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 viruses
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