Compared to other sciences, molecular biology is relatively young. Its “birth” can be associated with the research of Johann Gregor Mendel, who published a study on the inheritance in peas in 1865. The discovery of DNA – called nuclein in 1869 by Friedrich Miescher – was a turning point for the further development of molecular biology; and it marked the beginning of gene research at the chromosomal level (1). Scientists had intense debates about the true carrier for hereditary information in the chromosomes. The answer divided the scientific community. Is it DNA or proteins? Those who put DNA on a pedestal were on the right track. Thanks to the experiments of Oswald Avery‘s and Frederick Griffith research teams (2), it was shown that DNA is the carrier of genetic information (1,3)! But as happens in science, one answer led to countless other questions: What is the structure of DNA? How is the information encoded? How is the information in DNA used to make proteins?

Knowing the detailed structure of DNA has been the key to answer all these questions. The desire to know the mystery of DNA structure was awakened in young 23-year-old zoologist James Watson. He attended the lecture at the symposium where he first saw the XRD (X-ray diffraction) images of the DNA molecule obtained by Maurice Wilkins and Rosalinda Franklin (4). James’ meeting with Francis Crick was also fateful. Later, they worked together to build (albeit unsuccessfully for the first time) a model of the structure of DNA on the basis of XRD images. In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize (4). 

Interesting fact: Francis Crick hadn’t started working in biology until he was 31. Moreover, he got his Ph.D. 7 years later, in 1954 (5). So, keep calm. There is still enough time for either getting Ph.D. or doing whatever you like. 

The Watson-Crick model of DNA says that the DNA molecule is made up of 2 chains which together form a double helix. The strains consist of only three main components: a sugar component – ribose, a phosphate group, and a base. There are only 4 bases in DNA, namely adenine (A), thymine (T), cytosine (C), and guanine (G), from which A pairs with T and C pairs with G. Both chains are complementary, this means that if we know the sequence of consecutive bases of one strand, we can complement the sequence of the second one (3). 

The other questions are collectively answered by the central dogma of molecular biology. This concept was first used (1958) by Francis Crick (6,7). In short, the central dogma states that once the information is converted into the protein, there is no way back (6). 

So, what is the “flow” of biological information according to the central dogma? Let’s look at the answer from the kitchen. 

I like to compare the central dogma of molecular biology to the process of cooking. Let’s think about DNA as a very important cookbook full of recipes (genes). The cell protects the DNA in the chromosomes from unwanted damage by storing it in the nucleus (library). You don’t want to “get the cookbook dirty”, so you will not bring it to the kitchen (out of the nucleus). After all it is an inventory of recipes (genes) passed down from generation to generation. So, only “verified” molecules and proteins can enter or exit the nucleus. Recipes (genes) are therefore copied (transcribed) from the cookbook onto a “piece of paper” (mRNA) in the process called transcription. The mRNA molecule can exit the nucleus to the cytoplasm afterwards, where ribosomes (or in my case, chefs) are already waiting. In the process called translation, the ribosomes read what is written in the recipe and “cook” the protein. Molecules of mRNA are usually degraded relatively quickly (1,3,7) after translation. The rate of degradation of mRNA depends on its structure (3), but more on that sometime in the future. 

Interesting fact: Of course, it is also possible to “get permission to enter the nucleus” in an ‘illegal way’, which the flu virus or HIV can do very well. 

David Baltimore and Howard Martin Temin turned the existing central dogma in 1975, because they independently discovered RNA viruses capable of integrating their genetic information into the host cell’s DNA. During the life cycle of a virus, the viral RNA is transcribed into DNA by an enzyme called reverse transcriptase and then incorporated into the host genome by an enzyme called integrase. With the discovery of reverse transcriptase, the original structure of the central dogma was changed because of the “backward” information flow (from RNA to DNA). The central dogma still includes the inability to transfer information from proteins back to RNA or DNA (3,6,8).

In short: The central dogma gives us the direction of the flow of information from DNA to proteins. Information from DNA is constantly being transcribed into mRNA and then converted into protein by ribosomes in the process called translation.  The central dogma has been extended to include reverse transcription, the process in which information can be transcribed from RNA to DNA.

Thanks for reading this far!

Have a nice day!

Lucka

Sources:

1. Weaver, R. McGraw-Hill, 2012, 915 p.

2. Avery, O.T. et al. (1944). J Exp Med, 79, 137–158

3. Alberts, B. et al. W. W. Norton & Company, New York, NY London, 2022, 1552 p.

4. Watson, J. NobelPrize.org, https://www.nobelprize.org/prizes/medicine/1962/watson/biographical/

5. Crick, F. NobelPrize.org, https://www.nobelprize.org/prizes/medicine/1962/crick/biographical/

6. Ostrander, E. (2022). Genome.gov, https://www.genome.gov/genetics-glossary/Central-Dogma

7. Crick, F. (1970). Nature, 227, 561–563

8. The Nobel Prize 1975; NobelPrize.org, https://www.nobelprize.org/prizes/medicine/1975/summary/

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