DNA stands for deoxyribose nucleic acid DNA is a very large molecule made up of a long chain of sub-units The sub-units are called nucleotides Each

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2 DNA stands for deoxyribose nucleic acid DNA is a very large molecule made up of a long chain of sub-units The sub-units are called nucleotides Each nucleotide is made up of a sugar called deoxyribose a phosphate group - and an organic base 2

3 Ribose is a sugar, like glucose, but with only five carbon atoms in its molecule Deoxyribose is almost the same but lacks one oxygen atom Both molecules may be represented by the symbol 3

4 The most common organic bases are Adenine (A) Thymine (T) Cytosine (C) Guanine (G) 4

5 The deoxyribose, the phosphate and one of the bases Combine to form a nucleotide adenine deoxyribose 5

6 A molecule of DNA is formed by millions of nucleotides joined together in a long chain sugar-phosphate backbone + bases 6

7 In fact, the DNA usually consists of a double strand of nucleotides The sugar-phosphate chains are on the outside and the strands are held together by chemical bonds between the bases 7

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9 The bases always pair up in the same way They are linked by hydrogen bonds. Adenine and Thymine have two hydrogen bonds Adenine Thymine Cytosine and Guanine have three hydrogen bonds. Cytosine Guanine 9

10 adenine thymine cytosine guanine 10

11 Chargaff's rule Chargaff's rule states DNA of any species contains equal amounts of guanine and cytosine and also equal amounts of adenine and thymine. This pattern is found in both strands of the DNA. They were discovered by Austrian chemist Erwin Chargaff 11

12 James Watson and Francis Crick with, the help of Rosalind Franklin and others, determined the shape of the DNA molecule. The paired strands are coiled into a spiral called A DOUBLE HELIX 12

13 THE DOUBLE HELIX bases sugar-phosphate chain 13

14 Before a cell divides, the DNA strands unwind and separate Each strand makes a new partner by adding the appropriate nucleotides The result is that there are now two doublestranded DNA molecules in the nucleus So that when the cell divides, each nucleus contains identical DNA This process is called replication 14

15 The strands separate 15

16 Each strand builds up its partner by adding the appropriate nucleotides 16

17 The sequence of bases in DNA forms the Genetic Code A group of three bases (a triplet) controls the production of a particular amino acid in the cytoplasm of the cell The different amino acids and the order in which they are joined up determines the sort of protein being produced 17

18 For example Cytosine Adenine Codes for Valine Thymine 18

19 This is known as the triplet code Each triplet codes for a specific amino acid CGA - CAA - CCA - CCA - GCT - GGG - GAG - CCA - Ala Val Gly Gly Arg Pro Leu Gly The amino acids are joined together in the correct sequence to make part of a protein Ala Val Gly Gly Arg Pro Leu Gly 19

20 A sequence of triplets in the DNA molecule may code for a complete protein Such a sequence forms a gene There may be a thousand or more bases in one gene 20

21 The process of DNA typing converts DNA into a series of bands that can distinguish an individual Only a small percentage of DNA differs from one person to the next These parts of one s DNA are used to create a DNA profile The majority of DNA does not code for specific proteins and is repetitive, repeating the same sequence over and over 21

22 Polymerase Chain Reaction (PCR) A technique for making many copies of a specific piece of DNA Can amplify very minute quantities of DNA millions of times The steps of PCR 1. DNA is heated to separate and unzip it 2. Primers are added to combine with DNA strands 3. DNA polymerase (enzymes) and free nucleotides are added to rebuild separated strands 4. The DNA is cooled 5. The process is repeated several times 22

23 Short Tandem Repeats (STR) The latest method of DNA typing There are locations (loci) on a chromosome that contain short segments of 3 7 bases that repeat themselves STR s are less susceptible to degradation and can be recovered from bodies or stains that have been subject to extreme decomposition With the technology of PCR, one can extract and amplify a combination of different STR s 23

24 Mitochondrial DNA Another method of typing used for individual characterization Located outside a cell s nucleus and inherited only from the mother Not as useful as STR and is more costly than other DNA testing 24

25 Visualizing DNA through Electrophoresis In the lab, DNA molecules are cut by restriction enzymes into fragments of various sizes With electrophoresis, the resulting fragments are forced to move along a gel-coated plate under the influence of an electrical potential After the fragments have migrated across the gel, the gel can be stained to show the bands or fragments easily Comparisons can then be made, such as comparing a suspect s DNA to the DNA found on a crime scene 25

26 Combined DNA Information System (CODIS) CODIS maintains a database of DNA profiles from convicted offenders, unsolved crime scene evidence, and profiles of missing persons 26

27 Sources of DNA Skin Sweat Blood Mucus Saliva Tissue Semen Urine Hair Ear Wax Vaginal Cells Rectal cells 27

28 Collecting and Packaging Photograph evidence first Wear gloves at all times Biological Evidence Package each stained article separately in paper or a well-ventilated box Avoid using plastic or airtight containers because moisture could contribute to harmful bacteria and fungi growth Remove dried blood using a sterile swab moistened with distilled water Store biological evidence in the refrigerator or a cool location until it is delivered to the lab 28