How many different ‘one nucleotide’ codes do we have if there are only four different bases?
A U G C THERE ARE ONLY 4, one ribonucleotide codes.
How many different ‘two ribonucleotide’ codes can we come up with if there are only four
AA, AU, AG, AC, UA, UU, UG, UC, GA, GU, GG, GC, CA, CU, CG, CC = 16
How many different three nucleotide codes can we come up with?
There are 64 different three ribonucleotide combinations.
If the sequence of ribonucletides in a mRNA encodes the information to determine the order of
the amino acids in a polypeptide, what is the simplest number of ribonucleotides that could
determine the code?
a) one ribonucleotide
b) two ribonucleotides in a row
c) three ribonucleotides in a row. ********** correct answer
d) Four ribonucleotides in a row.
THE GENETIC CODE
What is the genetic code?
It is the order of the bases along the mature messenger RNA. And very importantly the unit
of code is a TRIPLET; that is the sequence of three consecutive bases in a mRNA, constitutes one
unit of code.
The sequence of three consecutive bases in a mRNA molecule, coding for a specific amino
acid, is called a codon.
Through careful experiments, done by earlier scientists we now know exactly what three bases
encode each of the different amino acids.
To examine the genetic code let's look at figure 12.6, page 264. As you can see from a careful
look at the table, there are 64 different possible triplet combinations of the four bases.
Since there are 64 different codons and only 20 different amino acids,
This means there is some redundancy in the code . In other words most amino acids are
encoded by more than just one codon. This is also sometimes the degeneracy in the genetic
code. The term degeneracy here means that more than one different codon is used to specify
the same amino acid.
So from Table 12.6 we see what the genetic code is. Please notice that most triplets encode an
amino acid, but three triplets encode encode stop translation. One important codon does double
duty. It encodes ‘start’ translation and the aminoacid methionine.
So there are 64-3= 61 codons that encode an amino acid.