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Lecture

lecture 5 for BGYA01

by OC4

Department
Biological Sciences
Course Code
BIOA01H3
Professor
Clare Hasenkampf

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Lecture 5 BGYA01 SEPT. 25, 2007
Now I want to continue our discussion of how DNA directs cellular activity.
DNA does not DIRECTLY control cellular activity, instead it acts INDIRECTLY through other
molecules.
DNA>>>>>>>>>>RNA>>>>>>>>protein. Proteins directly affect cellular activity.
transcription translation
Last class we looked at how DNA is used as a template to make RNA in a process known as
transcription.
There are 3 major types of RNA produced by the process of transcription:
transfer RNA (tRNA) - transfer RNA genes serve as the template for these
ribosomal RNA (rRNA) ribosomal RNA genes serves as the template for these
and messenger RNA (mRNA) genes encoding proteins serve as the temple for these.
All three types of RNA are needed to make protein, but only mRNA encode the information
for the sequence of the amino acids.
Now that we have looked at transcription it is time to turn begin to think about how mRNA
encodes the information to make protein.
TRANSLATION
The process of making protein is called translation.
We already know that proteins are polymers, and that the monomer units are the amino acids.
We already know that there are twenty different amino acids, each different amino acid with its
on unique R group, (as illustrated in Table 3.2, page 43).
Every different protein has its own shape, size and biological function.
The size, shape, and function of each protein is determined by the amino acids that make it up.
The precise order of the amino acids in the protein determines the proteins shape and
function.
How are the correct amino acids brought together for each specific protein?
We will discovertoday that it is the order of the bases of mRNA that encodes the information
for the order of amino acids of the protein. We have to see how the RNA codes for protein.
Lets think a little about math and code.
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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
different ribonucleotides?
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.
Clicker Question
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.
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TRANSLATING THE CODE
Now we know that 3 consecutive bases in the mRNA is a codon, and that the codons along the
mRNA molecules encodes the order of amino acids along a protein, but how is this
accomplished?
How is the information in codons (a nucleic acid) converted, translatedinto the language of
proteins (amino acids)?
In order to translate the code we need a complete set of tRNA molecules.
Earlier I said tRNAs are needed to make protein. To make any protein a complete set of tRNAs
are needed. Why?
It is because tRNAs are essential to the decoding process.
tRNAs are essential in translating the language of Nucleic acid (as expressed in an order of
bases) into the language of proteins (an order of amino acids).
There are another important set of translators, one working with each type of tRNA. These
second important translators are proteins and they are know as aminoacyltRNA synthetases.
A set of tRNAs and the set of aminoacyl tRNA synthetases together translate the genetic code.
Now lets see how this is done.
Lets look at the structure of these translator molecules to see how the translation of the code is
done.
tRNA molecules, being RNA, are made by the process of transcription.
There is a different gene that serves as the DNA template for each different tRNA.
Once tRNA is transcribed, internal base pairing occurs to give each tRAN molecule a
characteristic shape. Figure 12.8, page 266.
The single stranded transcript folds up on itself, using base pairing to form the mature transfer
RNAs, an elaborate cloverleaf structure. The rungs across the structure are places where there is
base pairing, and in fact this helps determine the folding pattern.
Diagrams often look like this (Figure 12.8 far left, but in 3 dimensions the two side loops fold
on the same side to look like 12.8 middle and far right.
Lets look at the regions of the tRNAs
One very important region of the tRNA is the anticodon. The three bases of the anticodon are
complementary to the three bases of a specific codon of a mRNA.
At the 3 end of the tRNA, at the opposite end from the anticodon, is an attachment site for a
specific amino acid. Every tRNA has 3 ACC as the first three ribonucleotides from the 3’ end.
As well see in a minute, an amino acid is attached to the adenine. (A).
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