Transcript Document

DNA Structure and Function
Chapter 8
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole,
Cengage Learning 2011.
Biology, Ninth Edition, by Solomon, Berg, Martin. Brooks/Cole, Cengage Learning 2011.
A Hero Dog’s Golden Clones
 Contest  Golden Clone Giveaway
• Dog died by DNA lives on in genetically identical
copies called clones
 Cloning
• Only 2% of implanted embryos result in a live birth
• If the clones survive, many have serious health
problems
• WHY? Because an adult cell uses only a faction of
the DNA found in an embryonic cell
• Why Clone? Study genetic diseases, form
replacement tissues or organs, save endangered
animals for extinction
DNA
 Cells contain coded genetic information in the
form of deoxyribonucleic acid (DNA)
 DNA is organized into informational units
(genes) which control a cell’s activities and are
passed on to its descendants
 When a cell divides, the information in DNA
must be replicated and the copies transmitted to
each daughter cell
Chromosomes, Mitosis, and Meiosis
 Each DNA molecule is packaged with proteins to form
a chromosome, containing hundreds or thousands of
genes
 A parent cell transmits one copy of every
chromosome to each of its two daughter cells by the
process of mitosis
 In organisms that reproduce sexually, meiosis
reduces the chromosome number in half to produce
eggs and sperm (gametes) – the original number is
restored in the zygote
Eukaryotic Chromosomes
• When a cell is not dividing  chromosomes are
present in an extended, partially unraveled form
called chromatin
• A structure that consists of DNA and associated
proteins, carries part or all of a cell’s genetic information
• During cell division  chromatin fibers condense
and chromosomes become visible as distinct
structures
• After chromosomes are duplicated, each consists of
two DNA molecules  sister chromatid
• Sister chromatid are connected to each other at the
centromere
Chromosomes
Fig. 10-1, p. 214
Histones and Nucleosomes
 In eukaryotic cells, DNA is packed in multiple
chromosomes which vary in size and number
among different species
 Positively-charged proteins (histones) associate
with negatively-charged DNA to form
nucleosomes
 Each nucleosome consists of a beadlike
structure with 146 base pairs of DNA wrapped
around a disc-shaped core of eight histone
molecules
Nucleosome Structure
DNA wound
around a cluster
of histone
molecules
Histone tails
Linker DNA
Nucleosome
(10 nm diameter)
(a) A model for the structure of a nucleosome. Each
nucleosome bead contains a set of eight histone molecules,
forming a protein core around which the double-stranded DNA
winds. The DNA surrounding the histones consists of 146
nucleotide pairs; another segment of DNA, about 60 nucleotide
pairs long, links nucleosome beads.
Fig. 10-2a, p. 215
DNA Released From Histone Proteins
 Histones are an
important part of the
regulation of gene
expression –
whether genes are
turned off or on
Organization of
a Eukaryotic Chromosome
1400 nm
700 nm
300 nm fiber
(looped
domains)
30 nm chromatin fiber
Condensed
chromosome
Scaffolding
Condensed chromatin protein
Extended chromatin
DNA wound
around a
cluster of
histone
molecules
Packed nucleosomes
Histone
10 nm
Nucleosomes
2 nm
DNA double helix
Stepped Art
Fig. 10-4, p. 216
Chromosome Number and Content
• Every species has a characteristic number of
chromosomes in the nucleus of each somatic
(body) cell
• Humans: 46 chromosomes
• Roundworm: 2 chromosomes
• Some ferns have more than 1000 chromosomes
• It is the information specified in the genes that
makes each species unique
Chromosome number
 Chromosome number
• The sum of all chromosomes in a cell of a given
type
 Diploid (2n)
• Having two of each type of chromosomes
characteristics of the species
• Human have 23 pairs of chromosomes
• One set from mom and one set from dad
 Haploid (1n)
• Having one of each type of chromosomes
characteristics of the species
Types of Chromosomes
 Karyotype
• Image of an individual’s complement of chromosomes
arranged by size, length, shape, and centromere
location
• Comparing the complement chromosomes can show
any extra or missing chromosomes and structural
abnormalities
 Autosomes
• Any chromosome other than sex chromosomes
 Sex Chromosome
• Member of a pair of chromosomes that differs
between males and females
Karyotype
KEY CONCEPTS
 In eukaryotic cells, DNA is wound around
specific proteins to form chromatin, which in turn
is folded and packaged to make individual
chromosomes
Discovery of DNA’s function
 Experimental tests using bacteria and
bacteriophages showed that DNA is the
hereditary material in living organisms
• Frederick Griffith’s experiments
• Oswald Avery’s experiments
• Hershey and Chase’s experiments
1920s: Frederick Griffith’s Experiments
• Two strains of pneumococcus bacteria:
• Smooth (S) strain causes death (virulent)
• Rough (R) strain does not cause disease
(avirulent)
• Mice injected with heat-killed virulent S cells did
