Standard Set 5. Genetics (Biotechnology)
Long before scientists identified DNA as the genetic material of cells, much was known about inheritance
and the relationship between various characteristics likely to appear from one generation to the next.
However, to comprehend clearly how the genetic composition of cells changes, students need to
understand the structure and activity of nucleic acids.
Genetic recombination occurs naturally in sexual reproduction, viral infection, and bacterial
transformation. These natural events change the genetic makeup of organisms and provide the raw
materials for evolution. Natural selection determines the usefulness of the recombinants. In recombinant
DNA technology speÂ¬cific pieces of DNA are recombined in the laboratory to achieve a specific goal.
The scientist, rather than natural selection, then determines the usefulness of the recomÂ¬binant DNA
5. The genetic composition of cells can be altered by incorporation of exogenous
DNA into the cells. As a basis for understanding this concept:
5. a. Students know the general structures and functions of DNA, RNA and protein.
Nucleic acids are polymers composed of monomers called nucleotides. Each nucleotide consists of three
subunits: a five-carbon pentose sugar, a phosphoric acid group, and one of four nitrogen bases. (For
DNA these nitrogen bases are adenine, guanine, cytosine, or thymine.) DNA and RNA differ in a number
of major ways. A DNA nucleotide contains a deoxyribose sugar, but RNA contains ribose sugar.
The nitrogen bases in RNA are the same as those in DNA except that thymine is replaced by uracil. RNA
consists of only one strand of nucleotides instead of two as in DNA.
The DNA molecule consists of two strands twisted around each other into a double helix resembling a
ladder twisted around its long axis. The outside, or up-rights, of the ladder are formed by the two sugar-
phosphate backbones. The rungs of the ladder are composed of pairs of nitrogen bases, one extending
from each upright. In DNA these nitrogen bases always pair so that T pairs with A, and G pairs with C.
This pairing is the reason DNA acts as a template for its own replication. RNA exists in many structural
forms, many of which play different roles in protein synthesis. The mRNA form serves as a template
during protein synthesis, and its codons are recognized by aminoacylated tRNAs. Protein and rRNA
make up the structure of the ribosome.
Proteins are polymers composed of amino acid monomers (see Standard Set 10 for chemistry in this
chapter). Different types of proteins function as enzymes and transport molecules, hormones, structural
components of cells, and antibodies that fight infection. Most cells in an individual organism carry the
same set of DNA instructions but do not use the entire DNA set all the time. Only a small amount of the
DNA appropriate to the function of that cell is expressed. Genes are, there-fore, turned on or turned off
as needed by the cell, and the products coded by these genes are produced only when required.
5. b. Students know how to apply base-pairing rules to explain precise copying of DNA during
semi conservative replication and transcripÂ¬tion of information from DNA into mRNA.
Enzymes initiate DNA replication by unzipping, or unwinding, the double helix to separate the two
parental strands. Each strand acts as a template to form a complementary daughter strand of DNA. The
new daughter strands are formed when complementary new nucleotides are added to the bases of the
nucleotides on the parental strands. The nucleotide sequence of the parental strand dictates the order of
the nucleotides in the daughter strands. One parental strand is conserved and joins a newly synthesized
complementary strand to form the new double helix; this process is called semiconservative replication.
DNA replication is usually initiated by the separation of DNA strands in a small region to make a â
€œreplication bubbleâ€� in which DNA synthesis is primed. The DNA strands progressively unwind and
are replicated as the replication bubble expands, and the two forks of replication move in opposite
directions along the chromosome. At each of the diverging replication forks, the strand that is conserved
remains a single, continuous â€œleadingâ€� strand, and the other â€œlaggingâ€� complementary strand
is made as a series of short fragments that are subsequently repaired and ligated together.
