READ: Structure of DNA

Introduction

Introduction

 DNA is the genetic code of life. What makes us "human" is the number and sequence of nucleotides in our genome. The human genome, as well as the genome of many other species has been sequenced. While 99.9% of the genes in every human are the same, the 0.1% variation makes each individual unique. When these small but unique parts of an organisms genome is known, it is possible to identify an unknown species by comparing the DNA sequence of the unknown to the known and make a match.

Essential Questions

1. How is DNA evidence analyzed?

2. What makes DNA evidence unique?

3. How can old or degraded samples be analyzed for DNA?

4. How are DNA profiles stored?

Module Minute

DNA analysis is very useful in Forensic Science to both implicate and exclude persons of interest in a crime. DNA analysis is unique in that each person, with the exception of identical twins, has their own individual DNA that can be used to identify them from the other 7 billion people on the planet. In addition, DNA can be extracted from sources many years later in many cases, which means that many formerly "cold cases" can be re-opened and possibly solved with newer technologies. DNA profiles are stored in a large database known as "CODIS" for the purpose of comparing unknown to known DNA samples at different points in time. While DNA evidence does not always lead to a suspect, it is one of the very best tools in the Forensic Investigator tool belt because of the amount of individual evidence it provides.

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Structure of DNA

Structure of DNA

The cell is the smallest unit of life. Within the cell is the nucleus or the "brain" of the cell. The nucleus is crucial because it contains all of the genetic information the cell needs to exist and to reproduce.

Chromosomes

In most types of cells, genetic information is organized into structures called chromosomes. Chromosomes are the biological structure by which hereditary information is physically transmitted from one generation to the next. Located in the cell nucleus, it consists of a tightly coiled thread of DNA with associated proteins and RNA. The genes are arranged in linear order along the DNA. Humans have 46 chromosomes (23 pairs); one set of chromosomes is inherited from each biological parent. A full set of chromosomes consists of 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes. Autosomal chromosomes are any chromosome that is not a sex chromosome. Sex chromosomes are either of a pair of chromosomes, usually designated X or Y, in the germ cells of most animals and some plants, that combine to determine the sex and sex-linked characteristics of an individual. XX results in a female and XY results in a male in mammals.

Chromosomes contain blocks of information called genes. A gene is the basic unit of heredity, or functional sequence of DNA, in a chromosome. Depending on the chromosome, there are as many as several hundred to thousands of genes on each chromosome. Each gene is the blueprint for a specific type of protein in the body. While we are very similar, no two people except identical twins, have identical DNA or identical genes. Often referred to as the "blueprint of life", DNA is the genetic material present in the nucleus of cells which is inherited from each biological parent. DNA, which is an abbreviation for Deoxyribonucleic acid, is a double helix, which means it consists of two strands composed of nucleotides that twist around each other. If you untwisted the strands, DNA would resemble a ladder. Each nucleotide has 3 parts:

• A nitrogenous base

• A deoxytribose sugar

• A phosphate groupe

The sugar and phosphate group are the same in all nucleotides, and together they form the "sides" of the ladder. Two complementary nitrogenous bases join together by hydrogen bonding in the center of the DNA to form the "rungs" of the ladder. The hydrogen bonds cause the DNA strand to twist and are what make DNA so stable. The four nitrogenous bases in DNA are:

• A = Adenine

• T = Thymine

• G = Guanine

• C = Cytosine

Purines and Pyrimidines are nitrogenous bases that make up the two types of nucelotide bases found in DNA and RNA. The purines found in DNA are Adenine (A) and Guanine (G) whereas Cytosine (C) and Thymine (T) are the pyrimidines. A,T,C and G are the molecular building blocks of DNA that only "pair up" or bond in specific base patterns. In DNA, A only pairs with T, and C only pairs with G. This is known as complementary base pairing.

DNA replication is a process in which DNA makes a copy of itself. This occurs inside our cells during a stage in the cell life cycle called interphase. DNA replication is "semi-conservative" which means that half of the original strand is always "saved" and the new strands form as complements. In this way, each new daughter cell receives one parent strand of DNA and one daughter strand of DNA.

 DNA is coiled so that it fits inside the cell. If you fully extended the DNA found in just one cell, the molecule would be about 1.7m long! If you uncoiled the DNA in all of the cells in your body, it would cover the distance from earth to the moon 6000 times! The function of DNA is to transmit information to the next generation of cells and provide information for synthesis of proteins that are necessary for cellular functions. In 1990, the United States partnered with many other nations as well as privately funded biotechnology companies, in an effort to determine the sequence of the human genome. The genome is all of the genetic material in the chromosomes of a particular organism; its size is generally given as its total number of base pairs. The goal of this project known as the Human Genome Project, was to determine the sequence of the 3 billion bases and identify the estimated 30,000 genes that make us human. In April of 2003, just 13 years later, the Human Genome Project was completed. At left is a picture of the first printout of the completed Human Genome presented in a series of books. The book collection contains more than 100 books! Each book is over 1,000 pages long with ATGC sequences printed in a font so small that it is barely legible. This display is found in the Wellcome Collection in London, England.

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