14.1 DNA Cloning
1. Cloning is the production of identical copies of DNA through some asexual means.
An underground stem or root sends up new shoots that are clones of the parent plant.
Members of a bacterial colony on a petri dish are clones because they all came from division of the same cell.
Human identical twins are clones; the original single embryo separate to become two individuals.
2. Gene cloning is production of many identical copies of the same gene.
If the inserted gene is replicated and expressed, we can recover the cloned gene or protein product.
Cloned genes have many research purposes: determining the base sequence between normal and mutated genes, altering the phenotype, obtaining the protein coded by a specific gene, etc.
Humans can be treated with gene therapy: alteration of the phenotype in a beneficial way.
Otherwise transgenic organisms are used to produce products desired by humans.
To clone DNA, scientists use recombinant DNA (rDNA) and polymerase chain reaction (PCR).
A. Recombinant DNA Technology
1. Recombinant DNA (rDNA) contains DNA from two or more different sources.
2. To make rDNA, a technician selects a vector.
3. A vector is a plasmid or a virus used to transfer foreign genetic material into a cell.
4. A plasmid is a small accessory ring of DNA in the cytoplasm of some bacteria.
5. Plasmids were discovered in research on reproduction of intestinal bacteria Escherichia coli.
6. Introduction of foreign DNA into vector DNA to produce rDNA requires two enzymes.
a. Restriction enzyme is a bacterial enzyme that stops viral reproduction by cleaving viral DNA.
The restriction enzyme is used to cut DNA at specific points during production of rDNA.
It is called a restriction enzyme because it restricts growth of viruses but it acts as a molecular scissors to cleave any piece of DNA at a specific site.
Restriction enzymes cleave vector (plasmid) and foreign (human) DNA.
Cleaving DNA makes DNA fragments ending in short single-stranded segments with “sticky ends.”
The “sticky ends” allow insertion of foreign DNA into vector DNA.
b. DNA ligase seals the foreign gene into the vector DNA.
Treated cells take up plasmids, and then bacteria and plasmids reproduce.
Eventually, there are many copies of the plasmid and many copies of the foreign gene.
When DNA splicing is complete, an rDNA (recombinant DNA) molecule is formed.
7. If the human gene is to express itself in a bacterium, the gene must be accompanied by the regulatory regions unique to bacteria and meet other requirements.
a. The gene cannot contain introns because bacteria do not have introns.
b. An enzyme called reverse transcriptase can be used to make a DNA copy of mRNA.
c. This DNA molecule is called complementary DNA (cDNA) and does not contain introns.
d. A laboratory DNA synthesizer can produce small pieces of DNA without introns.
B. The Polymerase Chain Reaction (PCR)
1. Polymerase chain reaction (PCR) can create millions of copies of a single gene or a specific piece of DNA in a test tube.
2. PCR is very specific—the targeted DNA sequence can be less than one part in a million of the total DNA sample; therefore a single gene can be amplified using PCR.
3. PCR uses the enzyme DNA polymerase to carry out multiple replications (a chain reaction) of target DNA.
4. PCR automation is possible because heat-resistant DNA polymerase from
Thermus aquaticus, which grows in hot springs, is an enzyme that withstands the temperature necessary to separate double-stranded DNA.
5. Three DNA analysis techniques are gel electrophoresis, short tandem repeat profiling, and “finger-printing.”
Gel electrophoresis separates DNA fragments according to their size and results in distinctive bands that can help identify individuals.
Short tandem repeat (STR) profiling is a technique used to analyze DNA.
Advantages include not being limited to using restriction enzymes.
Since the chromosomal locations of STRs are known, these are the only locations that are subjected to PCR and gel electrophoresis.
The band patterns are different in each person because each individual has their own number of STR repeats at different locations.
DNA fingerprinting is a technique of fluorescently labeling DNA fragments.
The STR are excited by a laser, emissions from DNA fragments are recorded in peaks and valleys.
6. PCR amplification and DNA analysis is used to:
a. Detect viral infections, genetic disorders, and cancer.
b. Determine the nucleotide sequence of human genes, and, because it is inherited.
c. Associate samples with DNA of parents.
d. Investigate evolutionary connectionis.
14.2 Biotechnology Products
1. Genetically engineered organisms can produce biotechnology products.
2. Organisms that have had a foreign gene inserted into them are transgenic.
A. Genetically Modified Bacteria
1. Bacteria are grown in large vats called bioreactors.
a. Foreign genes are inserted and the product is harvested.
b. Products on the market include insulin, hepatitis B vaccine, t-PA, and human growth hormone.