Students may visualize DNA by constructing models, and they can simulate semi conservative replication
by tracing the synthesis of the leading and lagging strands. The critical principles to teach with this activity
are that two double-stranded DNA strands are the product of synthesis, that the process is semi
conservative, that the antiparallel orientation of the strands requires repeated reinitiation on the lagging
strand, and that the only information used during synthesis is specified by the base-pairing rules.
RNA is produced from DNA when a section of DNA (containing the nucleotide sequence required for
the production of a specific protein) is transcribed. Only the template side of the DNA is copied. RNA
then leaves the nucleus and travels to the cytoplasm, where protein synthesis takes place.
5. c. Students know how genetic engineering (biotechnology) is used to produce novel
biomedical and agricultural products.
Recombinant DNA contains DNA from two or more different sources. BacteÂ¬rial plasmids and viruses
are the two most common vectors, or carriers, by which recombinant DNA is introduced into a host cell.
Restriction enzymes provide the means by which researchers cut DNA at desired locations to provide
DNA fragments with â€œsticky ends.â€� Genes, once identified, can be amplified either by cloning or by
polymerase chain reactions, both of which produce large numbers of copies. The recombinant cells are
then grown in large fermentation vessels, and their products are extracted from the cells (or from the
medium if the products are secreted) and purified. Genes for human insulin, human growth hormone,
blood clotting factors, and many other products have been identified and introduced into bacteria or other
microorganisms that are then cultured for commercial production. Some agricultural applications of this
technology are the identification and insertion of genes to increase the productivity of food crops and
animals and to promote resistance to certain pests and herbicides, robustness in the face of harsh
environmental conditions, and resistance to various viruses.
Students can model the recombinant DNA process by using paper models to represent eukaryotic
complementary DNA (cDNA), the activity of different restriction enzymes, and ligation into plasmid DNA
containing an antibiotic resistance gene and origin of DNA replication. To manipulate the modeled DNA
sequences, students can use scissors (representing the activity of restriction enzymes) and tape
(representing DNA ligase). If both strands are modeled on a paper tape, students can visualize how, in
many cases, restriction enzymes make staggered cuts that genÂ¬erate â€œsticky endsâ€� and how the
ends must be matched during ligation.
5. d.* Students know how basic DNA technology (restriction digestion by endonucleases, gel
electrophoresis, ligation, and transformation) is used to construct recombinant DNA molecules.
In recombinant DNA technology DNA is isolated and exchanged between organisms to fulfill a specific
human purpose. The desired gene is usually identified and extracted by using restriction enzymes, or
endonucleases, to cut the DNA into fragments. Restriction enzymes typically cut palindromic portions of
DNA, which read the same forward and backward, in ways that form sticky complementary ends. DNA
from different sources, but with complementary sticky ends, can be joined by the enzyme DNA ligase,
thus forming recombinant DNA.
DNA fragments of varying lengths can be separated from one another by gel electrophoresis. In this
process the particles, propelled by an electric current, are moved through an agarose gel. Depending on
the size, shape, and electrical charge of the particles, they will move at different rates through the gel and
thus form bands of particles of similar size and charge. With appropriate staining, the various DNA
fragments can then be visualized and removed for further analysis or recombination.
5. e.* Students know how exogenous DNA can be inserted into bacterial cells to alter their
genetic makeup and support expression of new protein products.
Bacteria can be induced to take up recombinant plasmids, a process called DNA transformation, and the
plasmid is replicated as the bacteria reproduce. Recombinant bacteria can be grown to obtain billions of
copies of the recombinant DNA. Commercially available kits containing all the necessary reagents,
restriction enzymes, and bacteria are available for experiments in plasmid DNA transformation. Although
the reagents and equipment can be expensive, various California corporations and universities have
programs to make the cost more affordable, sometimes providing reagents and lending equipment.
`Students should know that DNA transformation is a natural process and that horizontal DNA transfer is
common in the wild. An example of how humans have manipulated genetic makeup is through the
selective breeding of pets and of agricultural crops.