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DNA II – How does DNA replicate itself?
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!
https://www.youtube.com/watch?v=zdDkiRw1PdU
Errors
You are a replicating mistake, a lucky one. 1 error every 1000000000 nucleotides looks like nothing right?. Let us crunch some numbers:
A human cells contains approximately 1.15·10^(10) nucleotides (1). Hence, every time the DNA of a cell is replicated 11.5 mistakes are made. Now, you began as a single cell, while a human adult body has approximately 3.72 × 10^(13) (2), so something in the order of a thousand billions. * - this numbers are incredibly big, you cannot not picture in your head how a* thousand billions of something looks like. Biology is small but at the same time huge. Amazing -.
If you do the math, you only need around 40 replications to reach that number, but this is not true since there are cells that only divide once, cells that have a lifetime of days… The real number of divisions during a lifetime is close to a quadrillion (1015) according to Wikipedia. Even if the number is not correct, it is a good approximation. Therefore, the total number of replicating mistakes in a lifetime is about 11.5 quadrillions. Good, we should all be dead right? Wrong again.
An error or change in the DNA is known as a mutation. Most of the mutations fall in parts of the DNA that apparently do not encode for anything, thus having no effect. Others will cause changes in proteins that do not affect the function of the protein. From the rest, the majority will kill you, and the ones left will change the color of your eyes, the length of your arms, the shape of your skull, the levels of oxygen that your cells can take in, the facility with which you learn new languages…
Conclusions
The DNA is replicated using proteins that open the double chain helix and create a second copy, hence each of the cells resulting from the division keep a copy. During the replication errors, known as mutations are introduced. This mutations are the basis of EVOLUTION.
Hope you liked the article. Do you have questions? Comment them and see them answered! Just thought about something you would like to know? Ask me. Next article GMOs (lets start a fight!). Follow me to find out more about Biotechnology
Bibliography
1 - Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (1997). Molecular Biology of the Cell (Garland Science, New York, 2002).
2 - Bianconi, E., Piovesan, A., Facchin, F., Beraudi, A., Casadei, R., Frabetti, F. & Perez-Amodio, S. (2013). An estimation of the number of cells in the human body. Annals of human biology, 40(6), 463-471
Replies (14)
Great series, very well explained! As for the previous episode! (do you mind moving to the thermodynamics part? :D)
Very clear post, well documented and references are given. In short, this consists of an example on how any scientific post should be written.
As a bonus, and in addition to resteeming for exposure, we are awarding you a small 10 Steem Power deposit as a thank you for creating quality STEM related postings on Steemit. We hope you will continue to educate us all!
I would be more inclined to say that it matches them up like a puzzle piece. Then allows them to be glued in place
Polymerases make more than one error in 109 incorporations. High fidelity enzymes are in the 105-6 range, and repair polymerases (lacking proofreading nuclease domains ) are lower then that closer to 1 error in 10,000 incorporations.
The 109 statistic is more encompassing of replication as a whole, but that includes a variety of other repair mechanisms to catch errors.
Many uncaught deleterious mutations will induce apoptosis (cells are programmed to self destruct when shit goes haywire), however some do lead to cancer.
Nice job overall :)
That is definetely a more accurate and beautiful way to say it. I was trying to add a pinch of humour with that sentence though. But yes, I would also agree that your sentence is more correct. Do you study or work on something related to biology?
I am a research scientist at a small pharmaceutical. I have more than a few publications on DNA replication enzymes/complexes, from grad school and post doc.
Enjoyed your post and presentation, hope more will come.
Just one thing that I wanted to call your attention to. When you say "chosen by evolution", in my understanding it implies that evolution is a conscious process that sat down and made a decision. Yes, biologists use shorter formulations like that all the time to save time and space. "The less effective processes didn't continue and the more effective ones continued" seems much more cumbersome. But I suggest we use these cumbersome but more accurate expressions because otherwise we are talking just like there was intelligent design. But if evolution happened, then there was no intelligent design. So why not remove such phrasings from our language?
awesome! thanks for your comments!
completely agree. I used that phrasing because, as you say, it is easier to understand at a basic level. I did not want to start using more cumbersome expressions yet without having written an article about evolution and natural selection to which people could refer to understand better the process. The more articles I write the more accurate I will be since people will be able to refer to previous articles to understand the complexities. In short, my goal is to do what you say but previously building the basics so they do not get lost.
DNA's are indeed something amazing, its found that only a few grams of DNA can hold data equivalent to the whole digital data's stored in the world. I have made a post about it , you can check it out here
just read it! awesome post
Thanks buddy
Yo, @mguell love ur work. Do something on CRISPR-Cas9
great suggestion! I will work on it!
Great post, I thought it was really well explained. I would appreciate any feedback on my posts, especially this one on DNA https://steemit.com/science/@ovij/what-is-dna-why-do-we-need-it