Was it explained where sperm fractions or whole sperm cells come from, if not semen? I'm sorry, I just don't get it. JMO
I don't get it...but I'm going to start reading:what:
DNA Testing: An Introduction For Non-Scientists
An Illustrated Explanation
Analysis of Separated Sperm and Non-Sperm Fractions.
In order to perform DNA typing on sperm DNA, it is desirable to separate the sperm DNA from any other DNA that may be present. For example, in swabbed materials from a rape evidence kit, the swabs may contain non-sperm cells from the victim as well as sperm and non-sperm cells from the rapist. To accomplish separation of the sperm cells, a process known as differential extraction is often performed. This involves lysing (breaking open) the non-sperm cells followed by spinning (centrifugation) the mixture to remove the still unbroken sperm cells. To do this, chemicals, usually an enzyme called proteinase K (PROTEIN-ACE-K) (breaks down most proteins), and a mild detergent (breaks down cellular membranes) are added to the original mixture of sperm and non-sperm cells. The enzyme and the mild detergent can lyse most cell types but not sperm because the sperm cell membranes have cross-linking chemical bonds called disulfides (pronounced DI-SUL-FIDES). Actually, the illustration below is slightly incorrect because the proteinase K does remove most of the sperm tails. These were left in the illustration to assist in following what happens to the sperm.
When the treated mixture is spun in a centrifuge, the sperm are forced to the bottom of the tube because they are dense. On the other hand, the broken, non-sperm cells are not very dense so they stay higher in the tube. This higher portion is called the supernatant and after the first spin in the centrifuge, the supernatant can be removed. The supernatant is referred to in various ways but usually it is called the non-sperm or the E1 fraction. The pellet (the material at the bottom of the tube) is called the sperm or E2 fraction. Usually, the pellet will be re-suspended in fresh liquid and re-spun to help purify it away from non-sperm DNA.
Finally, the sperm fraction is lysed (the sperm cells broken open) by adding a chemical called DTT. The DTT breaks the disulfide bonds releasing the sperm DNA.
The description of this procedure so far is quite ideal. It works pretty much as described for fresh samples. Even with fresh samples however, some of the non-sperm DNA will be trapped in the sperm pellet. This can be a major problem if the amount of sperm is very low or if the samples are aged and degraded. Often male cells, most likely immature sperm or white cells may end up in the supernatant, variously called the “female” fraction or “non-sperm” fraction
There are illustrations in the link
http://www.scientific.org/tutorials/articles/riley/riley.html
