3.5 Genetic Modification Flashcards
What is PCR?
The polymerase chain reaction (PCR) is an artificial method of replicating DNA under laboratory conditions
What is PCR used for?
The PCR technique is used to amplify large quantities of a specific sequence of DNA from an initial minute sample
How many DNA molecules does each cycle of PCR produce?
Each reaction cycle doubles the amount of DNA – a standard PCR sequence of 30 cycles creates over 1 billion copies (230)
Where does PCR occur and why?
PCR occurs in a thermal cycler and uses variations in temperature to control the replication process
What is the first stage of PCR?
Denaturation – DNA sample is heated to separate it into two single strands (~95ºC for 1 min)
What is the second stage of PCR?
Annealing – DNA primers attach to the 3’ ends of the target sequence (~55ºC for 1 min)
What is the third stage of PCR?
Elongation – A heat-tolerant DNA polymerase (Taq) binds to the primer and copies the strand (~72ºC for 2 min)
What is done with the resulting DNA copies from PCR?
Once large quantities of DNA have been created, other laboratory techniques are used to isolate and manipulate the sequences
What is gel electrophoresis?
Gel electrophoresis is a laboratory technique used to separate and isolate proteins or DNA fragments based on mass / size
Where are samples placed in gel electrophoresis and what is done to them?
Samples are placed in a block of gel and an electric current is applied which causes the samples to move through the gel
What fragments will move faster?
Smaller samples are less impeded by the gel matrix and hence will move faster through the gel
What does the differing size of the pieces in DNA mean in electrophoresis?
This causes samples of different sizes to separate as they travel at different speeds
Is the process for separating DNA and proteins the same?
While both DNA and proteins are separated according to the same basic process, differences exist between the two protocols
What cuts DNA into pieces?
DNA may be cut into fragments using restriction endonuclease – different DNA samples will generate different fragment lengths
Why do DNA fragments separate?
Fragments separate because DNA is negatively charged due to the presence of a phosphate group (PO43–) on each nucleotide
What gel are DNA fragments placed in?
DNA samples are placed into an agarose gel and fragment size calculated by comparing against known industry standards
How can specific DNA sequences be identified in gel electrophoresis?
Specific sequences can be identified by incorporating a complementary radiolabelled hybridisation probe, transferring the separated sequences to a membrane and then visualising via autoradiography (Southern blotting)
What is the basis of protein separation by gel electrophoresis?
Proteins may be folded into a variety of shapes (affecting size) and have positive and negative regions (no clear charge)
What are proteins first treated with? gel electrophoresis
Proteins must first be treated with an anionic detergent (SDS) in order to linearise and impart a uniform negative charge
What are protein samples placed into? gel electrophoresis
Protein samples are placed into a polyacrylamide gel and sizes compared against known industry standards
How are target proteins identified via gel electrophoresis?
Separated proteins are transferred to a membrane and then target proteins are identified by staining with specific monoclonal antibodies (Western blotting)
What is DNA profiling?
DNA profiling is a technique by which individuals can be identified and compared via their respective DNA profiles
What is within an individual’s genome that is used for DNA profiling?
Within the non-coding regions of an individual’s genome there exists satellite DNA – long stretches of DNA made up of repeating elements called short tandem repeats (STRs)
WHy are short tandem repeats used for DNA profiling?
As individuals will likely have different numbers of repeats at a given satellite DNA locus, they will generate unique DNA profiles
When is DNA profiling commonly used?
DNA profiling is commonly used in criminal investigations (forensics) and to settle paternity disputes
What is the first step for DNA profiling?
A DNA sample is collected (e.g. from blood, semen, saliva, etc.) and then amplified using PCR
What is done with the collected sample? DNA Profiling
Satellite DNA (with STR sequences) are cut with specific restriction enzymes to generate fragments
What will differ in DNA profiling?
