2Insufficient evidence? Read the following article.Mr. Chan applied for permission for his son to come to HK to live with him, he had to provide evidence to prove his paternal relationship with his son. He underwent the blood group typing tests and obtained proof that his blood type matched with that of his son. However, Mr. Chan was told by the Immigration Department that the blood group typing result was an insufficient evidence.
3Insufficient evidence? Why was the blood group typing result not sufficient evidence?Any other tests recommend?
4Blood group typing is much less sensitive Blood group typing is much less sensitive. May use DNA fingerprint to provide biological evidence as a proof in a paternity test.
6Structural relationship among chromosomes, DNA and genes Cells are the basic building blocks of all living things.In a cell, DNA (deoxyribonucleic acid) is packaged in chromosomes within the nucleus.DNA is a large, polymeric molecule.A gene is a segment of DNA molecule of a chromosome. It is the basic unit of heredity. Genes determine the body characteristics of an organism.Each inherited characteristic is controlled by one or several genes.
7Look into DNADNA looks like an incredibly long twisted ladder. This shape is called a double helix.The sides of the ladder are a linked chain of 5- carbon sugars and phosphate (PO4) groups (called the backbone).The rungs connected to the 5-carbon sugars are known as bases.
8DNAEach rung is made up of two bases that link together. There are four bases - adenine (A), thymine (T), guanine (G) and cytosine (C).Because of their chemical nature, A will only link with T and G will only link with C. No base can join with itself (i.e. NO A-A /T- T /G-G / C-C base-pairs).
9Activity 8.1 What is DNA?Refer to the Pre-lesson Reading, finish Activity 8.1 to check your understanding on DNA.
10Activity 8.1Decide whether the following statements are TRUE or FALSE.There are 46 pairs of chromosomes in each nucleus of a human cell.Chromosomes are made of DNA and proteins.DNA determines the body characteristics of an organism.DNA may be extracted from red blood cells found in a blood sample.
11Activity 8.1The Hair and Teeth of the same person are composed of same DNA molecules.Identical twins have different base sequences of DNA .In the DNA structure, adenine (A) will only link with cytosine (C) and guanine (G) will only link with thymine (T).
12Activity 8.1Answers:a. Fb. Tc. Td. Fe. Tf. Fg. F
13Activity 8.1DNA has two strands. And the sequence of one strand determines the sequence of the other. If the base sequence on one strand of DNA is:GATCCTCATAWhat is the base sequence on the other strand?Answer: GATCCTCATACTAGGAGTAT
15Activity 8.2 DNA Extraction Objective of experimentTo extract DNA from strawberries with washing-up liquid.Apparatus and Materialswashing-up liquid (detergent)saltice-cold alcoholice bath
16Activity 8.2 Follow the procedures to extract DNA. Grind strawberries with a pestle in the mortar.Add about 10 mL washing-up liquid, 1/2 teaspoon salt and approximately 30 mL tap water to the grinded strawberries. Stir the mixture gently for 1 minute.Filter the mixture through a muslin cloth into a test tube, about 1/3 full, to separate strawberries from the clear liquid.Double up the volume of the mixture by adding alcohol slowly into the test tube.Leave the mixture undisturbed for 2 – 3 minutes in an ice-bath until no emergence of bubbles.Swirl a toothpick around the interface of the two layers formed inside the test tube. Long and stringy threads can be picked up.
17Activity 8.2 Questions (optional) What is the purpose of grinding the strawberries?To break down the cell wall, cellular membranes and nuclear membranes.Where is the DNA of the strawberries?In the cell nucleus.What is the function of detergent in step 2?Soap dissolves cell membranes so that DNA is liberated.
18Activity 8.2 Questions (optional) What is the function of salt in step 2?Salt solution helps the DNA strands come together.What happens when alcohol is added to the solution in step 4?DNA is soluble in water and will appear clear in water. But DNA is insoluble in non-polar solvent, such as alcohol. Therefore, DNA will precipitate (solidify and appear) out of alcohol. Of course, this is not pure DNA.
19Follow-up Discussion Write down your answers on a piece of paper.
20Activity 8.2 Follow-up Discussion Questions Do you think your results would be different if you use a vegetable or fruit other than strawberries, say, onion, kiwi fruit, broccoli, banana (without skin)? Explain.Do different organisms have the same DNA?At a crime scene what evidence would you collect in order to extract DNA? List as many as possible.
