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Science 21B
Presentation for Section 8.5
Translation
Slide 1
Think back to an earlier discussion of codes and how a coded message compares to a computer
code. How are these the same and how are they different? Both require translation of
information. However, the coded message reproduces the same information, whereas a computer
code produces actions. The translation of RNA into a protein is more like what happens with a
computer code. The information encoded in nucleic acids becomes functioning proteins.
Slide 2
The word transcription is not synonymous with translation. The root of transcribe is “to write,”
whereas the root of translate is “to transfer.”
What amino acid is coded by CUU? According to the diagram, you should see it is leucine. What
amino acid is coded for by AUG? That would be methionine.
Slide 3
What happens when a ribosome reads the codon AUG? It produces methionine, which is also a
start codon. If it is UGA, the translation is going to stop. It is a common misconception that there
are only twenty amino acids. The structure of an amino acid consists of a central carbon atom
bonded to an amino group (NH4), a carboxyl group (COOH), a hydrogen atom (H), and a side
group. An infinite number of side groups are possible, so is an infinite number of amino acids.
But only twenty are commonly found in proteins, however.
Slide 4
In 1961, Francis Crick and a group of colleagues performed an ingenious series of experiments
to demonstrate that the genetic code was a triplet code, consisting of three consecutive
nucleotides. They used a chemical to delete one, two, and three nucleotides from the DNA of a
bacteriophage and then looked at a gene downstream from the deletions to see if it was
transcribed correctly. They found that when one or two nucleotides were deleted, the reading
frame of the gene was shifted. When three nucleotides were deleted, the downstream gene was
read correctly. This also suggested that the reading of the code was continuous, without
punctuation interrupting the reading.
Slide 5
You might consider it to be odd to describe the genetic as a punctuation of stop and start codes.
The Latin word puctum means “point” and is derived form an older form meaning “to pierce or
puncture.” Punctuation, in a general sense, signifies an interruption. The word punctuate can also
be used to describe the act of placing stress or emphasis on a point.
Although most DNA is found in the nucleus, a small amount is located in the mitochondria and
chloroplasts. Scientists were shocked in the early 1980s when they determined the base
sequences of mitochondrial DNA and found a somewhat different genetic code than in nuclear
DNA. Since then, the genetic code of some protists, such as Paramecium, has been found to
differ. Thus, the genetic code is not quite universal.
Slide 6
The process of translation requires the transfer of information from one chemical form (nitrogencontaining bases) to another (amino acids), that is, from the language of nucleic acids to the
language of proteins. The translation is facilitated by an intermediate-transfer RNA. In contrast,
the language of complementary base pairing of nucleotides used to copy information in both
replication of DNA to DNA and transcription of DNA to RNA occurs directly. The language
does not change.
There are three binding sites in the ribosomes and the complementarity of the codon and
anticodon. What is the function of messenger RNA? It provides a copy of the genetic code.
Slide 7
What is the function of transfer RNA? It brings amino acids to the ribosome, in the proper order
What then is the function of the ribosomes? The ribosomes position messenger RNA so that it
can be read by transfer RNA. It breaks the bonds between the transfer RNAs and their attached
amino acids, and forms peptide bonds between amino acids, assembling the protein coded for a
gene.
Slide 8
The end of a transfer RNA molecule opposite the anticodon binds to a specific amino acid with
the help of the enzyme aminoacyl transfer RNA synthetase. There are 20 kinds of these enzymes
in a cell, one for each kind of amino acid. Each enzyme ensures that the correct amino acid is
attached to a transfer RNA having the anticodon corresponding to that amino acid. The enzymes
have two specific binding sites, one for a particular amino acid and the other for a particular
transfer RNA. This specificity is sometimes referred to as the second DNA code.
Slide 9
A transfer RNA molecule is a large subunit of the ribosome that catalyzes the formation of
peptide bonds between amino acids. In this role, it acts as an enzyme. This type of molecule is
called a “ribozyme.”
The small subunit holds onto the messenger RNA transcript. The large subunit has three sites
where the transfer RNA molecules can dock. It helps form the peptide bond between the amino
acids and helps break the bond between the amino acid and its carrier transfer RNA molecule.