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This project aims to answer these questions.
The genetic material of prokaryotic cells is organised in a single circular DNA molecule.
Study the structure of a bacterial cell in the following 3D scene.
The DNA macromolecules found in the nuclei of Eukaryoric cells form chromosomes (together with proteins).
Open the following 3D scene to learn about the structure of DNA and how it forms chromosomes. If you want to learn more, study the chemical structure of DNA.
The offspring inherit their genes from their parents.
The number of these genes varies greatly between species.
|Organism||Number of genes|
|water flea||31 000|
|fruit fly||14 000|
|intestinal bacterium||5 000|
Genetic material can change in a permanent way. In this case, the alteration of the DNA is permanent. This is called mutation.
If only a small segment of the gene, that is, only some nucleotides change, the mutation is a point mutation. Scientists assume that 100-200 point mutations can occur in a single person in a lifetime. Not all mutations have noticeable effects, that is, not all mutations are expressed in the phenotype of the individual and not all are inherited by the offspring.
Besides mapping the genome of species, scientists can also change the genes expressed in individuals. The technologies are called genetic engineering, gene therapy, genetic modification, etc.
The genome of organisms can contain short sequences that are palindromic, and certain enzymes can easily recognise them. For example, the restriction enzymes (DNA-cleaving enzymes) of bacteria can cleave DNA at specific recognition sites, called restriction sites, which are palindromic sequences of nucleotides. The EcoRI enzyme, for example, recognises the palindromic sequence GAATTC and cuts between the G and the A.
But what does CRISPR mean?
In 1987, repeated sequences were identified in the genetic material of E. coli bacteria, but the function of those was not yet known. Later similar sequences were found in the genome of other bacteria too. It turned out that these short sequences, containing 20–50 base pairs, originate from viruses.
When the offspring encounter the same virus, bacterial cells can 'remember' the viral infection and become immune to the given virus, similarly to the immune system of multicellular organisms.
Open the following 3D scene to see an introduction to the CRISPR-system.
The CRISPR/Cas9 system makes it possible to make targeted mutations at specific places in the cells, which is also why it is often referred to as 'DNA scissors'.
The three components of the original system are the following.
In addition, this system can target multiple genes at once, so it can be used to treat diseases that are caused by more than one gene. Modifications can be made within living cells too, not only in test tubes.
The CRISPR/Cas9 complex can be artificially engineered in laboratory circumstances and can be introduced into cells. During the production of the complex, it is possible to determine the DNA segment it will recognize, therefore it can also be used for high-precision modifications of the genes of living organisms. After the recognition, the complex cuts the gene, thus it will not be translated into a protein. This genetic engineering method is called gene knockout.
With the CRISPR/Cas9 complex, gene knockin is also possible. In this process, a DNA segment of a foreign gene must also be introduced into the cell together with the complex. Once the complex has cut the genetic material of the cell, the foreign DNA segment can bind to the cut ends and the new, foreign gene can be incorporated into the genome, or a new gene that carries a mutation can be substituted for the original one.
The technology may also be useful in creating more resistant plants, as well as to prevent epidemics and genetic diseases. Another promising possibility of using genetic modification is the eradication of antibiotic-resistant bacteria by removing the bacterial gene responsible for antibiotic resistance. It could also be used as a method of control for invasive plant species.
In December 2015, ethical questions related to the new technology were discussed in an international conference. The conference was organised by David Baltimore, Nobel-prize winning molecular biologist and held in Washington, in the US. An agreement was made that making permanent, hereditary changes in the human genome is irresponsible.
Genetically edited twin babies (Lulu and Nana) were born in China in a research funded by a private foundation in China, ignoring both Chinese and international laws regarding human genetic engineering. The researchers modified a gene in their genome to make their T-cells immune to HIV. Later it was announced that the genetic modification may have shortened the children's life expectancy.
Some clinical trials have already been approved in people with a terminal or incurable illness. For example, a trial will be launched for treating Leber’s congenital amaurosis by injecting the components of the gene-editing system directly into the eye. Previous clinical trials have used the CRISPR technique to edit the genomes of cells that have been removed from the body and then injected back. Scientists, however, fear that genetic engineering could lead to an unexpected disease in the patients.
The topic of 'designer babies' poses an ethical problem. That is, this technology would allow parents to pick a sex, eye colour, facial shape of their babies as well as the babies' susceptibility to illnesses and infections and even their psychological characteristics. The limit between preventing fatal illnesses and custom-picking characteristics for our offspring is not clear.