Presentation on theme: "Chapter 14 DNA: The Genetic Material. Question? u Traits are inherited on chromosomes, but what in the chromosomes is the genetic material? u Two possibilities:"— Presentation transcript:
Chapter 14 DNA: The Genetic Material
Question? u Traits are inherited on chromosomes, but what in the chromosomes is the genetic material? u Two possibilities: u Protein u DNA
Qualifications u Protein: u very complex. u high specificity of function. u DNA: u simple. u not much known about it (early 1900’s).
For testing: u Name(s) of experimenters u Outline of the experiment u Result of the experiment and the importance of the result
Griffith u Pneumonia in mice. u Two strains: u S - pathogenic u R - harmless
Result u Something turned the R cells into S cells. u Transformation - the assimilation of external genetic material by a cell.
Problem u Griffith used heat. u Heat denatures proteins. u So could proteins be the genetic material? u DNA - heat stable. u Griffith’s results contrary to accepted views.
Avery, McCarty and MacLeod u Repeated Griffith’s experiments, but added specific fractions of S cells. u Result - only DNA transformed R cells into S cells. u Result - not believed.
Hershey- Chase 1952 u Genetic information of a virus or phage. u Phage - virus that attacks bacteria and reprograms host to produce more viruses.
Bacteria with Phages
Phage Components u Two main chemicals: u Protein u DNA u Which material is transferred to the host?
Used Tracers u Protein - CHONS, can trace with 35 S. u DNA - CHONP, can trace with 32 P.
Experiment u Used phages labeled with one tracer or the other and looked to see which tracer entered the bacteria cells.
Result u DNA enters the host cell, but the protein did not. u Therefore: DNA is the genetic material.
Chargaff u Studied the chemical composition of DNA. u Found that the nucleotides were in certain ratios.
Chargaff’s Rule u A = T u G = C u Example: in humans, A = 30.9% T = 29.4% G = 19.9% C = 19.8%
Why? u Not known until Watson and Crick worked out the structure of DNA.
Watson and Crick u Used X-ray crystallography data (from Rosalind Franklin) u Used model building. u Result - Double Helix Model of DNA structure. (One page paper, 1953).
Book & Movies u “The Double Helix” by James Watson- His account of the discovery of the shape of DNA u Movie – The Double Helix
DNA Composition u Deoxyribose Sugar (5-C) u Phosphate u Nitrogen Bases: u Purines u Pyrimidines
DNA Backbone u Polymer of sugar-phosphate. u 2 backbones present.
Nitrogen Bases u Bridge the backbones together. u Purine + Pyrimidine = 3 rings. u Constant distance between the 2 backbones. u Held together by H-bonds.
Chargaff’s Rule u Explained by double helix model. u A = T, 3 ring distance. u G = C, 3 ring distance.
Watson and Crick u Published a second paper (1954) that speculated on the way DNA replicates. u Proof of replication given by others.
Replication u The process of making more DNA from DNA. u Problem: when cells replicate, the genome must be copied exactly. u How is this done?
Models for DNA Replication u Conservative - one old strand, one new strand. u Semiconservative - each strand is 1/2 old, 1/2 new. u Dispersive - strands are mixtures of old and new.
Meselson - Stahl late 1950’s u Grew bacteria on two isotopes of N. u Started on 15 N, switched to 14 N. u Looked at weight of DNA after one, then 2 rounds of replication.
Results u Confirmed the Semiconservative Model of DNA replication.
Replication - Preview u DNA splits by breaking the H-bonds between the backbones. u Then DNA builds the missing backbone using the bases on the old backbone as a template.
Origins of Replication u Specific sites on the DNA molecule that starts replication. u Recognized by a specific DNA base sequence.
Prokaryotic u Circular DNA. u 1 origin site. u Replication runs in both directions from the origin site.
Eukaryotic Cells u Many origin sites. u Replication bubbles fuse to form new DNA strands.
DNA Elongation u By DNA Polymerases. u Adds DNA triphosphate monomers to the growing replication strand. u Matches A to T and G to C.
Energy for Replication u From the triphosphate monomers. u Loses two phosphates as each monomer is added.
Problem of Antiparallel DNA u The two DNA strands run antiparallel to each other. u DNA can only elongate in the 5’--> 3’ direction.
Leading Strand u Continuous replication toward the replication fork in the 5’-->3’ direction.
Lagging Strand u Discontinuous synthesis away from the replication fork. u Replicated in short segments as more template becomes opened up.
Priming u DNA Polymerase cannot initiate DNA synthesis. u Nucleotides can be added only to an existing chain called a Primer.
Primer u Make of RNA. u 10 nucleotides long. u Added to DNA by an enzyme called Primase. u DNA is then added to the RNA primer.
Priming u A primer is needed for each DNA elongation site.
Okazaki Fragments u Short segments ( bases) that are made on the lagging strand. u All Okazaki fragments must be primed. u RNA primer is removed after DNA is added.
Enzymes u Replaces RNA primers with DNA nucleotides. u DNA Ligase - joins all DNA fragments together.
Other Proteins in Replication u Helicase - unwinds the DNA double helix. u Single-Strand Binding Proteins - help hold the DNA strands apart.
u Lets see replication in action! u ence/anthropology/stein2003/ stein.html ence/anthropology/stein2003/ stein.html
DNA Replication Error Rate u 1 in 1 billion base pairs. u About 3 mistakes in our DNA each time it’s replicated.
Reasons for Accuracy u DNA Polymerase self-checks and corrects mismatches. u DNA Repair Enzymes - a family of enzymes that checks and corrects DNA.
DNA Repair u 50+ different DNA repair enzymes known. u Failure to repair may lead to Cancer or other health problems.
Example: u Xeroderma Pigmentosum - Genetic condition where a DNA repair enzyme doesn’t work. u UV light causes damage, which can lead to cancer.
Xeroderma Pigmentosum Cancer Protected from UV
Thymine Dimers u T-T binding from side to side causing a bubble in DNA backbone. u Often caused by UV light.
Excision Repair u Cuts out the damaged DNA. u DNA Polymerase fills in the excised area with new bases. u DNA Ligase seals the backbone.
Problem - ends of DNA u DNA Polymerase can only add nucleuotides in the 5’--->3’ direction. u It can’t complete the ends of the DNA strand.
Result u DNA gets shorter and shorter with each round of replication.
Telomeres u Repeating units of TTAGGG ( X) at the end of the DNA strand (chromosome) u Protects DNA from unwinding and sticking together. u Telomeres shorten with each DNA replication.
u Serve as a “clock” to count how many times DNA has replicated. u When the telomeres are too short, the cell dies by apoptosis.
Implication u Telomeres are involved with the aging process. u Limits how many times a cell line can divide.
Telomerase u Enzyme that uses RNA to rebuild telomeres. u Can make cells “immortal”. u Found in cancer cells. u Found in germ cells. u Limited activity in active cells such as skin cells
Comment u Control of Telomerase may stop cancer, or extend the life span.
NEWS FLASH u The DNA of Telomeres is actually used to build proteins. u These proteins seem to impede telomerase. u Feedback Loop??
Summary u Know the Scientists and their experiments. u Why DNA is an excellent genetic material. u How DNA replicates. u Problems in replication.