 Now let's talk about the genetic code. This is basically the code that translates the nucleotide information into the protein or amino acid information. The three messenger RNA nucleotides that form a codon. These three nucleotides basically imply that what which amino acid will be placed in a particular protein at that region. This genetic code is complementary and anti-parallel to corresponding DNA. Genetic code is basically the sequence of nucleotides of messenger RNA. The three nucleotides at a time, three adjacent nucleotides of the messenger RNA, they basically specify a particular amino acid. That's basically what we are talking about. Genetic code relates codons to their specific amino acids in proteins. This important features of genetic code is that it is universal and redundant but not ambiguous. So what do I mean by that universal is that pretty much all organisms, they use the same genetic code. There are few minor exceptions, but pretty much the genetic code is universal. So if we take a messenger RNA from a human and put it in a bacteria, bacteria will make the same protein a human cell would because the genetic code is almost universal redundant. Redundant meaning that more than one codons specify one particular amino acid on the screen. For example, for leucine, we have six codons. All these codons specify leucine, wherever these codons appear in messenger RNA, leucine will be placed in the resulting protein. It is not ambiguous, meaning that these codons will only result in leucine. It does not mean, by ambiguous we mean sometimes a codon will specify leucine, sometimes it will specify serine, sometimes it will specify proline. That doesn't happen. It is redundant but not ambiguous. It is always specific. There's also start and stop codons. Here I have placed the whole genetic code on the screen. So you can see we have first letters here, the second letters here and the third letters of the genetic code. It's a three letter code and the combinations are here. Here you would, I would like to point out that AUG is codes for the thionine and this is also the start code. This is a codon which tells where to start making the protein from on the messenger RNA. We'll talk more about that. There are also three stop codons. These three stop codons whenever they appear in messenger RNA, they basically tell the ribosomes to stop making protein when they hit this stop codon. So how was genetic code deciphered? How did people know which three nucleotides specify which particular amino acid in a protein? There are simple experiments initially that basically people were able to synthetically made poly nucleotides of the same nucleotide. For example, poly U chain, uracil as you know is one of the nucleotides in the messenger RNA. When they made this, artificially made this messenger RNA and placed in some extract with the machinery which could translate it, convert it into protein, all the amino acids in the protein were phenylalanine. So basically meaning that three U's basically code for phenylalanine. Similarly, three A's code for lysine, three C's code for proline. So gradually people were able to come up with complete the genetic code which three nucleotide sequence or the codon specifies which amino acid. Now we will talk about the adapter because adapter is the molecule that interacts with the codons. And as we have mentioned earlier, adapter has to have three functions. It has to carry an amino acid on one part, one domain of this adapter should carry a amino acid. It should be able to associate with messenger RNA because it has to interact with the codons. It has to be able to associate with messenger RNA. It should be able to bind ribosomes because these organelles are the ones responsible for synthesizing proteins. So tRNA is the adapter. It is as the name implies, it is an RNA molecule but we put a small t here meaning that it's a transfer RNA. So we have talked about one type of RNA was messenger RNA which is carrying the message from the nucleus to the cytoplasm. This is the transfer RNA. It is about 75 to 18 nucleotides long and it performs all these functions. So nucleotides, this tRNA, all these, we have several tRNAs each carrying a specific amino acid and can interact with a specific codon. It has a sort of a little complex three-dimensional structure which we look at. That structure is due to the internal hydrogen bonding. I'll show you that. Confirmation, the three-dimensional shape or placement of atoms in three-dimensional space of this molecule allow it to bind the ribosomes. The three prime end of the tRNA, we know exactly what three prime end is. The basically implying the three prime hydroxyl of the last nucleotide of this molecule is the place where amino acids are bound. Midpoint of this molecule, tRNA, has an anti-codon. Anti-codon is basically the part of this molecule that interacts with the codon present on the messenger RNA. So you see the beauty in it. This molecule has nucleotides. These nucleotides have a sequence which can bind specific codon on the messenger RNA and on other tip it is carrying an amino acid. So this is basically the functions of the adapter or the transfer RNA. I have to again point out that the interaction of codon and anti-codons is anti-parallel. All nucleotide interactions are anti-parallel for our purposes. Here is the three-dimensional structure. Here you can see the internal three hydrogen bonding which is forming between the nucleotides. We know that A's can have hydrogen bond with the C's, A's can have hydrogen bonds with U's and G's can have hydrogen bonds with C's. So this basically results in formation of this structure, three-dimensional structure and the three-prime end is where the three-prime hydroxyl is. This is the part where amino acid will be bound and just so that you know the abbreviation we will use in later slides for tRNA is this. So we have seen the structure of tRNA and next we will talk about how this, what is the role of this RNA, tRNA in protein synthesis and first of all how it gets to carry the amino acid. We will look at that later.