THE GENETIC CODE

                            THE GENETIC CODE

 This is the genetic code that translates

a set of three RNA bases into amino

acids and it forms the third part of the

central dogma which is translation of

RNA to protein the way that the genetic

code is set up is that each codon which

is a unit of three bases represents some

amino acid sequence and it needs to be

set up in this way because you need to

have three in order to cover the diverse

group of amino acids that we have access

to four to the third equals 64 we have

four different RNA bases and we have

three different opportunities within one

codon because each codon has three

separate bases within it that equals 64

which gives us enough diversity to cover

all of the 20 amino acids that we need

to cover if it were only two part codons

then we would end up with four to the

second which is 16 and that would not

allow us to encode for the 20 amino

acids that we have here and so the way

that you read this table and I I'll tell

have to be responsible for this or be

able to draw it the way that you read

these is by looking at the first

position so this column represents the

first position of the three part codon

and you read these codons from the five

prime end to the three prime end you

have the first part of it here the

second one makes the grid that we have

here so it for example this would be C u

this would be C C and then the third

position is represented on this right

side and so that's how you read this and

you interpret a chart like this that if

you encounter it on the MCAT it will be

provided to you and the what you'll

notice is that as you're looking through

this you'll start to see that a lot of

different

codons a lot of different three base

combinations code for the same amino

acid and it's partly because we have 64

possible combinations but only 20 amino

acids and this helps us arrive at a

vocabulary term you might hear and that

is that the genetic code is 

degenerative so for example female

phenylalanine can be coded with you you

you or you you see leucine we have six

different ways that you can encode for

that and it could be you you a see you G

and so on and the degenerative code

means that there are several different

three member codons that can code for

the same amino acid another vocabulary

term you might hear with this genetic

code is that it's unambiguous and what

that means is that if you know a three

base codon that will only encode one

amino acid there's no variation there

there's no ambiguity hence it's

unambiguous if you have let's just say a

C G then we have a C and G down here

that is threonine and nothing else

whenever you have a CG that will encode

the amino acid threonine and so this

genetic code is something that has been

conserved throughout much of evolution

and nearly all species have this

universal genetic code that they share

the nice thing is that for the MCAT you

will not be responsible for memorizing

all of the different codons and what

they represent you'll have to be able to

work with codons and what happens when

there are errors in replication or when

a mutation occurs and you'll have to be

able to recognize a few very important

codons one is a u G that codes for

methionine which is the start codon that

is always the starting

point where the ribosome begins to

translate your mRNA into a protein it

will always start with methionine and

that will be a ug the other ones that

you need to know are the three codons

that signal stop to stop codons and

those stop codons are going to be you a

a u a G and UGA there's a mnemonic for

this and the mnemonic that people like

to use is you are annoying you go away

you are gone so you a a UGA and UAG you

and that's a good way to recognize these

stop codons when they come up so as long

as you know that AUG start codon and

that it represents methionine and that

you have the three different stop codons

you a a UGA and UAG then you're good to

go as far as specific codons and what

they represent remember that it's

degenerative that multiple different

codons can encode the same amino acid

but remember also that it's unambiguous

there is no ambiguity when you know the three bases in your codon then you are

able to pinpoint exactly which amino

acid it will encode and that reduces

some of the guesswork and greatly it

increases the adherence of the ribosome

to the desired instructions of your DNA

and so this is essentially the genetic

code and once again you're not

responsible for knowing what all of

these codons represent only know the

start codon and the three stop codons

and then differ you chart like this and

features like the degenerate ivities of

it and the unambiguous nature of this

and this is something that applies

throughout millions or billions of years

of different organisms and it's

something that is at the heart of life

and DNA can produce proteins that are

essential to eukaryotic cell function.