- [Instructor] The genome of an organism
is inscribed in its DNA,
or in the case of some viruses, RNA.
Now the portion of the genome that codes for a protein
or an RNA is called a gene.
And those genes that code for proteins
are composed of trinucleotide units called codons.
Each coding for a single amino acid.
So in other words, each set of three bases is a codon.
So let's take one gene in the genome.
So I'll draw a little chromosome here, here's the gene.
This gene gets transcribed
into the corresponding RNA polymer.
And in prokaryotes, this RNA can function
as the mRNA or messenger RNA.
But in eukaryotes, the transcribe needs to be processed
first before it processes a mature mRNA.
This mRNA then gets translated by a ribosome
into a chain of amino acids,
otherwise known as a polypeptide.
In this mRNA, each set of three nucleotides is a codon
and corresponds to a particular amino acid.
So the question is, how exactly do you get from a chain of
nucleotides in RNA to a chain of amino acids in a protein?
Well, the answer to that question is the genetic code.
So, here's the genetic code shown in table form.
As you can see, the table shows
which amino acid each of the 64 codons.
For example, let's take this codon here, CCG.
You can see that it corresponds to proline.
And if we take a look at another codon here, say CGA,
you can see that it corresponds to arginine.
The genetic code is the set of rules
by which information coded within genetic material
is translated into proteins by living cells.
Biological decoding is accomplished by the ribosome,
which links amino acids in an order specified by mRNA
using transfer mRNA molecules that read
the mRNA three nucleotides at a time,
using their anticodon loop.
And then carry the corresponding amino acid
to be added to this grown polypeptide chain.
The genetic code is highly similar among all organisms.
And because the vast majority of genes are encoded
with exactly the same code.
This particular code is often referred to
as the standard genetic code,
although there have been some variant codes proposed.
We have four different bases
and possible combinations of three bases at a time.
There are four to the third, or 64,
different codon combinations possible,
with a triplet codon of three nucleotide.
So all 64 codons are assigned to either an amino acid
or a stop signal.
If for example, an mRNA sequence,
say we'll just make one up here,
and we take this mRNA sequence, and the reading frame
starts with the first U,
because it's by convention read 5 prime to 3 prime.
There are three codons, namely UCA, GCA,
CGG, and CAG.
Each of which specifies one amino acid,
therefore this 12 base RNA sequence will be translated
into an amino sequence that is four amino acids long.
Why don't you go ahead and take a second
and see if you can decode this particular strand of RNA?
If you came up with serine, alanine, arginine,
and glutamine, then you'd be correct.
Now, a codon is defined by the initial nucleotide
from which translation starts.
For example, let's take our mRNA sequence.
If read from the first position, it contains the codons
UCA, GCA, CGG, and CAG.
If read from the second position, however,
you get a totally different combination of codons.
You get CAG, CAC, and GGC.
And if read from the third position,
you get yet a different set of codons.
So every sequence can thus be read
in three different reading frames,
each of which produces a different amino acid sequence.
And with double stranded DNA,
there are six possible reading frames.
Three in the forward orientation, on one strand,
and three reverse on the opposite strand.
So to avoid any confusion, the actual frame
in which a protein sequence is translated,
is defined by a start codon,
which is usually the AUG codon in the mRNA sequence,
which you can see here codes for methionine.
Translation starts with a chain initiation codon
or start codon, typically AUG which codes for methionine.
There are alternative start codons,
depending on the organism.
Now, unlike stop codons, the start codon alone
is not sufficient to begin the process of translation.
Nearby sequences, such as the Shine Dalgarno sequence
and bacteria, as well as initiation factors
are also required to start translation.
Now, the three stop codons are UAA, UAG, and UGA.
Stop codons are also known as termination
or nonsense codons.
They signal the release of the nascent polypeptide
from the ribosome because there's no corresponding tRNA
that has anticodons complementary to these stop signals.
And so a release factor binds to the ribosome instead.
An important concept that comes up with regards
to genetic code is the idea
that it is redundant but not ambiguous.
And just FYI, another way to refer to redundancy
is with the word degeneracy,
so you might see that sometimes.
So what exactly does this mean?
Well, for example, although codons CUC and CUA
both specify for lucine, this is the example of redundancy,
neither of them specifies any other amino acid.
This is where the code is not ambiguous.
The practical consequence of redundancy
is that some errors in the genetic code
as only a silent mutation
or an error that would not affect the ultimate coding
of the protein because the overall hydrophobicity
or hydrophillicity is maintained
by equivalent substitutions of amino acids.
For example, a codon of, say NAN,
where N stands for any nucleotide,
tends to encode average size hydrophilic residues,
whereas NUN, tends to code for those amino acids
that are hydrophobic.
And NCN generally yields amino acid residues
that are small in size and moderate in hydrophobicity.
And these tendencies may actually result
from the shared ancestry of the aminoacyl tRNA synthetases
related to these codons.