Chapter 11 DNA and Its Role in Heredity Biology 101 Tri-County Technical College Pendleton, SC

Start with a Review Generic nucleotide contains 5-carbon sugar, one of four nitrogenous bases, and a phosphate group DNA has deoxyribose and ATGC RNA has ribose and AUGC Numbering carbons in ringed sugar… “At the Grand Canyon” will save your sack lunch more times than you care to count…

Dr. Ruth is right on… Remember how to manipulate the mnemonic device and pairing and size will not be a problem Chalk talk time on ATGC Purines are double-ringed nitrogenous compounds Pyrimidines are single-ringed nitrogenous compounds

World will never be the same… Griffin’s transformation experiments Hershey & Chases bacteriophage work Watson and Crick **Chargaff’s Rule Strands always equidistant; 2 H bonds between A & T, and 3 H bonds between G & C Known as “complementary base pairing” Specific pairing is KEY to replicating genetic material

DNA Visual

Structure of DNA DNA is double-stranded helix Right-handed; twists to the right Stands are antiparallel Covalent bonds link sugar, base, and phosphate Hydrogen bonds holds strands together Remember, H bonds NOT very strong, but DNA has so many, it is quite stable

DNA Replication Strands separate and each strand serves at template (mold) for making new complementary strand When replication is over…there will be TWO molecules of DNA Each molecule will be composed of one old strand and one new strand Meselson and Stahl’s work confirmed semiconservative replicaiton is correct model

Replication Requirements DNA must act as template for complementary base pairing Four deoxyribonucleoside triphosphates (dATP, dTTP, dGTP, dCTP), must be present DNA polymerase needed to bring substrates to template and catalyze reaction Source of chemical energy needed

Process of Replication Occurs in two stages DNA locally denatured (unwound) New nucleotides linked by covalent bonding to each growing strand Current model suggests huge protein replication complex and DNA moves through the complex All chromosomes have at least one sequence of nucleotides recognized by replication complex

Process, cont. Sequence called origin of replication Forms replication bubble with 2 replication forks DNA helicase opens up double helix Single-stranded binding proteins keep two strands separated RNA primase makes primer strand needed to get replication underway

Process, cont. DNA polymerase III adds nucleotides to primer, proofreads the DNA, and repairs it Leading/lagging strands and Okazaki fragments DNA Polymerase I responsible for removing RNA primer and replacing it with DNA DNA ligase responsible for linking Okazaki fragments together and for linking any repaired strands back together

Replication Visual

Replication Forks Visual

Lagging Strand Visual

Proofreading and Repair DNA polymerase(s) “proofs” each nucleotide as it is added to growing strand Repair enzymes designed for 2 basic functions Mismatch repair (should be AT and is AC for example) Excision repair works for mismatched pairs, chemically modified bases, or points where strand has more bases than the other

Proofreading, cont. Cuts out errors, DNA polymerase and DNA ligase synthesize and seal up new piece to replace the excised one DNA ligase responsible for linking segments of strand back together after repairs are made Error rate of one base in 10 6 ; after repair, the error rate reduced to one base in 10 9

Proofreading Visual