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Chapter 16 DNA The Molecular Basis of Inheritance

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Presentation on theme: "Chapter 16 DNA The Molecular Basis of Inheritance"— Presentation transcript:

1 Chapter 16 DNA The Molecular Basis of Inheritance

2 molecule responsible for all cell activities
DNA molecule responsible for all cell activities and contains the genetic code Genetic Code method cells use to store the program that is passed from one generation to another

3 DISCOVERY OF THE GENETIC CODE
1928: Frederick Griffith Transformation

4 1928: Frederick Griffith Experiment 1
1. Grew 2 strains of bacteria on plates - smooth colonies- caused disease (virulent) - rough edge colonies-did not cause disease (avirulent) 2. Injected into mice Results: - smooth colonies: died - rought colonies: lived Conclusion: bacteria didn’t produce a toxin to kill mice Experiment 1

5 Experiment 2 1. Injected mice with heat killed virulent strain
non -virulent strain + heat killed virulent strain Results: - heat killed: lived - mixed strains: mice developed pneumonia Conclusion: heat killed virulent strain passed disease causing abilities to non virulent strain

6 Cultured bacteria from dead mice and they grew virulent strain.
After Experiment Cultured bacteria from dead mice and they grew virulent strain. Griffith hypothesized that a factor was transferred from heat killed cells to live cells . TRANSFORMATION

7 DNA was transforming factor
1944: Avery (et al) 1. Repeated Griffith’s experiment with same results. - result: transformation occurred 2. Did a second experiment using enzymes that would destroy RNA. 3. Did third experiment using enzymes that would destroy DNA. - result: no transformation CONCLUSION DNA was transforming factor

8 Virus composed of DNA core and protein coat that infect bacteria
1952: Hershey / Chase - studied how viruses (bacteriophage) affect bacteria. Bacteriophage Virus composed of DNA core and protein coat that infect bacteria

9 Hershey Chase Experiment
1. They labeled virus protein coat with radioactive sulfur 2. They labeled virus DNA with radioactive phosphorous Result observed that bacteria had phosphorous *** virus injected bacterial cells with its phosphorous labeled DNA*** Conclusion DNA carried genetic code since bacteria made new DNA animation

10 DISCOVERY OF STRUCTURE OF DNA

11 x ray crystallography evidence:
Early 1950’s: Rosalind Franklin (English) x ray crystallography evidence: X pattern showed that fibers of DNA twisted and molecules are spaced at regular intevals on length fiber. Maurice Wilkins: x ray diffraction, worked with Franklin

12 Chargaff (American biochemist)
Same time period: Chargaff (American biochemist) Chargaff’s Rule:

13 1953 Watson (American) & Crick (English)
**double helix model** won Nobel prize in 1962 Discovered the double helix by building models to conform to Franklin’s X-ray data and Chargaff’s Rules

14 DNA A – T - may have 1000’s of nucleotides in 1 strand
- double strand of nucleotides - may have 1000’s of nucleotides in 1 strand (very long molecule) - bases join up in specific (complementary) pairs: complementary pairs (base pairing rules) 1 purine bonds with 1 pyrimidine on one rung of the ladder connected by a weak H bond C - G A – T Order of nucleotides not important, proper complementary bases must be paired.

15 STRUCTURE OF DNA Backbone Phosphate + Deoxyribose sugar (5 C) Rungs
4 Nitrogenous bases - Purines Adenine A Guanine G - Pyrimidines Thymine T Cytosine C D bases attached to sugar E. bases attached to each other by weak H bond

16 Nucleotide Structure Purines Pyrimidines Sugar Base Phosphate

17 DNA makes up Chromosomes (chromatin packing)

18 DNA Comparison Prokaryotic DNA Double-stranded Circular One chromosome
In cytoplasm No histones Supercoiled DNA Eukaryotic DNA Double-stranded Linear Usually 1+ chromosomes In nucleus DNA wrapped around histones (proteins) Forms chromatin

19 REPLICATION OF DNA Process of duplication of DNA
Before cell can divide a new copy of DNA must be made for the new cell - Semiconservative replication: each strand acts as a template (pattern) for new strand to be made End Result: one old strand, one new daughter strand

20 DNA REPLICATION

21 Models of DNA Replication

22 Discovery of Replication Model
Meselson and Stahl Cultured E coli with heavy isotope 15N for many generations. Transferred to light isotope 14N Sampled after first and second replication Removed bacteria and extracted DNA Centrifuged to separate different density DNA

23 Meselson and Stahl Compared results to models of replication
1st replication: Hybrid DNA (14 and 15) -Eliminated conservative model 2nd replication: Light and hybrid DNA - Eliminated dispersive model - Supported semi-conservative model of replicaiton.

