Briefly describes how different sized DNA fragments are sepa

Briefly describes how different sized DNA fragments are separated by gel electrophoresis. What is the purpose of a DNA ladder? What does STR stand for? Suppose two possible alleles for the VWA locus are 5 and 15. What is the difference between these two alleles?

Solution

8. Gel electrophoresis is a process of separating fragments of DNA based on their size. All DNA molecules have same charge per mass, so they are separated on the basis of their size only. In gel electrophoresis, a gel is used which is normally made out of a polysaccharide called agarose. One end of the gel box(containing the gel inside) is connected to a positive electrode and the other end to a negative electrode. DNA molecules have a negative charge, because of the phosphate groups in their sugar - phosphate backbone. So they move through the matrix of the gel towards the positive pole when the power is turned on. Shorter pieces travel faster than the longer ones. So the shortest pieces of DNA will be close to the positive end and the longest pieces near the negative end. The DNA fragments can be visualized by staining the DNA with a DNA binding dye like Ethidium Bromide (EtBr) and then placing them under UV light.

9. A DNA ladder is used to determine the size and and quantity of the given DNA fragments. It is also used to estimate the size of an unknown fragment by comparing it to the size of the known fragments.

10. Full form of STR is Short Tandem Repeat

11. VWA is a STR locus in which particular unit of nucleotides are repeated many times. The difference between these two alleles are that the nucleotide in one is repeated 5 times and in the other 15 times.