In my previous post DNA I I explained the very basics of DNA. Here I will try to explain one of the most complex processes in life, the replication of DNA. Welcome to the unveiling of one of the greatest mysteries in biology!

DNA has two special properties, it is significantly stable and easy to replicate. As a result, it was chosen by evolution (a topic that will be explained in coming articles) as the biologic information carrier. But first of all, why does DNA have to be replicated?

The cell cycle

Most cells follow a simple cycle, they divide, grow, perform their function (or just live) until they can divide again (Figure 1). Replicating the DNA is the way cells have to give their information to the future generations, to perpetuate their lineage if you will. The relevance of this resides in the concept of evolution which, as I said, will be explained in a future article. Right now, what concerns you is that when cells divide, a process known as mitosis in mammalian cells, and the two resulting cells need to be able to survive. By replicating the DNA, each cell receives a copy of the manual containing all the information required to survive. More or less like a biology survival guide written by Macgyver.



Figure 1. The cell cycle. Source: http://schoolbag.info/biology/living/57.html .

The replication process

The whole replication process is based on the affinity properties of the nitrogenous bases of the DNA. -Wow, science gibberish! If you are lost or you find an unknown technical word along this post, check my previous post, which contains a brief explanation of the concepts mentioned in this post.- Hence, you only need to put an adenine (A) close to a T (thymine) and they will bind together by themselves (magic? No, thermodynamics but we are not going down that rabbit hole…yet).

- Oh that is easy! – Not so fast lad. DNA is very sensitive in a single chain form that is why is always found in a double chain form, because it is a lot more stable (Figure 2). Hence, every single nucleotide is already paired and stable. Even if a matching nucleotide gets close, an already bound couple will not separate and exchange couples. So how did biology solve this problem?



Figure 2. DNA in a double helix form. Source: http://www.sciencebuddies.org/blog/2013/04/celebrating-dna-and-the-history-of-the-double-helix.php .

The replicating machinery

Proteins. The answer is almost always proteins. Remember, proteins are the robots that fulfil DNA orders. Let me introduce you then one of the most, if not the most famous of such robots, the DNA polymerase. This protein acts together with a whole bunch of other proteins to actually open the DNA separating the two chains. Once the chains are open, the DNA polymerase gets in there and starts replicating the DNA. Since now the nucleotides are not couples they are susceptible to be bound to nearby matching nucleotides. Actually, it is more accurate to say that the DNA polymerase smashes them together.

The DNA polymerase not only creates a new chain by adding together one nucleotide after the other one but also checks if it is doing a good job between adding a new nucleotide (Figure 3). A-M-A-Z-I-N-G. It can add close to a thousand nucleotides per second, only making one mistake every 10^(9) nucleotides (1). So almost no mistakes right? WRONG. Question everything!

Take a look at this cool video of how it works!