Molecular Biology is the study of what goes on inside our cells on a very tiny (molecular) level. It can be thought of as a branch of biology (the study of life and living organisms) or biochemistry (the study of chemical processes related to organisms). Specifically, Molecular Biology is concerned with the structure and function of these core factors:
Another key aspect of Molecular Biology is to understand how processes involving these factors are controlled and what happens when something goes wrong. A classical example of this is cancer- which I’ll talk more about in an upcoming post.
So a little more detail…
DNA: deoxyribonucleic acid
Often described as the ‘code of life’, DNA is a molecule that acts as the ‘blueprint’ of information to make us who we are. Almost all of our cells have exactly the same DNA, which is stored in the nucleus or control center of the cell.
Primarily comprised of 4 different base-pairs (nucleobases), this may seem like a very simple code, but the different combinations of these 4 bases (abbreviated to A, C, G & T) are what make us unique individuals. Each cell has around 3 billion base-pairs which are collectively called our genome and the order of these bases is known as our DNA sequence. Most of this DNA sequence is the same between all humans- only around 3 million (or 1 in 1,000) base-pairs differ between people. The parts that differ are called variants.
In 1953 Francis Crick and James Watson ‘solved’ the structure of DNA. Below is a simple illustration of the the double helix and below this, a photograph of Watson and Crick with a more detailed (huge scale) model of what our DNA looks like.
RNA: ribonucleic acid
RNA is another molecule involved in making us who we are. Both DNA and RNA are nucleic acids but with some differences, such as the fact that while DNA is present in 2 strands (ie. double helix) RNA is single stranded. There are also different types of RNA, but the one often referred to as RNA is messenger RNA (mRNA). As the name suggests, it carries the ‘blueprint’ message from DNA onwards. RNA also has 4 bases, but Thymine is replaced with Uracil. The process of generating RNA from DNA is known as transcription.
Amino Acids & Proteins
The messenger RNA is then used as a template to make proteins by combining different amino acids in certain sequences. Three RNA bases form a codon, with each codon matching an amino acid. Since there are 4 bases with 3 possible positions in the codon, there are 64 possible codons. As there are only 20 amino acids, this means there is redundancy in the genetic code, with most amino acids coded by several different codons.
Here’s a link to a quick animation that shows this process, known as translation. Different proteins are then used to make cells which are combined to make different tissues like muscle and skin.
Of our large 3 billion base-pair genome, only 2% of the DNA sequence is actually comprised of genes that go on to make proteins. Humans have around 20,000 of these genes and they are referred to as ‘coding-regions’ because they contain the ‘code’ to make proteins. The other 98% is referred to as ‘non-coding’ DNA and was previously thought to be ‘junk DNA‘. We now know that some of this ‘junk’ is actually very important in controlling how genes are used to make proteins.
This is the process by which information in the ‘coding’ part of your DNA is transcribed to RNA and then translated to proteins. This process is directed or regulated by many factors, two of the most important being Transcription Factors and Epigenetic Mechanisms (more about these in my next post).
Importantly, this flow of genetic information only goes one way in humans:
DNA ⇒ RNA ⇒ Protein
In 1958 Francis Crick termed this one-way flow the central dogma of molecular biology. If you’re keen on molecular biology history then here’s a link to his paper On Protein Synthesis.
Our cells often divide to make new cells (old cells wear out and die), when this is about to happen the DNA forms into 23 pairs of chromosomes, like in the familiar image below.
However most of the time (when a cell is not dividing or preparing to divide) our DNA is not in the form of chromosomes, instead it is more loosely ‘packaged’ in the nucleus. I’ll talk a lot more about this packaging in my post on epigenetics.
I won’t go into the details of DNA replication here, but I will provide a tiny insight into how this happens. As I mentioned before, DNA is present in our cells as a double helix, which means there are two strands of the DNA sequence that are connected. These strands are complementary, with an A on one side always connected (bound) to a T on the other side and the same with C and G. In the last lines of their (amazingly succinct) 1953 paper, Watson and Crick briefly describe how this complementary pairing of the two DNA strands provides a simple yet very effective way of coping DNA when a cell divides: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”
This is just one of the many ways that science blows my mind– by helping me understand the amazingly elegant simplicity of nature. I hope this has been a helpful overview of what molecular biology entails. Please let me know if something is unclear or if you’d like more details on a particular aspect and I can point you in the direction of some relevant material! My next post will describe epigenetics- one of the mechanisms that controls how genes are expressed.