Suppose a researcher wants a cell to produce a
particular protein—say, Green Fluorescent Protein (GFP). Now that we understand
the central dogma, the researcher's path is pretty straight forward. First, the researcher
would need a copy of the gene, usually from an existing source, like
jellyfish DNA. There are many ways to insert the gene into a cell, and more
ways are being explored. Let's say for now that the researcher puts the GFP
gene on a plasmid (a circular piece
of DNA that is self-replicating in a cell). To cut-and-paste a piece of DNA,
scientists use restriction enzymes,
which are like molecular scissors. They recognize specific sequences of the DNA
alphabet and sever double stranded DNA in predictable ways. DNA ligase is like the glue, that bonds
strands back together. 
The researcher then puts the plasmid in solution
with the cells of interest, and uses electrophoresis (a quick electrical pulse)
to move the plasmid into the cells. We have DNA with the correct sequence of
As, Gs, Ts, and Cs. From here, the cell should automatically make mRNA copies,
and translate those into proteins. Under a black light, the cells would glow
green.
Usually, the cells need a little bit more
help to glow noticeably green. To increase the number of mRNA copies of our
gene, researchers can use promoters.
Promoters are sequences of DNA upstream
of our gene of interest. A promoter attracts RNA polymerase. In other words,
the sequence of DNA in a promoter tends to stick better to a polymerase
molecule than random sequence. By keeping polymerase in the neighborhood longer
and more often, more mRNA is created, which leads to more copies of the
fluorescent protein. If a researcher wants even more control over gene
expression, there are promoters with effective, molecular on/off switches. Most
have proteins that bind to them, changing the way polymerase interacts with the
promoter, which can potentially increase or decrease transcription. Proteins
that bind to DNA and affect transcription are called transcription factors.
Also included in the promoter region are ribosomal binding sites. A ribosome recognizes mRNA by attaching to specific regions at
the beginning of a gene. By changing the ribosomal binding site, it's possible
to control how efficiently mRNA is translated into protein.
Multi-Layered Engineering
If your head is spinning with all the jargon, don't
worry about it. Use these introductory posts as a resource for later on. Don't
feel pressured into making flash cards. The main point you should take away right
now is this: it is possible to engineer cellular processes at many levels. We
can change inputs, change DNA, RNA, and proteins. At every level we have a
chance to alter the final output. This is a big part of biotechnology.
It is also
worth noting that we have only scratched the surface of molecular biology. Just
know that the central dogma is an oversimplification. In nature, it works
backwards and forwards. RNA can be transcribed into DNA. RNA can perform enzymatic functions. Organisms can have slightly different genetic codes—e.g.
the three-letter codes in some organisms might code for different amino acids
than they do in us. As a matter of fact, the mitochondria in our cells have
their own genes and genetic code, separate from the rest of our body! There are
more proteins, elements of genetic control, and complex RNA splicing than we
have time to mention. But, don't view that as a deterrent. That's why we have
molecular biology textbooks. That's why research continues. The complexity of
life on earth is exactly why biotechnology is a booming sector—the
possibilities are endless!
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