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Lecture 3

BISC 101 - Lecture 3 - DNA Replication.docx

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Simon Fraser University
Biological Sciences
BISC 101
Christopher Kennedy

Biological Science 101 – General Biology Lecture Three: DNA Replication Textbook: Chapter 16, pg. 305-319 Fig. 16.5-16.19 DNA: Deoxyribonucleic Acid. - Genetic material that makes up traits of an individual species. - Passed on to next generation. - Contained inside the nucleus. Structure of DNA Molecule: - Sugar (Deoxyribose):  Adenine  Thymine  Cytosine  Guanine - Nitrogen base. - Phosphate group. A=T (30% each) G=C (20% each) Adenine and Guanine = Purines Thymine and Cytosine = Pyrimidines Watson & Crick – 1953: - Revealed the structure of DNA. - Discovered the double-helix shape (two DNA strands). DNA Replication: - Semi-conservative model: Two strands separate, each strand serves as a template to produce a complementary strand. Details of Replication: 1) Origins of replication - Specific DNA sequence. - Proteins attach. - Causes strands to separate. - DNA polymerase (enzyme) adds new nucleotides. Energy for Replication: - Nucleotides occur as triphosphates (3 Phosphates), 2 Phosphates are released and energy is produced. Orientation of DNA Strand: - Anti-parallel. - Opposite directions. Replication: 1) Nucleotides added only to the 3’ end. - New strand goes from 5’ -> 3’. - First strand is called leading strand. Okazaki Fragments: Fragments that are joined together by DNA ligase. - Joins fragments of nucleotides from 100-200 nucleotides long. Roles of polymerase DNA Polymerase can only add nucleotides to an existing strand. - It must have a starting point in order to attach nucleotides to the strands of DNA. Primer: A nucleotide the initiates elongation of the strand, it is added by the primase enzyme. Each Okazaki fragment must have a primer. Involvement of other enzymes 1) Primase – Adds primer. 2) Polymerase III – Adds nucleotides. 3) Polymerase I – Converts RNA into DNA in primer. 4) Ligase – Fuses Okazaki fragments together. 5) Helicase – Unwinds the double-helix. - Single Stranded Binding Protein = Holds strands apart. DNA Repair DNA Replication occurs at 50 nucleotides per second. - Mistakes taking place. - An error is 1 in 100,000 bases (mismatched bases). - Errors are corrected by DNA polymerase. - Error rate is then reduced by 1 in 10 billion nucleotides. - Errors can result in mutation (e.g. Cancer and etc.) DNA Damaged in the Cell, Causes are: - X-Rays. - Ultra-violet Light. - Mutagens/Carcinogens (Chemicals). DNA Repair: 1) DNA Polymerase checks for damage and repairs DNA in the cells. 2) Nuclease: Cuts out damaged DNA. 3) Polymerase: Adds nucleotides. 4) Ligase: Seals the pieces together. Telomeres Telomeres are short repeated nucleotides. - They are noncoding. - They provide protection to the ends of the chromosomes. E.g. TTAGGG – repeated about 100-1000 times. Telomerase: Adds nucleotides (telomeres) to the ends of the chromosomes. - Active in gametes (sperm and egg cells). - High in newbor
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