i know grim topic but this is good for every woman to know
Rapists snared by sperm-free semen
07 March 2007
Exclusive from New Scientist Print Edition. Subscribe and get 4 free issues.
Linda Geddes
Dividing the sexesPolice investigating sexual assaults could be aided by a new technique that allows DNA to be collected from semen samples even when no sperm are present.
A common problem for forensic scientists hoping to use DNA fingerprinting to identify assailants in sexual assault cases is that the quantity of male DNA in swabs taken from the woman is often tiny compared to the amount of hers present. "The female DNA profile is so strong in the analysed sample that the male DNA is swamped," says Andy Hopwood of the Forensic Science Service in Birmingham, UK. This means that conventional methods of amplifying small amounts of DNA don't work, because the female DNA would be amplified as well. "We're looking for ways to isolate out the male component," Hopwood says.
One way of doing this is using a method called preferential lysis, in which enzymes are used to destroy the membranes of ordinary cells, leaving only the more robust sperm cells behind. However, in some cases the assailant may not produce any sperm - either because of a medical condition or because he has had a vasectomy.
"It is surprisingly common," says Hopwood. "The semen may have come from a vasectomised male in around 10 to 15 per cent of the cases we deal with. We got to thinking: 'What else is there in this sample that we could look for?'"
“The semen may have come from a vasectomised male in around 10 to 15 per cent of the cases we deal with”Semen doesn't only contain sperm, but often immune and epithelial cells as well. Until recently, these would have been virtually impossible to distinguish from female cells present in a swab. Now Hopwood's team has done just that by combining a technique called laser microdissection (LMD), which enables single cells to be extracted from a microscope slide, with fluorescence in-situ hybridisation (FISH), a method that can be used to highlight chromosomes carrying a particular DNA sequence.
Hopwood's team used fluorescent tags specific to repetitive areas of DNA on the X and Y chromosomes. When added to cells and viewed under a fluorescence microscope, these tags made the X chromosomes glow red and the Y chromosomes green. This made it possible to distinguish between male cells containing an X and a Y chromosome, and female cells containing two X chromosomes.
The team then used LMD to mark the coordinates of male cells on a microscope slide and cut them out from the plastic membrane they were sitting on. The researchers transferred the cells to a collecting tube and used DNA amplification to increase the amount of male DNA. They were then able to compare the DNA against a database of DNA profiles in order to search for the assailant.
Using this technique, full male DNA profiles have so far been obtained from vaginal swabs taken up to 24 hours after sexual intercourse, even when no sperm were present.
The FSS gets around 90 cases a year in which the new technique could be useful, says Keith Elliott, one of Hopwood's team. "These are really difficult cases where you have a sample that's semen positive, but sperm negative," he says. In January, the technique was put into practice for the first time, helping UK police to charge a suspected rapist when his DNA profile was found to match the male DNA recovered from the victim. The case is currently going through the UK courts.
From issue 2594 of New Scientist magazine, 07 March 2007, page 16
07 March 2007
Exclusive from New Scientist Print Edition. Subscribe and get 4 free issues.
Linda Geddes
Dividing the sexesPolice investigating sexual assaults could be aided by a new technique that allows DNA to be collected from semen samples even when no sperm are present.
A common problem for forensic scientists hoping to use DNA fingerprinting to identify assailants in sexual assault cases is that the quantity of male DNA in swabs taken from the woman is often tiny compared to the amount of hers present. "The female DNA profile is so strong in the analysed sample that the male DNA is swamped," says Andy Hopwood of the Forensic Science Service in Birmingham, UK. This means that conventional methods of amplifying small amounts of DNA don't work, because the female DNA would be amplified as well. "We're looking for ways to isolate out the male component," Hopwood says.
One way of doing this is using a method called preferential lysis, in which enzymes are used to destroy the membranes of ordinary cells, leaving only the more robust sperm cells behind. However, in some cases the assailant may not produce any sperm - either because of a medical condition or because he has had a vasectomy.
"It is surprisingly common," says Hopwood. "The semen may have come from a vasectomised male in around 10 to 15 per cent of the cases we deal with. We got to thinking: 'What else is there in this sample that we could look for?'"
“The semen may have come from a vasectomised male in around 10 to 15 per cent of the cases we deal with”Semen doesn't only contain sperm, but often immune and epithelial cells as well. Until recently, these would have been virtually impossible to distinguish from female cells present in a swab. Now Hopwood's team has done just that by combining a technique called laser microdissection (LMD), which enables single cells to be extracted from a microscope slide, with fluorescence in-situ hybridisation (FISH), a method that can be used to highlight chromosomes carrying a particular DNA sequence.
Hopwood's team used fluorescent tags specific to repetitive areas of DNA on the X and Y chromosomes. When added to cells and viewed under a fluorescence microscope, these tags made the X chromosomes glow red and the Y chromosomes green. This made it possible to distinguish between male cells containing an X and a Y chromosome, and female cells containing two X chromosomes.
The team then used LMD to mark the coordinates of male cells on a microscope slide and cut them out from the plastic membrane they were sitting on. The researchers transferred the cells to a collecting tube and used DNA amplification to increase the amount of male DNA. They were then able to compare the DNA against a database of DNA profiles in order to search for the assailant.
Using this technique, full male DNA profiles have so far been obtained from vaginal swabs taken up to 24 hours after sexual intercourse, even when no sperm were present.
The FSS gets around 90 cases a year in which the new technique could be useful, says Keith Elliott, one of Hopwood's team. "These are really difficult cases where you have a sample that's semen positive, but sperm negative," he says. In January, the technique was put into practice for the first time, helping UK police to charge a suspected rapist when his DNA profile was found to match the male DNA recovered from the victim. The case is currently going through the UK courts.
From issue 2594 of New Scientist magazine, 07 March 2007, page 16