DNA: The Molecule of Life

The Importance of DNA Every cell in your body contains a full set of DNA (deoxyribonucleic acid) Only molecule capable of replicating itself DNA codes for a series of chemical messages and regulates the production of protein Contained in 46 chromosomes in the human body Made of base pairs (an estimated 3 billion) in humans 100 000 genes on the chromosomes DNA ensures the continuity of life New combinations of genes and mutations to the DNA may cause for biodiversity and variation

Chromosomes Long thin threads of genetic material, protected by a cell nucleus Roughly half nucleic acids and half protein Proteins are made of different sequences of amino acids, of which there are about 20 By changing a single amino acid, a different protein can be created Genes = segments of DNA that code for specific proteins WHY ARE PROTEINS SO IMPORTANT?? http://publications.nigms.nih.gov/structlife/chapter1.html

James Watson and Francis Crick Rosalind Franklin used a technique called x-ray defraction to help determine the structure of the DNA molecule Other scientists gathered evidence that nucleotide base pairs were in definite proportion to one another With this data, Watson and Crick suggested that the nitrogen bases where arranged in pairs, and the pairs were held together by a sugar-phosphate compound, which has been dubbed “backbone” This created a 3-D model of what DNA looked like

Structure of DNA Because of many contributions by research scientists, DNA structure has been well defined DNA is made of a double helix, or spiraling ladder, where nucleotide base pairs act like rungs on the ladder The nucleotide pairs are attracted to one another because of a hydrogen bond The four nitrogen base pairs are adenine, thymine, cytosine and guanine Guanine always pairs and bonds with cytosine (G-C) Thymine always pairs and bonds with adenine (A-T)

Nucleotide The basic structure of nucleic acid is a nucleotide Nucleotides are arranged in long chains, which eventually form chromosomes They are made of a sugar phosphate structural support, attached to one of four nitrogen base units

Deoxyribose sugar + phosphate group + nitrogenous base Nucleotides: Deoxyribose sugar + phosphate group + nitrogenous base Purine pair with Pyrimidine A, G T, C http://www.nelson.com/ABbio20-30/student/protect/media/dna_close-up.html

Practice Problems A sample of DNA contains A and C nucleotides in the following proportions A = 34% C = 16% What are the proportions of G and T nucleotides in this sample? 2. Use Chargaff’s rule to complete the following table 3. Draw a linear stretch of a double stranded DNA molecule about 20 base pairs long, with a nucleotide composition that corresponds to the nucleotide composition of the table above. Nucleotide Proportion (%) A 24 C G T

Replication of DNA Only molecule capable of replicating itself During replication the hydrogen bonds separate the base pairs The parent strand acts as a template, and new base pairs, which are complimentary to the parents code, join the parent strand Each newly created strand is half the parents DNA and half the new DNA This causes the DNA to be semi-conservative http://www.nelson.com/ABbio20-30/student/protect/media/replicating_dna.html

Semiconservative Replication During replication, the new DNA strands are “half old, half new” Each parent strand is a mold or template An enzyme, known as polymerase, fuses the new bases to the old template Free nucleotides are derived from the food you eat http://www.youtube.com/watch?v=teV62zrm2P0&feature=related

Replication- Overview DNA helicase Unzips double helix (breaks the hydrogen base pair bonds) Single strands serve as TEMPLATE to build the complimentary strands DNA polymerase III Builds new strands from templates ( 5’ to 3’) LEADING STRAND vs LAGGING STRAND Leading strand – continuous Lagging strand – formed in Okazaki fragments Start with RNA primer DNA Polymerase I replaces primer with appropriate DNA nucleotides DNA ligase joins fragments RESULT: Formation of 2 identical DNA molecules Both with 2 strands http://www.youtube.com/watch?v=teV62zrm2P0&feature=related

Errors in Replication (mutations) Genetic mistakes are very infrequent 1 mistake (addition, deletion or substitution) per million would mean 3000 errors in one translation, which would cause enormous mutations DNA polymerases acts as quality control by proofreading newly synthesized strands – they backtrack to incorrect nucleotide, cut it out and replace it with the correct base Mutations can be triggered by environmental factors, such as drugs, exposure to radiation

PRACTICE PROBLEMS DNA Template Strand GCC CTA TAA CGC

PRACTICE PROBLEMS 5’ TACTTTGGCCCAGAG 3’ DRAW THE COMPLIMENTARY STRAND

Replication - worksheet Reading for tomorrow - 18.2 - 636 - 642