From DNA to proteins
DNA is my latest fascination. So I am planning to do a few posts on the basics of DNA. Through this, I want to just get the basics across correctly, so that in future, we can talk about advanced topics like Genetic diseases, genetic therapy and stem cell research. More than with other posts, this series will be more of a self-learning exercise.
I assure you that what follows is very simple to understand, with just a little focus. The DNA is made up of 4 fundamental elements represented by the characters A, C, T and G. Each of the characters A, C, T and G represent 4 different molecules, called Adenine, Cytosine, Thymine and Guanine. These belong to a group of molecules called nucleotides. You don’t need to remember the names. But assume that these are the only letters of the genetic alphabet. In comparison, the English language has 26 letters in its alphabet.
The entire DNA is a sequence of these nucleotides A, C, T and G. Thus the entire instruction set needed to build our body is made of this alphabet. The important thing about the DNA is that it exists in a double strand. This is the picture that is most common in people’s minds when you talk of DNA. These strands are complementary, in the sense that if one strand has say A G G C T as its sequence, then the other strand will have the sequence T C C G A. That is all A’s are linked to T’s and vice-versa. Similarly all C’s are linked to G’s and vice-versa. Thus, whenever there is an A in one strand, the corresponding position in the other strand will be T. If it is C in one strand, the corresponding position would have a G. Thus, given any one strand you can say what the sequence of the other strand is. A and T form a pair, and similarly C and G form a pair. Given below explains the structure of the DNA. The shape, of course, is that of the famous double helix, like a coiled ribbon.
Now, a combination of three such nucleotides is called a codon. Thus C-T-A is a codon and T-T-A is another codon.These can be considered analogous to words in a language, like how R-E-D represents something and B-A-T represents something else. Of course the English language can have words of any length, whereas the genetic words have a fixed length of three.
The way English words come together to form meaningful sentences, the genetic words (the 3 letter codons) too, come together to form a meaningful string of codons. This string of codons is called a gene. Thus a gene is nothing but a string of codons. But what does the word “meaning” mean in genetics. What is the “meaning” of a gene?
We have all been taught that proteins are the building blocks of our body. That is, they are the fundamental elements that come together to build our body. But what is a protein? A protein is a sequence of molecules called amino acids joined together. There are many different amino acids present. Different amino acids are brought together to make a protein.
We can make the connection now. We saw that the gene is nothing but a string of codons, and the protein is a string of amino acids. The link must now be obvious. Each codon (the 3 letter combination) maps to an amino acid. Thus suppose, you have a gene that has codons C1 to C10, its “meaning” is a protein that has a string of amino acids AA1 to AA10, where C1 maps to AA1, C2 maps to AA2 and so on.
Let me add an important detail to this picture so that the picture becomes slightly more accurate. In the process of DNA getting converted to protein there is an intermediate type of molecule called RNA. It is the RNA that, so to say, reads the DNA and brings the correct amino acid for the codon read, into place, so that a sequence of such amino acids can form a single protein. An important difference between DNA and RNA is that instead of A being paired up with T, in the RNA, A is paired up with U (Uracil). That is, whenever there is an A on the DNA the corresponding position on the RNA is occupied by the nucleotide Uracil, represented by the alphabet U.
In fact, the mapping of different codons to Amino Acids, can be seen in this table.
A pictorial representation of what we discussed is given below.
That is it for this time. It is good to take it slow. In the next post we will see where these genes reside in our body and how they are organised.