Chapter 8
DNA: The Molecule of
Heredity
8-1 DNA
 1928
Frederick Griffith: How does bacteria
cause pneumonia?
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Isolated two strains of pneumonia bacteria in
mice; only one of them caused disease
Disease strain had smooth colonies; the
harmless strain had rough colonies
When injected with the disease strain, mice
got sick and died; w/ the harmless strain, mice
stayed healthy
Griffith cont
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Next Griffith heat-killed the disease strain and
injected it into mice; the mice stayed healthy
Next he mixed the heat-killed strain with the
harmless strain and injected mice; mice
developed pneumonia and died
Killed bacteria passed on a GENE to the
harmless bacteria which then developed the
disease causing capability
Griffith’s experiment
DNA
 1944
Oswald Avery repeated Griffith’s
experiment and determined that DNA
stores and transmits genetic information
from one generation to the next
Avery repeated the experiment
Avery’s conclusion
DNA
 1952
Alfred Hershey and Martha Chase
proved the importance of the chemical
nature of DNA
 Conducted experiments using bacterial
viruses
 Proved that the viral DNA was the part that
caused disease
Hershey and Chase’s
experiment
8-2 Chargaff’s Rules
 Erwin
Chargaff discovered the
percentages of nucleotides in DNA
 Amounts of guanine = amounts of cytosine
 Amounts of adenine = amounts of thymine
Rosalind Franklin
 1950s
used X-ray diffraction to study DNA
 X-ray created scattered patterns as a
result of the reflection of the X-rays
Francis Crick and James Watson
 1953
developed the double helix model of
DNA
 Used the X-ray diffraction patterns from
Franklin’s studies to help determine the
structure
Watson and Crick
12-2 Structure and Function of
DNA
 The
components of DNA are deoxyribose ,
a phosphate group, and a nitrogen base.
 Nitrogen base- an organic ring structure
that contains one or more atoms of
nitrogen.
DNA Molecule
Four Possible Nitrogen Bases
 Adenine
(A)
 Guanine (G)
 Cytosine (C)
 Thymine (T)
 This allows for four nucleotides, each
containing one of these four bases
 These base pairs match as follows:
 (A) with (T) & (G) with (C)
Chains of Nucleotides
 Nucleotides
do not exist as individual
molecules, they combine to form long
chains to produce one molecule
 The two chains are held by hydrogen
bonds between the bases
 This structure resembles a ladder
 The shape of DNA is known as a double
helix because it looks like a twisted ladder
Importance of Nucleotide
Sequence
 The
genetic material of living things is
made of DNA, they are different because
of the order of the nucleotides in the DNA
strands of the organism
 The sequence of nucleotides forms the
unique genetic information of an organism
 The more closely related the more alike
the DNA strands are
8-3 Replication of DNA
 DNA
replicates because every time a cell
divides, it must make a copy of its
chromosomes, so each cell can have a
complete set
 Without replication, species could not
survive and individuals could not
successfully grow and reproduce
How DNA Replicates
 During
replication, each strand serves as a
pattern to make a new DNA molecule
 It begins as an enzyme breaks the
hydrogen bond between nitrogen bases
that hold the two strands together
 The action “unzips” the DNA molecule
 As the DNA unzips, free nucleotides from
the surroundings in the nucleus, bond to
the single strands by base pairing
DNA Replication Cont.

Another enzyme bonds these new nucleotides
into a chain
 This process continues until the entire molecule
has been unzipped and replicated
 As a result a new strand is formed that is a
complement of one of the original, or parent
strand
 The result is the formation of two DNA
molecules, each identical to the original DNA
molecule
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8-4 DNA to Protein

Genetic Code: The sequence of nitrogen base
along one of the two strands for the synthesis of
proteins
 There are 20 different amino acids. But DNA
only contains 4 bases.
 A single base can’t represent a single amino
acid because that system would code for only 4
different amino acids
 A sequence of 3 bases provides more than 20
combinations needed to code for all amino acids
Codon: each set of 3 nitrogen
bases representing an amino acid
 The
DNA code is often called a triplet code
 64 combinations are possible when a
sequence of three bases is used, 64
different codons are in the genetic code
 The order of nitrogen bases in DNA will
determine the order of the amino acids in
a protein
Codon cont.
