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 S- phase 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: Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA T Nucleotides CG A GC A A T T A A A A C C C T T G G A C A A A A A A CC C CG T G T T T T G T T T G G

DNA Replication om/watch?v=teV62zrm 2P0 om/watch?v=teV62zrm 2P0 DNA replication reviewreview

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

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. DNA Replication

Two Types of Mutations Base Changes – A change in the sequence of nucleotide bases – Most are silent (do not change a protein) Chromosomal Abnormalities (macro- mutations) – 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