Chapter 16 The Molecular Basis of Inheritance Question
Chapter 16 The Molecular Basis of Inheritance
Question? • Traits are inherited on chromosomes, but what in the chromosomes is the genetic material? • Two possibilities: • Protein • DNA
Qualifications • Protein: • very complex. • high specificity of function. • DNA: • simple. • not much known about it (early 1900’s).
For testing: • Name(s) of experimenters • Outline of the experiment • Result of the experiment and the importance of the result
Griffith - 1928 • Pneumonia in mice. • Two strains: • S - pathogenic • R - harmless
Griffith’s Experiment
Result • Something turned the R cells into S cells. • Transformation - the assimilation of external genetic material by a cell.
Problem • Griffith used heat. • Heat denatures proteins. • So could proteins be the genetic material? • DNA - heat stable. • Griffith’s results contrary to accepted views.
Avery, Mc. Carty and Mac. Leod - 1944 • Repeated Griffith’s experiments, but added specific fractions of S cells. • Result - only DNA transformed R cells into S cells. • Result - not believed.
Hershey & Chase -1952 • Genetic information of a virus or phage. • Phage - virus that attacks bacteria and reprograms host to produce more viruses.
Bacteria with Phages
Phage Components • Two main chemicals: • Protein • DNA • Which material is transferred to the host?
Used Tracers • Protein - CHONS, can trace with 35 S. • DNA - CHONP, can trace with 32 P.
Experiment • Used phages labeled with one tracer or the other and looked to see which tracer entered the bacteria cells.
Result • DNA enters the host cell, but the protein did not. • Therefore: DNA is the genetic material.
Picture Proof
Chargaff - 1947 • Studied the chemical composition of DNA. • Found that the nucleotides were in certain ratios.
Chargaff’s Rule • A=T • G=C • Example: in humans, A = 30. 9% T = 29. 4% G = 19. 9% C = 19. 8%
Why? • Not known until Watson and Crick worked out the structure of DNA.
Watson and Crick - 1953 • Used X-ray crystallography data (from Rosalind Franklin) • Used model building. • Result - Double Helix Model of DNA structure. • (One page paper, 1953).
Rosalind Franklin
Book & Movies • “The Double Helix” by James Watson- His account of the discovery of the shape of DNA • Movie – The Double Helix
DNA Composition • Deoxyribose Sugar (5 -C) • Phosphate • Nitrogen Bases: • Purines • Pyrimidines
DNA Backbone • Polymer of sugar-phosphate. • 2 backbones present.
Nitrogen Bases • Bridge the backbones together. • Purine + Pyrimidine = 3 rings. • Constant distance between the 2 backbones. • Held together by H-bonds.
Chargaff’s Rule • Explained by double helix model. • A = T, 3 ring distance. • G = C, 3 ring distance.
Watson and Crick • Published a second paper (1954) that speculated on the way DNA replicates. • Proof of replication given by others.
Replication • The process of making more DNA from DNA. • Problem: when cells replicate, the genome must be copied exactly. • How is this done?
Models for DNA Replication • Conservative - one old strand, one new strand. • Semiconservative - each strand is 1/2 old, 1/2 new. • Dispersive - strands are mixtures of old and new.
Replication Models
Meselson – Stahl, late 1950’s • Grew bacteria on two isotopes of N. • Started on 15 N, switched to 14 N. • Looked at weight of DNA after one, then 2 rounds of replication.
Results • Confirmed the Semiconservative Model of DNA replication.
Replication - Preview • DNA splits by breaking the Hbonds between the backbones. • Then DNA builds the missing backbone using the bases on the old backbone as a template.
Origins of Replication • Specific sites on the DNA molecule that starts replication. • Recognized by a specific DNA base sequence.
Prokaryotic • Circular DNA. • 1 origin site. • Replication runs in both directions from the origin site.
Eukaryotic Cells • Many origin sites. • Replication bubbles fuse to form new DNA strands.
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. mcgrawhill. com/olcweb/cgi/pluginpop. cgi? i t=swf: : 535: : /sites/dl/free/0072 437316/120076/micro 04. swf: : DNA %20 Replication%20 Fork
Video • http: //www. bing. com/videos/sear ch? q=You. Tube+DNA+Replicatio n+Process&Form=VQFRVP&adl t=strict#view=detail&mid=1 B 8 F 7 75 F 1 A 027059094 E 1 B 8 F 775 F 1 A 027059094 E
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 nucleotides 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? ?
Assignments • Read Chapter 17, Chapter 9 in Hillis • Discussion Board – the double helix video – today • Chapter 16 – Monday
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. • H 1 - 5 th 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
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