Site author Richard Steane
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HOW DNA CONTROLS PROTEIN SYNTHESIS BY MEANS OF A BASE CODE
Control of protein synthesis
Most of the time when a cell is not dividing, it is performing a series of activities under the control of the DNA in its nucleus. In order to do this, information from certain portions of the DNA in the chromosomes must be taken out into the cytoplasm, to be used to make (synthesise) control proteins (enzymes, etc) for the cell.
There are 2 parts to this process: transcription and translation.
The 2 strands of the DNA molecule are temporarily split by enzymes which cause a short part to be copied into a similarly short section of RNA molecule. The copying is along the same lines as already explained, (A for T, G for C, C for G) except that a different base called U (uracil) replaces T (thymine). Also, RNA is only made of a single strand, and it contains a different sidechain subunit.
The RNA copy from one section of DNA, which usually corresponds to a single gene, is called messenger RNA (mRNA).
What will be the sequence of bases on the mRNA strand if the strand of DNA to be transcribed has the following base sequence?
C A T G A G C G C G A T,
> GUA CUC GCG CUA
Transcription: RNA made according to base sequence in DNA
30 base pairs (10 triplets) shown for example - actual genes are usually hundreds or thousands of base pairs in length
The two strands of DNA - shown here in black and grey - separate (under the influence of the enzyme RNA polymerase). Messenger RNA - here red - forms on one - black - strand of DNA. The other strand - grey - does not take part in the process.
The strand of messenger RNA (mRNA) formed then leaves the nucleus and passes into the cytoplasm. The opened-up section of DNA re-forms into a double helix, as before.
Messenger RNA then passes out of the nucleus and travels to small structures called ribosomes in the cytoplasm. Here the message it contains is interpreted, and a protein is built up, bit by bit, from its individual subunits - amino - acids, which are in the cytoplasm.
There are 20 different amino acids, with rather formidable names. Although they differ greatly in size and chemical properties, they all have a similar section by which they may be linked, to form a polypeptide chain, which will then coil to make a protein.
Each section of 3 bases in the messenger RNA strand is called a triplet, which carries enough information to identify the next amino acid which will be added to the developing polypeptide chain. The actual amino acids that are added as a result of the particular sequence of bases has been found out as a result of experiments. It has been discovered that there are several different triplet codes for each amino acid, as well as special ones to signify the start and end of the polypeptide chain.
This base code seems to be the same in practically all living organisms, which confirms its fundamental significance in the organisation of life. It also explains how it is sometimes possible to take sections of DNA corresponding to genes from one organism and transfer them to another organism in which they may still work. This is the basis of genetic engineering.
The genetic code
The genetic codes for each amino acid
| RNA triplet
|| RNA triplet
3lc* = 3-letter code for amino acid
- do not confuse with DNA/RNA triplets
The information above is included here for reference only. Do not worry about the details!
Different varieties of another form of RNA (transfer RNA) and a variety of enzymes are involved in recognising the messenger RNA triplet codes, due to the way in which one RNA strand can pair up with a complementary part of another.
Working together, these bring in the individual amino acids one by one in the correct order for assembly into the protein. For example, the triplet CCC in messenger RNA pairs up with its counterpart triplet GGG in transfer RNA, which will result in the amino acid proline being added to the polypeptide chain.
So a protein is the final product of the the gene made up of DNA.
The overall process may be summarised as follows:
Translation: Protein made according to base sequence in RNA
As messenger RNA (mRNA) - red - passes through the ribosome - grey, it causes a protein to be made (synthesised) by joining together various amino acids - green - in a particular order.
A different combination of 3 mRNA bases, also called a triplet, codes for each one of the 20 amino acids. Each triplet in mRNA causes a corresponding transfer RNA (tRNA) molecule - blue - to bring in the appropriate amino acid.
This occurs because the triplet of 3 bases in mRNA, also called a codon, pairs up inside the ribosome with the corresponding 3 bases in tRNA. also called an anticodon.
When the transfer RNA has delivered the amino acid to the growing polypeptide chain, it leaves the ribosome, returns to the cytoplasm and picks up another amino acid.
As the ribosome moves along the mRNA strand, the synthesis process continues until it reaches the stop code which causes amino acid addition to cease. The polypeptide is then released, and it may fold into its final protein structure. The messenger RNA may enter another ribosome and repeat the protein synthesis process, or it may be broken down and its sub-units may be re-used.
During translation, the following stages are taking place:
(1) Messenger RNA strand passes through ribosome
(2) Triplet CCC codes for the next amino acid to be brought into position in the ribosome
(3) Transfer RNA brings in the appropriate amino acid (proline)
(4) Amino acid added to polypeptide chain
(5) Transfer RNA released to pick up amino acid to be recycled
(6) "Used" messenger RNA strand may pass on to another ribosome
(7) Process repeats with next triplet code until:
(8) Triplet UAA causes translation to stop
(9) If it is "long", the polypeptide chain folds into the shape of the final protein
Summary - base conversions
| Transfer RNA
This topic has connections with other units on:-
Amino acids commonly found in proteins
All 20 Amino acids visible in 3-D on a single page
Amino acid structure
Amino acid condensation
3-D interactive structures of Biological molecules