Textbook Notes (270,000)
CA (160,000)
McMaster (10,000)
BIOLOGY (700)
Chapter 2

BIOLOGY 1A03 Chapter Notes - Chapter 2: Rna Tie Club, Marshall Warren Nirenberg, George Gamow


Department
Biology
Course Code
BIOLOGY 1A03
Professor
Dr.Da Silva
Chapter
2

This preview shows pages 1-2. to view the full 6 pages of the document.
Theme 2: From Gene To Protein
Module 2: The Genetic Code
1) The Genetic Code:-
Deciphering the Code in the mRNA:
Transcription is an extremely valuable way of creating multiple copies of the gene and it
can be further regulated by RNA stability or compartmentalization
Proteins are made from 20 different amino acids, but there are only four nucleotides in
mRNA: adenine (A), cytosine (C), guanine (G), and uracil (U)
But in order to make proteins, first the transcribed nucleotide code and translated it into
the language of proteins
2) Defining a Codon:-
Does one nucleotide make a Codon?
After the discovery of the structure of DNA by James Watson, Francis Crick and Rosalind
Franklin, researcher became eager to understand how to read the hidden messages
contained within DNA molecules
Researchers understood that to be able to crack the genetic code, it is important to
determine how many specific nucleotide bases code for a single amino acid
A physicist, named George Gamow suggested the number of nucleotides that would be
necessary to code for one amino acid
In 1954, he founded the RNA Tie Club
This club consisted of 20 members (one for each amino acid), and four honourary
members (one for each nucleotide in nucleic acids)
Gamow first reasoned that if one nucleotide coded for one amino acid, then the
DNA could only code for four distinct amino acid residues which is much less than
the required 20 amino acids that the cells utilize
So he determined that a 1-base code would not be enough to code for all 20 amino
acids
Do two Nucleotides code for an Amino acid?
Gamow rationalized that a 2-base code would not be sufficient to code for the 20 amino
acids of the cell
This would give us 4 × 4 = 16 combinations or 16 distinct doublets which is still not
enough since all of the possible doublets would only code for 16 amino acids
Do three nucleotides code for an Amino Acid?
find more resources at oneclass.com
find more resources at oneclass.com

Only pages 1-2 are available for preview. Some parts have been intentionally blurred.

If three nucleotides (a triplet) were to make up the code for each amino acid then there
would be 4 x 4 x4 = 64 distinct combinations of three nucleotides
Since only 20 distinct codes are needed, it is the 3-base code that is the smallest group of
nucleotides that will accommodate the need to code for 20 amino acids
This means that multiple unique triplets would code for the same amino acid, a
redundancy in the code
More nucleotides could also work (the code might be comprised of groups of 4 or 5
nucleotides) but this is unnecessary and would increase the redundancy in the code
3) Deciphering the Code:-
A simple mRNA:
To decipher how the mRNA code can be translated into functional proteins, researchers
needed to take simple mRNA sequences and see what they coded for
At the same time that the RNA Tie Club was formed, two non-members (Marshall
Nirenberg and Johann Matthaei) used a cell-free system to decipher the first letter of
the code in 1961
Nirenberg and Matthaei placed all the components that they believed were necessary for
protein synthesis (RNA template, nucleotides, ribosomes, amino acids and an energy
source)
In these experiments, they were able to determine what specific amino acids a particular
RNA nucleotide template could give rise to
They started by making a very simple nucleic acid made up of a string of uracils and
they discovered that this simple nucleic acid produced a repeated simple polypeptide
sequence that contained identical amino acids (repeated sequences of phenylalanine)
But the question remained; how many uracils coded for a single phenylalanine? Was it
one, two, more?
A simple mRNA:
Follow-up research used a nucleic acid with alternating uracils and cytosines
It was found that this strand of nucleotides was always translated into a polypeptide string
of alternating serine and leucine amino acids
These results confirmed that three nucleotides make up what is referred to as a codon,
that will code for a specific amino acid in a protein
More simple mRNAs were constructed and in each case, a novel sequence of 3
nucleotides seemed to code for a distinct amino acid
These simple mRNAs were used to determine the specific amino acid encoded in each
triplet codon (example: UCA codes for serine, CAU codes for histidine, AUC codes for
isoleucine)
4) The Standard Code:-
find more resources at oneclass.com
find more resources at oneclass.com
You're Reading a Preview

Unlock to view full version