not die
• Mice injected with a mixture of heat-killed
virulent S cells and live avirulent R cells died –
Griffith isolated living S cells from the dead mice
The Transforming Principle
 Permanent genetic change in which the
properties of one strain of dead cells are
conferred on a different strain of living cells is
called transformation
 Scientists hypothesized that some chemical
substance (a transforming principle) was
transferred from the dead bacteria to the living
cells and caused transformation
Griffith’s Transformation Experiments
Griffith’s Experiments
1940s: Oswald Avery’s Experiments
 Avery separated the contents of S cells into
lipids, proteins, polysaccharides, and nucleic
acids (DNA and RNA)
 Only nucleic acids caused transformation
 This was the first definitive demonstration that
DNA is the genetic material
1950s: Hershey and Chase’s Experiments
 Alfred Hershey and
Martha Chase worked
with viruses that infect
bacteria (phages or
bacteriophages)
 Only the genetic
material of a phage
enters the bacterium
Bacteriophage
Phage genetic
material
Bacterial cell
(E. coli)
Fig. 12-2, p. 265
Hershey and Chase’s Experiments
 In one experiment, phage proteins were labeled
with 35S, a radioactive isotope of sulfur
 In another experiment, phage DNA was labeled
with 32P, a radioactive isotope of phosphorus
 Infected bacteria contained 32P, not 35S
 Hershey and Chase concluded that phages
inject their DNA into bacterial cells, leaving most
of their protein outside
Hershey and Chase’s Experiments
Fig. 12.4, p.189
35S
remains
outside cells
Virus particle
coat proteins
labeled with 35S
DNA being
injected into
bacterium
Virus DNA
labeled with 32P
32P
remains
inside cells
Labeled DNA
being injected
into bacterium
Fig. 12.4, p.189
KEY CONCEPTS
 Starting in the 1920s, evidence began to
accumulate that DNA is the hereditary material
Watson and Crick’s DNA Model
Key Concepts:
DISCOVERY OF DNA’S FUNCTION
 In all living cells, DNA molecules store
information that governs heritable traits
Historical DNA Discoveries
 Many scientists
contributed to our
knowledge of DNA
structure and function
Table 12-1, p. 267
Discovery of DNA Structure
 DNA consists of two strands of nucleotides,
coiled into a double helix
 Each nucleotide has
• A five-carbon sugar (deoxyribose)
• A phosphate group
• A nitrogen-containing base (adenine, thymine,
guanine, or cytosine)
DNA Structure
• Each DNA building block is a nucleotide
consisting of the pentose sugar deoxyribose, a
phosphate, and one of four nitrogenous bases
• The nitrogenous base is attached to the 1′
carbon of the sugar, and the phosphate is
attached to the 5′ carbon
• Four nitrogenous bases:
• Two purines: adenine (A) and guanine (G)
• Two pyrimidines: thymine (T) and cytosine (C)
DNA Structure (cont.)
 Nucleotides are linked by covalent bonds to form
an alternating sugar–phosphate backbone
 The 3′ carbon of one sugar is bonded to the 5′
phosphate of the adjacent sugar to form a 3′, 5′
phosphodiester linkage
 The 5′ end has a 5′ carbon attached to a
phosphate and the 3′ end has a 3′ carbon
attached to a hydroxyl group
Nucleotide Subunits of DNA
 Note the polarity of the
polynucleotide chain,
with the 5′ end at the
top and the 3′ end at
the bottom
Fig. 12.5, p.190
Fig. 12.5, p.190
Fig. 12.5, p.190
Fig. 12.5, p.190
Base Pairing
 Bases of two DNA strands pair in only one way
• Adenine with thymine (A-T)
• Guanine with cytosine (G-C)
 The DNA sequence (order of bases) varies
among species and individuals
Chargaff’s Rules
• Erwin Chargaff found a relationship among DNA
bases that turned out to be an important clue to
DNA structure
• Chargaff ’s rules
• In double-stranded DNA molecules, the number
of purines equals the number of pyrimidines
• The number of adenines equals the number of
thymines (A = T), and the number of guanines
equals the number of cytosines (G = C)
2-nanometer diameter overall
0.34-nanometer distance
between each pair of bases
3.4-nanometer
length of each
full twist of the
double helix
The pattern of base
pairing (A with T,
and G with C) is
consistent with the
known composition
of DNA (A = T,
and G = C).
In all respects shown here, the
Watson–Crick model for DNA
structure is consistent with the
known biochemical and x-ray
diffraction data.
Fig. 12.6, p.191
Complementary Base Pairing
• Because A only pairs with T, and G only pairs
with C, the sequences of bases in the two chains
exhibit complementary base pairing
• The sequence of nucleotides in one chain
dictates the complementary sequence of
nucleotides in the other
• Example: If one strand has the sequence 3′GCTAC-5′, then the other strand must have the
complementary sequence 5′-CGATG-3′
Base Pairing and Hydrogen Bonding
Adenine
Thymine
Deoxyribose
Guanine
Deoxyribose
Deoxyribose
Cytosine
Deoxyribose
(b) Hydrogen bonding between base pairs adenine (A) and thymine (T) ( top )
and guanine (G) and cytosine (C) ( bottom ). The AT pair has two hydrogen
bonds; the GC pair has three.
