PROKARYOTIC PROTEIN SYNTHESIS
Because of the way their DNA is organised, the whole work of the cell proceeds differently in prokaryotes.
Bacterial DNA is found in one large, single, circular chromosome as well as several smaller loops called
plasmids. The larger chromosome loop is highly condensed (supercoiled) to fit its 1100 m length into the
2 m length of the bacteria. This supercoiling also seems to be important for the DNA to function properly
(recombination, gene expression and regulation of gene expression will not occur without it). Prokaryotes
do not undergo mitosis or meiosis (they don’t form spindles) but the DNA can replicate itself and when it
does so a specific enzyme attaches the chromosome to the cell membrane as an anchor to allow the
chromosome and its copy to separate. Plasmids have been found to carry genes that confer resistance to
drugs like antibiotics and genes coding for enzymes needed in the fermentation process (particularly
important in cheese making). Each plasmid can replicate independently of the main chromosome.
Fig. 1 Eukaryote transcription
The
DNA
inside
these
chromosomes carries genetic
information in genes.
Once
transcribed, eukaryote mRNA
carries only one gene but
bacteria can have many genes
on the same mRNA (the operon
which has genes for several
functional proteins plus the
genes which regulate them). So
prokaryotes are polycistronic1
(each mRNA can code for
several different proteins) while
eukaryotes are monocistronic.
Remember that the Central Dogma tells us that the code on DNA is transcribed onto mRNA and is then
translated into a protein. Most of the prokaryote genome2 is used but much of the eukaryote genome is not
transcribed or translated (the “junk” DNA).
Transcription begins with RNA polymerase making the mRNA strand. In eukaryotes there are 3 different
RNA polymerases (I in the nucleolus to make rRNA, II & III in the nucleus to make mRNA & tRNA
respectively). In prokaryotes there is only one type of RNA polymerase to make all these different RNAs.
RNA is unstable in both eukaryotes and prokaryotes – it quickly breaks down in the cytoplasm.
Before mRNA can leave the
nucleus to make a protein it has
to be processed into a usable
form.
Eukaryotic mRNA
processing does three things.
First it removes introns, the
noncoding stretches inside the
gene. A poly-A tail (up to 200
adenine nucleotides) is added
at the 3’ end to help a finished
ribosome reattach at the start
again.
Molecules of mRNA
without this poly-A tail are
Fig. 2 Prokaryote transcription and translation
broken down rapidly in the
nucleus. And at the 5’ end a cap (a guanine and 3 phosphates) is added to help bind the ribosome onto
mRNA and to distinguish this RNA from others in the cell. As bacteria have no introns their mRNA does
not need processing and neither is a cap or a poly-A tail added.
Translation begins with mRNA attaching to a ribosome. The ribosomes central to this process are often
smaller in prokaryotes than in eukaryotes.
One ribosome binds and begins to translate mRNA. As this
ribosome passes along the mRNA another ribosome attaches to the free end and begins to make a second
polypeptide. When many ribsosomes bind to the same mRNA, they form polysomes.
1
2
A cistron is the length of DNA that codes for one functional protein.
The genome is the entire DNA on all the chromosomes in an organism.
Proteins are therefore made very quickly in prokaryotes (20 amino acids per second, compared with 3
amino acids per second in eukaryotes). This also means less mRNA is needed in prokaryotes.
In prokaryotes, translation of an mRNA molecule often begins before its transcription is complete (see Fig.
2). This is possible because mRNA molecules are both synthesised and translated in the same 5’ to 3’
direction and because there is no nuclear membrane to separate the 2 processes. The same simultaneous
processes are also seen in mitochondria & chloroplast DNA.
Once made, the polypeptide chain separates from the ribosome and is released into the cytoplasm. There
it will be folded and/or joined with other polypeptides to make a functional protein.
This all works well for bacteria because of their relative simplicity. There is less DNA, fewer genes and a
smaller variety of proteins to be made. Life is pared back to the bare essentials. This simplicity also helps
explain another difference that we have seen. The operon sequence that switches genes on & off works
well in bacteria where transcription and translation take place side by side. Within the single bacterial loop
of DNA control genes can be placed beside or very near the working genes. As this is not possible in
eukaryotes, regulation of their gene expression is much more complex.
Questions
1. In table form briefly describe in your own words the key differences between protein synthesis in
prokaryotes & eukaryotes.
2. Give definitions for each of these terms:
A. Polycistronic
B. Polysome
C. Poly-A tail