Slide 2 UNIT 3: Molecular Genetics Chapter 5: The Structure and
Function of DNA Chapter 6: Gene Expression Chapter 7: Genetic
Research and Biotechnology Slide 3 Chapter 5: The Structure and
Function of DNA What are the structures and functions of DNA and
RNA? Scientists now know that DNA, shown here in its uncondensed
form, carries genetic information that defines many of an organisms
traits (including behaviours) and its predisposition for certain
diseases. UNIT 3 Chapter 5: The Structure and Function of DNA Slide
4 5.1 DNA Structure and Organization in the Cell Before DNA was
established as the genetic material in cells, scientists knew: UNIT
3 Chapter 5: The Structure and Function of DNA Section 5.1 there
was a connection between chromosomes and inherited traits the
genetic material had to control the production of enzymes and
proteins the genetic material had to be able to replicate itself
with accuracy and still allow mutations to occur Slide 5
Identifying DNA as the Material of Heredity: Griffith Frederick
Griffiths experiments with Streptococcus pneumoniae in 1928
established DNA as the material of heredity. Two forms of the
bacterium were used: UNIT 3 Chapter 5: The Structure and Function
of DNA Section 5.1 The S-strain was highly pathogenic but could be
made non-pathogenic by heating it. The R-strain was non-
pathogenic. Continued Slide 6 Identifying DNA as the Material of
Heredity: Griffith Griffith discovered that mice died after being
injected with a mixture of heat-killed S-strain and living R-strain
bacteria. UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1 Griffith called this phenomenon the transforming
principle because something from the S- strain transformed the
R-strain into deadly bacteria. Slide 7 Identifying DNA as the
Material of Heredity: Avery, MacLeod, and McCarty Canadian-American
microbiologist Oswald Avery and his group built on Griffiths work
to identify the molecules in the S-strain that caused the
transformation. Avery, MacLeod, and McCarty prepared identical
extracts of the heat-killed S-strain and subjected each extract to
one of three enzymes: UNIT 3 Chapter 5: The Structure and Function
of DNA Section 5.1 one that destroyed proteins one that destroyed
RNA, and one that destroyed DNA. Continued Slide 8 Identifying DNA
as the Material of Heredity: Avery, MacLeod, and McCarty Each
enzyme-treated extract was then mixed with live R-strain cells. The
only extract that did not allow transformation of the R-strain to
the pathogenic S-strain was the one treated with the DNA-destroying
enzyme. Avery and his colleagues concluded that DNA was the
hereditary material. UNIT 3 Chapter 5: The Structure and Function
of DNA Section 5.1 Slide 9 Identifying DNA as the Material of
Heredity: Hershey and Chase In 1952, Americans Alfred Hershey and
Martha Chase ruled out protein as the hereditary material. Their
experiments used T2 bacteriophages, which consist of nucleic
material surrounded by a protein coat. Hershey and Chase used two
different radioactive isotopes to track each molecule ( 35 S for
proteins and 32 P for DNA). UNIT 3 Chapter 5: The Structure and
Function of DNA Section 5.1 Continued Slide 10 Identifying DNA as
the Material of Heredity: Hershey and Chase In their first
experiment: UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1 a virus with DNA radioactively labelled with 32 P was
allowed to infect bacteria. After agitation and separation,
radioactivity was found in the bacteria pellet but not in the
liquid medium. Continued Slide 11 Identifying DNA as the Material
of Heredity: Hershey and Chase UNIT 3 Chapter 5: The Structure and
Function of DNA Section 5.1 a virus with its protein coat
radioactively labeled with 35 S was allowed to infect bacteria.
After agitation and separation, radioactivity was found in the
liquid medium but not in the bacteria pellet. The results proved
that viral DNA held the genetic material needed for viruses to
reproduce. In their second experiment: Slide 12 Determining the
Chemical Composition and Structure of DNA DNA was discovered in
1869 by Fredrich Miescher. By isolating the nuclei of white blood
cells, he extracted an acidic molecule he called nuclein. In the
early 1900s, Phoebus Levene isolated two types of nucleic acid: RNA
and DNA. In 1919, he proposed that both were made up of individual
units called nucleotides. Each nucleotide was composed of one of
four nitrogen- containing bases, a sugar, and a phosphate group.
UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1
Slide 13 The Chemical Composition of the Nucleotides, DNA, and RNA
UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 DNA
has the nucleotides adenine (A), guanine (G), cytosine (C), and
thymine (T). RNA has the nucleotides adenine (A), guanine (G),
cytosine (C), and uracil (U). In later years, other scientists
confirmed and extended Levenes work. DNA and RNA are both made up
of a combination of four different nucleotides. Nucleotides are
often identified by referring to their bases: Continued Slide 14
The Chemical Composition of the Nucleotides, DNA, and RNA UNIT 3
Chapter 5: The Structure and Function of DNA Section 5.1 The
general structure of a DNA nucleotide includes a phosphate group, a
deoxyribose sugar group, and a nitrogen-containing base.