Fig. 12-9b, p. 270
Key Concepts:
THE DNA DOUBLE HELIX
 A DNA molecule consists of two chains of
nucleotides, hydrogen-bonded together along
their length and coiled into a double helix
 Four kinds of nucleotides make up the chains:
adenine, thymine, guanine, and cytosine
Key Concepts:
THE DNA DOUBLE HELIX (cont.)
 The order in which one kind of nucleotide base
follows the next along a DNA strand encodes
heritable information
 The order in some regions of DNA is unique for
each species
12.3 Watson, Crick, and Franklin
 Rosalind Franklin’s research
produced x-ray diffraction
images of DNA
• Helped Watson and Crick
build their DNA model, for
which they received the
Nobel Prize
Rosalind Franklin’s Experiments
 Rosalind Franklin used X-ray diffraction to
determine 3-D structure and measurements of
the DNA molecule
 Her images showed that DNA has a helical
structure with three major types of regular,
repeating patterns – indicating that nucleotide
bases are stacked like rungs of a ladder
 Using Franklin’s information, James Watson and
Francis Crick began to build scale models of
DNA components and fit them together to
correlate with experimental data
RESEARCH METHOD:
How X-Ray Diffraction Works
Key Concepts:
THE FRANKLIN FOOTNOTE
 Like any race, the one that led to the discovery
of DNA’s structure had its winners—and its
losers
James Watson and Francis Crick
 Watson and Crick’s
DNA model consisted
of two polynucleotide
chains arranged in a
coiled double helix
 The sugar–phosphate
backbones of the two
chains form the
outside of the helix
Fig. 12-7, p. 269
3-D Model of the
DNA Double Helix
 Each pair of bases is
0.34 nm from the
pairs above and
below
 Exactly 10 base pairs
are present in each
full turn of the helix
DNA Replication and Repair
 DNA Sequence
• Order of nucleotides bases in a strand of DNA
 DNA Replication
• Process by which a cell duplicates its DNA before it
divides
 A cell replicates its DNA before dividing
• Enzymes unwind the double helix
• DNA polymerases assemble complementary DNA
strands on templates from free nucleotides
• DNA ligase seals gaps in new DNA strands
 Product  Two double-stranded DNA molecules
• One strand of each is new
Semiconservative DNA Replication
 The DNA model suggests how the sequence of
nucleotides in DNA could be precisely copied
• DNA replication
 Because nucleotides pair with each other in
complementary fashion, each strand of the DNA
molecule serves as a template for synthesizing the
opposite strand
 Two identical DNA double helices are produced,
each consisting of one original strand from the
parent molecule and one newly synthesized
complementary strand (semiconservative
replication)
Semiconservative DNA Replication
Details of DNA Replication
Fig. 12.9, p.193
direction of
unwinding
direction
of
synthesis
New DNA is assembled
continuously on only one
of the two parent template
strands. It is assembled on
the other parent template
strand in short fragments.
DNA ligase seals the gaps
between the fragments.
Why discontinuous assembly? DNA
synthesis occurs only in the 5’ to 3’
direction. Free nucleotides can be
added only to the —OH group at
the 3’ end of a growing strand.
Fig. 12.9, p.193
Part of a parent
DNA molecule, with two
complementary strands of
base-paired nucleotides.
Replication starts.
The strands are unwound
at many sites along
the molecule’s length.
Each of the two parent
strands guides the assembly
of new DNA strands from free
nucleotides, according to
base-pairing rules.
Any gaps between bases
of the “new” DNA are joined
to form a continuous strand.
The base sequence of each
half-old, half-new DNA
molecule is identical
to that of the parent.
Stepped Art
Fig. 12-9, p.193
Replication Errors
 DNA repair mechanisms fix DNA damaged by
chemicals or radiation
 Proofreading by DNA polymerases corrects
most base-pairing errors
 Uncorrected errors are mutations
• Mutations  permanent change in the sequence
of DNA
Key Concepts:
HOW CELLS DUPLICATE THEIR DNA
 Before a cell divides, enzymes and other
proteins copy its DNA
 Newly forming DNA strands are monitored for
errors, most of which are corrected
 Uncorrected errors are mutations
12.5 Cloning
 Clones
• Genetically identical individuals
• Produced by artificial twinning, nuclear transfers
 To clone an adult animal
• Cell’s DNA must be reprogrammed to function
like an embryonic cell and direct development
Methods of Cloning
 Reproductive Cloning
• Produces genetically identical individuals
• Somatic cell nuclear transfer  to clone adults
• Type of reproductive cloning in which genetic
material is transferred from an adult somatic cell
into an unfertilized, enucleated egg.
• Transfer the NUCLEUS
 Therapeutic Cloning
• Use of SCNT to produce human embryos for
research purposes
Cloning Methods
Nuclear Transfer
Key Concepts: DNA AND
THE CLONING CONTROVERSIES
 Knowledge about the structure and function of
DNA is the basis of several methods of cloning
Animation: DNA close up
Animation: DNA replication details
Animation: Griffith's experiment
Animation: Hershey-Chase experiments
Animation: How Dolly was created
Animation: Subunits of DNA