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DNA The key to understanding the flow of information in living things.

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Presentation on theme: "DNA The key to understanding the flow of information in living things."— Presentation transcript:

1 DNA The key to understanding the flow of information in living things

2 Early in the 20th century, scientists were trying to find the molecule of inheritance. When T. H. Morgan’s group showed that genes are located on chromosomes, the two components of chromosomes, ______ and _________ -became candidates for the genetic material The key factor in determining the genetic material was choosing appropriate experimental organisms The role of DNA in heredity was first discovered by studying ________ and later the _________ that infect them

3 The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 Griffith worked with two strains of a bacterium, one _____________ and one ____________ When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became _____________________ This phenomenon is called _________________ How is it defined?

4 Living S cells (control) Living R cells (control) Heat-killed S cells (control) Mixture of heat-killed S cells and living R cells Mouse dies Mouse healthy Living S cells RESULTS GRIFFITH EXPERIMENT What does this mean?


6 In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming substance was DNA The conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria R Why did biologists remain skeptical? What do these enzymes do?

7 Bacterial cell Kjhkh 100 nm Such viruses, called _________________ (phages), are widely used in molecular genetics research More evidence for DNA as the genetic material came from studies of viruses that infect bacteria

8 HERSHEY-CHASE EXPERIMENT Batch 1: Batch 2: In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2

9 Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) To determine the source of genetic material in the phage, they designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection

10 Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA Centrifuge They concluded that the injected DNA of the phage provides the genetic information


12 This evidence of diversity also made DNA a more credible candidate for the genetic material Sugar–phosphate backbone 5 end Nitrogenous bases Thymine (T) Adenine (A) Cytosine (C) Guanine (G) DNA nucleotide Sugar (deoxyribose) 3 end Phosphate _______________’s rules state that in any species there is an equal number of __ and __ bases, and an equal number of __ and __ bases It was learned that DNA is a polymer of nucleotides, each consisting of … In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next

13 Fig. 12.3 O N N CH C C NH 2 cytosine (C) 3 C C2C2 C 1 OHOP O O H HH HH OH CH 3 O HN N C CH C C OHOP O O H HH HH OH HN N N C CH O C C C N H2NH2N C 2 C 2 C 1 C 1 OHOPO O guanine (G) phosphate H HH HH OH N N N HC CH NH 2 C C C N 4 3 C 2 C 1 5 O O O O O O H HH HH OH c. Chargaff’s data DNA Composition in Various Species (%) Species Homo sapiens (human) Drosophila melanogaster (fruit fly) Zea mays (corn) Neurospora crassa (fungus) Escherichia coli (bacterium) Bacillus subtilis (bacterium) 31.0 27.3 25.6 23.0 24.6 28.4 31.5 27.6 25.3 23.3 24.3 29.0 19.1 22.5 24.5 27.1 25.5 21.0 18.4 22.5 24.6 26.6 25.6 21.6 ATGC a. Purine nucleotidesb. Pyrimidine nucleotides nitrogen-containing base sugar = deoxyribose thymine (T) adenine (A) HOPO CH 2 5 5 5 C 4 C 4 C 4 C C 3 C 3 C

14 Maurice Wilkins and Rosalind Franklin were using a technique called ________________________to study molecular structure Franklin produced a picture of the DNA molecule using this technique

15 In 1953, _________ and _______ introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA Hereditary information is encoded in DNA and reproduced in all cells of the body What can DNA encode?

16 Space-filling model Hydrogen bond 3 end 5 end 3.4 nm 0.34 nm 3 end 5 end Partial chemical structure Key features of DNA structure 1 nm Franklin had concluded that there were two ___________________________ __________-_____________ backbones, with the nitrogenous bases paired in the molecule’s interior

17 Purine + purine: Pyrimidine + pyrimidine: Purine + pyrimidine: At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width In the end, pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray

18 Watson and Crick reasoned that the pairing was more specific, dictated by the base structures: This came to be: They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C) The Watson-Crick model also explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C

19 A T G C TA TA G C A T G C T A T A G C Parent molecule Separation of strands DNA Replication Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication

20 A T G C TA TA G C Parent molecule AT GC T A T A GC “Daughter” DNA molecules, each consisting of… Separation of strands A T G C TA TA G C A T G C T A T A G C Notice that the parent strands are now serving as templates for the new strands In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules

