DNA The key to understanding the flow of information in living
things Slide 2 Early in the 20th century, scientists were trying to
find the molecule of inheritance. When T. H. Morgans 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 Slide 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? Slide 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? Slide 5 Slide 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? Slide 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 Slide 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 Slide 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 Slide 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 Slide 11 Slide 12 This
evidence of diversity also made DNA a more credible candidate for
the genetic material Sugarphosphate 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 Slide 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. Chargaffs 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 Slide 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 Slide 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?
Slide 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 molecules interior Slide
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 Slide 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
Chargaffs rules: in any organism the amount of A = T, and the
amount of G = C Slide 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 Slide 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
Slide 21 Parent cell First replication Second replication
Conservative model Semiconservative model Dispersive model Watson
and Cricks ______________ ______________ 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) Slide 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 Slide 23 DNA
REPLICATION: A CLOSER LOOK Slide 24 The copying of DNA is
remarkable in its speed and accuracy More than a dozen enzymes and
other proteins participate in DNA replication Slide 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 Slide 26 replication fork replication
bubble parental strand daughter strand new DNA duplexes replication
is occurring in two directions replication is complete origin Slide
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
Slide 28 For the protein that does the copying, it cant 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 (510 nucleotides long), and
the 3 end serves as the starting point for the new DNA strand Slide
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? Slide 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 Slide 31 Fig.
16-15a Origin of replication Primer Overall directions of
replication Slide 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
_____ ______________ Slide 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
Slide 34 Slide 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 Slide 36 Slide 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 Slide 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 Slide 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 Slide 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 Slide 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 Slide 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
