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Transcript
Gene Expression & DNA Replication • DNA must be copied so that the DNA is the same in all cells • Differences in gene expression give rise to: – different cells – multicelluar organisms DNA Replication • DNA Synthesis during Sphase of Interphase (will discuss w/ mitosis) • H-bonds between 2 strands are broken • Two Strands separate • Each strand is a template for the other DNA Replication • The Watson-Crick model of DNA replication: A T A T C G C G G C G A T A T A T Parental molecule of DNA A C C Nucleotides Both parental strands serve as templates T A T A T G C G C G C G C G C T A T A T A T A T A Two identical daughter molecules of DNA DNA Replication • https://www.youtube.c om/watch?v=teV62zrm 2P0 • DNA replication review DNA Replication • Helicase breaks hydrogen bonds to separate strands • DNA Polymerase pairs new bases and creates two new strands Semi-conservative • DNA segments connected by ligase When does the cell replicate DNA? • Before cell division so that new cells have enough DNA to divide • Cells divide in embryogenesis, wound repair, cell replacement DNA Structure Review • Covalent bonds hold a strand together • Hydrogen bonds hold two strands together – A pairs with T – C pairs with G • Each strand is used as a template to replicate DNA DNA Replication • If the DNA is not replicated correctly, mistakes occur. • Polymerase and other enzymes can correct mistakes. • If they are not corrected, they result in mutation, a permanent change in DNA. Two Types of Mutations • Base Changes – A change in the sequence of nucleotide bases – Most are silent (do not change a protein) • Chromosomal Abnormalities (macromutations) – Large changes in whole regions of DNA – Most are lethal Base Mutations • Incorrect base-pairings that are not fixed – Mutagens increase the frequency • Next replication includes these changes • Source of new alleles (new variation) – A few are beneficial (make a better protein) – Most are silent (make no change in a protein) – Some are harmful (most that change a protein) Mutations • Ask for examples of mutant human genes • Bad mutations include -> – sickle cell anemia, hemophilia, color blindness. • Can you find a mutation in this picture? Can you find two mutations? (There are at least two.) Mutations: • blue eyes & white skin (occurred long ago). • some say … DNA Repair • DNA polymerases often insert incorrect bases – DNA mismatch • Proofreading enzymes correct mistakes DNA Repair • Enzymes recognize damaged DNA • Cut the backbone on either side • Damaged DNA is removed • DNA is corrected