21 Parent cell First replication Second replication Conservative model Semiconservative model Dispersive model Watson and Crick’s ______________ ______________ model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand Competing models were: Conservative model (the two parent strands rejoin) Dispersive model (each strand is a mix of old and new)

22 region of parental DNA double helix region of replication: new nucleotides are pairing with those of parental strands region of completed replication old strand new strand daughter DNA double helix A A A A A A A A A A A A A A A A A A A A T T T T T T T T T T T T T G G G G G G G G G G G G G G G G C C C C C C C C C C C C C old strand new strand


24 The copying of DNA is remarkable in its speed and accuracy More than a dozen enzymes and other proteins participate in DNA replication

25 Getting Started: STEP-BY-STEP Replication begins at special sites called ____________ ____ ______________, where the two DNA strands are separated, opening up a replication “bubble” – A eukaryotic chromosome: __________________________ – A prokaryotic ring of DNA:____ Replication proceeds in both directions from each origin, until the entire molecule is copied

26 replication fork replication bubble parental strand daughter strand new DNA duplexes replication is occurring in two directions replication is complete origin

27 5 5 53 3 3 At the end of each replication bubble is a _____________ _____, a Y- shaped region where new DNA strands are elongating ___________ are enzymes that untwist the double helix at the replication forks ____________________ ____________________ binds to and stabilizes single-stranded DNA until it can be used as a template _________________ corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands

28 For the protein that does the copying, it can’t start on its own. It needs some help from a beginning stretch of nucleotides that are laid down The initial nucleotide strand is a short _______ primer An enzyme called ___________ can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template The primer is short (5–10 nucleotides long), and the 3’ end serves as the starting point for the new DNA strand

29 Enzymes called _____ ________ catalyze the elongation of new DNA at a replication fork Most of these enzymes require a ______________ and a DNA template strand What are the rates of elongation of a prokaryotic vs. a eukaryotic cell?

30 The antiparallel structure of the double helix (two strands oriented in opposite directions) affects replication DNA polymerases add nucleotides only to the free ____________of a growing strand; therefore, a new DNA strand can elongate only in the ____________direction Along one template strand of DNA, the DNA polymerase synthesizes a _____________ ___ continuously, moving toward the replication fork ANTIPARALLEL ELONGATION

31 Fig. 16-15a Origin of replication Primer Overall directions of replication

32 To elongate the other new strand, called the ________________, DNA polymerase must work in the direction away from the replication fork This strand is synthesized as a series of segments called _________ __________, which are joined together by _____ ______________

33 Lagging Strand synthesis Template strand 5 5 3 3 RNA primer 3 5 5 3 1 1 3 3 5 5 Okazaki fragment 1 2 3 3 5 5 1 2 3 3 5 5 1 2 5 5 3 3 Overall direction of replication


35 Proofreading and Repairing DNA DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides In _________________ of DNA, repair enzymes correct errors in base pairing DNA can be damaged by chemicals, radioactive emissions, X-rays, UV light, and certain molecules (in cigarette smoke for example) In ________________________________________, a ___________ cuts out and replaces damaged stretches of DNA


37 Ends of parental DNA strands Leading strand Lagging strand Last fragment Previous fragment Parental strand RNA primer Removal of primers and replacement with DNA where a 3 end is available Second round of replication New leading strand New lagging strand Further rounds of replication Shorter and shorter daughter molecules 5 3 3 3 3 3 5 5 5 5 Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes The usual replication machinery provides no way to complete the 5 ends, so repeated rounds of replication produce ____________ DNA molecules

38 Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called ________________ They do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules It has been proposed that the shortening of telomeres is connected to aging

39 If chromosomes of stem cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce An enzyme called ______________________ catalyzes the lengthening of telomeres in stem cells The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist

40 DNA double helix (2 nm in diameter) Nucleosome (10 nm in diameter) Histones Histone tail H1 DNA, the double helix Nucleosomes, or “beads on a string

41 30-nm fiber Chromatid (700 nm) LoopsScaffold 300-nm fiber Replicated chromosome (1,400 nm) 30-nm fiber Looped domains (300-nm fiber) Metaphase chromosome

42 Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis Loosely packed chromatin is called ___________ During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into ___________________ Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions

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