2. Transgenic bacteria have been produced to protect and improve the health of plants.
a. Frost-minus bacteria protect the vegetative parts of plants from frost damage.
b. Root-colonizing bacteria receive genes from bacteria for insect toxin, protecting the roots.
c. Bacteria that colonize corn roots can be endowed with genes for insect toxin.
3. Transgenic bacteria can degrade substances.
a. Bacteria selected for ability to degrade oil can be improved by bioengineering.
b. Bacteria can be bio-filters to prevent airborne chemical pollutants from being vented into the air.
c. Bacteria can also remove sulfur from coal before it is burned and help clean up toxic dumps.
d. Bacteria can also be given “suicide genes” that cause them to die after they have done their job.
4. Transgenic bacteria can produce chemical products.
a. Genes coding for enzymes can be manipulated to catalyze synthesis of valuable chemicals.
b. Phenylalanine used in artificial sweetener can be grown by engineered bacteria.
B. Genetically Modified Plants
1. Plant cells that have had the cell wall removed are called protoplasts.
2. Electric current makes tiny holes in the plasma membrane through which genetic material enters.
3. The protoplasts then develop into mature plants.
4. Foreign genes now give cotton, corn, and potato strains the ability to produce an insect toxin and soybeans are now resistant to a common herbicide.
5. Plants are being engineered to produce human proteins including hormones, clotting factors, and antibodies in their seeds; antibodies made by corn deliver radioisotopes to tumor cells and a soybean engineered antibody can treat genital herpes.
C. Genetically Modified Animals
1. Animal use requires methods to insert genes into eggs of animals.
a. It is possible to microinject foreign genes into eggs by hand.
b. Vortex mixing places eggs in an agitator with DNA and silicon-carbide needles that make tiny holes through which the DNA can enter.
c. Using this technique, many types of animal eggs have been injected with bovine growth hormone (bGH) to produce larger fishes, cows, pigs, rabbits, and sheep.
2. Gene pharming is the use of transgenic farm animals to produce pharmaceuticals; the product is obtainable from the milk of females.
a. Genes for therapeutic proteins are inserted into animal’s DNA; animal’s milk produces proteins.
b. Drugs obtained through gene pharming are planned for the treatment of cystic fibrosis, cancer, blood diseases, and other disorders.
D. Cloning Transgenic Animals
1. For many years, it was believed that adult vertebrate animals could not be cloned; the cloning of Dolly in 1997 demonstrated this can be done.
2. Adult vertebrate cloning would require that all genes of an adult cell be turned on again.
3. Cloning of mammals involves injecting a 2n nucleus adult cell into an enucleated egg.
4. The cloned eggs begin development in vitro and are then returned to host mothers until the clones are born.
E. Applications of Transgenic Animals
1. Scientists are using transgenic animals to illustrate that maleness is due to a section of DNA called SRY (the sex determining region of the Y chromosome).
2. Transgenic animals are also being used to investigate various treatments for diseases by eliminating genes from these animals.
3. Organ transplant from transgenic animal donors to human recipients is called xenotransplantation.
14.3 Gene Therapy
1. Gene therapy involves procedures to give patients healthy genes to make up for a faulty gene.
2. Gene therapy also includes the use of genes to treat genetic disorders and various human illnesses.
3. There are ex vivo (outside body) and in vivo (inside body) methods of gene therapy.
A. Ex Vivo Gene Therapy
1. Children with severe combined immunodeficiency (SCID) underwent ex vivo gene therapy.
a. Lacking the enzyme ADA involved in maturation of T and B cells, they faced life-threatening infections.
b. Bone marrow stem cells are removed, infected with a retrovirus that carries a normal gene for the enzyme ADA, and returned.
c. Use of bone marrow stem cells allows them to divide and produce more cells with the same genes.
d. Patients who undergo this procedure show significant improvement.
2. Gene therapy includes treatment of familial hypercholesterolemia where liver cells lack a receptor for removing cholesterol from blood.
a. High levels of blood cholesterol make the patient subject to fatal heart attacks when young.
b. A small portion of the liver is surgically removed and infected with retrovirus with normal gene for receptor.
c. This has lowered cholesterol levels following the procedure.
In Vivo Gene Therapy
1. Cystic fibrosis patients lack a gene for trans-membrane chloride ion carriers; patients die from respiratory tract infections.
a. Liposomes, microscopic vesicles that form when lipoproteins are in solution, are coated with healthy cystic fibrosis genes and sprayed into a patient’s nostrils.
b. Various methods of delivery are being tested for effectiveness.
2. A gene for vascular endothelial growth factor (VEGF) can be injected alone or within a virus into the heart to stimulate branching of coronary blood vessels.
3. Another strategy is to make cancer cells more vulnerable and normal cells more resistant, to chemotherapy.
4. Injecting a retrovirus containing a normal p53 gene—that promotes apoptosis—into tumors may stop the growth of tumors.
