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Introduction to some basic biology

Introduction to some basic biology
Forensic biology is the study of blood and body fluids in the context of the investigation of crime. If we think back to the discussion in week one of the six Ws, we see that the context of forensic biology is mainly addressing the questions of who and what, as highlighted here in the investigative star. Most major crimes are crimes against the person, from assault through attempted murder to rape and murder. Many of these events will result in the shedding of one or more body fluids, which can be linked to the donor by DNA testing. And the very presence of these outside of the body of the donor can provide information as to what happened.
Some examples include, a burglar attempts to enter a house by breaking a window and cuts himself on the glass leaving a blood stain. Analysis of the stain can provide information about who the blood came from. A man is attacked by two youths intending to rob him. He fights back and is stabbed. Bloodshed from the stab wound may be transferred to one or both assailants. And analysis of stains on their clothes can provide a link between the assailants and victim. If the knife is discarded near the scene but found by the investigators, blood on it will provide a link to the crime, the what.
And swabs from the handle can be analysed and provide a link to the person who used it, the who. A young man is in a state of extreme depression and commits suicide by shooting himself in the temple. He falls down dead, dropping the gun. He is an illegal immigrant and friends, also illegal immigrants, find the body and dump it and the gun in a wooded area in a nearby park in order to conceal the death. The body and gun are discovered and in reconstructing events blood spatters on the right hand of the deceased corroborate other data that resulted in a conclusion of suicide and not of murder, the what.
Before we go into detail on how forensic biology can tell us about what took place and who was involved, we will revise the biochemistry of the main body fluids involved– blood, semen, vaginal secretions, and saliva. The average human body contains four to six litres of blood, approximately 45% of which consists of cells with the remainder being an aqueous solution of salts, proteins, and various other chemicals. The volume of blood is distributed approximately 58% in veins, 13% in arteries, 12% in the pulmonary vessels, 9% in the heart, and 8% in the arterioles and capillaries. There are three distinct types of cellular material in blood.
The red coloured bi-concave discs known as erythrocytes, or RBCs, white blood cells or leukocytes, and platelets or thrombocytes, which are not cells but small, colourless discs that are involved in blood clotting. There are about 5 million erythrocytes per micro-litre of blood. They give blood its colour and are unique in the body in that they have no nucleus. There are about 9,000 leukocytes per microlitre of blood. These nucleated cells play a vital role in antibody production, are larger than erythrocytes, and are further classed as granulocytes, lymphocytes, and monocytes. The leukocytes are the source of the DNA used for DNA profiling of blood stains.
When blood is shed from the body, a complex biochemical chain reaction takes place which results in the protein fibrinogen being converted to fibrin, which traps platelets to form a clot and staunch the bleeding. The same reaction takes place in blood withdrawn into a syringe unless an anticoagulant such as heparin or EDTA is added. The non-cellular fragment of whole, uncoagulated blood is called plasma. And the non-cellular fraction of whole, coagulated blood is called serum. The validity of the examples given above depends on the material examined being blood, no matter whether the testing is directed to identification of the person from whom the blood came or reconstruction of the circumstances in which it was deposited.
This is achieved by applying a logical cascade. Could it be blood? If so, could it be human? And can we confirm this to the exclusion of any other likely explanation? The sequence begins with a screening or presumptive test. For example, the examination of the seat covers from the car begins with a visual inspection followed by swabbing with dry filter paper and the addition of the Kastle-Meyer presumptive test reagent and hydrogen peroxide. The rapid development of a strong pink colour is a positive reaction for blood. However, some vegetable materials will also display peroxidase activity and tomato ketchup will also give a dark red colour.
Traditionally, a positive screening test was followed by a test specific for human serum or human haemoglobin, which would confirm that the material is blood and is of human origin. Today, the screening and confirmatory test can be combined using a technique similar to those used for home pregnancy testing. You can find more information and the basis for one of these tests in the resources section. Since DNA testing includes features that are specific for human DNA, many laboratories go straight from the screening test to DNA profiling. Semen is the name given to the fluid produced by the male sex organs and ejaculated from the penis. It contains a cellular component, spermatozoa, carried in a fluid component, seminal plasma.
A normal ejaculate is about three to four millilitres in volume, and contains somewhere between 70 and 150 million sperm. Human sperm are of a unique size and shape, consisting of a head, tail, and mid piece. The head is about 4.5 micrometres long by 2.5 micrometres wide. The tail is about 40 micrometres long. The characteristic morphology can be observed by microscopy, which is one of the techniques that can be used to identify a stain or a sample taken from a body cavity as containing semen. The DNA through which paternal genes are passed to the offspring when an egg is fertilised is contained in the head.
It is the source of the material used in DNA profiling of samples in the investigation of sexual offences. Seminal plasma is an aqueous solution of salts, proteins, and various other chemicals originating from the seminal vesicles and the prostate gland. Of particular forensic interest are the proteins Acid phosphatase AP, and P30, or prostate-specific antigen, also referred to as PSA, produced in the prostate. Acid phosphatase is an enzyme that is widely distributed in human tissues, but which is found in particularly high concentrations in semen. PSA is not unique to the prostate, but levels in other tissues are much lower than in semen, which means that it can be used as a confirmatory test for semen stains even from vasectomised men.
PSA levels in blood are used clinically for screening for prostatic cancer, one of the rare times that forensic science has contributed to medicine rather than the other way around. Although seminal fluid is regarded as an aqueous solution, it will contain cells shed by the epithelial linings of the various ducts through which it travels, from formation in the seminal vesicles, prostate, and bulbo-urethral glands. Blood and semen dominate the body fluids of forensic interest, but others are also encountered. Vaginal secretions can be DNA-profiled to provide the identity of the woman. Saliva associated with bite marks, or with postage stamps or envelopes can likewise produce a DNA profile.

We’re going to start with some basic biology and how we can use this in criminal investigation. Keep in mind the information you already know about the Case Study as we look at the materials and in particular what aspects of biological evidence may be important.

The video opens by illustrating how forensic biology can answer some of the questions contained in the 6Ws tool.

Note the variation on the presentation of the 6Ws, by using a device called the “Investigative Star” which shows how each of the Ws can link to any and all of the others, emphasising that the tool is shorthand for an iterative process. There is then a brief review of the biochemistry of blood – many of you will be familiar with this, but those who are not should make sure that they understand the basics and the terminology used.

Probably the most important part of this section is that describing how blood stains are examined at the scene and in the laboratory, with the sequence of visual inspection -> presumptive or screening test -> confirmatory test. To see how these can be condensed into one test, view the video on HemaTrace in the ‘see also’ resource section below.

Finally, there is a very brief overview of other body fluids, mainly semen.

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Introduction to Forensic Science

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