Fundamental #1: DNA to mRNA
to Protein
I talk a lot about “gene
expression” in this blog. What is a
gene? What is actually being
“expressed”?
A gene is a small section of
a chromosome that carries the instructions necessary to make a protein (there
are exceptions to everything I’m saying, but we won’t get into those now). A gene is made up of the molecule DNA. DNA is made of two strands of nucleic acids
linearly hooked together. There are 4
types of nucleic acids, which are abbreviated by their first letter (A, C, G,
or T).
The sequence of a gene tells
the cell what type of protein to make.
Proteins are made of amino acids (like tryptophan or cysteine, etc)
attached in a linear strand. Proteins can
fold into intricate shapes which help them perform various functions around the
cell. If DNA is the instruction manual,
then proteins are the workers. They can
act as enzymes which speed up chemical reactions (like even in the process of
making more proteins or DNA). Other
proteins act as scaffolding or as intercellular signals or for transport within
the cell.
Because DNA is inside the
nucleus and proteins are made in the cytoplasm, there has to be an intermediate
messenger in this process and that is mRNA (m = messenger). RNA is also made of nucleic acids, but has
only one strand. When a gene is ready to
be expressed, enzymes will make a copy of the DNA sequence in a single strand
of mRNA, in a process called transcription.
We say that the mRNA is “complimentary” to the DNA sequence. The mRNA goes out of the nucleus and
encounters a ribosome, which is the factory that translates the nucleic acid
code into the amino acids of proteins. A
sequence of three nucleotides will be translated into one particular amino
acid.
 |
| The "Central Dogma" of Molecular Biology |
All the cells in our body
have the same sequence of DNA, which includes all of our genes. However, only a subset of these genes is
expressed depending on the cell type, the developmental age and environmental
factors. There’s no point in expressing
the gene for eye color in any other cells besides the iris. It’s up to every cell to figure out which
genes to transcribe and when. Gene
expression can be regulated by signals from other cells (like hormones). Environmental factors can also change which
genes are expressed through a process known as epigenetics. The regulation of transcription and translation
is a fundamental mechanism our bodies use to adapt and change. We are able to learn and remember things only
because our brain cells are adjusting expression of particular genes.
Side note: Like I said, this
is a simplified description of molecular biology. Whole textbooks are devoted to this subject
and there are many exceptions to the rule.
For instance, RNA can do a lot more than just act as a messenger. RNA can actually act as an enzyme too and it plays
an important role in the ribosome during translation. There are also tons of small inhibitory RNAs
that will “silence” gene expression. The
“central dogma” of molecular biology is always getting more and more
complicated.
