To biology students everywhere, the principles of the central dogma of molecular biology are only too familiar. Today I’m going to take you through a basic overview of these principles. I’m going to explain what the central dogma is and each of the steps that it describes.
Ready? Here we go.
What is it?
The term ‘Central Dogma’ was coined in 1958 by Dr Francis Crick, credited as one of the co-discoverers of the structure of DNA (along with Dr Rosalind Franklin). If you were to look up the central dogma in a textbook or online, you would be told that it describes ‘the flow of genetic information.’
Well, cool. But what does that mean?
As I’ve said in our sidebar ‘On Mutation,’ the instructions for life are contained in a long polymer called DNA. In mammals, this polymer is linear. DNA is a great instruction manual- it’s descriptive, compact and can be easily copied and passed down to later generations. However, just like you can’t eat your grandma’s cookbook, you can’t run life on DNA alone. For the information in the DNA to be turned into cells and muscle and life, it has to be transcribed, then translated. I mean that in both a biological and a literal way.
At the most basic level, the central dogma looks like this:
DNA –transcribed–> RNA –translated–> Protein
The Pieces:
DNA:
DNA stands for deoxyribonucleic acid. Earlier, I said DNA is a long polymer. I also said that in mammals, DNA is linear. Linear just means that it is a line- it has a beginning and an end. A polymer is a type of chemical compound that’s made up of molecules joined together in a long and repeating pattern. You can think of a metal chain as a polymer made up of single links. By that same logic, DNA is a polymer made up of molecules called nucleotides. A nucleotide has four components:
- A sugar (deoxyribose)
- A phosphate
- A base
The sugar (deoxyribose) links to the phosphate and the base links to the sugar. All three of these together make up a nucleotide. The nucleotide links up with a neighbouring nucleotide and so on until we have DNA. There are four nucleotides- differentiated by the base that they carry: Adenine (A), Guanine (G), Cytosine (C) and Thymine (T).
If you zoomed in on a long strand of nucleotides, you’d see what looks like a ladder that’s been cut in half (lengthwise). A full DNA molecule consists of two strands of nucleotides bonded together, then twisted into the famous double helix shape. How do the two stands stay together? This is done through something called complementary base pairing. What that means is that each of the nucleotide bases can bind to one other nucleotide base. In other words, A can form bonds with T and C can form bonds with G.
The pattern of the nucleotides in DNA is what contains the information important to life. When DNA is copied or when it is transcribed to RNA, the pattern of the nucleotides is used to make the new genetic molecule. Again, this is thanks to the complementary base pairing. When the DNA replication or transcription machine sees an “A,” it puts a “T” into the molecule it’s making. When it sees a “C,” it puts in a “G.” This allows for DNA to be copied or transcribed very, very quickly.
Notice I keep saying ‘transcribed.’ Transcription is the act of copying the information contained in DNA to RNA. Converting DNA to RNA is like taking a photocopy of a single recipe out of your grandma’s cookbook. Now there’s only one page to transport (instead of the whole book), and when you’re done with that page you can throw it out. There’s no danger of spilling egg on the whole book, or worse, ripping out a page.
Here’s a nice overview image showing the nucleotide structure (bottom), complementary base pairing (top right) and the double helix (top left):
RNA:
RNA stands for ribonucleic acid The structure of RNA is very similar to the structure of DNA, with two key differences:
- The sugar used by RNA is ribose
- RNA uses uracil (U) instead of thymine (T), in RNA adenine binds with uracil and cytosine binds with guanine.
RNA can be single or double-stranded depending on what it’s being used for. Since we’re talking about the central dogma, let’s focus on messenger RNA (mRNA). Messenger RNA is a single-stranded RNA molecule that contains the information held in a small section of the genetic code. Like we said earlier, an RNA molecule is a photocopy of a cookbook (the genetic code). DNA is unwound at the correct spot and is copied (transcribed) into mRNA using complementary base pairing. The new mRNA molecule is then processed (which we’re not going to talk about today), and transported to a 3D printing facility called a ribosome. The ribosome then reads the instructions in the mRNA and uses those instructions to make proteins.
Proteins:
Proteins are the coolest things in the world, and I will leave it there. There are entire courses taught on the nuances of protein science and if I get started now we’re going to be here for a while.
Simply put, proteins are also polymers. They are made up of building blocks called amino acids (so if a protein is a chain, an amino acid is the link). Proteins are made by manufacturing facilities called ribosomes.
Ribosomes work by reading the bases in the mRNA and translating that nucleotide base sequence into an amino acid sequence. This step is like reading the photocopy of your grandmother’s recipe and following the recipe to put the correct ingredients into the bowl. In this analogy, the ingredients are the amino acids. If you haven’t guessed it- moving from mRNA to protein is called translation. How do ribosomes ‘know’ what amino acid to put in? They use the genetic code, which has been nicely decoded in this table:
Ribosomes read bases in sequences of three and each of those sequences correspond to a different amino acid. In our recipe analogy, you are the ribosome. The letters on the photocopy of the recipe book (RNA) are the bases. The letters F-L-O-U-R correspond to the ingredient ‘flour,’ which is (in this very loose analogy), our amino acid. We now throw flour in the bowl.
As amino acids are joined together, they begin to form a growing polypeptide chain (our batter). This polypeptide chain then begins to form complex and functional structures and multiple polypeptides may come together (the batter becomes a muffin). The result is a protein – a machine made out of amino acids. Proteins do everything from catalyzing reactions to copying DNA. Proteins can be processed, altered or broken by proteases- proteins that are designed to cut up other proteins. In our grandma’s recipe book analogy, the protein is the hot and delicious muffin you get at the end of the process.
That’s my quick overview of the central dogma. We’ve gone from grandma’s recipe book (DNA), to a photocopy of our desired recipe (RNA), to a delicious muffin (protein). In scientific terms, DNA is transcribed into RNA which is translated into proteins. The whole process is so much more complicated and intricate than this summary suggests and it’s worth looking into for anyone with an interest in biology. The central dogma of molecular biology describes how life works at a fundamental level.
And if you ask me, that’s pretty darn cool.
