Transgenesis
Definition/ Purpose: Transgenesis is a process where genes from one individual are transferred into the genome of another (the second organism can be a different species). Through the process of transgenesis, the organisms created will express a new characteristic. These new characteristics are passed onto the next generation when the original transgenic organism reproduces. Transgenesis allows people who have a specific purpose in mind to create organisms with the best possible traits for the purpose. Transgenic organisms can be used for testing treatments for diseases, modifying the genome of crops and improving livestock (such as sheep). Much more accurate method of genetic modification than letting mutations naturally occur and even more accurate than selective breeding.
Case Study: An example of transgenesis is transgenic sheep. In recent years transgenic sheep have been created with specific new traits in mind that make them good for different purposes.
For example genes have been transferred into sheep to make the transgenic individual have more meat on its body, or produce more milk, or have more wool (like the sheep on the slide). This benefits the people creating sheep using transgenesis as they will be able to get sheep with new, desirable qualities. Growth hormone gene inserted. How it’s done. Advantages.
Methods: Transgenic organisms can be achieved through multiple methods. Two of these methods are PCR and restriction enzymes and ligation. The first method, PCR (polymerase chain reaction) is used to produce large quantities of DNA from a small sample so there is enough DNA to work with. Scientists use a small machine to carry out PCR and obtain many identical copies of the original DNA sample, which can then be used to create transgenic organisms. The process is performed including a heat tolerant enzyme (taq). In order to replicate the DNA during PCR, some requirements need to be met. Scientists need DNA polymerase, free nucleotides, template DNA strands and primers to start the DNA replication. The DNA is heated (denatured/ DNA strands separated) and cooled repeatedly for 20-30 cycles and the amount of DNA is amplified. PCR is a good method to use for transgenesis as it means scientists can take a small sample of DNA and amplify it to use in transgenesis. E.g. Inserting a growth hormone into salmon to increase the size of the fish. However there are some drawbacks to PCR (transgenesis) that occur in contrast to selective breeding. PCR can only be used with small fragments of DNA not long pieces which limits the amount of work scientists can do with it. Also the taq enzyme used in PCR cannot proofread the copying or correct errors when they occur. This means there is a risk that there could be an error in the DNA that the scientists use in transgenesis. Also there is a risk that the scientists could contaminate the DNA with their own DNA making it unable to be used in transgenesis.
A second method for transgenesis is restriction enzymes and ligation. Restriction enzymes cut DNA into specific nucleotide sequences. This allows DNA to be cut at specific locations, as DNA is easier to work with in smaller amounts. There are many different restriction enzymes, each with a specific location on a gene at which it works. Some restriction enzymes make DNA fragments form sticky ends that have free nucleotide bases at each end which means different pieces of DNA can be joined by matching each sticky end up with complementary sticky ends. Other restriction enzymes create DNA fragments with blunt ends which have no free nucleotide bases on the end. These blunt ends can be joined to other blunt end DNA fragments. While sticky ends need to be joined to complementary sticky ends, blunt ends have the ability to be joined to any other blunt ends even if they are not complementary. This is useful for scientists as it expands the number of different types of transgenic organisms it is possible to make. It is also helpful for scientists to have access to many different types of restriction enzymes for transgenesis as it allows the DNA to be cut at many different locations and makes more possibilities of what organisms could be created using transgenesis. These sequences have the ability to be joined to other sequences later on. Different restriction enzymes can be used to create DNA fragments with different lengths. DNA can be joined to other DNA fragments if the same restriction enzyme has been used, and this is ligation. When two different fragments of DNA are mixed, complementary ends form base pairs and there is now one new set of DNA. The two kinds of DNA are joined together are mixed and complementary ends form base pairs. When fragments of DNA are joined it is called annealing. E.g. Some goats have been transgenically created to have spiders silk protein present in its milk. Spiders silk is one of the strongest, yet lightest substances on the planet and have many uses to humans.
Implications: Transgenesis has implications on different things. These include implications on genetic biodiversity of a population and on the health and survival of individuals.
First, the implications on the genetic biodiversity of a population. Genetic biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species. Things that impact on genetic biodiversity in a population can cause an increase or decrease in alleles present in a gene pool. In some cases, transgenesis causes an increase in genetic biodiversity as there are new alleles being created and added to a population’s gene pool. E.g transgenic sheep. Transgenesis allows scientists to improve desirable features in livestock and crops and the introduced genes can be inherited by following generations meaning the gene pool of a population is changed. Maintaining genetic biodiversity in a population is important as it reduces the chance of inbreeding and recessive alleles being expressed. Using marker assisted selection is better than using only phenotypes to select organisms for selective breeding. It is much quicker to identify if that individual has traits of interest. In addition, some phenotypes are connected to multiple genes and so they may be inherited together which may not be obvious until it’s too late. PAGE 144 BIOZONE?
There are also implications on the health and survival of transgenic individuals, which is often not too positive. Another implication of transgenesis is the health and survival of individuals. Side effects to a transgenic organism can be positive or negative. What impact transgenesis has on an organism depends on which gene or genes have been added during transgenesis, and it also depends on the organism’s environment. For example, research animals created through transgenesis may have been specifically created to have a genetic defect for study. This means the organisms quality of life is low, and it also lowers the organism's chance of survival. “Being used as mere instruments for human benefit and interests.” (Jones, 2006).
