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 H2O 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.

Complete set of genetic information 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, -NH2; (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 bondThe 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" (NH3

+) at one end; and the carboxyl terminal or "C-terminal" (COO-) at the other.

carboxylterminal

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 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

12.7

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