21Activity 8.2Answers:DNA can be easily extracted from many different plants. The amount of DNA extracted depends upon many factors, including the number of cells crushed, and whether the cells can be easily broken apart.Though all DNA molecules are made up of similar chemical components, different organisms have different DNA molecules due to the different base sequences of the bases — A, T, G, and C, which makes a specific organism have distinctive characteristics.Blood (white blood cells), saliva, skin scalp, hair and etc.Note that DNA is found in nearly every cell in the body. But one significant exception is red blood cells (lack cell nuclei).
23Collection of DNA Evidence Before the collection of evidence close-ups of any biological evidence should be photographed and its location relative to the entire crime scene needs to be recorded through notes, sketches, and photographs.All clothing from both the victim and suspect should be collected and sent to the laboratory for examination. Why?
24Collection of DNA Evidence Answer:Because the criminal may have wiped his or her hands on materials not readily apparent to the investigator. In addition, blood may exist in less-than-obvious places.
25Activity 8.3 Collection & Preservation of DNA Evidence Guess (i) the possible locations of DNA on the collected evidence from a crime scene and (ii) the possible sources of DNA.EvidencePossible locations of DNA on the evidenceSources of DNAe.g. EyeglassesEar piecesSweat, skinBaseball batFacial tissueDirty laundryUsed cigaretteBlanket, pillow, sheetBite markFingernail, partial fingernail
26Possible locations of DNA on the evidence Activity 8.3AnswersEvidencePossible locations of DNA on the evidenceSources of DNAEyeglassesEar piecesSweat, skinBaseball batHandleSweat, skin, bloodFacial tissueSurface areaMucus, blood, sweat, semenDirty laundryBlood, sweat, semenUsed cigaretteCigarette buttSalivaBottle, can, glassSides, mouthpieceSaliva, sweatBlanket, pillow, sheetSweat, hair, semen, urine, salivaBite markPerson’s skinFingernail, partial fingernailScrapingsBlood, sweat, tissue
27Preservation of DNA Evidence Paper bags or other “breathable” containers should be used. Because biologic evidence must never be packaged in airtight containers because moisture can build up, which promotes the growth of bacteria that can degrade DNA.All precautions against contamination must be taken. These includewearing face marks, shoe covers and gloves;using tools such as tweezers to collect evidence.
28Precautions in handling biological evidence All body fluids must be assumed to be infectious; hence, wearing disposable latex gloves while handling the evidence is required to prevent being infected.
30Restriction EnzymesA common use of restriction enzymes is to generate a "fingerprint" of a particular DNA molecule.DNA may be cut by some special naturally-occurring proteins called restriction enzymes.Each restriction enzyme only identifies and “cuts” at a very specific sequence (recognition site) in the DNA strand.Restriction enzymes typically recognise a symmetrical sequence of DNA, such as the site GAATTC.
31Example 1Read to the leftRead to the rightThe top strand is the same as the bottom strand when you read backwards.When the enzyme EcoRI cuts the strand between G and A, it leaves overhanging chains:These are termed "sticky ends" because the base pairs formed between the two overhanging portions will glue the two pieces together, even though the backbone is cut.
32Example 2 GGATCC CCTAGG G GATCC CCTAG G Another symmetrical sequence of DNA, GGATCC:GGATCCCCTAGGWhen the enzyme Bam HI cuts the strand between G and G, it leaves overhanging chains:G GATCCCCTAG G
33Activity 8.4 Restriction Enzymes In the worksheet, different DNA samples with their restriction sites are given. Answer the following questions:How many times does the specified base sequence appear?How many fragments would result from a restriction digestion of the DNA sample with the given enzyme?List all the fragments formed.
34Activity 8.4 Answers: a. The strand is cut into four pieces. b. Fragments produced:Hae III,Bam HI
36ElectrophoresisDNA fragments of different sizes can be separated by using gel electrophoresis.DNA fragments carry negative charges. When there is a current flow, DNA fragments move towards the anode.Source: Science Education Section, CDI, EDB.
38Outline of Electrophoresis Source: Science Education Section, CDI, EDB.DNA fragments are loaded into “wells” in a gel. The gel floats in a buffer solution within a chamber between two electrodes.When an electric current is passed through the chamber, negatively charged fragments move towards the positive terminal.Shorter DNA fragments (smaller size) move faster than the longer ones (bigger size).In a given time, DNA fragments are separated into bands according to their size.“wells”Source: Science Education Section, CDI, EDB.
