Molecular Genetics
Quick links to sub-topics on this page:
The discovery of the structure of DNA
The discovery of the structure of DNA involved many scientists, but ultimately only 3 were awarded the Nobel prize in 1962 in physiology or medicine.
James Watson (b. 1928), Francis Crick (1916–2004), and Maurice Wilkins (1916–2004)
Watson and Crick's paper on the double helix structure of DNA was published in the journal Nature on 25 April 1953.
James Watson
b. 1928
Francis Crick
1916 - 2004
Maurice Wilkins
1916 - 2004
The controversy
Rosalind Franklin (1920 - 1958) worked in the same laboratory belonging to Maurice Wilkins.
Her research in X-ray crystallography together with her Ph.D student (named Raymond Gosling) resulted in the famous "Photo 51". This was a key piece of information that allowed Watson & Crick to deduce the structure of DNA.
Rosalind Franklin died at the age of 37 from ovarian cancer, four years before the Nobel Prize was awarded to the three men (Watson, Crick & Wilkins) .
Photo 51
This famous photo is an x-ray diffraction image of a DNA molecule. It is the final clue that allowed Watson & Crick to deduce that DNA was actually a double helix.
Their competitor Linus Pauling (who previously won unshared Nobel Prizes in Chemistry and Peace) proposed a triple helix model, which was incorrect. Thus losing the race in elucidating the structure of DNA.
Forgotten hero of DNA
Although Rosalind Franklin is now widely credited for Photo 51, the forgotten hero of the story is Raymond Gosling.
He was the PhD student working in Rosalind Franklin's laboratory who was responsible for taking the actual Photo 51, but unfortunately is not often given credit for doing so.
Read more about him and the timeline of the discovery of the structure of DNA here.
Central Dogma of Molecular Biology
The term central dogma was coined by Francis Crick, one of the scientists who was awarded the Nobel Prize for the discovery of the structure of DNA. The central dogma of molecular biology explains how genetic information encoded in the base sequence of a DNA molecule can be transcribed into mRNA, and subsequently translated into a protein product.
The DNA molecule consists of two polynucleotide strands, held together by hydrogen bonds between the base pairs.
Of the two strands, one is designated as the template strand; this strand is transcribed during transcription. The mRNA formed has a base sequence that is complementary to the template strand.
The other DNA strand that is not transcribed is known as the coding strand. The base sequence on the coding strand is identical to the sequence on the mRNA transcribed. With one major difference where all the T has been replaced by U in the mRNA.
Some clarifications
Since I have been receiving a number of emails with regards to the concept of the central dogma, I thought I would use my page to address some of these questions.
Question: Is the central dogma the same thing as protein synthesis?
Answer: The central dogma seeks to explain how the genetic information (in the form of the base sequence or the order of bases in a DNA molecule) is able to guide protein synthesis. This involves a two step process: transcription and translation. The DNA sequence from the template strand is "copied" during the process of transcription to produce a mRNA strand, whose base sequence are complementary to the original DNA strand it copied.
The base sequence on the mRNA strand then guides the ribosome to synthesise the appropriate protein during the process of translation.
The term protein synthesis is quite general; it can refer to the process of translation, but it can also delve deeper into the actually mechanics of protein synthesis that occurs inside a cell. Details of how RER, Golgi bodies, transport vesicles function together to make protein molecules inside a cell are the finer details of protein synthesis.
The mRNA Codon Table
A codon (or triplet code) refers to a specific sequence three nucleotides found on either a DNA molecule, or more commonly the mRNA molecule. Each codon will specify for a specific amino acid that will be added to the growing polypeptide (or protein) chain that is being made by the ribosome during translation.
Ribosomes
Ribosomes "reading" the codon on the mRNA, at the same time tRNA molecules will bring in the amino acid specified by the codon on the mRNA which will be added to the growing polypeptide chain.
If you have not realised by now, polypeptides are large molecules that are made up of amino acids (subunits). Amino acids are joined together by covalent bonds known as peptide bonds, to form the large polypeptide molecule.
Question: If a protein molecule consists of 100 amino acids, what is the minimum number of bases found in that gene?
Answer: Since every amino acid requires a triplet code, the minimum number of bases is 3 x 100 = 300 bases. This is in theory.
Controlling gene expression
In reality, every gene has a promoter region that is a stretch of 100 to 1000 base pairs long upstream to the base sequences that are transcribed.
The promoter region itself is known as "non-coding", meaning the base sequence here does not code for any amino acids.
Instead, the promoter region plays an important role in gene expression, controlling the binding of RNA polymerase to the DNA molecule. Thus, controlling the rate of transcription, and eventually the rate of translation.
Golden Rice is approved!
Golden Rice is one of the classic examples of genetic engineering of crops. Genes that code for enzymes that enable the synthesise of beta-carotene are transferred into the rice genome with the aid of biotechnology. The original source of these genes come from daffodil and a microorganism in soil.
This multi-gene biochemical pathway that exists in Golden Rice allows beta-carotene to be synthesised, where when consumed will be converted into vitamin A inside the human body.
Multi-gene biochemical pathway
Enzymes (font in red) are protein molecules. This means that there must be an appropriate gene that codes for that specific enzyme.
The gene (for the enzyme) phytoene synthase originates from the daffodil plant.
While the gene (for the enzyme) crt1 originates from the soil bacterium Erwinia uredovora.
These genes do not occur naturally in the rice genome, and have been transferred into Golden Rice using biotechnology.
If successfully transferred and the enzymes are expressed, the rice endosperm will be able to synthesise beta-carotene. This results in the orange appearance of Golden Rice.
Growing number of countries that approve Golden Rice
Click on the images which are links that will bring you to a different page with a news article containing details on the approval of Golden Rice (finally!) in the various countries around the world.
I am excitedly waiting for Bangladesh to be added to the list, however there seems to be some delays stated here and here.
Philippines (2019)
Vitamin A deficiency is prevalent among much of the population in the Philippines.
I am very glad that Golden Rice has been approved, and I hope that cultivation and consumption by the masses will happen in the near future.
Australia & New Zealand (2017)
FSANZ (Food Standards Australia New Zealand) has approved the import and sale of Golden Rice, as long has it is labelled as "genetically modified" on the package.