the central dogma of molecular biology tells us that the flow of genetic information in all living cells including our own human cells is from DNA to RNA to proteins now what we mean by that is if our cell wants to actually synthesize proteins it must first take that DNA molecule and copy the gene the genetic information found in the gene into the RNA molecule and what this does is it essentially makes a copy of that genetic information via process known as transcription now it's not to difficult to actually imagine how transcription takes place because both DNA molecules and RNA molecules use the same exact language they consist of the same exact monomers we call nucleotides now when we go from RNA to proteins things are a bit more complicated and that's because RNA and proteins consist of two different languages the language used by RNA are these nucleotides but the language used by the proteins are amino acids and nucleotides and amino acids are two completely different types of molecules so the question is in the process of translation how do the ribosomes actually know to basically create that specific sequence of amino acids if the RNA consist of these different molecules we call nucleates well the ribosomes of the cell basically use a system called the genetic code so the cells of our body and the cells of other living organisms use the system called the genetic code and the genetic code is used to basically translate that sequence of nucleotides on the RNA molecule into its corresponding sequence of amino acids on that polypeptide chain now now this is basically our genetic code and we'll see exactly what it actually means in just a moment now there are four facts that you have to know about the genetic code fact number one is a three nucleotide sequence known as a codon is used to encode each amino acid fact number two is the code does not actually overlap fact number three the code is read continuously there is no punctuation in the code and number four the genetic code is degenerative so let's begin by discussing what fact number one means so the question is when we essentially go from the RNA molecule to the protein how exactly do we pair up the nucleotide sequence to the amino acid sequence is it one to one now what do we mean by one to one so does a single nucleotide correspond to a specific amino acid now would that even be possible we have four different types of nucleotides and 20 different types of amino acids and what that means is if this was the case if each nucleotide corresponded to a specific amino acid because we only have four nucleotides we would only be able to produce four amino acids and that clearly doesn't work because in our body we have 20 unique amino acids so there are 20 amino acids but only four nucleotides therefore a single nucleotide cannot use cannot be used to encode for a specific amino acid so that system does not work well if if the relationship is not to one is not one: one what about 2: one can a sequence of two nucleotides correspond to a specific amino acid so let's calculate this mathematically so remember we want to get a number that is 20 or greater now if this was nucleotide number one there are four possibilities for nucleotide number one now if this is nucleotide number two remember we have a pair a sequence of two nucleotides also we have four possibilities now the total number of possibilities is simply the uh the product of these two quantities and 4 * 4 gives us a value of 16 possibilities and we have 20 amino acids and because this is less than 20 once again that means a pair of nucleotides cannot be used to basically build a single amino acid so pairs of nucleotides cannot be used as well because that would only produce 16 unique possibilities and it turns out that cells use triplet nucleotide sequences known as codons to encode for that amino acid because if we multiply 4 * 4 * 4 in our triplet that would give us a value greater than 20 it would give us 64 now we'll see why that's important in just a moment when we discuss fact number four so for example the nucleotide sequence u u u where u u you are uril uril uril this corresponds to the specific amino acid pheny alanine while uug corresponds to the specific amino acid Lucine and we have many more examples as we'll see in just a moment now fact number two the code does not actually overlap now what does that mean well when we take the RNA sequence the RNA molecule and place it into our ribosome of the cell the ribosome reads the codons without actually overlapping those codons so what exactly do we mean by that well let's take a look at these two examples in this particular case as the ribosome reads our codons there is overlap between the codons but in this case there's no overlap and it turns out that this is how the ribosome actually reads that sequence of nucleotid so we have the sequence of nucleotides this is the five Prime n this is the three prime and and we have 12 nucleotides as shown now when the ribosome reads our nucleotide sequence it reads it via these codons so triplets so we have 1 two three and this green section is codon number one next it moves on to this sequence so first it's GCC that's the green one and then it's CCU that's the orange one the third sequence is cuu that's the purple one and notice that these three codons the green codon the orange codon and the purple codon all have overlapping sections and that's because in this case the ribosome essentially moves one nucleotide over every time it reads that arate chain now this is not how our ribosome reads it the ribosome reads it like this essentially every time it reads A codot it moves over three nucleotide sequences so we have GCC then we move over three units then we go U then we move over again a single codon so three units c g and none of these actually have the overlapping regions and that's exactly what we mean by fact number two now let's move on to fact number three the code is read continuously and what that means is it essentially goes sequentially one codon after the neck so it never skips codons it never uses codons as pauses it reads them continuously from beginning to end and fact number four the genetic code is degenerative now what do we mean by that well remember in fact number one we said that we have a sequence of 1 2 3 nucleotid so a codon and this is basically used to correspond to those amino acids now we have 4 * 4 * 4 gives us 64 possibility so there are 64 unique possibilities for our kodon and that's way more than the number of amino acids we have 20 amino acids and 64 codons and what that basically means is we have these codons that are different but can correspond to the same exact amino acid and that's exactly what we mean by the code being degenerative so codons consist of three nucleotides since there are four types of nucleotides there are four * 4 * 4 so 64 unique codons and this is more than the 20 possible amino acids which means that most amino acids AR encoded by several codons for instance if we examine Lucine Lucine contains six different codons unique codons that essentially encode for that specific Lucine amino acid and this means all these different codons so we have 1 2 3 4 five six so UA uu cuu CU u c uh cuu C and C UA all these six different codons will create the same exact amino acid namely Lucine and these different codons that encode for the same exact amino acid are known as synonyms now what's the big deal about uh this this idea that the genetic code is degenerative well what it means is the reason the code is the generative is because that actually minimizes the number of mutations that arise in our cell so what do we mean by that well let's suppose we have the following case so this is our mRNA molecule and it consists of these three codons so cgc UCC uua cug now let's suppose that some type of mutation arises in this mRNA molecule and instead of producing this a we essentially produce this G so the mutation basically takes place on this red uh nucleotide now when this RNA molecule is placed into that ribosome that ribosome will begin to read the codons one at a time so first it reads cgc now what is cgc well we can use this table to basically determine what that amino acid is so the first nucleotide is this column the second nucleotide is this row and the third nucleotide is this column as well so we we have C and that's here then we have G and that's here so that's in this box and then we have C which means we have looseing so c u c oh I'm sorry C okay the second one is C the uh the first one is C the second one is G so we're in here uh so it's c g and then we have C so this is Arginine then we have U CC so it's U C and C see that Serene then we have so initially without that mutation we were supposed to have uua so U UA gives us Lucine now there was a mutation in the third amino acid in in the third nucleotide on this codon and it went from a to G now because the code is degenerative what we see is this amino acid will still be the same amino acid that is produced used in this case so uua codes for Lucine and so will uu so u u will also code for Lucine so even though there was a mutation in that nucleotide sequence because the code is degenerative what that means is the amino acid will still be that same amino acid and the protein that produced will still be that fully functional protein so we see that the fact that that the code is degenerative means that this will minimize the effects of mutations and once again what this table describes is all the different types of amino acids that can be formed from some specific sequence of nucleotides and notice for most of these amino acids we have several different types of codons that essentially code for a specific type of amino acid and notice that some of these sequences for example u a a u a g and u g a they code for a stop signal and what that means is as the ribosome is reading our mRNA molecule when it gets to that specific stop sequence that will tell it to basically stop the process of translation and that basically means that's the end of that protein synthesis process