 For
any one codon there can only
be one amino acid
 The code is said to be universal
because the codons represent the
same amino acids in all
organisms
Transcription-from DNA to RNA
 RNA
structure:
RNA is a nucleic acid
Differs from DNA structure in 3 ways:
1. RNA is usually composed of a single
strand rather than a double strand
2. RNA also contains a different type of
sugar molecule, ribose instead of
deoxyribose
3. RNA also contains four nitrogen
bases, but rather than thymine RNA contains
Uracil (U)
Making RNA Transcription:
 Transcription:
the process by which
enzymes make an RNA copy of a DNA
strand
 This process is similar to DNA replication
except the main difference is that the
process results in the formation of a
single-stranded RNA molecule
 This RNA copy carries information from
the DNA out into the cytoplasm of the cell
RNA by Transcription
 This
is called messenger RNA (mRNA)
 mRNA carries the information for making a
protein chain
 Some portions of DNA code for the RNA
that makes up ribosomes, where proteins
are synthesized
 This type of RNA is called ribosomal RNA
(rRNA)
 rRNA helps to produce enzymes needed
to bond amino acids together during
protein synthesis
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8-5 Translation –RNA to Protein
 Translation:
the process of converting
information in a sequence of nitrogen
bases in mRNA into a sequence if amino
acids that make up protein
 This occurs on ribosomes, and involves a
third kind of RNA
 Transfer RNA (tRNA): brings amino acids
to the ribosomes so they can be
assembled into proteins
Translating the mRNA code:
 Correct
translation of the code depends
upon the joining of each mRNA codon with
the anticodons of the proper tRNA
molecules
 The end result of translation is the
formation of the large variety of proteins
that make up the structure of organisms
and help them function
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8-7 Genetic Changes
– A Change in DNA
 Mutation – any mistake or change in the
DNA sequence
 Point Mutation – a change in a single base
pair in DNA (EX) The dog chased the car.
 Changing a single letter in this sentence
changes the entire meaning, a change in a
single nitrogen base can change the entire
structure of the protein.
 Mutation
The effects of point
mutations
mRNA
Normal
Protein
Stop
Replace G with A
mRNA
Point
mutation Protein
Stop
Mutations cont.
 Frameshift
mutation: a mutation when a
single base is added or deleted from DNA
 EX: a mRNA strand has a new amino acid
added to the protein for every codon on
the mRNA strand; now a single base is
deleted from that strand; this new
sequence is now translated into mRNA,
but it is out of position by one base; this
causes every codon to be out of position
by one base; this would cause every
amino acid to be changed
Frameshift mutations
Deletion of U
Frameshift
mutation
mRNA
Protein
Chromosomal Mutations
 Mutation
affecting gene distribution to
gametes during meiosis, most commonly
by deletions, insertions, inversions, or
translocations
 Some ways they are described as
mutating include: parts of the
chromosomes are broken off or lost during
mitosis or meiosis, chromosomes break
and then rejoin incorrectly, or the parts join
backwards or even to the wrong
chromosome
Effects of chromosomal mutation
 Occur
in living organisms, but they are
especially common in plants
 Affect the distribution of genes to gametes
during meiosis
 Gametes that should have a complete set
of genes may end up with extra copies of
some genes or a complete lack of certain
genes
 Few chromosomes mutations are passed
on to the next generation because the
zygote usually dies
Types of chromosomal mutation
 Deletions:
occur when part of a
chromosome is left out
 Insertions: occur when a part of a
chromatid breaks off and attaches to its
sister chromatid. The result of genes on
the same chromosome.
 Inversions: occur when part of a
chromosome breaks out and is reinserted
backwards
 Translocation: occur when part of one
chromosome breaks off and is added to a
different chromosome
Errors in Disjunction
 Many
chromosomal mutations result from
the failure of chromosomes to separate
properly during meiosis
 Nondisjunction: the failure of homologous
chromosomes to separate properly during
meiosis
In one form of nondisjunction, 2
kinds of gametes result:
 Trisomy:
the presence of an extra
chromosome
 Triploidy: involves a total lack of
separation of homologous chromosomes
(this condition is rare in animals, but
frequent in plants)
 Monosomy: absence of a chromosome
(these usually do not survive)
Causes of Mutation
 Spontaneous
mutation: mutations that
occur at random
 Environmental agents also cause
mutations, such as exposure to x-rays,
ultraviolet light, radioactive substances, or
certain chemicals
 Mutations often result in sterility or the lack
of normal development in an organism
 In human gametes mutations may cause
birth defects or in body cells it may cause
cancer