DNA Structure and Function

 Methods of inheritance uncertain  Suspected that it was due to proteins (great variety)  Some evidence of DNA as passing on genes in viruses  1953—James Watson, Francis Crick  Described DNA model for inheritance

 Deoxyribonucleic acid (DNA)  Nucleotide—monomer unit of nucleic acid  5-carbon sugar (ribose or deoxyribose)  Nitrogen base  Phosphate group

 Adenine  Guanine  Thymine  Cytosine

 ALWAYS match with specific base pairing  A—T, T—A  G—C, C—G  Each chain bonded covalently  Base pairs bonded with hydrogen bonds

 Polarity of molecules determine shape  Double helix

 Interphase—S phase  Makes one copy, doubles amount of DNA  Semiconservative replication—in each copy, one strand of the helix is carried over from the original strand

 Step 1—helicase attaches to DNA  Step 2—helicase “unzips” hydrogen bonds between base pairs  This causes double helix to “unwind”

 Step 3—DNA polymerase forms new, complementary strand of DNA from free nucleotides  Moves along entire length of DNA

 Step 3b—DNA ligase binds short stretches of new DNA on “lagging” strand  Okazaki fragments—short sequences of DNA on lagging strand

 Step 4—helicase separates from DNA  Two new strands now formed  Sister chromatids!!!

 Brief Replication Overview Brief Replication Overview  Brief DNA Replication Brief DNA Replication  Detailed DNA Replication Detailed DNA Replication

 Rare, but sometimes mismatched base pairs  DNA polymerase usually catches these & corrects  If can’t correct mismatch, protective mechanisms stop cell division (Ch. 9, slide 19)  Repair enzymes can repair some damage  Can “snip” out damage or mismatch

 Isolate DNA from one organism  Create another organism from DNA  Second organism genetically identical to original

 Embryo cloning  Fertilized egg separates early in division  Natural = identical twins

 Adult cloning  1—Start with somatic cell of organism  2—Remove nucleus of the cell  3—Take mature egg from female  4—Remove nucleus from egg  5—Implant donor nucleus into egg (fused by shock)  6—New cell begins to divide to early embryo  7—Implant embryo into surrogate mother  8—If goes to full term, clone is born

 Theraputic cloning  Somatic cell DNA into stem cell  Stem cell is undifferentiated, can turn into almost any tissue  Potentially regenerate organs

 Applications  Livestock  Pets  Humans???  Organs

 Risks  Egg doesn’t always accept new DNA  New DNA isn’t always used properly  Ethical issues