CENTRAL DOGMA The Central Dogma of genetics is: DNA is transcribed to RNA which is translated to protein.  Protein is never back-translated to RNA or DNA; and except... click here to read more	Eventually the mature mRNA finds its way to a ribosome, where it is translated into a protein, another biological polymer. In prokaryotic cells, which have no nuclear compartment, the process of transcription and translation may be linked together. In eukaryotic cells, the site of transcription (the cell nucleus) is usually separated from the site of translation (the cytoplasm), so the mRNA must be transported out of the nucleus into the cytoplasm, where it can be bound by ribosomes. The mRNA is read by the ribosome as triplet codons, usually beginning with an AUG, or methonine codon downstream of the ribosome binding site. Complexes of initiation factors and elongation factors bring aminoacylated transfer RNAs (tRNAs) into the ribosome-mRNA complex, matching the codon in the mRNA to the anti-codon in the tRNA, thereby adding the correct amino acid in the sequence encoding the gene. Translation takes place in three stages: Initiation, Elongation, Termination. Initiation Initiation of translation begins as the small subunit of the ribosome slides down the strand of RNA until it reaches a certain sequence. The ribosome is made of protein and ribosomal RNA (rRNA). The ribonucleotide sequence the ribosome is looking for on the RNA strand is AUG (adenine, uracil, guanine). Once it finds this sequence, the large subunit of the ribosome joins the small subunit. An initiator tRNA then binds to what is called a P (promoter) site on the ribosome. The initiator tRNA has either a methionine or a modified methionine (in bacteria) as the amino acid attached to it. Elongation The elongation phase starts when the anticodon of another tRNA molecule is able to form base pairs with the next three ribonucleotides that come after the AUG. Each group of three ribonucleotides on the strand of RNA is called a codon. This tRNA moves into what is called the A site on the ribosome. Next, a peptide bond is formed between the amino acid of this tRNA and the methionine from the initiation phase. After this first peptide bond is formed, the ribosome slides down three ribonucleotides to the next codon on the RNA molecule. The most recently added tRNA then moves into the position of the P site. Another tRNA molecule, one that can base pair with this next codon, then moves into the A site. A peptide bond is then formed between its amino acid and the amino acid of the preceding tRNA, and again, they change positions and the most recently added tRNA moves into the P site. The tRNA separates from its amino acid as it moves away from the ribosome, leaving behind just the string of amino acids held together with peptide bonds. Termination Translation ends when one of three stop codons, UAA, UAG, or UGA, enters the A site of the ribosome. There are no aminoacyl tRNA molecules that recognize these sequences. Instead, release factors bind to the P site, catalyzing the release of the completed polypeptide chain and separating the ribosome into its original small and large subunits.
DNA REPLICATION Prior to cell division, the DNA material in the original cell must be duplicated so that after cell division, each new cell contains the full amount of DNA material. The process of DNA duplication is... click here to read more
TRANSCRIPTION Transcription is the process through which a DNA sequence is enzymatically copied by an RNA polymerase to produce a... click here to read more
TRANSLATION Eventually the mature mRNA finds its way to a ribosome, where it is translated into a protein, another biological polymer. In prokaryotic cells, which have no nuclear compartment, the process... click here to read more

Last updated on 23 March 2008

Copyright © 2008 Ivan & Luther Corp UTM.2SCB