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 groupDNA has deoxyribose and ATGCRNA has ribose and AUGCNumbering 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 experimentsHershey & Chases bacteriophage workWatson and Crick**Chargaff’s RuleStrands always equidistant; 2 H bonds between A & T, and 3 H bonds between G & CKnown as “complementary base pairing”Specific pairing is KEY to replicating genetic material

DNA Visual

Structure of DNADNA 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 ReplicationStrands 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 pairingFour deoxyribonucleoside triphosphates (dATP, dTTP, dGTP, dCTP), must be presentDNA polymerase needed to bring substrates to template and catalyze reactionSource of chemical energy needed

Process of Replication

Occurs in two stagesDNA 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 RepairDNA polymerase(s) “proofs” each nucleotide as it is added to growing strandRepair enzymes designed for 2 basic functionsMismatch 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 106; after repair, the error rate reduced to one base in 109

Proofreading Visual