Essay --

Short Tandem Repeats are nucleic acid sequences in short strings, usually1-7 bases long, (ex. AGAT) which repeat over and over (ex. AGATAGATAGAT†¦). STRs can be subject to duplication, inversion or deletion, which may alter their sequence, if only by one base pair. They can be typed by PCR. The small size of STRs makes them easy and efficient to amplify, and allows testing to be done even on small pieces of DNA. This means STR typing can be performed on lower quality samples, more quickly (24 – 48 hours), and with smaller sample sizes (1 ug – 10 ng). Accordingly, STR typing is a great method for both forensic and paternity testing. The FBI developed the Combined DNA Index System (CODIS), a database of DNA profiles, using 13 STR markers. Similar to databases in other countries, CODIS catalogs DNA from repeat criminals, allowing agencies to search for a match between crime scene DNA and profiles in their catalog, in an attempt to identify a perpetrator. STR alleles are distinguished from each other by how many times their base sequence repeats (ex. how many times AGAT is repeated). PCR distinguishes how many base sequences are present in an allele by attaching a fluorescent marker to each base sequence (ex. AGAT), making it possible to count the number of repeats in a product. The process is typical of PCR. In the initial amplification step, sample is incubated with primers, polymerase, and buffer. This procedure is concurrently performed on control DNA from a standard. Next, the DNA is added to electrophoretic gel, along with formamide, allelic ladders (various alleles of the locus being sought), and internal size standards (markers of molecular weight). Once the gel has run, computers analyze the products. Analytic software det... ...ng with the APC itself). MHC Class I cells can be any nucleated cell within the body. If the MHC is Class II, it binds with extracellular antigens, such as parasites, bacteria and toxins. These extracellular antigens need to be inside the APCs, before they can bind with MHC. This requires APCs with MHC Class II to be phagocytic cells that swallow up the antigenic material. MHC Class II bearing APCs can be B-cells, dendritic cells, langerhans cells, or macrophages. Lysosomes inside these cells contain enzymes that break the antigens down into peptides. Once presented, the MHCs communicate with CD4+ receptors on T-cells. T-cells with these receptors are destined to become helper T-cells, once they are activated. At this point, the helper T-cells can release cytokines and coordinate the immune response by activating cytotoxic T-cells, other helper T-cells and B-cells.