3.5 - Genetic modification and biotechnology

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Gel electrophoresis

involves the separating of charged molecules in an electric field, according to their size and charge

samples are placed in wells cast in a gel

the gel is immersed in a conducting fluid and an electric field is applied

molecules in the same that are charged will move through the gel

opposite charges move in opposite directions

proteins may be positively or negatively charged so can be separated according to their charge

the gel used consists of a mesh of filaments that resists the movement of molecules in a sample

DNA molecules from eukaryotes are too long to move through the gel, so they must be broken up into smaller fragments

all DNA molecules carry negative charges so move in the same direction during gel electrophoresis, but not at the same rate

small fragments move faster than large ones, so they move farther in a given time

gel electrophoresis can be used to separate fragments of DNA according to their size

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DNA amplification by PCR

the polymerase chain reaction is used to make large numbers of copies of DNA

only a very small amount of DNA is needed at the start of the process - in theory just a single molecule

within an hour or two, millions of copies can be made

this makes it possible to study the DNA without the risk of using up a limited sample

for example:

DNA extracted from fossils can be amplified using PCR

small amounts of DNA from blood, semen or hairs can also be amplified for use in forensic investigations

PCR is not only used to copy the entire set of DNA molecules in a sample such as blood or semen

white blood cells contain all chromosomes of the person whose blood it is, the sperm cells in a sample of semen contain a man's entire genome

instead, PCR is used to copy specific DNA sequences

a sequence is selected for copying using a primer that binds to the start of the desired sequence

the primer binds by complementary base pairing

the selectivity of PCR allows particular desired sequences to be copied from a whole genome or even greater mixture of DNA

one test for the presence of genetically modified ingredients in foods involves the use of a primer that binds to the genetically modified DNA

any such DNA present is amplified by the PCR, but if there is none present the PCR has no effect

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DNA profiling

a sample of DNA is obtained, either from a known individual or from another source such as a fossil or a crime scene

sequences in the DNA that vary considerably between individuals are selected and are copied by PCR

the copied DNA is split into fragments using restriction endonucleases

the fragments are separated using gel electrophoresis

this produces a pattern of bands that is always the same with DNA taken from one individual, this is the individual's DNA profile

the profiles of different individuals can be compared to see which bands are the same and which are different

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Genetic modification

is the transfer of genes from one species to another

this is possible because the genetic code is universal

so when genes are transferred, the amino acid sequence translated from them is unchanged - the same polypeptide is produced

genes have been transferred from eukaryotes to bacteria

example: the transfer of the gene for making human insulin to a bacterium; this was done so that large quantities of this hormone can be produced for treating diabetics

genetic modification has been used to introduce new characteristics to animal species

example: goats have been produced that secrete milk containing spider silk protein; spider silk is immensely strong, but spiders could not be used to produce it commercially

genetic modification has also been used to produce many new varieties of crop plant

these are known as GM crops

example: genes from snapdragons have been transferred to purple rather than red, the production of folder rice involved the transfer of three genes (two from daffodil plants and one from a bacterium) so that the yellow pigment is produced in the rice grains

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