DNA Elongation By DNA Polymerases such as DNA pol III Adds DNA triphosphate monomers to the growing replication strand. Matches A to T and G to C.

Energy for Replication From the triphosphate monomers. Loses two phosphates as each monomer is added.

Problem of Antiparallel DNA The two DNA strands run antiparallel to each other. DNA can only elongate in the 5’--> 3’ direction.

Leading Strand Continuous replication toward the replication fork in the 5’-->3’ direction.

Priming DNA pol III cannot initiate DNA synthesis. Nucleotides can be added only to an existing chain called a Primer.

Primer Make of RNA. 10 nucleotides long. Added to DNA by an enzyme called Primase. DNA is then added to the RNA primer.

Priming A primer is needed for each DNA elongation site.

Lagging Strand Discontinuous synthesis away from the replication fork. Replicated in short segments as more template becomes opened up.

Okazaki Fragments Short segments (100-200 bases) that are made on the lagging strand. All Okazaki fragments must be primed. RNA primer is removed after DNA is added.

Enzymes DNA pol I - replaces RNA primers with DNA nucleotides. DNA Ligase - joins all DNA fragments together.

Other Proteins in Replication Topoisomerase – relieves strain ahead of replication forks. Helicase - unwinds the DNA double helix. Single-Strand Binding Proteins - help hold the DNA strands apart.

Video http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA%20Replication%20Fork

Video http://www.youtube.com/watch?v=teV62zrm2P0

DNA Replication Error Rate 1 in 1 billion base pairs. About 3 mistakes in our DNA each time it’s replicated.

Reasons for Accuracy DNA pol III self-checks and corrects mismatches. DNA Repair Enzymes - a family of enzymes that checks and corrects DNA.

DNA Repair Over 130 different DNA repair enzymes known. Failure to repair may lead to Cancer or other health problems.

Example: Xeroderma Pigmentosum -Genetic condition where a DNA repair enzyme doesn’t work. UV light causes damage, which can lead to cancer.

Xeroderma Pigmentosum Cancer Protected from UV

Thymine Dimers T-T binding from side to side causing a bubble in DNA backbone. Often caused by UV light.

Excision Repair Cuts out the damaged DNA. DNA Polymerase fills in the excised area with new bases. DNA Ligase seals the backbone.

Problem - ends of DNA DNA Polymerase can only add nucleuotides in the 5’--->3’ direction. It can’t complete the ends of the DNA strand.

Result DNA gets shorter and shorter with each round of replication.

Telomeres Repeating units of TTAGGG (100- 1000 X) at the end of the DNA strand (chromosome) Protects DNA from unwinding and sticking together. Telomeres shorten with each DNA replication.

Telomeres

Telomeres Serve as a “clock” to count how many times DNA has replicated. When the telomeres are too short, the cell dies by apoptosis.

Implication Telomeres are involved with the aging process. Limits how many times a cell line can divide.

Telomerase Enzyme that uses RNA to rebuild telomeres. Can make cells “immortal”. Found in cancer cells. Found in germ cells. Limited activity in active cells such as skin cells

Comment Control of Telomerase may stop cancer, or extend the life span.

NEWS FLASH The DNA of Telomers is actually used to build proteins. These proteins seem to impede telomerase. Feedback Loop??

Chromatin Packing 1. Nucleosomes 2. 30-nm Chromatin Fibers 3. Looped Domains 4. Chromosomes Focus on #1 & 4

Nucleosomes "Beads on a String”. DNA wound on a protein core. Packaging for DNA. Controls gene reading

Protein Core Two molecules of four types of Histone proteins. H1- 5th type of Histone protein attaches the DNA to the outside of the core.

Chromosomes Large units of DNA. Similar to "Chapters" in the Book of Life.

Summary Know the Scientists and their experiments. Why DNA is an excellent genetic material. How DNA replicates. Problems in replication. Chromatin packing