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Chapter 12 Genetic Engineering and the
Molecules of Life
How can we benefit from genetically
engineered crops?
What have we learned from this?
How does genetic engineering work?
Corn is Susceptible to the European Corn Borer
Corn can be genetically modified to:
• Produce its own insecticide (therefore less pesticides are used)
• Resist herbicides
12.1
How Can Corn be Made to Resist Herbicide or Create its
Own Insecticide?
Each cell of the corn plant has a complete set of instructions on how
to grow and reproduce
• This information passes from generation to generation and is called
the genome
• Genes are short sections of instructions that govern specific
reactions, chemicals, or events in the cell
• If a gene is changed, then an inheritable trait changes (such as
making corn produce a new chemical, such as an insecticide)
• A soil bacterium (Bacillus thuringiensis) has the genes to make
insecticidal proteins, so by taking a gene out of the bacteria and
inserting it into the corn plant, we have corn plants that produce
an insecticidal protein
12.1
The Chemistry of Heredity
What are we made of?
• All genetic information is stored in the nucleus of the millions of
cells in the body.
• Each nucleus contains chromosomes, 46 compact structures of
intertwined molecules of DNA, and about 30,000 genes,
components that convey one or more hereditary traits.
• DNA is a special template written in a molecular code on a tightly
coiled thread that carries all genetic information.
12.2
What makes up DNA?
DNA is made of fundamental
chemical units, repeated over
and over.
Each unit is composed of three
parts: nitrogen-containing
bases, the sugar deoxyribose,
and phosphate groups.
Adenine (A), Guanine (G),
Cytosine (C), and Thymine
(T) are the bases.
12.2
Nucleotides
A combination of a base, phosphate group, and a deoxyribose
sugar is a nucleotide.
This
nucleotide is
an adenosine
phosphate.
Any of the
four bases
can be used
to form a
nucleotide.
A covalent bond
exists between the
phosphate group
and the sugar.
Another covalent bond is
present between the ring
nitrogen of the base and a ring
carbon of the sugar.
12.2
What does a segment of DNA look like?
A typical DNA molecule
consists of thousands of
nucleotides covalently
bonded in a long chain.
The phosphate groups are
responsible for linking
each nucleotide.
A phosphate group of one
nucleotide reacts with an
–OH group present on the
deoxyribose ring of another
nucleotide, forming and
eliminating a H2O
molecule.
This –OH group reacts with the
phosphate group of another nucleotide
12.2
The Double Helix of DNA
X-Ray Diffraction pattern of a hydrated
DNA molecule taken in 1952.
Rosalind Franklin- her
data was used by Watson
and Crick (below)
This technique uses the fact that a
molecule’s electrons diffract X-Rays at
particular angles and the resulting pattern,
like the one above, can be used to solve
the structure of a crystal.
12.3
The Double Helix of DNA
Using Rosalind Franklin’s X-ray
diffraction data, Watson and Crick
proposed a molecular model for DNA.
This model had a double strand of
repeating nucleotides. Complementary
base pairing (AT, CG) is held in place
by hydrogen bonds (shown in red).
The nature of the base pairing required
that the two strands be coiled in the
shape of a double helix.
12.3
Complementary Base Pairs
Adenine hydrogen bonds with thymine and cytosine with guanine
in DNA
12.3
Chargaff’s Rules
Erwin Chargaff’s research showed that for all humans, the percentage of adenine
in DNA is almost identical to the percentage of thymine.
Similarly, the percentages of guanine and cytosine are almost equal.
From this, Chargaff concluded that the bases always come in pairs; adenine is
always associated with thymine and guanine is always associated with cytosine.
Thus, Chargaff’s rule states: %A = %T and %G = %C
12.3
DNA Replication
The process by which copies of DNA
are made is called replication.
The original DNA double helix
partially unwinds and the two
complementary portions separate.
Each of the strands serves as a
template for the synthesis of a
complementary strand.
The result is two complete and
identical DNA molecules.
Complete set of genetic
information packaged into
chromosomes packed into the cell
nucleus.
12.3
Cracking the Chemical Code
The 3 billion base pairs in each human cell provide the
blueprint for producing a human being.
The specific sequence of base pairing is important in
conveying the mechanism of how genetic information is
expressed.
The expression is seen through proteins.
Through directing the synthesis of proteins, DNA can control
the characteristics of an individual, including inherited
illnesses.
12.4
Proteins are made of amino acids. The general formula for an amino acid includes
four groups attached to a carbon atom: (1) a carboxylic acid group, -COOH; (2) an
amine group, -NH2; (3) a hydrogen atom, -H; and (4) a side chain designated as R:
There are 20 naturally
occurring amino acids
that make up proteins
They differ from one another by the different R groups
12.4
Two amino acids can link together via a peptide bond:
The two molecules join,
expelling a molecule of water
Peptide bond
The process may repeat itself over and over, creating a
peptide chain.
Once incorporated into the peptide chain, the amino acids
are known as amino acid residues.
12.4
Codons: How are they relevant in genetic expression?
The order of bases in DNA determines the order of amino acids in a protein.
Because there are 20 amino acids present in the proteins, the DNA code must
contain 20 code “words”; each word represents a different amino acid.
The genetic code is written in groupings of three DNA bases, called codons.
The diagram shows possible codons, determined according to the base
sequence of the nucleic acid strand. The expression of the genetic
information is then seen through the specific proteins assigned.
12.4
The primary structure of a protein is its linear
sequence of amino acids and the location of any
disulfide (-S-S-) bridges.
12.5
The secondary structure of a protein is the folding
pattern within a segment of the protein chain.
12.5
N-terminal
carboxyl
terminal
The sequence is
characterized by the
amino terminal or
"N-terminal" (NH3+) at
one end; and the
carboxyl terminal or
"C-terminal" (COO-) at
the other.
Tertiary structure of the enzyme, chymotrypsin
12.5
The function of a protein is dependent on its shape or threedimensional structure.
Small changes in the primary structure can have dramatic effects
on its properties.
Sickle cell anemia is an example of a
condition that develops when red blood cells
take on distorted shapes due to an error in the
amino acid sequence.
Because these cells lose their normal shape,
they cannot pass through tiny openings in the
spleen and other organs.
Some of the sickled cells are destroyed and anemia results. Other
sickled cells can clog organs so badly that the blood supply to them
is reduced.
12.5
The Process of Genetic Engineering
• When a species is genetically engineered, the DNA in the cell is modified.
• When the genes are changed, the proteins synthesized by the genes is
modified.
• When the cell grows and develops, a plant with new characteristics from the
different DNA is generated.
• Before genetic engineering, when humans selected for plants with certain
characteristics or crossbred different strains, genes were manipulated.
The genetic traits for modern corn were selected over time, starting with an early
ancestor, teosinte (below).
12.6
A representation of
genetic engineering
12.6
Genetic Engineering
• Genetic engineering is transgenic where an organism is created
by the transfer of genes across species.
• Genetic engineering can also be used to do the same thing as
crossbreeding, just more efficiently and faster.
Transgenic rice with virus-resistance
12.6
Other Reasons for Genetic Engineering
• Make crops more resistant to disease, tolerant of stresses (salt, heat,
or drought)
• Develop soybeans that produce high yields of biofuel per acre
• Use of enzymes to create new drugs
• Develop vaccines that grow in edible products
Developing strains of algae for new biofuels
12.7
Genetically-Engineered Agriculture
Transgenic Plants
Virus resistant transgenic rice
Frankenfood?
Greenpeace activists dumping
papaya during a Bangkok protest.
12.8
Where do we go from here?
12.8