39Electrophoresis demonstration The steps for Electrophoresis are demonstrated in the following video:(Video Clips: DNA Gel Electrophoresis)
40DNA Analysis Technique Restriction Fragment Length Polymorphism (RFLP)
41RFLP It was the first commercial technique of DNA analysis. It can be used in paternity cases or criminal cases to determine the source of a DNA sample.
42RFLP Steps It uses restriction enzymes to cut DNA. DNA fragments of different lengths are produced.
43RFLPUsing gel electrophoresis, the fragments are separated so that fragments with different lengths can be separated. This provides a pattern of bands.
44RFLPCover specific radioactive probes over the gel. The probes contain a match for the DNA sequence that the test is looking for.
45RFLPPut a film under the gel to record. Allow signal exposure in dark room.DNA fingerprints are now ready for analysis and comparison.For animation, you may browse(Virtual Lab. Topic: 7. RFLP)
46Another DNA Analysis Technique To learn more, you may surf the following websites:Polymerase Chain Reaction (PCR)（http://users.ugent.be/~avierstr/principles/pcr.html）Short Tandem Repeats (STRs)（http://www.cstl.nist.gov/biotech/strbase/intro.htm）Mitochondrial DNA (mDNA)（
47RFLP Vs PCRAdvantage of using RFLP (more often used in forensic work):Can individualise a specimen to a narrow portion of the population-possibly one person in billionsDisadvantage of using RFLP:Requires a larger sample than PCR; more time-consuming and labour intensive; uses radioactive reagents that require special lab. Procedures.
48PCR Vs RFLP Advantages of using PCR: Faster and simpler to use and may be applied to exceedingly tiny samples-as small as a billionth of a gram of DNADisadvantage of using PCR:Results of PCR are less dramatic than those of RFLP, being discriminatory on the order of one individual in thousands rather than the potential billions with RFLP(Crime Science: Methods of Forensic Detection (p.203))
49What is the use of DNA fingerprinting? More than 99% of the base sequence in DNA is the same in all humans. Less than 1% is unique to each individual.Scientists make use of this difference to generate DNA fingerprints which are specific to different individuals.
51Warm-up exercise In the following simulation, decide which two fingerprints match each other;drag the right-most DNA fingerprint over the suspects’ fingerprints to find a match.
52Activity 8.5 DNA Fingerprint Analysis Case 1Mr. Chan’s family consists of mom, dad and four kids. The parents have one daughter and one son together, another daughter is from the mother’s previous marriage, and the other son is adopted. Here are the DNA analysis results:Which child is adopted? Why?Which child is from the mother’s previous marriage? Why?Who are the own children of Mr and Mrs Chan?
53Activity 8.5 Answers: Child 4 is adopted. Child 2 is the child from the mother’s previous marriage.Child 1 and Child 3 are own children of Mr and Mrs Chan.
54Activity 8.5Case 2A blood sample from a crime scene was collected. DNA samples of the victim and the potential suspects (June, Scarlet and John) were also collected for DNA analysis. The DNA profile is shown. Now, you should be able to identify the potential murderer.
55Activity 8.5Answers:All of the DNA fragments of Scarlet can be found in the crime scene sample making her the most likely suspect.
56Activity 8.5 Extended discussions: Both June and Scarlet have the same DNA fragments (“8” and “12”), why?Why DNA evidence must be combined with the traditional forms of evidence such as eyewitness accounts?
57Activity 8.5Answers:This pattern may arise if the two women are related or if this pattern were common in population.Someone’s DNA is found at a crime scene does not mean that they committed the crime because of the following reasons:(i) The DNA sample may be contaminated by the environment.(ii) The sample may be a mixture of more than one person’s DNA.(iii) The DNA evidence may be degraded or broken down.
58Activity 8.6 Ethical and Non-ethical Issues Write down your answers on a piece of paper.
59Activity 8.6 Group Discussions Do white blood cells give more accurate test result for paternity than cheek cells?What are the legal and ethical issues on the use of DNA fingerprinting in society?DNA fingerprinting can be used to convict someone of a crime. What would be the difficulty in using this technique to identify criminals if the suspects were father and son / twin brothers?Other than forensic science, list out the other uses of DNA analysis. Describe each use briefly.
60ReferencesNickell, J. and Fischer, J.F. (1999). Crime Science: Methods of Forensic Detection. Kentucky: The University Press of Kentucky.