Chapter 10: DNA

Dna review What is dna? Who discovered the structure of dna and when? Deoxyribonucleic acid Carries all genetic information Made of nucleotides that have 1 of 4 base pairs that match up (a-t and c-g) Who discovered the structure of dna and when? James Watson and francis crick (using Rosalind franklins research) in 1950s

Dna review Explain the structure of dna Double helix double stranded shape Explain how dna is responsible for coding for amino acids (proteins) Dna codes for certain amino acids and then amino acids in a chain with a start codon and a stop codon form a protein.

Dna review How do diseases like sickle cell arise? The dna sequence coded for a different amino acid so instead of coding for glutamate it codes for valine Now the hemoglobin cells are misshaped and look like “sickles” and can clot easily Explain the process of dna replication The dna strand “unzips” and creates a new strand by semiconservative replication using dna polymerase How does this process relate to the technique of Polymerase chain reaction (pcr)? A lab can take a small piece of dna and replicate it many times in order to have a larger sample to test

Dna typing What is a tandem repeat? Repeating sequences that seem to act as filler or spacers between the coding regions of dna They don’t affect outward appearance or control any other genetic function, but they can identify individuals from others Does all the dna in your genome code for stuff? No…see above

Dna typing Explain what a restriction fragment length polymorphism (rflp) is. These are repeated fragments of dna. There can be thousands of repeated segments of 15-30 How can these rflps be separated by electrophoresis Dna is cut by restriction enzymes and placed on a gel plate. Electric current is sent through the gel and the dna fragments migrate across the plate. Smaller fragments move faster so the pieces separate out by size

Dna typing Explain southern blotting When fragments of dna are transferred to a nylon membrane in the same way you would transfer an ink line onto a blotter. The nylon sheet is treated with radioactively labeled probes containing a base sequence complementary to the rflps being identified How do you compare a rlfp for a match Each sequence has a 1/100 frequency in a population. So if you have 4 probes then your combined frequency would be 100 million in a population What famous case used this technique? Bill Clinton (semen on dress)

pcr Explain the specific steps to pcr Heat the dna strands to 94°C Then add primers to separated strands and allow the primers to combine with the strands by lowering the test-tube temperature to 60°C Then add the dna polymerase and a mixture of free nucleotides (A, C, T, G) to the separated strands Heat tube to 72°C and then polymerase enzyme directs the rebuilding of a double- stranded dna molecule This completes the first cycle and doubles the number of dna strands from one to two.

pcr What are the advantages of using pcr? Shorter strands and therefore less susceptible to environmental degredation Can amplify minute quantities so forensics is not limited by the sample Why don’t we use rlfps anymore? They were too long and tended to break apart or degrade

Str typing What is a short tandem repeat? How does it differ from a rflp? They are locations (loci) on the chromosome that contain short sequence elements that repeat themselves within the dna molecule They are shorter than rflp procedure so are ideal candidates for pcr

Str typing What is multiplexing and how does it help to determine an individual based on their dna? A technique that simultaneously detects more than one dna marker in a single analysis Hundreds of strs are found in humans. The more you can characterize, the smaller the percentage of the population from which they can originate. How many strs does the us use to determine a person’s identity? How do they compare them? 13 Codis What is the alternative to gel electrophoresis? Why is this method preferred? Capillary electrophorensics Its carried out in a thin glass column rather than on the surface of a coated plate

Str typing What is the amelogenin gene and what is it used for in str typing? A genetic locus that’s useful for determining sex How can you tell the difference between the x and y str? The x is 6 bases shorter than the y When a male is tested, it is two bands as one is shorter. A female will show one band since both xs are the same length. What is the y str typing used for? Why is it useful in sexual assault cases? To identify males with the 20 different y str markers identified To swab when multiple males are involved What is the major significance of dna typing? Now vital in solving violent crimes and sexual offenses Dna is impartial, implicating the guilty and exonerating the innocent

Mitochondrial dna What are mitochondria? Cell structures found in all human cells. They produce the energy for the body Where is mitochondrial dna found in the cell? Inside the mitochondria (in loops) How does mtdna compare to nuclear dna in terms of copy number, information, and analysis? It is in a circular loop with enough a, c, t, and g to make 37 genes Inherited straight from the mother When is mtdna used for investigative purposes? When nuclear dna is not possible as it does not come close to the extent that str analysis does.

Codis What is codis and what is it used for Combination dna index system Computer software program developed by the fbi Maintains local, state, and national databases of dna profiles Used to compare dna types recovered from crime-scene evidence to those of convicted sex offenders and other convicted criminals

Dna collection and packaging What is touch dna and how does it relate to epithelial cells? Touch dna is dna from skin (epithelial) cells transferred onto the surface of an object by simple contact Explain the proper procedures for collecting and packaging dna Collecting: Photographed first, no disturbing of pattern, wear latex gloves Packaging: not packaged in plastic or airtight because residual moisture could contribute to growth of dna destroying bacteria and fungi Each article separate and in a paper bag or well ventilated box Why do you need a dna reference sample? It only has value when an analyst can compare each of the dna types to a sample from a victim or a suspect

How do you prevent contamination of dna specimens?