How does Information get out of the Nucleus? - Transcription
What substrate(s) is an RNA molecule synthesized from, and what kind of reaction is involved?
See the macromolecules page if you forget.
The monomers are nucleotides, of course (see the ribonucleotides). They differ from the
deoxyribonucleotides in DNA in two ways. They contain the sugar ribose instead of deoxyribose,
and they have U instead of T as one of the four bases.
What controls the sequence of nucleotides in an RNA molecule
What catalyzes the reactions that join the nucleotides together into a new RNA molecule?
The portion of a DNA molecule that codes for one protein (a protein is typically around 100 to
1000 amino acids long) is called a gene. Copies of the gene that differ slightly from each other
are called alleles.
The gene is transcribed to produce a messenger RNA. The nucleotides in the messenger RNA
have bases that are complementary to the bases in the DNA; this is how the DNA controls the
sequence of nucleotides in the messenger RNA. Once the RNA molecule which codes for a
protein (messenger RNA or mRNA) is synthesized, it comes loose from the DNA sense strand
(the one it is complementary to) and leaves the nucleus.
Out in the cytoplasm, the mRNA then binds to a ribosome, a large complex of rRNA and
proteins, which acts like a very complex enzyme.
How does a nucleic acid code for a protein? - Translation
Now comes the most interesting part. The sequence of the mRNA was determined by a simple
chemical interaction between complementary bases, so that where the DNA had an A, the RNA
got a U, and so on. How can an RNA molecule determine the sequence of amino acids in a
protein? First of all, can one nucleotide in RNA stand for one amino acid in a protein?
If one RNA nucleotide stood for one amino acid, how many different amino acids could be used
in proteins? (Hint: how many different nucleotides are there in RNA?)
What if you could have a code in which two RNA nucleotides stood for one amino acid. How
many different amino acids could this code specify? (Hint: how many two -nucleotide
combinations are possible?)
Instead, 3 mRNA nucleotides form a codon or code word that stands for one amino acid. How
many different such codons are there?
But the amino acids don't bond to the mRNA directly. A chemical adapter is necessary to
connect them. What is the chemical adapter which connects the mRNA to an amino acid?
Transfer RNAs (tRNAs) are small RNA molecules that have two special features: on one end, a
sequence of three nucleotides (the anticodon) complementary to a particular mRNA codon;
and on the other end, the specific amino acid that the mRNA codon stands for.
tRNA (illustration from Johnson, The Living World)
Actually, the paired strands of RNA form helices, so
the shape of a tRNA is more like this. This tRNA
has the anticodon UAC and the amino acid
methionine.
In translation, a mRNA is bound to the ribosome. A tRNA binds to the mRNA's first codon by
complementary base pairing.
If the first codon on the mRNA is AUG, what will be the anticodon on the first tRNA that binds?
A second tRNA binds to the second codon on the mRNA. When two tRNAs are bound, the
ribosome then acts like an enzyme, doing the following things:
-it breaks the bond between the first tRNA and
its amino acid. -it joins that amino acid to the
amino acid on the second tRNA. -it moves down
the mRNA three nucleotides (that is, to the next
codon).
The first tRNA is released into the cytoplasm. It has no
amino acid on it. How does it get another one? What
controls which amino acid it gets?
For each kind of tRNA, there is a specific enzyme which binds to it and to the amino acid that
goes on it, and bonds them together. These synthetase enzymes control what amino acid
each codon will stand for; they determine the genetic code. It's important to note that the
groups where the tRNA and amino acid are attached are the same for all tRNAs and all amino
acids. Any amino acid could be hooked onto any tRNA. Only the specific synthetase enzymes
cause a particular tRNA always to carry the same amino acid.
The synthetase has an active site that fits
a particular tRNA and the amino acid that
goes with it.
There's a different synthetase enzyme for
each different tRNA in the cell.
Note that the amino acid, like all amino
acids, has an OH in the carboxyl group.
The tRNA has, on the last nucleotide's
sugar, an OH -- just like every RNA
molecule does.
The synthetase catalyzes a synthesis
reaction, joining the amino acid to the
tRNA.
The "charged" tRNA is released.
The synthetase enzyme does this out in the cytoplasm. Meanwhile, on the ribosome, another
tRNA binds to the third codon by complementary base pairing. The ribosome performs its
functions again, but this time what is attached to the tRNA on the second codon? It's not just an
amino acid this time.
The ribosome this time does the following:



it breaks the bond between the previous tRNA and the growing peptide chain.
it joins that peptide chain to the amino acid on the next tRNA.
it moves down the mRNA three nucleotides (that is, to the next codon).
After this the process repeats until one of the three stop codons (UAA, UAG, or UGA) is reached.
No tRNA has an anticodon to match these, and at this point translation stops. The mRNA is
released (and can be translated again), and the new protein molecule is released. The protein
molecule formed in this way has a sequence of amino acids that has been determined by the
sequence of nucleotides in the mRNA, which was determined by the sequence of nucleotides in
the DNA.
Suppose the DNA contains a mutation (an accidental change in the DNA sequence). Will the
protein coded for be different? Explain. (use the back of the page)