Gene Expression
Genes are units of DNA that contain the coded instructions for the synthesis of functional
molecules - mostly proteins (and some RNAs). The functional molecules do the cell's
work. Genes (segments of DNA) are not "functional" in the sense of doing the cell's
work.
Genes are more complicated than we once thought. Before the human genome project
found out otherwise, it was estimated that humans needed over 100,000 genes to do all
that is being done in our unimaginably complex bodies. Jaws were dropping all over the
scientific community when it was determined that there are actually only about 22,000
genes in humans. The immediate inference (now documented experimentally) was that
one gene could provide the instructions for more than one protein. So much for the old
"Central Dogma of Biology" (proposed in those terms by Francis Crick) of ONE GENE >
ONE PROTEIN.
If you don't learn anything else about multicelluar organisms this year, make sure you
learn this: What makes one cell different from another is not the genes it contains, but
what it does with those genes. Cell specialization starts with which genes are being
expressed (turned on) in the cell. And now we know that a single gene can code for more
than one molecule - based on the needs of the cell and environmental influences. Your
brain cells contain the exact same set of genes as your liver cells, muscle cells, skin cells,
etc. The mechanisms of turning genes on and off is called Gene Regulation, or
Epigenetics (the newer term).
When a cell calls on a gene to be expressed, the process is called … Gene Expression.
Before we realized that there were numerous functional RNAs as well, the concept of
gene expression was more or less the synonymous with protein synthesis. Functional
proteins (mostly enzymes) far outnumber functional RNAs, so you will still see gene
expression simply referred to as "protein synthesis" in many references.
When a gene is expressed, an enzyme (itself a protein) called RNA Polymerase unzips
the DNA helix and makes a complimentary RNA molecule. If the gene sequence is 3'
ATCGGCTA5', the complimentary RNA transcript will be 5'UAGCCGAU3'. (Now go
back and read the last sentence again until you see it clearly.) Note that Uracil takes the
place of Thymine in RNA. If the RNA is a functional molecule in its own right, that
might be the end of it. Most of the time, however, the RNA transcript is an intermediate
molecule that heads off to a ribosome to be translated into a protein.
The Genetic Code:
In the 1960's, after Watson and Crick discovered the structure of DNA, things started
happening pretty fast. The genetic code (how does a polynucleotide provide instructions
for the making of a polypeptide?) was THE puzzle to solve. It was solved by a group of
scientists, most notably Gamow and Nirenberg. Nirenberg lived until 2010, so he was
witness to the significance of his discovery (in case you were wondering, he won a Nobel
Prize for his work). What was discovered immediately after the code was broken was
even more astonishing: The Genetic Code is universal. A human gene can be expressed
by a bacterium BECAUSE the code is universal. Even viruses use the same code. The
implications for evolution were profound beyond belief, but there it was for all to see.
Breaking the code:
There are 4 nucleotides in RNA (AUCG), and 20 amino acids. How can 4 'letters' spell
out the instructions for 20 amino acids? The math heads quickly figured out that it would
take 3 sequencial nucleotides for each amino acid. 4x4x4=64 (16 is not enough).
Additionally, having more than the necessary 20 allows for some wiggle room in
translating the code. Just as there are synonyms for "dry" (arid, parched, dehydrated,
desiccated), there are synonyms for many of the 3 letter "codons" that specifiy an amino
acid. The genetic code contains no spaces, punctuation, or other formatting in the DNA.
It's all in the sequence. THATSPRETTYIMPRESSIVEIFYOUASKME. When the RNA
transcript gets to a ribosome, it has to be just right… a perfect transcript. The ribosome
takes 3 letters at a time (one codon) and brings in the designated amino acid to add to the
protein.
The RNAs of Gene Expression: The RNA transcript (copy) of a gene is called
messenger RNA (mRNA). Like all RNAs of eukaryotic cells, mRNA is produced in the
nucleus. The ribosome is a 2-part molecular machine, with each part composed of
ribosomal RNA (rRNA) and proteins. Proteins are assembled at the complex when the 2
sub units (small subunit and large subunit… SSU and LSU) of the ribosome come
together with the mRNA. The third RNA involved in protein synthesis is transfer RNA
(tRNA). tRNAs deliver the 20 amino acids to the translational complex (ribosome and
mRNA). tRNAs recognize the codons in the mRNA using anticodons. For example, the
codon UUU on mRNA calls for the amino acid Phenylalanine. The tRNA carrying the
anticodon AAA (complementary to UUU) transfers phenylalanine. The codon GCU is
complimentary to the anticodon CGA, etc.
It all gets more complicated from here, but this is as far as we go at this time. Please let
me know if you hunger for more details, and I can point you in the right direction.
Biology at the molecular level is truly amazing.
Technologies align: Since the 1980s, molecular genetic technology and computer
technology have been advancing together. The world's first computer was built in 1946.
Computer technology and genetic technology advanced independently of one another for
a few decades during their infancy. In the 1970s and especially the 1980s, computers
became essential components of DNA research - the technologies merged. Sequencing
(like the Human Genome Project) would not be possible without computer technology.
Even now, genetic data is coming in faster than it can be analyzed. With each advance in
computer technology, there is a corresponding advance in our understanding of genetics.
In many labs, robots (controlled by computers) have taken the place of human scientists.
All of this in less than 50 years!