DNA Elongation By DNA Polymerases such as DNA
DNA Elongation Ø By DNA Polymerases such as DNA pol III Ø Adds DNA triphosphate monomers to the growing replication strand. Ø Matches A to T and G to C.
Energy for Replication Ø From the triphosphate monomers. Ø Loses two phosphates as each monomer is added.
Problem of Antiparallel DNA Ø The two DNA strands run antiparallel to each other. Ø DNA can only elongate in the 5’--> 3’ direction.
Leading Strand Ø Continuous replication toward the replication fork in the 5’-->3’ direction.
Priming Ø DNA pol III cannot initiate DNA synthesis. Ø Nucleotides can be added only to an existing chain called a Primer.
Primer Ø Make of RNA. Ø 10 nucleotides long. Ø Added to DNA by an enzyme called Primase. Ø DNA is then added to the RNA primer.
Priming Ø A primer is needed for each DNA elongation site.
Lagging Strand Ø Discontinuous synthesis away from the replication fork. Ø Replicated in short segments as more template becomes opened up.
Okazaki Fragments Ø Short segments (100 -200 bases) that are made on the lagging strand. Ø All Okazaki fragments must be primed. Ø RNA primer is removed after DNA is added.
Enzymes Ø DNA pol I - replaces RNA primers with DNA nucleotides. Ø DNA Ligase - joins all DNA fragments together.
Other Proteins Replication in Ø Topoisomerase – relieves strain ahead of replication forks. Ø Helicase - unwinds the DNA double helix. Ø Single-Strand Binding Proteins - help hold the DNA strands apart.
Video Ø http: //highered. mcgraw- hill. com/olcweb/cgi/pluginpop. cgi? it=swf: : 5 35: : 535: : /sites/dl/free/0072437316/120076 /micro 04. swf: : DNA%20 Replication%20 For k
Video Ø http: //www. youtube. com/watch? v=te. V 62 zr m 2 P 0
DNA Replication Error Rate Ø 1 in 1 billion base pairs. Ø About 3 mistakes in our DNA each time it’s replicated.
Reasons for Accuracy Ø DNA pol III self-checks and corrects mismatches. Ø DNA Repair Enzymes a family of enzymes that checks and corrects DNA.
DNA Repair Ø Over 130 different DNA repair enzymes known. Ø Failure to repair may lead to Cancer or other health problems.
Example: Ø Xeroderma Pigmentosum -Genetic condition where a DNA repair enzyme doesn’t work. Ø UV light causes damage, which can lead to cancer.
Xeroderma Pigmentosum Cancer Protected from UV
Thymine Dimers Ø T-T binding from side to side causing a bubble in DNA backbone. Ø Often caused by UV light.
Excision Repair Ø Cuts out the damaged DNA. Ø DNA Polymerase fills in the excised area with new bases. Ø DNA Ligase seals the backbone.
Problem - ends of DNA Ø DNA Polymerase can only add nucleuotides in the 5’--->3’ direction. Ø It can’t complete the ends of the DNA strand.
Result Ø DNA gets shorter and shorter with each round of replication.
Telomeres Ø Repeating units of TTAGGG (100 - 1000 X) at the end of the DNA strand (chromosome) Ø Protects DNA from unwinding and sticking together. Ø Telomeres shorten with each DNA replication.
Telomeres
Telomeres Ø Serve as a “clock” to count how many times DNA has replicated. Ø When the telomeres are too short, the cell dies by apoptosis.
Implication Ø Telomeres are involved with the aging process. Ø Limits how many times a cell line can divide.
Telomerase Ø Enzyme that uses RNA to rebuild telomeres. Ø Can make cells “immortal”. Ø Found in cancer cells. Ø Found in germ cells. Ø Limited activity in active cells such as skin cells
Comment Ø Control of Telomerase may stop cancer, or extend the life span.
NEWS FLASH Ø The DNA of Telomers is actually used to build proteins. Ø These proteins seem to impede telomerase. Ø Feedback Loop? ?
Chromatin Packing 1. Nucleosomes 2. 30 -nm Chromatin Fibers 3. Looped Domains 4. Chromosomes Focus on #1 & 4
Nucleosomes Ø "Beads on a String”. Ø DNA wound on a protein core. Ø Packaging for DNA. Ø Controls gene reading
Protein Core Ø Two molecules of four types of Histone proteins. Ø H 1 - 5 th type of Histone protein attaches the DNA to the outside of the core.
Chromosomes Ø Large units of DNA. Ø Similar to "Chapters" in the Life. Book of
Summary Ø Know the Scientists and their experiments. Ø Why DNA is an excellent genetic material. Ø How DNA replicates. Ø Problems in replication. Ø Chromatin packing
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