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Lecture 13

Lecture 13 – DNA in Forensic Science- Watson-Crick Discover the Structure and Function of DNA

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Anthropology 2235A/B
Eldon Molto

Lecture 13 – DNA in Forensic Science: Watson-Crick Discover the Structure and Function of DNA  1953: Watson and Crick publish “Molecular Structure of Nucleic Acids: A Structure for Deoxyribonucleic Acid” in Nature o Formed the basis for understanding heredity at molecular level including how protein are made via a pathway between DNA – mRNA – messenger RNA and transfer RNA o Was thought that most of our DNA was used to code for proteins, but we know that less than 5% of our genome is made up of structural genes (exons) o Watson and Crick postulated that the base pairings they found (A-T and C-G) have more important implications  DNA Structure and Reactivity o The tightly wound helix protects the bases which are connected by a weak hydrogen bond o DNA is hydrophobic and water/humidity is a potential destroyer of the molecule o Desiccation units are used in the field to dry evidentiary samples o T-A and C-G called the base pair rule o Each unit of DNA is called a nucleotide consisting of a base, sugar and phosphate  The latter of the two are always the same, only the bases vary o Helical structure unwinds during cell division and the complementary strands attract new bases that are joined by an enzyme called DNA polymerase  The Nucleus o Contains the majority of an organisms DNA o Contains two complete copies of nuclear DNA o Cell’s chromosomes contain approximately 3 billion base pairs of DNA o Only 2-3% of the genome make proteins o Nuclear DNA is found on chromosomes and is packaged around proteins called histones o DNA loci (STRs) used in forensic analysis are however outside the protection of histones and are very vulnerable to postmortem damage  Development of Sequencing o 1977 Fred Sanger of Cambridge a method for sequencing DNA o The method uses an enzymatic procedure to synthesize DNA chains of varying lengths, stopping the DNA replication by a dummy base. Each of the resulting fragment lengths is then analyzed by electrophoresis and a base sequence is elucidated. This method forms the basis for all subsequent DNA sequencing technologies (VNTRs and mtDNA etc.)  Sequencing Technology o Today we know that the human genome is made up of over 3 billion base pairs on the 23 chromosomes o In order to identify the gene loci we had to develop was of
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