Fragment length will differ between individuals due to the variable length of their short tandem repeats
What separates the fragments in DNA profiling?
The fragments are separated using gel electrophoresis and the resulting profiles are compared
What must the DNA profile show if a conviction is to occur, DNA Profiling?
Suspects should be a complete match with the DNA sample taken from the crime scene if a conviction is to occur
What does the number of loci used to generate a unique profile depend on?
The number of loci used to generate a unique profile depends on the size of the population being compared
How can DNA profiling be used to determine paternity?
Children inherit half their chromosomes from each parent and thus should possess a combination of parental fragments
In other words, all fragments produced in the child should also be produced by either the mother or father
How does a gene determine a particular trait?
A gene determines a particular trait by encoding for a specific polypeptide in a given organism
What does the universality of the genetic code allow for in terms of genetic modification?
Because the genetic code is (almost) universal, an organism can potentially express a new trait if the appropriate gene is introduced into its genome
What is genetic modification and what is created?
The transfer of genes between species is called gene modification, and the new organism created is called a transgenic
WHat are the 4 key steps of gene transfer?
Isolation of gene and vector (by PCR)
Digestion of gene and vector (by restriction endonuclease)
Ligation of gene and vector (by DNA ligase)
Selection and expression of transgenic construct
1.1. How can DNA be isolated from cells? GT (gene transfer)
DNA can be isolated from cells by centrifugation – whereby heavier components such as nuclei are separated
1.2. What is done to the gene of interest? GT.
The gene of interest can then be specifically amplified via the polymerase chain reaction (PCR)
1.3 What is an alternate way of generating the gene sequences? GT
Gene sequences can also be generated from mRNA using reverse transcriptase – these DNA sequences (cDNA) lack introns
1.4 What is a vector?
A vector is a DNA molecule that is used as a vehicle to carry the gene of interest into a foreign cell
1.5 What is a common vector in gene transfer?
Bacterial plasmids are commonly used as vectors because they are capable of autonomous self-replication and expression
1.6 What MAY be done to bacterial plasmids? GT
These plasmids may be modified for further functionality (e.g. selection markers, reporter genes, inducible expression promoters)
1.7 What are other types of vectors used in gene transfer?
Other types of vectors include modified viruses and artificial chromosomes
2.1 What must be done to incorporate the gene of interest into the vector? GT
In order to incorporate a gene of interest into a vector, both must be cut with restriction enzymes at specific recognition sites
2.2 What do restriction enzymes do? GT
Restriction enzymes cleave the sugar-phosphate backbone to generate blunt ends or sticky ends (complementary overhangs)
2.3 How will the vector and gene be joined? GT
Scientists will often cleave the vector and gene with two different ‘sticky end’ restriction endonucleases (double digestion) to ensure the gene is inserted in the correct orientation and to prevent the vector from re-annealing without the desired insert
3.1.Where is the gene of interest inserted? GT
The gene of interest is inserted into a plasmid vector that has been cut with the same restriction endonucleases
3.2 Why do the gene of interest and plasmid join together? GT
This occurs because the sticky ends of the gene and vector overlap via complementary base pairing
3.3 What is a recombinant construct and how is it formed? GT
The gene and vector are then spliced together by the enzyme DNA ligase to form a recombinant construct
3.4 What enzymes joins the desired gene and plasmid together? GT
DNA ligase joins the vector and gene by fusing their sugar-phosphate backbones together with a covalent phosphodiester bond
4.1 What is done with the recombinant construct? GT
The recombinant construct (including the gene of interest) is finally introduced into an appropriate host cell or organism
4.2 How can the process of introducing the recombinant construct into the organism be achieved?
This process can be achieved in a variety of ways and is called transfection (for eukaryotes) or transformation (for prokaryotes)
4.3 What is a common way to identify which cells have successfully incorporated the human gene?
Antibiotic selection is commonly used in order to identify which cells have successfully incorporated the recombinant construct