA
Ribosomal RNA, or rRNA, makes up the ribosome in conjunction with structural proteins. The ribosome is the most numerous organelle (small functional unit) of the living cell. Ribosomal RNA is made up of two units: a large subunit and a small subunit. Protein synthesis takes place between these two units. The units enclose three binding sites -- labeled A, P and E -- that accommodate three tRNA molecules and a strand of mRNA during the synthetic process. Unlike mRNA and tRNA, rRNA does not carry a complementary codon or anticodon.
Transcription
Protein synthesis begins with the untwisting of the DNA molecule and a temporary separation of its two halves. Free mRNA nucleotides line up against complementary bases on one half of the DNA molecule, the bases forming weak hydrogen bonds with each other. With the help of RNA polymerase, a sugar-phosphate backbone forms along the mRNA nucleotides. The weak hydrogen bonds between the DNA and mRNA complementary pairs then break, releasing a single strand of mRNA from its DNA template. The DNA molecule reforms; after processing to remove extraneous regions (introns) in the mRNA strand, the mRNA leaves the nucleus. In prokaryotic cells, which lack a nucleus, transcription takes place in the cytoplasm of the cell and the last processing step does not occur.
Translation
In the cytoplasm of a prokaryotic cell -- or across a membrane in the eukaryotic cell -- translation begins when a small and a large subunit of rRNA form around an mRNA strand. The rRNA complex begins to move along the mRNA strand. One at a time, tRNA molecules, each carrying a single amino acid, enter the rRNA complex, bind to an exposed complementary codon on the mRNA molecule, release their amino acid to a growing string of amino acids that were released by previous tRNA molecules, and are then in turn released from the rRNA. When the rRNA reaches a stop code on the mRNA strand, it stops reading the mRNA strand and binding tRNA molecules and releases the string of amino acids, now a polypeptide. This polypeptide may be itself a protein; in conjunction with several other polypeptides, it may form a protein. Several rRNA complexes may read and process a single mRNA strand at one time, producing multiple copies of a single polypeptide or protein simultaneously.