Epigenetics – IV. The Dance of Life (1 of 3)
Date Posted: January 14th, 2010
There is at any given moment million of chemical reactions
taking place in our bodies. You can liken it to a carefully
choreographed dance, an exquisite ballet comprising a
phenomenally talented corps. I like to think of it as the dance
of life; were it to stop, life will stop with it! So who are the
performers and what exactly do they do?
DNA (Deoxyribonucleic Acid)
A DNA molecule is very long and very thin, yet it fits
inside a much smaller (cell) nucleus and is folded up in the
chromosome in a very precise manner. DNA is a linear polymer
made out of four different building blocks, the nucleotides. The
sequence of the nucleotides is in effect the genetic information
that will allow the cell to carry out its work. Each nucleotide
is composed of three parts: The first part is a nitrogenous base
known as Purine [Adenine (A) and Guanine (G)] or Pyrimidine
[Cytosine (C) and Thymine (T)]; the second part is a Sugar,
Deoxyribose; the final part is a Phosphate Group. The
nitrogenous base provide any given nucleotide with its identity
and it is referred to by its base, i.e. A,C,G,T. (This in effect
was the information provided by The Human Genome Project, an
endless number of A,C,G,T; billions of them!) One DNA strand can
be up to several hundred million nucleotides in length. Note
that T match with A and C with G.
Picture a technical setting with technicians assembling the
components of an intricate device. They are guided in their work
by an instruction manual, it’s their bible. This is in fact the
role of DNA inside the cell; it provides the cell with the
instruction it needs to do its work. However, the “technician”
is RNA (Ribonucleic Acid); it translates the information into a
medium that can be used directly by the cell.
Note that all cells for a given individual contain the same
genetic information. However, for any given organ, only certain
genes are expressed, the rest are silenced (they are inactive).
For example liver cells will only produce the proteins allocated
to the liver, not those of the heart or the brain! This gene
silencing process has been discussed in the Overview.
RNA (Ribonucleic Acid)
RNA differs from DNA in that it includes in the second
position of the Ribose Ring a Hydroxy (-OH) group. Put in a
simpler way, Thymine (T) does not occur in RNA and is replaced by
Uracil (U). Thus the coding for RNA is A,C,G,U.
RNA has three functions: a) It serves as the messenger that
tell the cell (the ribosomes) what proteins to make [Messenger
RNA (mRNA)]; b) It’s part of the structure of the ribosome, the
protein/RNA complex that synthesizes proteins based on the coding
carried over by the mRNA [Ribosomal RNA (rRNA)]; and c) it
obtains the Amino Acids (AA) (the constituents of the proteins)
needed by the ribosome [since it transfers to the cell the needed
AA, it is called Transfer RNA (tRNA)].
Looking upon it in a more simplified way, the DNA provides
the mRNA with a “shopping list” of AA and instructions as to how
to put them together (like in a recipe where you first list the
needed ingredients, followed by the required methodology to cook
this particular dish). The rRNA does the “cooking” after the
tRNA had obtained and transferred to the cell the needed AA.
The process gets too technical after that and goes beyond
the scope of this simplified narrative. I am, however, including
one important point.
The required genetic information is transcribed from the DNA
to the mRNA. This happens at a specific site on the DNA called a
promoter. Each gene has its own promoter(s). Transcription ends
at a terminator sequence on the DNA. The transcripts can be
between 300 to 50,000 nucleotides long and contain the
instructions needed to make the protein in question. Generally,
the transcripts need to be processed before they can be used as a
blueprint for a protein. This is done by removing intervening
sequences (introns) in the genes.