Compare and contrast:
Transgenesis is much faster as it can be time consuming for selective breeders as they may have to wait many generations to get the offspring expressing the desired traits. Both selective breeding and transgenesis have the ability to improve organisms both plant and animals from their wild states. Transgenesis is much more recent and controversial than selective breeding. Through transgenesis we are able to make organisms resistant to things such as disease and droughts and pests and we can get higher yields. However, there can be unknown side effects and ethical issues. Both selective breeding and transgenesis have the ability to decrease genetic diversity, which is not a good thing, however it can be argued that this is more likely to occur in transgenesis as they have the power to make many of the same genetic organisms with the same new characteristics whereas mutations can randomly occur during selective breeding and also they can only access phenotypes that already exist in that species (however, it should be noted that inbreeding can be a problem). They both may lead to problems with individual animals e.g. selectively breed chickens sometime can’t stand and walk properly as they have too much meat on them. Also because selective breeding means populations will be similar genetically, diseases can easily wipe out the population. Depending on the way they are used both can either increase or decrease biodiversity.
Selective breeding and transgenesis are two processes of genetic modification of organisms by humans. They both have different methods involved and different implications on other areas of life.
Selective Breeding
Definition/ purpose: Selective breeding has been around for many years, it is the process where humans select animals and plants to breed based on the traits the individual organisms have. They select the two organisms to breed in hopes the offspring will express the best of both the organism's genes. Selective breeding can take many generations to get to the point where offspring have the most desirable and/ or useful characteristics.
Case Study: An example of selective breeding is easy care sheep. The purpose of the easy care sheep is to produce animals which save the farmers time and energy and money in care. The sheep are bred for economic purposes and to be more productive. Easy care sheep are selectively bred to have short tails, short wool and no horns. These features mean the sheep need less maintenance than non selectively bred sheep. E.g. the sheep with short tails means the farmers do not have to deal with docking lambs tails each year and short wool means they save time they would have had to spend leaving. Wool is no longer worth that much money so farmers no longer need to breed sheep to have long wool to then sell. Breeders will want this offspring to be the most biologically fit, by having the best traits. This will be the things such as shorter tails and wool. By having these 2 traits, the sheep will be easier to care for. The less wool they have then means less build up of faeces which reduces the sheep's risk of flystrike. This lowers costs for farmers as they do not have to pay to help get their sheep healthy again. Selective breeding is used to create easy care sheep as heritability of the traits selected for can be measured so farmers have almost a template of how to selectively breed their sheep to have the traits they want. Improved time management. Phenotypes can be selected for quicker and can be sure organism will have it.
Methods: Selective breeding can be done by many methods including phenotypic selection, marker assisted selection, line breeding, crossbreeding, screening for genes and gene mapping.
Marker assisted selection is one method used in selective breeding. Genetic markers are used to locate the allele that is of interest to the breeder. This means a desirable trait can be selected for and undesirable traits can be selected against. This is useful as identification of traits can be done at an early age so that breeders do not have to keep that organism until it is at an age where it can breed and see if it does actually have the trait they desire. The technique can help identify individuals with desirable genes early on so things like breeding programmes can be focused on those individuals with desirable combination of genes. E.g. if there is a marker that is known to be attached to a recessive allele the breeders will know how to avoid breeding organisms with those alleles and passing them on. E.g. fish?
Screening for genes is a second method of selective breeding. This methods involve DNA probes testing for specific genes that may or may not be present in a particular organism. E.g. The Nelson Seafood Research facility developed new species of fish to provide more for the market and make breeders more confident that the fish they would get would be what they could sell. This was needed as new species would take the pressure off supplying the main species. For example snapper. They used a breeding programme that employed selective breeding techniques to make sure the new breed expressed the best traits e.g. good colour and length. A DNA probe will be complementary to the gene being screened for, so that it can be identified. These probes will be fluro coloured so that they can be seen on a gel.
Implications: There are implications of selective breeding on many different areas of life. Two implications of selective breeding in particular are on the evolution of populations and on the genetic biodiversity of populations.
Selective breeding comes with a risk of changing the evolution of the species we selectively breed. Breeders are breeding different species together (for example sheep), which runs the risk of losing some genes from the gene pool. Once these genes have then been lost, it is extremely hard to gain them back.
The process of selective breeding alters which traits are expressed in offspring, with selected ones only being displayed.
Genetic diversity refers to the different number of genetic traits in a population, making it more diverse. This also refers to losing alleles from a gene pool, making a population less diverse. In sheep, the genetic biodiversity would have increased in some places as new sheep are being created, This adds more alleles, such as short tails and no horns, to the overall gene pool. This in turn means that selective breeding can and will increase genetic biodiversity of a population. Selective breeding over many generations removes genes that code for undesirable characteristics or introduces genes that code for desirable characteristics. This is achieved by humans selectively breeding organisms with desirable characteristics. The growth of specific breeds has caused specific genotypes to more frequently appear in the gene pool of a population. New alleles e.g. short tails, no horns. Can result in a population having a very small gene pool. This is because only a select number of organisms from the population are used to create offspring, ususally a large number of offspring.