Genetics and the Intricacies of Gene Manipulation and Gene Control
Erasmus :To understand this topic, I think you need to pick up some basic knowledge (of genetic language). So, I’ll present some definitions and some opinions. Then we will look at a few operons and how they work. Lastly we will look at methods of controlling genes and their operation. We will try to understand gene control is such a complex area? Then we can imagine the future.
Kinkajou: What is an operon?
- An operon is a piece of DNA defined by a stop codon and a start codon
- Which contains a DNA segment coding for a protein (gene)
- Which contains a segment of DNA which functions to control the operation or translation of the protein contained therein.
Kinkajou : What’s the difference between Polycistronic and monocistronic?
Erasmus :A cistron is also known as a gene.
So a polycistronic operon contains the DNA code for multiple proteins (from the genes contained therein). This type of structure is more common in prokaryotic type organisms.
So a monocistronic operon contains the DNA code for a single protein (gene). This type of structure is more common in eukaryotic type organisms.
The DNA coding for a single gene does not need to occur in one contiguous segment. It can occur in several separate subunits within the operon. The subunits are known as exons. Exons are expressed (are translated into proteins).
Exons are separated by pieces of DNA called introns which may or may not be functional, to a variable extent
Eukaryotic organisms typically have each gene structured into a number of separate exons, separated by introns. Transcription can occur with each exon being expressed separately and combined later OR with each exon being expressed in an RNA sequence and then combined (undergoing trans-splicing to create more modest RNA sequences that are translated separately).
Prokaryotic organisms typically have each gene containing the entire DNA required to produce the expressed protein as a contiguous sequential piece of DNA. There are no segments. There are no introns.
Dr Xxxxx : I severely reject the argument that prokaryotic DNA is primitive. Prokaryotic life has been present on earth for 3.4 billion years. The generation time of prokaryotic organisms is of the order of 30 minutes often. Prokaryotes are capable of evolving at a rate of at least a hundred times faster than many more complex organisms, especially multicellular animals.
The fact that prokaryotes formed a primitive form of life 3.4 billion years ago, does not mean they have not also evolved into much more complex organisms. The rate of growth of most single celled prokaryotes and evolution of the billions of years, would guarantee that this type of organism could evolve to a much greater complexity than eukaryotic organism.
This belief that eukaryotes are modern and prokaryotes are primitive and will lead to some very dangerous assumptions when analysing cell DNA sequences. Just because humans are complex does not necessarily imply that all aspects of their DNA are complex. There is probably much more selective pressure on the DNA of a prokaryote to increase complexity, improve efficiency and to have complex modifiers for operation of genes. (cf eukaryote).
It is best not to describe the differences between prokaryotic and eukaryotic as a difference of primitive versus modern.
The key differentiating factor between eukaryotes and prokaryotes is that eukaryotes contain much more DNA, which needs to be packed onto histone proteins and combined to form a chromosome (because there are so much more DNA). Introns are present in eukaryotic DNA. These must be bypassed in some method to allow the production of proteins.
Packing of eukaryotic DNA is critical to allow it to fit within the nucleus of the cell. This places constraints on the structure of the DNA. The need for the DNA strand to wrap around its packing proteins (histones), means that pieces of the DNA are not accessible for translation. It makes more sense for eukaryotic DNA to contain exons and introns. With appropriate spacing, exons can be expressed and introns can be “not” translated or expressed if packing constraints limit the access of the polymerase to the DNA segment.
The situation is different in the DNA of prokaryote. There are less packing constraints and less organisation of the DNA into the typical chromosomal structure of eukaryotes. It makes much more sense for efficiency reasons to evolve multiple highly organised contiguous pieces of DNA genes within a single operon.
In the event of a mutation in a prokaryote which critically affects an operon’s structure, the entire operon may cease to work. This is likely to be lethal. This results in rapid selection of only viable cells with complete genetic information for survival. The speed of replication guarantees that such a death is unimportant for the survival of the species.
In a eukaryote a mutation in such a multi-gene operon would be a total disaster, probably leading to an organism’s death. The low replication rate of higher eukaryotic organisms (especially multicellular organisms such as mammals), would impact on the probability of survival of the species. It makes much more sense for a eukaryote to have a single regulatory segment controlling a single expressed gene (protein).
Also the need to pack the DNA material into chromosomes interferes with efficiency of translating genetic DNA material where there are multiple genes in a single operon. Large multi-gene operons are more likely to be constrained by histone proteins causing critical folds in DNA operons and consequently isolating regulatory DNA segments (intron) from possible action on their expressed gene.
Erasmus and Kinkajou: Strongly held beliefs!
Dr Xxxxx :In short, the differences between prokaryotic polycistronic DNA and eukaryotic monocistronic relate more to packing constraints affecting regulation and operation, than any consideration of primitive versus modern.
Monocistronic Eukaryotic Gene Organisation
mRNA Translation prokaryote vs eukaryote