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Monday 29 April 2013

NUCLEIC ACIDS STORE AND PROCESS CODED INFORMATION

A bit has already been said about how nucleic acids - or DNA, one type of nucleic acid - hold the information from which proteins are made.  Obviously, there's more to it than that.
Nucleic acids are another type of polymer, another long string of repeating subunits.  The subunits in this case are nucleotides, which are themselves made up of three pieces:  a five-carbon ribose sugar - ribose or deoxyribose, which is where the names of the 2 types come from;  a phosphate group that actually links the string of sugars together;  and one of five possible nitrogenous bases, attached to the sugar and capable of cross-linking to bases on other strings.  Three of the five bases, adenine, cytosine, and guanine, are found in both DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid);  the remaining two are similar, but thymine (not to be confused with thiamine, the vitamin) is found only on DNA while uracil takes the same "spot" on RNA.
In RNA, the polymer is a single string, commonly called a strand;  DNA is double-stranded, held by hydrogen bonds in a helix and cross-connected between bases on strands "running" in opposite directions, so there is a double helix.  The cross connections are very particular:  adenine on one side only bonds to thymine on the other (and vice versa), while cytosine only bonds to guanine.  This means that if you know which base is on one side, you can accurately predict which one is on the other.  Adenine will only link to thymine in DNA and uracil in RNA;  cytosine only links to guanine (and vice versa).  This set-up of complementary strands produces excellent copying potential:  copies of DNA are made by separating strands and then building new ones across from each old one, and the new strands will be duplicates of the "peeled off" strands.
Briefly, with details elsewhere, DNA carries the genes, codes from which proteins can be built.  Genes exist as stretches of DNA (or multiple stretches that can be combined) on very long structures called chromosomes.  Chromosomes carry more than just genes (and any given chromosome will house hundreds to thousands of separate genes):  one of the cutting-edge aspects of genetics is figuring out what all of the "extra" DNA is there for.  In a story that is sadly all too common, until recently the extra DNA was called "Junk DNA," which shows how a common hypothesis following, "we can't figure out why this is here," is "well, then it must have no purpose."  Much of that unknown has been explained:  there are templates for many functional RNA molecules, remnants of viral DNA long shut off, and many bits that are essentially molecular parasites, existing just to reproduce and spread among the genome.
The coding process depends upon codons, three-base sequences along one strand of the DNA that convey three basic codes:  a) one codon for where the gene code starts;  b) various codons that can each be translated into an amino acid in the protein sequence;  c)  one codon for where the gene sequence ends.  This means that any coding gene has three times more bases in the sequence (+ 6, for the start and stop codons) than the protein will have amino acids.  Except, of course, that this is biology, and the truth is much more complicated and confusing then that.
While DNA for the most part has a single basic form and function, RNA has a range of roles, mostly involving the middle steps between DNA sequence and protein sequence.  When a protein must be made in a cell, the two strands of the gene responsible are separated.  On the actual code strand, a strand of messenger RNA (mRNA) is made in a step called transcription.  The mRNA then moves to where the actual protein will be made (ribosomes, cell structures which themselves have RNA in their construction), and a small type of RNA performs most of the next step, translation.  It is transfer RNA (tRNA), with a codon-based connector on one end and an amino acid carrier on the other, that allows base sequence code to become amino acid sequence reality.
RNA molecules can be active beyond their coding and decoding roles, but not much is known yet about that aspect of the molecules.  Recent research hints that short-strand RNA molecules, called microRNAs, may be used by many types of cells to block the replication of RNA viruse

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