24 Steps of Replication Enzyme DNA helicase attaches to DNA molecule at origins of replication and breaks H bonds so strand unwinds - single strand binding proteins bind to unpaired bases to keep them from re-binding - topoisomerase relieves additional strain on forward DNA by breaking and rejoining DNA strands - replication forks: two areas on either end of the DNA where double helix separates - forms replication bubble: “bubble” under electron microscope

25 2. enzyme RNA Primase lays down an RNA primer on DNA strand
- RNA primer segment signals beginning of replication 3. Enzyme DNA polymerase III moves along each of DNA strand and adds complementary bases of nucleotides floating freely in nucleus A. DNA polymerase III begins synthesis at RNA primer segment B. DNA polymerase I replaces RNA primers with DNA nucleotides -

26 DNA Directionality antiparallel strands
3’ 5’ 5’ 3’

27 - Directionality: DNA polymerase III reads the template in the 3’ to 5’ direction
Daughter DNA strand (since it is complementary) must be synthesized in the 5’ to 3’ direction Strands are antiparallel.

28 But if there exist no DNA polymerases
capable of polymerizing DNA in the 3' to 5' direction, how could this be?

29 Discontinuous synthesis
- synthesis only occurs when a large amount of single strand DNA is present - daughter DNA is then synthesized in 5’ to 3’ direction - leading and lagging strands: - leading strand – continuously synthesized DNA strand - lagging strand - delayed, fragmented, daughter DNA - Okazaki fragments discontinuous fragmented DNA segments

30 D. DNA ligase stitches together Okazaki fragments into a single, unfragmented daughter molecule E. enzyme chops off RNA primer and replaces it with DNA

31 Okazaki fragment animation
DNA polymerase III catalyzes formation of H bonds between nucleotides of template and newly arriving nucleotides which will form daughter DNA Once all DNA is copied, daughter DNA detaches Animation DNA Replication Fork Okazaki fragment animation

32

33 End Replication Problem
On one end, RNA primer cannot be replaced with DNA because it is a 5’ (DNA polymerase III can only read from 3’ to 5’) Causes daughter DNA’s to be shorter with each replication (cell division)

34 Solution to End Replication Problem
telomeres: repeated units of non-coding short nucleotide sequences (TTAGGG) at ends of DNA - become shorter with repeated cell divisions - once telomeres are gone, coding sections of chrom are lost and cell does not have enough DNA to function ***telomere theory of aging*** - telomerase: special enzyme that contains an RNA template molecule so that telomeres can be added back on to DNA (rebuilds telomeres) ** found in: Cancer cells - immortal in culture Stem cells ** not found in most differentiated cells

35 Telomeres and Telomerase

36 Speed of Replication Multiple replication forks- replication occurs simultaneously on many points of the DNA molecule Would take 16 days to replicate 1 strand from one end to the other on a fruit fly DNA without multiple forks Actually takes ~ 3 minutes / sites replicate at one time Human chromosome replicated in about 8 hours with multiple replication forks working together

37 Accuracy and Repair DNA polymerases proofread as bases are added
- can remove damaged nucleotides and replace with new ones for accurate replication Mismatch repair: special enzymes fix incorrect pairings Nucleotide excision repair: Nucleases cut damaged DNA DNA polymerase and ligase fill in gaps RNA does not have this ability- reason RNA viruses mutate so much

38 Nucleotide Excision Repair
Errors: Pairing errors: 1 in 100,000 nucleotides Complete DNA: 1 in 10 billion nucleotides

39 Importance of DNA Controls formation of all substances in the cell by the genetic code Directs the synthesis of specific strands of m RNA to make proteins RNA (Ribonucleic acid) Another nucleic acid takes orders from DNA Used in protein synthesis


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