DNA profiling 2

Question 1

How is the sample prepared for electrophoresis?

Answer 1

• DNA fragments are put into wells in agarose gel strips
  
  • contain buffering solution
  • in first and last wells DNA framgents of known length are used to preovide a reference for fragment sizing

Question 2

How does electrophoresis occur?

Answer 2

• when electric current passed through palte
  
  • DNA fragments in wells at the cathode (-ve) move through the gel towards the +ve anode at the other end
  • due to -ve phosphate groups
• rate of movement depends on the mass or length of DNA fragments
  
  • the gel has a mesh structure that resists movement of particles
  • small particles move more than larger
  • when faster small fragments reach the anode end of the gel the electric current is switched off

Question 3

WHat happens to the gel after the current is stopped?

Answer 3

• placed in alkaline buffer solution to denature the DNA fragments
• the two DNA strands of each fragment separate, exposing the bases

Question 4

When is PCR used?

Answer 4

• solving crimes and only very tiny amounts of DNA may be available

Question 5

What is PCR generally?

Answer 5

• version of the natural processs by which DNA is replicated
• allows scientist to produce a lot of DNA from a small sample

Question 6

What is needed to be added to allow a DNA sample to be amplified?

Answer 6

• excess of four nucleotide a bases
  
  • in the form of deoxynucleoside triphosphates
• small primer DNA sequences and the enzyme DNA polymerase are mixed in a vial that is placed in a PCR machine (thermal cycler)

Question 7

What need to be controlled in a PCR machine?

Answer 7

• temperature at programmed intervals
  
  • triggering different process

Question 8

What is a primer?

Answer 8

• short strand of RNA or DNA
• serves as starting point for DNA syntehsis
• required for DNA replicated because enzymes that catalyse this process, DNA polymerases, can only add new nucleotides to an existing strand of DNA

Question 9

What happens in stage 1 of PCR?

Answer 9

• separating strands
• temp increased to 90-95 degress for 30 seconds
• denature the DNA by breaking the H-bonds holding the strands together, so they separate

Question 10

What is step 2?

Answer 10

• primers added
• temperature decreased to 55-60
• primers anneal (bind) to the ends of the DNA strand
• need for replicateion to occur

Question 11

WHat is step 3?

Answer 11

• temp increased to 72-75 for at least 1 min
• optimum temp for DNA polymerase to work best
• DNA polymerase adds bases to the primer, bulding up the complementary strands of DNA so producing double-stranded DNA identical to the original sequence

Question 12

How is DNA used in forensic science?

Answer 12

• used in criminal investigation
• PCR and DNA profiling peroformed on traces of DNA at crime scenece
  
  • e.g/ saliva, hair roots, blood , semen
• compared to sample from suspect of from criminal DNA database
• useful for evidence

Question 13

What are the other uses of DNA profiling?

Answer 13

• proving paternity
  
  • e.g. in immigration cases to prove relationships
• identifying species
  
  • more accurate than older methods
• identifying evolutionary relationships
• identifying people who are at risk of diseases
  
  • gene markers for diseases can be observed