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Video transcript
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.