DNA, Genes, and
7.1 Molecule of Heredity
A. Genetic Material Transforms Cells
Sutton and Boveri formulated the
chromosome theory of heredity (genes
carry information coding for traits) – 1903.
Griffith, 1928, tested if DNA or proteins
compose genes. He used bacteria in mice
to test his hypothesis.
Griffith concluded that the transforming
material (DNA) was genetic material not
proteins. See page 158
1944, Avery and his colleagues discovered
that “naked” DNA from the bacteria could
act as hereditary information.
B. DNA or Protein?
Chase and Hershey in 1953 used
bacteriophages (viruses that infect bacteria)
to finalize the DNA puzzle.
Chase and Hershey radioactively marked the
DNA to show that the DNA from the
bacteriophage entered the bacteria and not
7.2 – DNA Structure and Replication
A. Nucleotides and Bases
The structure of DNA needed to be
determined next because structure of an
object is related to its function.
DNA is made up of nucleotides.
Each nucleotide has three components:
(1) a five carbon sugar - deoxyribose, (2)
a phosphate group, and 1 of 4
The four DNA bases are: adenine (A),
guanine (G), cytosine (C), and thymine (T).
Two are purines: A and G; two are
pyrimidine: C and T.
In the 1950’s, Chargaff discovered that A =
T and G = C; so he determined that these
base types must connect together.
B. Double Helix
Watkins and Franklin used x-ray
photographs of DNA to support Watson and
Photos supported that the DNA molecule
was a double helix, resembling a twisted
Watson and Crick also found the two
components of the nucleotides: sugar and
phosphate which form the sides of the
The bases that Chargaff linked together
form the “rungs” of the ladder.
See page 160 and 161 to see the structure.
Since A attaches to T and G attaches to C, if
you know the sequence of bases on one
side of the DNA molecule, you will always
know the sequence on the other side.
C. DNA Replication
All genetic material has the capacity to copy
DNA copies itself by the process of
replication. (recall that this occurs prior to
See page 162
Replication occurs in 3 basic steps.
1. DNA unzips – Enzymes split apart base
pairs and unwinds the DNA double helix.
2. Bases pair up – Free nucleotides in the cell
find their complementary bases along the
exposed or unzipped sides of the DNA.
3. Backbone bonds – The sugar and
phosphate backbone is assembled along
the complementary strands on both sides
of the DNA.
Thus two DNA strands are made; each
strand contains an “original half” and a
The enzyme responsible for replication is
called the DNA polymerase.
7.3 – Linked Genes
Biologists hypothesize that one gene
contains information to code for many
A chromosome is made of thousands of
genes; a gene is made of thousands of
Experiments by Bateson/Punnett and
Morgan showed that some genes that
code for different traits were located on
the same chromosome.
Their discoveries along with Sutton’s work,
expanded Mendel’s laws.
Linked genes – are genes that are located
on the same chromosome; they are
inherited together instead of independently.
Recombination of linked traits was noted in
Morgan’s experiments with fruit flies.
Recombination occurs when the linked traits
were not seen in the offspring as they
appeared in the parents.
Recombination occurs during meiosis when
crossing over occurs.
Crossing over occurs when tetrads
exchange parts of their “legs” reshuffling
the combination of traits.
Crossing over is only one type of
Scientists use the frequency of
recombination between two traits that
appear on the same gene to determine their
More frequent recombination between traits
indicates that they are located farther apart
on the same gene.
Continuing this process allows scientists to
“map” the genes on a chromosome
(determining their specific location on the
7.4 Sex Linkage
A. Sex Chromosomes
Sex chromosomes determine whether the
offspring of an organism are male or
Some plants and animals have sex
All non-sex chromosomes are called
Humans and other mammals have two sex
chromosomes: X and Y.
Females have two X chromosomes (XX);
males have an X and a Y chromosome (XY).
During meiosis, sex chromosomes
All eggs have “X” chromosomes.
Half of the sperm have “X” chromosomes
and half of the sperm have “Y”
Therefore, males or sperm determine the
gender of the offspring.
B. Sex-Linked Traits
Sex-linked genes occur on only the “X” or
on the “Y” chromosomes.
The traits that the sex-linked genes code for
are called sex-linked traits.
Morgan first discovered sex-linked genes in
fruit flies. Later other scientists discovered
sex-linked genes in humans.
Red-green color blindness is caused by an
X-linked recessive allele. Thus it is seen
more frequently in males.
Hemophilia is also an X-linked recessive
allele. Hemophilia causes excessive
bleeding after minor injuries.
C. Sex-Limited and Sex-Influenced Traits
Sex-limited traits are only expressed in the
presence of sex hormones and are only
observed in one sex or the other.
Sex-limited traits are controlled by genes on
the autosomes. They only appear when the
appropriate sex hormone is present (thus
not seen in children).
Sex-limited traits include beard growth in
men and milk production in women. This
also includes bright feather colors in male
Sex-influenced traits are also controlled by
These traits are expressed in both genders,
but they are expressed differently in the
For example, baldness is dominant in males
due to their male hormones and is recessive
in females due to their female hormones.
7.5 The Human Gene Map
Scientists have developed several
techniques to diagnose and treat genetic
Karyotypes are photographs of all of an
organism’s chromosomes. Karyotypes
allow scientists to study the shape,
structure, and size of an organism’s
Analyses of the genes and the base pair
sequences is called a genome study. The
genome is the base sequence of all of the
DNA in an organism.
B. Issues in Biology
The Human Genome Project is a world-wide
effort to map the entire human genome
(sequencing all the DNA in the human
They wish to locate the genes responsible
for diseases and other traits.
Those who oppose the Human Genome
Project question the usefulness of the
C. Changes in the Genome
Genetic counselors use karyotypes to find
certain chromosomal abnormalities.
Nondisjunction occurs when chromosomes
fail to separate during cell division. This can
cause the cell to die but not the organism.
Abnormal gametes can form.
Monosomy occurs when a zygote has only
one copy of a particular chromosome. The
embryo normally dies in this case.
Trisomy occurs when a zygote has three
copies of a particular chromosome. The
embryo normally dies in this case too.
Nondisjunction occurs in both autosomes
and in sex chromosomes.
Example: Down syndrome is trisomy of
chromosome 21. Individuals with this are
mildly to severely developmentally
Trisomy in sex chromosomes include XXX or
XXY. Monosomy of the sex chromosomes
is XO, usually.
Polyploidy occurs when nondisjunction of all
chromosomes pairs happens.
This occurs during the process of gamete
production. This causes the resulting
offspring to have multiple sets of
Polyploidy in animals yields death.
Polyploidy in plants can produce a more
robust plant: larger flowers, more fruit, etc.
Polyploidy in plants can occur during
meiosis resulting in self fertilization, when a
polyploidy gamete cross fertilizes with a
normal gamete, etc.
25% -50% of all plant varieties are
polyploidy, including many things that you