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Chapter 13
Selective breeding is a technique of choosing specific traits that are desirable, and breeding organisms with those traits
so the next generation has them. Great examples are domesticated dogs and cats. They are all the same species, but
look different due to selective breeding over thousands of years.
One example of selective breeding is hybridization. This is the crossing of individuals with different traits to get offspring
that have the traits of each parent. For example, one set of corn plants has been proven to be disease resistant. Another
set of plants have been proven to produce a lot of ears of corn. By crossing these two plants, you get a disease resistant
plant with increased production.
Another example of selective breeding is inbreeding. This is continued breeding of closely related organisms with similar
traits to maintain those traits generation to generation. For example, always breeding poodles to poodles to get a poodle.
Sometimes it can be harmful if two genetically similar organisms have recessive genes for a genetic defect. For example,
dalmations are known for hearing problems that have resulted from inbreeding for generations.
Genetic engineering is the intentional changing of an organism's DNA to increase variation.
Scientists use 3 methods to do this:
1. DNA Extraction
2. Cutting DNA
3. Separating DNA
Once scientists have the DNA, they can read it, change it, or copy it.
To read it, they take a single strand of DNA that has an unknown base sequence and put it in a test tube. They add DNA
polymerase (the enzyme that copies DNA) and the 4 nucleotide bases. Some of the bases have a chemical dye added to
them. By reading the colored bases on the new copied strand, they can figure out the sequence on the original strand.
To change it, short sequences of DNA made in the laboratory can be joined to the DNA of an organism. The DNA
molecules are called recombinant DNA, because it is DNA combined for different sources.
To copy it, scientists use a technique called polymerase chain reaction. The gene that is wanted on the DNA is sectioned
out using primers (short pieces of complementary DNA). DNA polymerase starts and ends copying at these primers,
making repeated exponential copies of the gene as the DNA is heated and cooled.
These little sections of DNA can not work by themselves. They need to become a part of the DNA molecule in an
organism. When a cell takes in DNA from outside of the cell, and the new DNA becomes a part of the cell's DNA, it's
called transformation. To transform bacterial cells, the new DNA pieces are placed in a plasmid- a small, circular DNA
molecule that occurs naturally in some bacteria. These plasmids are then put into a solution with bacteria. Some bacteria
absorb the plasmid, and those bacteria are transformed.
Plant cells can be transformed in 3 ways:
1. By removing the cell wall, some plants will absorb the new DNA on their own.
2. Plasmids can be used, and then the bacteria inserted into a plant cell.
3. DNA can be injected directly in some plant cells.
Animal cells are transformed in ways similar to plant cells. An egg cell may be large enough to have DNA injected directly
into the nucleus. Once inside, repair enzymes help insert the DNA fragment into the chromosomes of the cell.
When transformation is successful, recombinant DNA is integrated into one of the chromosomes of the cell.
What do we do with this technology?
Transgenic species- Species who have a gene from another species within their DNA. For example, putting the gene
that codes for glowing in fireflies into a tobacco plant.
Human genes have been added to bacteria. These transgenic bacteria are used to make mass quantities of human
proteins such as insulin, human growth hormone, and clotting factor.
Transgenic animals have been created to improve the food supply. By adding extra copies of growth hormone genes,
animals grow faster. Mice have been produced with human genes that make their immune system act like that of a
human. This allows scientists to study the effects of diseases on the human immune system.
Another use, that affects our daily lives, are transgenic crops, also called genetically modified foods. These crops can
make their own insecticide or are resistant to weed killers. Some also contain vitamins needed for human health. In the
year 2000, 52% of soybeans and 25% of corn grown in the US was transgenic.
A clone is a member of a population of genetically identical cells that were produced from a single cell. A single cell from
an individual is fused with an egg from another individual. The egg contains the genetic makeup of the one individual,
rather than genetic material from 2 parents. The egg is placed in the uterus of another individual, and the offspring is an
EXACT replica of the donor of the single cell.
Chapter 14
Humans have 46 chromosomes. Two of these, X and Y, are sex chromosomes. Females have two X chromosomes.
Males have one X and one Y. All human eggs carry 1 X chromosome. Sperm cells carry either an X or a Y (about half and
half). This ensures that about 1/2 of the offspring will be female and 1/2 male.
All other human chromosomes are called autosomes. Genes controlling blood type were some of the first human genes
to be identified. Red blood cells carry two antigens- A & B. Antigens are molecules the immune system can recognize.
This is where our blood types come from AA is A, BB is B, AB is AB, and if there is no antigen, the blood type is O.
The Human Genome Project is an ongoing effort to analyze the human DNA sequence. The goal is to locate genes on our
chromosomes and use that information to help diagnose and treat disease.