What have we learned from this? How does genetic engineering ...

roachavocadoBiotechnologie

14 déc. 2012 (il y a 8 années et 7 mois)

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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?


12.2



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

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.

What makes up DNA?

Nucleotides


A combination of a base, phosphate group, and a deoxyribose
sugar is a
nucleotide
.




A covalent bond
exists between the
phosphate group
and the sugar.



This
nucleotide is
an adenosine
phosphate.

Any of the
four bases
can be used
to form a
nucleotide.


Another covalent bond is
present between the ring
nitrogen of the base and a ring
carbon of the sugar.

12.2

A typical DNA molecule
consists of thousands of
nucleotides covalently
bonded in a long chain.

The phosphate groups are
responsible for linking
each nucleotide.


12.2

What does a segment of DNA look like?

A phosphate group of one
nucleotide reacts with an

OH group present on the
deoxyribose ring of another
nucleotide, forming and
eliminating a H
2
O
molecule.

This

OH group reacts with the
phosphate group of another nucleotide

The Double Helix of DNA

X
-
ray diffraction pattern of a hydrated
DNA molecule taken in 1952.


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

Rosalind Franklin


her
data was used by
Watson
and
Crick
(below)

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.


12.3

Thus, Chargaff’s rule states:
%A = %T and %G = %C

DNA Replication

12.3

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.

A complete set of genetic
information is packaged into
chromosomes

packed into the cell
nucleus.

12.4

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,

NH
2
; (3) a hydrogen atom,

H; and (4) a side chain designated as R:

They differ from one another by the different R groups

There are 20 naturally
occurring amino acids
that make up proteins.

Two amino acids can link together
via

a peptide bond:

12.4

Peptide bond

The two molecules join,
expelling a molecule of water.

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.


12.5



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






The sequence is
characterized by the
amino terminal

or

"
N
-
terminal
" (NH
3
+
) at
one end, and the
carboxyl terminal

or

"
C
-
terminal
" (COO

) at
the other.

carboxyl

terminal

Tertiary structure of the enzyme, chymotrypsin

N
-
terminal

12.5



The function of a protein is dependent on its shape or three
-
dimensional 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.

12.6

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 are
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 (bottom).

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

12.7



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

Genetically
-
Engineered Agriculture

Transgenic Plants

12.8

Frankenfood?

Greenpeace activists dumping
papaya during a Bangkok protest.

Virus resistant transgenic rice

Where do we go from here?

12.8