Hey.
So you may have seen another video in which I
talk about the central dogma of molecular biology,
and this being the work of Francis Crick and James Watson,
who delineated the structure of DNA
and determined the flow of information to be from DNA
to RNA to protein.
And while this is the traditionally held view
of the flow of information in living organisms,
there have also been a lot of new discoveries that extend
or even contradict the central dogma.
And so I'm going to expand on some of those ideas here.
And so this is the central dogma revisited.
Before I go on and touch on those new ideas though,
I would like to highlight the legacy
of another famous scientist whose work was so, so
critical to elucidating the double helix structure of DNA.
And that was the work of Rosalind Franklin, seen here.
And her contributions often go overlooked,
of which I am just as guilty.
And Franklin's work on the X-ray diffraction images of DNA
and her interpretation of that data
were key to confirming the helical structure of DNA.
And so whenever there's a mention of Watson and Crick,
Rosalind Franklin deserves a shout out as well.
So now on to the conflicting versions
of the central dogma of molecular biology.
As I said before, the traditional held view
of the central dogma was that the flow of information
occurs from DNA to RNA to protein.
And DNA goes to DNA via replication,
from DNA to RNA via transcription, and from RNA
to protein via translation.
And basically proteins perform virtually
all the tasks within living cells
and make up the very structure of those cells.
It would be premature though to think that information flows
in these pathways only and that proteins
are the final expression of the information encoded in our DNA.
So now that we have a firm foundation what
the central dogma was as it was conceived originally,
we can explore some of the newer discoveries
that have been made that expand on this concept.
And the first idea is that of reverse transcription,
in which information flows backward, if you will,
from RNA to DNA.
And reverse transcriptase is an enzyme
that generates complementary DNA or cDNA from an RNA template.
And reverse transcriptase is needed
for the replication of retroviruses,
including HIV, is probably the most well known one.
And these retroviruses use the enzyme to reverse
transcribe their RNA genomes back
into DNA, which is then integrated into the host genome
and replicated along with it.
So this is the first idea-- it was discovered back
in 1970-- that violated the central dogma.
It was very unpopular at first, saying that information
can actually flow from RNA back to DNA.
Now another example of an alternate pathway
for information flow is demonstrated
by a group of viruses called RNA viruses.
Now, these viruses have their genetic material stored as RNA,
as opposed to DNA.
And they can have their genome directly used
by host cell replication machinery
as if it were messenger RNA and then
translated directly into protein.
Or they can have their RNA serve as a template for another RNA
strand, that is then used for protein translation.
Some well-known examples of RNA viruses
are the coronavirus, which was responsible for the SARS
epidemic; the influenza virus responsible for the flu,
that you have to get a shot for every year,
and also paramyxovirus, which is the virus
responsible for measles.
Now, the next idea that represents a deviation
from the central dogma as it was originally conceived
is the discovery of what is called noncoding RNA or ncRNA.
Noncoding RNA is a functional RNA molecule
that skips this last step of being translated into a protein
and can directly perform functions
within the cell as an RNA molecule.
Now, two examples of functional RNA molecules
that you might be familiar with are transfer RNAs and also
ribosomal RNA, both of which are used
for the translation of messenger RNAs into proteins.
Now a final consideration that I'll
be bringing up that sort of expands on the central dogma
is the field of epigenetics, which
is the study of heritable changes in gene activity
that are not caused by changes in DNA sequence.
So unlike in simple genetics where changes in phenotype
are based on changes in genotype,
epigenetics describes the mechanism
where the same DNA sequence can be modified,
resulting in a different phenotype
without changes to the underlying DNA sequence.
Now, some examples of mechanisms that produce such changes
are DNA methylation and also histone modification.
And this can help explain why you
can have the same DNA in each of the cells of your own body,
but those cells don't necessarily
look or behave the exact same way.
For example, the DNA contained in the nucleus of, say, one
of your muscle cells is the same DNA
that's contained in one of your skin cells,
the same DNA in the nucleus of those cells,
but these two cells are different
because the expression of that DNA
is modified by these epigenetic mechanisms.
And these epigenetic mechanisms allow the transcription
of only certain genes within the genome,
depending on the type of cell.