S-Cool Revision Summary
S-Cool Revision Summary
The Eukaryotic Genome
The term genome refers to all of the alleles possessed by an organism. While the amount of DNA for a diploid cell is constant within a species, the differences can be great between species.
Most, but not all structural eukaryote genes contain introns. Although transcribed, these introns are excised (cut out) before translation.
The number of introns varies with the particular gene, even occurring in tRNAs, rRNAs and viral genes!
Some of the base sequences in the introns and intergenic portions of the genome contain sites that regulate gene expression.
The importance of these sites is that they allow genes to produce various forms of the protein for in different tissues or at different times in the development of the organisms.
Evidence from E.coli
The single chromosome of the common intestinal bacterium E.coli is circular and contains some 4.7 million base pairs.
The chromosome replicates in a bi-directional method, producing a figure resembling the Greek letter theta.
The promoter is the part of the DNA to which the RNA polymerase binds before opening the segment of the DNA to be transcribed.
In the late 1950's, Fancois Jacob and Jacques Monod proposed the operon model of prokaryotic gene regulation.
Operons can either be inducible (promoters) or repressible according to the control mechanism.
In the 250 structural genes of E.coli, seventy-five different operons have been identified.
When a repressor molecule is present, this blocks the process of RNA polymerase, so preventing any transcription at this site.
One piece of evidence suggest that the influence of the regulator genes may be thousands of base pairs away from the promoter gene, unlike the close sequences found in E.coli.
Eukaryotes have a number of gene control mechanisms not found in prokaryotes. One of these is called DNA methylation.
The structure of DNA was found by Watson and Crick in 1953, and only in 1956 was it first known that humans had 23 pairs of chromosomes. The subsequent interval of time has bee very active in the file of Genetics.
In 1977, Fred Sanger invented the first DNA sequencing process. He opened up the possibility of working-out the entire sequence of the bases in the human DNA, a total of some three thousand million (three billion or 3 x 109).
To sequence a gene, the first difficult part is to obtain the gene. Once obtained, there are laborious manual methods, or those using machines do the process.
The manual methods involves electrophoresis; a process that separates fragments of DNA by using an electrical current.
One of the main project goals is to address the ethical, legal and social issues (ELSI) that may arise from the project. 3-5% of the HGP budget has been set-aside for this purpose.
Knowledge about the effects of DNA variations amongst individuals can lead to diagnosis and possibly treatment of the thousands of disorders that afflict humans all over the world.
Learning about the DNA of other organisms can lead to a better understanding of their natural capabilities in solving challenges in health care, energy sources, agriculture and environmental conservation.