Molecular Biology Quiz

Study Notes

Enzymes and proteins like DNA polymerase are involved in DNA replication, unwinding DNA and assembling new DNA strands
PCR (polymerase chain reaction) is a technique used to replicate small amounts of DNA, overcoming the limitation of sample size in forensic investigations

Restriction enzymes are chemicals that cut DNA into fragments
Recombinant DNA involves inserting a cut fragment of DNA into a foreign DNA strand, usually that of a bacterium
This process allows for the production of substances like insulin and growth hormone, as the bacteria multiply replicating the altered DNA

Electrophoresis is a technique similar to TLC (thin-layer chromatography)
Substances like DNA are forced across a gel-coated plate under the influence of an electrical potential
This separates the substances based on fragment size

RFLP uses a restriction enzyme to cut DNA into fragments, which are then sorted by electrophoresis
Smaller fragments move faster on the gel plate than larger ones
Fragments are transferred to a nylon membrane in a process called Southern blotting
To visualize RFLPs, the nylon membrane is treated with radioactive probes with base sequences complementary to the RFLPs
The sheet is exposed to X-Ray film for several days, revealing bands where the radioactive probes have stuck to the fragments
This process creates a DNA fragment pattern, typically showing two bands (one from each chromosome)
Comparing band sets between multiple specimens can be used for identification, with higher levels of discrimination achieved by using multiple probes
A single band in a suspect's lane could indicate that they are homozygous for that particular gene

PCR enables amplification of minute DNA samples
PCR relies on the knowledge of how DNA strands naturally replicate within a cell
Using the low copy number DNA protocol, analysis of samples containing as few as 18 cells is possible
The process involves heating DNA to separate the strands, adding primers to hybridize with the strands, and then copying the DNA through cycles that take approximately two minutes each

STRs are typically 3 to 7 base repeating sequences, making them ideal for PCR amplification
The entire STR strand being less than 450 bases makes them less susceptible to degradation
The amelogenin gene, used in commercial STR kits, is a marker for gender determination

Hundreds of different STRs can be found in human genes
STRs are highly variable between individuals
Multiplexing involves the simultaneous extraction and amplification of multiple STRs for more accurate identification

US crime laboratories have standardized on 13 STRs for entry into the national database (CODIS)

Biological evidence should not be packaged in airtight containers to prevent the growth of bacteria and fungi, which could damage DNA
Each stained article should be packaged separately in a paper bag or well-ventilated box
Whole blood for DNA typing should be placed in a vacuum tube with the preservative EDTA
Dried blood should be removed with a sterile swab moistened with distilled water and air-dried
All biological evidence should be refrigerated until delivery to the laboratory
Standard/reference DNA specimens such as blood or buccal swabs should also be collected