UNIT 3: Molecular Genetics
Chapter 5: The Structure and Function
of DNA
Chapter 6: Gene Expression
Chapter 7: Genetic Research and
Biotechnology
UNIT 3 Chapter 5: The Structure and Function of DNA
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
organism’s traits (including
behaviours) and its predisposition
for certain diseases.
UNIT 3 Chapter 5: The Structure and Function of DNA
5.1 DNA Structure and
Organization in the Cell
Before DNA was established as the
genetic material in cells, scientists
knew:
•
•
•
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
Section 5.1
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Identifying DNA as
the Material of Heredity: Griffith
Frederick Griffith’s experiments with Streptococcus pneumoniae
in 1928 established DNA as the material of heredity.
Two forms of the bacterium were used:
•
•
The S-strain was highly
pathogenic but could be
made non-pathogenic by
heating it.
The R-strain was nonpathogenic.
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
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.
Griffith called this phenomenon
the transforming principle
because something from the Sstrain transformed the R-strain
into deadly bacteria.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Identifying DNA as the Material of
Heredity: Avery, MacLeod, and McCarty
Canadian-American microbiologist Oswald Avery and his
group built on Griffith’s 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:
•
•
•
one that destroyed proteins
one that destroyed RNA, and
one that destroyed DNA.
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
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
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 (35S for proteins and 32P for DNA).
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Identifying DNA as the Material of
Heredity: Hershey and Chase
In their first experiment:
•
a virus with DNA radioactively labelled with 32P was allowed
to infect bacteria. After agitation and separation, radioactivity
was found in the bacteria pellet but not in the liquid medium.
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Identifying DNA as the Material of
Heredity: Hershey and Chase
In their second experiment:
• a virus with its protein coat radioactively labeled with 35S
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
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 nitrogencontaining bases, a sugar, and a phosphate group.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
The Chemical Composition of the
Nucleotides, DNA, and RNA
In later years, other scientists confirmed and extended
Levene’s work. DNA and RNA are both made up of a
combination of four different nucleotides.
Nucleotides are often identified by referring to their bases:
•
•
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).
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
The Chemical Composition of the
Nucleotides, DNA, and RNA
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Chargaff’s Rule: Closing in on
the Structure of DNA
Erwin Chargaff was inspired by Avery, MacLeod, and
McCarty’s work on DNA and launched a research program to
study nucleic acids. By the late 1940s, he had reached two
conclusions:
•
•
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 Chargaff’s rule.
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Chargaff’s Rule: Closing in on
the Structure of DNA
In DNA, the percent composition of adenine is the same
as thymine, and the percent composition of cytosine is the
same as guanine.
UNIT 3 Chapter 5: The Structure and Function of DNA
Pauling Discovers a Helical
Structure for Proteins
In 1951, Linus Pauling discovered
that many proteins have helixshaped structures. Scientists,
including James Watson and
Francis Crick, used this
information when deducing the
structure of DNA.
Section 5.1
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Franklin Determines a
Helical Structure for DNA
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:
• 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.
(A) Rosalind Franklin was a British chemist who was
hired to work alongside Maurice Wilkins at the X-ray
diffraction facilities at King’s 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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Watson and Crick Build a ThreeDimensional Model for DNA
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:
•
a twisted ladder, which they called a double-helix. The sugarphosphate molecules make up the sides or “handrails” of the
ladder, and the bases make up the “rungs” of the ladder by
protruding inwards.
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Watson and Crick Build a ThreeDimensional Model for DNA
•
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
The Modern DNA Model:
The DNA Double Helix
Structural features of DNA:
• 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 sugarphosphate backbones in each strand is
constant.
• The bases of each nucleotide are
attached to each sugar and face
inward.
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
The Modern DNA Model:
The DNA Double Helix
•
•
•
•
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
The DNA Double Helix
Section 5.1
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
The Structure and Organization
of Genetic Material
To relate DNA’s 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:
• 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
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 organism’s 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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
DNA of Prokaryotic Cells
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…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
DNA of Prokaryotic Cells
Prokaryotic DNA must be tightly packed so that it can fit
in the nucleoid. DNA packing is achieved through coiling,
compacting, and supercoiling.
(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.
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
DNA of Prokaryotic Cells
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…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
DNA of Prokaryotic Cells
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.
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.
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
DNA of Eukaryotic Cells
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.
The compacting of DNA in eukaryotic cells is achieved through
different levels of organization.
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
DNA Organization in Eukaryotic Cells
Histones: a family of proteins that associates with DNA
Nucleosome: the condensed structure formed when doublestranded 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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1
Variation in the Eukaryotic Genome
The size and number of genes in the eukaryotic genome vary
a great deal. There is no correlation between an organism’s
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.1 Review
Section 5.1
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2
5.2 DNA Replication
All life depends on the ability of cells to reproduce. Successful
reproduction includes the transfer of the parent’s 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.
UNIT 3 Chapter 5: The Structure and Function of DNA
DNA Replication
In the mid-1950s, three competing
models of DNA replication were
proposed:
•
•
•
conservative model
semi-conservative model
dispersive model
The conservative model results in one new
molecule and conserves the old. The semiconservative 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.
Section 5.2
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2
Meselson and Stahl Determine the
Mechanism of DNA Replication
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” 14N and “heavy” 15N. 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…
UNIT 3 Chapter 5: The Structure and Function of DNA
Meselson and Stahl Determine the
Mechanism of DNA Replication
Section 5.2
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2
DNA Replication Phase 1: Initiation
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…
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2
Initiation
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•
•
•
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2
Elongation: Part I
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 preexisting 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…
UNIT 3 Chapter 5: The Structure and Function of DNA
Elongation: Part I
Section 5.2
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2
Elongation: Part II
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2
Termination
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Important Enzymes in
DNA Replication
Section 5.2
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2
Errors During DNA Replication
A human cell can copy its entire DNA in a few hours, with
an error rate of about one per 1 billion nucleotide pairs.
Errors naturally occur
during replication.
Mispairing of bases
and strand slippage are
two types of errors that
cause either additions
or omissions of
nucleotides.
Incorrect pairing (mispairing) of
bases is thought to occur as a
result of flexibility in DNA structure.
Continued…
UNIT 3 Chapter 5: The Structure and Function of DNA
Errors During
DNA Replication
Strand slippage during
DNA replication can cause
the addition or omission of
nucleotides in newly
synthesized strands, which
represent errors.
Section 5.2
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2
Correcting Errors During DNA Replication
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.
UNIT 3 Chapter 5: The Structure and Function of DNA
Comparing DNA Replication in
Eukaryotes and Prokaryotes
Section 5.2
UNIT 3 Chapter 5: The Structure and Function of DNA
Section 5.2 Review
Section 5.2
UNIT 3 Biology Connections
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.
STSE