INTRODUCTION TO MOLECULAR BIOLOGY
Molecular biology is the study of biology at a molecular level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, including the interactions between DNA, RNA and protein biosynthesis and learning how these interactions are regulated.
Writing in Nature, William Astbury described molecular biology as:
"... not so much a technique as an approach, an approach from the viewpoint of the so-called basic sciences with the leading idea of searching below the large-scale manifestations of classical biology for the corresponding molecular plan. It is concerned particularly with the forms of biological molecules and is predominantly three-dimensional and structural - which does not mean, however, that it is merely a refinement of morphology - it must at the same time inquire into genesis and function."
CENTRAL DOGMA OF MOLECULAR BIOLOGY
The central dogma of molecular biology was first enunciated by Francis Crick in 1958 and re-stated in a Nature paper published in 1970. The central dogma of molecular biology deals with the detailed residue-by-residue transfer of sequential information. It states that such information cannot be transferred back from protein to either protein or nucleic acid.
In other words, "once information gets into protein, it can't flow back to nucleic acid." The dogma is a framework for understanding the transfer of sequence information between sequential information-carrying biopolymers, in the most common or general case, in living organisms.
There are 3 major classes of such biopolymers:
DNA and RNA (both nucleic acids), and protein. There are 9 conceivable direct transfers of information that can occur between these. The dogma classes these into 3 groups of 3: 3 general transfers (believed to occur normally in most cells), 3 special transfers (known to occur, but only under specific conditions in case of some viruses or in a laboratory), and 3 unknown transfers (believed to never occur). The general transfers describe the normal flow of biological information: DNA can be copied to DNA (DNA replication), DNA information can be copied into mRNA (transcription), and proteins can be synthesized using the information in mRNA as a template (translation).
GENERAL TRANSFERS OF BIOLOGICAL SEQUENTIAL INFORMATION
Table of the 3 classes of information transfer suggested by the dogma
General Special Unknown
DNA -- DNA RNA -- DNA protein -- DNA
DNA -- RNA RNA -- RNA protein -- RNA
RNA -- protein DNA -- protein protein -- protein
As the final step in the Central Dogma, to transmit the genetic information between parents and progeny, the DNA must be replicated faithfully. Replication is carried out by a complex group of proteins that unwind the super helix, unwind the double-stranded DNA helix, and, using DNA polymerase and its associated proteins, copy or replicate the master template itself so the cycle can repeat DNA -- RNA -- protein in a new generation of cells or organisms.
TRANSCRIPTION AND TRANSLATION
Transcription is the process by which the information contained in a section of DNA is transferred to a newly assembled piece of messenger RNA (mRNA). It is facilitated by RNA polymerase and transcription factors. In eukaryote cells the primary transcript (pre-mRNA) is often processed further via alternative splicing. In this process, blocks of mRNA are cut out and rearranged, to produce different arrangements of the original sequence. Eventually, this mature mRNA finds its way to a ribosome, where it is translated. 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 initiator 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. As the amino acids are linked into the growing peptide chain, they begin folding into the correct conformation. This folding continues until the nascent polypeptide chains are released from the ribosome as a mature protein. In some cases the new polypeptide chain requires additional processing to make a mature protein. The correct folding process is quite complex and may require other proteins, called chaperone proteins. Occasionally, proteins themselves can be further spliced; when this happens, the inside "discarded" section is known as an intein.
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