Nucleotides in RNA have the same basic structure, except a ribose
sugar group is used. The sugar groups differ by a hydroxyl group at
the 2 carbon. Both DNA and RNA contain the same purine bases and
the cytosine pyrimidine base. However, thymine is only present in
DNA, and uracil is only present in RNA. Slide 15 Chargaffs Rule:
Closing in on the Structure of DNA UNIT 3 Chapter 5: The Structure
and Function of DNA Section 5.1 There is variation in the
composition of nucleotides in different species. Regardless of the
species, DNA maintains certain nucleotide proportions. That is, the
amount of A and T nucleotides are equal and the amount of C and G
nucleotides are equal. This constant relationship is known as
Chargaffs rule. Erwin Chargaff was inspired by Avery, MacLeod, and
McCartys work on DNA and launched a research program to study
nucleic acids. By the late 1940s, he had reached two conclusions:
Continued Slide 16 Chargaffs Rule: Closing in on the Structure of
DNA UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1
In DNA, the percent composition of adenine is the same as thymine,
and the percent composition of cytosine is the same as guanine.
Slide 17 Pauling Discovers a Helical Structure for Proteins UNIT 3
Chapter 5: The Structure and Function of DNA Section 5.1 In 1951,
Linus Pauling discovered that many proteins have helix- shaped
structures. Scientists, including James Watson and Francis Crick,
used this information when deducing the structure of DNA. Slide 18
Franklin Determines a Helical Structure for DNA UNIT 3 Chapter 5:
The Structure and Function of DNA Section 5.1 DNA has a helical
structure. The nitrogen bases are on the inside of the DNA helix,
and the sugar-phosphate backbone is on the outside. In the early
1950s, Rosalind Franklin and Maurice Wilkins used X-ray diffraction
to analyze DNA samples. Franklin captured high-resolution
photographs and, using mathematical theory to interpret them,
determined the following: (A) Rosalind Franklin was a British
chemist who was hired to work alongside Maurice Wilkins at the
X-ray diffraction facilities at Kings College. (B) In the
diffraction image of DNA that she produced, the central x-shaped
pattern enabled researchers to infer that DNA has a helical
structure. Slide 19 Watson and Crick Build a Three- Dimensional
Model for DNA UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1 a twisted ladder, which they called a double-helix. The
sugar- phosphate molecules make up the sides or handrails of the
ladder, and the bases make up the rungs of the ladder by protruding
inwards. In the early 1950s, Watson and Crick began working on a
description of the structure of DNA using the results and
conclusions of their peers. In 1953, they published a paper that
proposed a structure with the following features: Continued Slide
20 Watson and Crick Build a Three- Dimensional Model for DNA UNIT 3
Chapter 5: The Structure and Function of DNA Section 5.1 The
distance between the sugar-phosphate backbones remains constant
over the length of a molecule of DNA. An A nucleotide on one strand
always sits across from a T nucleotide (and C across from G) in
order to maintain constant distance. These are called base pairs.
Different sequences of base pairs can exist, which accounts for the
differences between species. Slide 21 The Modern DNA Model: The DNA
Double Helix UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1 The double helix is composed of two polynucleotide
strands that twist around one another. Each strand has a backbone
of alternating phosphate groups and sugars. The distance between
the sugar- phosphate backbones in each strand is constant. The
bases of each nucleotide are attached to each sugar and face
inward. Structural features of DNA: Continued Slide 22 The Modern
DNA Model: The DNA Double Helix UNIT 3 Chapter 5: The Structure and
Function of DNA Section 5.1 The two strands are complementary due
to complementary base pairing of A with T and C with G. Hydrogen
bonds link each complementary base pair. Each strand has a 5 end
and a 3 end. The 5 and 3 come from the numbering of the carbons in
the deoxyribose sugar. The two strands are antiparallel, where the
5 end from one strand is across from the 3 end of the complementary
strand. The sequence of a DNA strand is written in the 5 to 3
direction. Slide 23 The DNA Double Helix UNIT 3 Chapter 5: The
Structure and Function of DNA Section 5.1 Slide 24 The Structure
and Organization of Genetic Material UNIT 3 Chapter 5: The
Structure and Function of DNA Section 5.1 how DNA is organized in
the cell, and how its genome (the total genetic material of an
organism) is arranged into distinct functional regions on DNA To
relate DNAs primary structure (how nucleotides are linked) and
secondary structure (how two strands of nucleotides form a double
helix) to DNA function, it is necessary to consider: The functional
unit of DNA is a gene, which is a specific sequence of DNA that
codes for proteins and RNA molecules. The majority of DNA in an
organisms genome does not contain genes and, instead, has
non-coding regions. These regions may contain regulatory sequences,
which are sections of DNA that regulate the activity of genes.
