B IOLOGY 2121 INDEPENDENT NOTES
PROTEIN SYNTHESIS
Background
Remember that DNA contains all of the information needed to produce proteins that are critical parts of cells, serve as hormones,
important blood component, etc. This information will be used to produce these proteins on the surface of ribosomes in the
cytoplasm. One problem however. DNA cannot be moved from the nucleus. There must be a process that allows for the information
from the DNA in the nucleus to be transferred to the ribosomes in the cytoplasm.
Transcription
Transcription is defined as the process in which the genetic information (genes) in DNA is copied. At specific points instead of DNA
replication occurring, this critical information is copied into mRNA. mRNA stands for ‘messenger’ RNA. This process is similar to
DNA replication. The enzyme helicase breaks the hydrogen bonds between the double strands of DNA. RNA nucleotides are brought
in that base pair correctly with the DNA bases. There is one important difference. The base Uracil replace Thamine in RNA. So the
base pair rules for RNA are as follows:
G bonds with C and A bonds with U.
As you can see, the transcription process above shows the base pairing between a strand of DNA and mRNA. T bonds with A, C
bonds with G, etc. Either strand of DNA may be used as the template for transcription.
In this diagram, this shows another enzyme that plays an important role in transcription. RNA polymerase brings in the correct base
that is a part of the nucleotide to bind to the corresponding DNA nucleotide and base.
Translation
After transcription takes place, a process called translation then follows. Translation is defined as the process of converting the
language of DNA into the language of amino acids. In other words, mRNA which now contains the critical information for making a
protein, is a ‘blueprint’ for synthesizing the correct amino acids on the ribosomes.
The process begins with the newly synthesized mRNA leaving the nucleus through the nuclear pores. mRNA then makes its way to
the ribosomal surface (free ribosomes or attached to the ER). mRNA then attaches itself to the ribosome. tRNA then begins to bring in
the correct amino acid to the surface of the ribosome to base pair or bond with the correct mRNA base.
This is a diagram of tRNA. Remember this contains the base uracil just like mRNA. Notice the amino acid attached to the upper part
of the tRNA molecule. This amino acid will become a part of the polypeptide chain that will make up the protein. Also notice the
dark green bottom section. There are 3-bases, CGG. Notice the term codon. A codon is a triple base sequence on the surface of the
mRNA. Remember the information on mRNA codons comes from the DNA sequence (indirectly via transcription).
This image shows the process of codon recognition. The sequence on tRNA UCU (called the anticodon) base pairs with the sequence
AGA (codon). The important point here is that this base pairing between the mRNA codon and tRNA anticodon assures that the
correct amino acid is brought to the surface of the ribosome.
Notice the growing chain or sequence of amino acids.
Base pairing between tRNA and mRNA assures that
the correct amino acid in the sequence is assembled.
Once brought to the surface and base pairing occurs, a
chain of amino acids is produced. This chain or
sequence is referred to as the primary structure of the
polypeptide (protein).
The diagram shown at the left demonstrates the entire
process from beginning to end.
Other Important Information
Not all of the genetic information contained in DNA is
copied into mRNA. Each gene contains a number of
triplets that are not needed to build a functional protein.
The mRNA strand is modified during transcription.
Some, non-important information is spliced out. The
most important parts are left in the mRNA. The excess
information that is cut out is called introns and the
important information that remains are called exons.
Using the Genetic Code
The genetic code is a tool that you can use to take the information found in the mRNA codons to identify the amino acid that
corresponds with this information.
Each triplet codon sequence corresponds to an
amino acid. For some amino acids there are
more than one triple sequences (redundancy).
Let’s translate a mRNA sequence:
AUG CCU AAA (Methionine, Proline, Lysine)
Try this:
DNA strand: AAA GGG CCC – transcribe this into mRNA
mRNA:
UUU CCC GGG
Translation - Phe, Pro, Gly