1. Genetics in the 21st century concerns genomics: the study of genomes of humans and other organisms.
A. Sequencing the Genome
The Human Genome Project has produced a record of all the base pairs in all our chromosomes.
The task took 13 years to learn the sequence of the three billion base pairs along the length of our chromosomes.
Most of the DNA regions that differed among individuals are called single nucleotide polymorphisms (SNPs)
Many SNPs have no effect and others could contribute to enzymatic differences affecting the phenotype.
Following functional genomics, structural genomics investigates base sequences and the number of genes in humans.
Structure of the Eukaryotic Genome
1. Eukaryotic chromosome structure is much more complex than prokaryotic chromosome structure.
2. Generally speaking, more complex organisms have more complex genes with more and larger introns.
3. Introns are currently regarded as gene expression regulators and determine which genes are expressed and how they are spliced.
1. Intergenic sequences are DNA sequences that occur between genes.
2. Generally speaking, as the complexity of an organism increases, so does the proportion of its non-protein-coding DNA sequences.
3. Most of the human chromosomes are comprised of intergenic sequences, genes represent 1.5% of human’s total DNA, and the remainder represents “junk DNA,” which scientists believe serve many important functions.
4. Three types of intergenic sequences found in the human genome are: repetitive elements, transposons, and unknown sequences.
5. Repetitive DNA elements occur when the same sequence of two or more nucleotides are repeated many times along the length of one or more chromosomes.
a. They make up nearly half of the human genome and scientists believe that their true significance have yet to be discovered.
b. Repetitive DNA elements occur as tandem repeats—meaning the repeated sequences are next to each other on the chromosome.
c. One type of tandem repeat sequence, short tandem repeats (STRs), are a standard method in forensic science for identifying one individual from another and also determining family relationships.
d. Another type of repetitive DNA element is called an interspersed repeat—meaning the repetitions may be placed intermittently along a single chromosome, or across multiple chromosomes.
e. Interspersed repeat are common and thought to play a role in the evolution of new genes.
Transposons are specific DNA sequences that move with and between chromosomes.
Their movement may increase or decrease the expression of neighboring genes.
They are among the 40% of the human genome consisting of the same short sequence of DNA continuously repeated.
They are noncoding sequences that play regulatory functions, and could thus be considered part of epigenetic inheritance.
7. Unknown DNA sequences were once dismissed as junk DNA since scientists could not identify any function they have.
a. Recently scientists believe observed that many of the unknown sequences are transcribed into RNA.
b. This RNA may carry out regulatory functions more easily than proteins at times.
Revisiting the Definition of a Gene
1. A modern definition of a gene, suggested by Mark Gerstein and associates, “…is a genomic sequence (either DNA or RNA) directly encoding functional products, either RNA or protein.”
2. This definition recognizes that the genetic material involved does not have to be DNA, and that coding does not only mean DNA sequencing, but the gene product could be RNA or protein as well.
Functional and Comparative Genomics
1. The emphasis on genome structure is both on functional and comparative genomics.
2. Functional genomics aims to understand the exact role of the genome in cells or organisms.
a. To meet the aim of functional genomics, DNA microarrays can be used (see Science Focus box).
b. DNA microarrays rapidly identify a person’s complete genotype; this is called the genetic profile.
3. Comparative genomics aims to compare the human genome to the genome of other organisms.
a. This type of genomics has revealed little difference between DNA sequence of our bases and those of many other organisms.
Proteomics is the study of the structure, function, and interaction of cellular proteins.
The information obtained from proteomic studies can be used in designing better drugs, and to correlate drug treatment to the particular genome of the individual.
Bioinformatics is the application of computer technologies to the study of the genome.
2. Information obtained from computer analysis of the genome can show relationships between genetic profiles and genetic disorders.
H. DNA Microarray Technology (Nature of Science reading)
1. DNA microarray technology is being used to identify genes associated with gene tissues, and identify links between disease and chromosomal variation.
2. DNA microarray technology techniques begin with labeling the test mRNA with a fluorescent dye added to the chip.
3. The tagged mRNA can then be followed by identifying the genes that exhibit the fluorescent dye.
I. Copy Number Variations (Evolution reading)
1. Scientists have recently discovered small duplications and deletions in chromosomes, referred to as copy number variations (CNVs).
2. The change in genes may arise from fork stalling and template switching.
3. Some CNVs may be linked to diseases such as HIV, Lupus, and autism.
4. Evolutionarily speaking, it may be advantageous for species to have multiple copies of genes because if one copy of an allele does not function properly, the duplicate allele may be able to restore normal function.
5. In addition, having two normal alleles may free the extra gene copy to accumulate mutations, which could ultimately lead to the formation of a new gene.