Slide 25 DNA of Prokaryotic Cells UNIT 3 Chapter 5: The Structure
and Function of DNA Section 5.1 In prokaryotes such as bacteria,
the bacterial chromosome consists of a circular, double-stranded
DNA molecule. More than one copy of the bacterial chromosome may
exist in a bacterial cell. Since prokaryotes do not have a nuclear
membrane, each bacterial chromosome is packed in the region of the
cell called the nucleoid. The DNA of prokaryotic cells such as E.
coli is packed near the centre of the cell in the nucleoid, which
is not segregated by membranes from the rest of the interior of the
cell. Continued Slide 26 DNA of Prokaryotic Cells UNIT 3 Chapter 5:
The Structure and Function of DNA Section 5.1 Prokaryotic DNA must
be tightly packed so that it can fit in the nucleoid. DNA packing
is achieved through coiling, compacting, and supercoiling.
Continued (A) The circular chromosomal DNA molecule can be
compacted through (B) the formation of looped structures. (C) The
looped DNA can be further compacted by DNA supercoiling. Note: the
coloured balls represent proteins involved in supercoiling. Slide
27 DNA of Prokaryotic Cells UNIT 3 Chapter 5: The Structure and
Function of DNA Section 5.1 DNA supercoiling is the formation of
additional coils in the structure of DNA due to twisting forces. It
is controlled by the enzymes topoisomerase I and topoisomerase II.
Antibacterial drugs have been developed to specifically block these
enzymes and inhibit bacterial survival. Some prokaryotes also have
plasmids, which are small, circular or linear DNA molecules that
often carry non-essential genes. Plasmids are not part of the
nucleoid. They can be copied and transmitted between cells or
incorporated into the chromosomal DNA and reproduced during cell
division. Continued Slide 28 DNA of Prokaryotic Cells UNIT 3
Chapter 5: The Structure and Function of DNA Section 5.1 This is a
partial map of the E. coli genome. There are actually thousands of
genes in the genome, very few of which are non-essential. The
majority of prokaryotic genomes are composed of regions that
contain either genes or regulatory sequences. Regulatory sequences
are sections of DNA sequences that determine when certain genes and
associated cell functions are activated. Most prokaryotes are
haploid, meaning they only have one set of chromosomes and
therefore carry only one copy of each gene. Their genomes carry
very little non-essential DNA. Slide 29 DNA of Eukaryotic Cells
UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1
Eukaryotic DNA is a double-stranded, linear molecule that is
tightly compacted in the nucleus of eukaryotic cells. The total
amount of DNA in eukaryotic cells is much greater than in
prokaryotic cells. Therefore, eukaryotic DNA undergoes greater
compacting than prokaryotic DNA. Continued The compacting of DNA in
eukaryotic cells is achieved through different levels of
organization. Slide 30 DNA Organization in Eukaryotic Cells UNIT 3
Chapter 5: The Structure and Function of DNA Section 5.1 Histones:
a family of proteins that associates with DNA Nucleosome: the
condensed structure formed when double- stranded DNA wraps around
an octamer of histone proteins NOTE: Organization as a distinct
chromosome (shown in D) occurs during cell division. Otherwise, the
DNA is less condensed and is called chromatin. Slide 31 Variation
in the Eukaryotic Genome UNIT 3 Chapter 5: The Structure and
Function of DNA Section 5.1 The size and number of genes in the
eukaryotic genome vary a great deal. There is no correlation
between an organisms complexity and genome size. There is also
variation in the molecules that genes produce. Genes can code for
RNA molecules, in addition to proteins. (A) Lungfish have 40 times
more DNA per cell than a human cell. (B) Rice has 30 000 more
protein-coding genes than a human. (C) C. elegans has the same
number of genes as humans but less DNA. Slide 32 Section 5.1 Review
UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1
Slide 33 5.2 DNA Replication UNIT 3 Chapter 5: The Structure and
Function of DNA Section 5.2 All life depends on the ability of
cells to reproduce. Successful reproduction includes the transfer
of the parents DNA to offspring. The life cycle of a cell is
referred to as the cell cycle. For most cells, the cell cycle can
be represented as in the diagram shown here. The process of copying
one DNA molecule into two identical molecules is called DNA
replication. DNA is copied during S phase of interphase in the cell
cycle. Slide 34 DNA Replication UNIT 3 Chapter 5: The Structure and
Function of DNA Section 5.2 In the mid-1950s, three competing
models of DNA replication were proposed: The conservative model
results in one new molecule and conserves the old. The semi-
conservative replication model results in two hybrid molecules of
old and new strands. The dispersive model results in hybrid
molecules with each strand being a mixture of old and new strands.
conservative model semi-conservative model dispersive model Slide
35 Meselson and Stahl Determine the Mechanism of DNA Replication
UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2
Matthew Meselson and Franklin Stahl reasoned that they could test
the models if they could distinguish between parent and daughter
strands of DNA. They used two different isotopes of nitrogen to
label the DNA: light 14 N and heavy 15 N. These isotopes were
chosen for two reasons: Nitrogen is a component of DNA and would be
incorporated into newly synthesized daughter strands. Light and
heavy forms of nitrogen would allow separation of different strands
of DNA based on how much isotope was present. DNA with the heavy
nitrogen would be more dense than DNA with light nitrogen.
