Why is blood collected at a crime scene not a good source of nuclear DNA?
The correct answer and explanation is:
Correct Answer:
Blood collected at a crime scene is not always a good source of nuclear DNA because it may be degraded or contaminated, and red blood cells do not contain nuclei, which limits the amount of nuclear DNA available.
Explanation (300 words):
Nuclear DNA is found within the nucleus of cells and carries the genetic blueprint that is unique to each individual (except identical twins). While blood might seem like an ideal source of DNA due to its frequent presence at crime scenes, it has limitations, particularly regarding nuclear DNA.
Blood is composed primarily of red blood cells (RBCs), white blood cells (WBCs), and platelets. Red blood cells make up about 99% of all blood cells and do not contain nuclei, meaning they lack nuclear DNA. The only blood components that contain nuclei (and therefore nuclear DNA) are white blood cells, which are present in much smaller quantities (less than 1% of total blood volume). This makes the amount of nuclear DNA in a blood sample relatively low compared to other tissue types.
In addition to the low concentration of nuclear DNA, environmental conditions at crime scenes can degrade DNA quickly. Exposure to heat, moisture, UV light, bacteria, or chemicals can break down the DNA molecules, making it difficult or impossible to get a usable DNA profile.
Another issue is contamination. Blood samples at crime scenes may be contaminated with DNA from other individuals (like first responders or suspects), making it harder to isolate the victim’s or perpetrator’s nuclear DNA.
Therefore, while blood can still be useful in DNA analysis—especially for mitochondrial DNA, which is more abundant and more stable than nuclear DNA—it is not always reliable as a source of nuclear DNA. Investigators often prefer other biological materials, such as buccal cells, bone, teeth, or semen, for a more reliable nuclear DNA profile.