Continued Slide 36 Meselson and Stahl Determine the Mechanism of
DNA Replication UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2 Slide 37 DNA Replication Phase 1: Initiation UNIT 3
Chapter 5: The Structure and Function of DNA Section 5.2
Semi-conservative replication has three phases: initiation,
elongation, and termination. In initiation, a portion of the DNA is
unwound to expose the bases for base pairing. Unwinding starts at a
specific nucleotide sequence called the origin of replication.
Circular prokaryotic DNA has a single origin of replication while
linear eukaryotic DNA often has thousands. Continued Slide 38
Initiation UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2 Initiator proteins bind to the DNA and begin the
process of unwinding. Helicase enzymes cleave the hydrogen bonds
that link the complementary base pairs. Single-strand-binding
proteins help to stabilize the unwound strands. Topoisomerase II
(in prokaryotes) relieves strain on the double helix that is
generated from unwinding. A replication bubble forms with a
Y-shaped replication fork at each end. Slide 39 Elongation: Part I
UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 Two
new strands are assembled using the parent DNA as a template. DNA
polymerase III catalyzes the addition of new nucleotides to create
a complementary strand to the parent strand. However, it can only
attach new nucleotides to the free 3 hydroxyl end of a pre-
existing chain of nucleotides. Only one parent strand has a free 3
hydroxyl end. The strand that is synthesized continuously in the 5
to 3 direction from this parent strand is called the leading
strand. Continued Slide 40 Elongation: Part I UNIT 3 Chapter 5: The
Structure and Function of DNA Section 5.2 Slide 41 Elongation: Part
II UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2
The lagging strand is formed in short segments away from the
replication fork in a discontinuous manner. This requires primase
to synthesize an RNA primer. Once the primer is attached to the
parental strand, DNA polymerase III extends the strand by
synthesizing DNA fragments called Okazaki fragments. DNA polymerase
I removes the primers and fills in the space by extending the
neighbouring DNA fragment. DNA ligase then joins the Okazaki
fragments to create a complete strand. Slide 42 Termination UNIT 3
Chapter 5: The Structure and Function of DNA Section 5.2
Termination occurs once the synthesis of the new DNA strands is
complete. The two new DNA molecules separate from each other, and
the replication machine is dismantled. At each replication fork
lies a complex of proteins and DNA that make up the replication
machinery. The image shown here is a simplified version of the
molecules involved. Slide 43 Important Enzymes in DNA Replication
UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2
Slide 44 Errors During DNA Replication UNIT 3 Chapter 5: The
Structure and Function of DNA Section 5.2 A human cell can copy its
entire DNA in a few hours, with an error rate of about one per 1
billion nucleotide pairs. Incorrect pairing (mispairing) of bases
is thought to occur as a result of flexibility in DNA structure.
Continued Errors naturally occur during replication. Mispairing of
bases and strand slippage are two types of errors that cause either
additions or omissions of nucleotides. Slide 45 Errors During DNA
Replication UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2 Strand slippage during DNA replication can cause the
addition or omission of nucleotides in newly synthesized strands,
which represent errors. Slide 46 Correcting Errors During DNA
Replication UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2 DNA polymerase I and II have proofreading abilities.
These enzymes recognize and correct errors in newly synthesized
strands of DNA. This method repairs about 99% of the mismatch
errors that occur during replication. Mismatch repair is done by a
group of proteins that can recognize and repair deformities in
newly synthesized DNA that feature the mispairing of bases. Errors
that remain after DNA polymerase proofreading or mismatch repair
are considered mutations once cell division occurs. Slide 47
Comparing DNA Replication in Eukaryotes and Prokaryotes UNIT 3
Chapter 5: The Structure and Function of DNA Section 5.2 Slide 48
Section 5.2 Review UNIT 3 Chapter 5: The Structure and Function of
DNA Section 5.2 Slide 49 UNIT 3 Biology Connections STSE Biobanking
is the collection and analysis of physical specimens from which DNA
can be derived from a wide sampling of individuals. A goal of
biobanking is to provide researchers with open access to millions
of high-quality tissue samples collected from people around the
world. While biobanks offer the potential for gaining insight into
the interaction between genes, environmental factors, and disease,
they also present difficult legal and ethical challenges.
