Ch. 20 Biotechnology

DNA cloning yields multiple copies of a gene or other DNA segment

• Gene cloning and other techniques, collectively termed DNA technology, can be used to manipulate and analyze DNA and to produce useful new products and organisms.

• In genetic engineering, bacterial restriction enzymes are used to cut DNA molecules within short, specific nucleotide sequences (restriction sites), yielding a set of double-stranded DNA fragments with single stranded sticky ends.

DNA cloning

• The sticky ends on restriction fragments from one DNA source can base-pair with complementary sticky ends on fragments from other DNA molecules; sealing the base-paired fragments with DNA ligase produces recombinant DNA molecules.

Cloning a eukaryotic gene in a bacterial plasmid

Cloning a eukaryotic gene in a bacterial plasmid

DNA cloning yields multiple copies of a gene or other DNA segment

• Cloning vectors include plasmids and bacterial artificial chromosomes (BACs). Recombinant plasmids are returned to host cells, each of which divides to form a clone of cells. Collections of clones are stored as genomic or complimentary DNA (cDNA) libraries. Libraries can be screened for a gene of interest using nucleic acid hybridization with a nucleic acid probe.

DNA cloning

• Several technical difficulties hinder the expression of cloned eukaryotic genes in bacterial host cells. The use of cultured eukaryotic cells (such as yeasts, insect cells, or cultured mammalian cells) as host cells, coupled with appropriate expression vectors, helps avoid these problems.

DNA cloning

• The polymerase chain reaction (PCR) can produce many copies of (amplify) a specific target segment of DNA in vitro, using primers that bracket the desired sequence and a heat-resistant DNA polymerase.

DNA technology allows us to study genes

• DNA restriction fragments of different lengths can be separated by gel electrophoresis. Specific fragments can be identified by southern blotting, using labeled probes that hybridize to the DNA immobilized on a “blot” of the gel. Historically, restriction fragment length polymorphisms (RFLPs) were used to screen for some disease-causing alleles such as the sickle-cell allele.

DNA technology allows us to study genes

• Relatively short DNA fragments can be sequenced by the dideoxy chain termination method, which can be performed on automated sequencing machines. The rapid development of faster and cheaper methods is ongoing.

DNA technology allows us to study genes

• Expression of a gene can be investigated using hybridization with labeled probes to look for specific mRNAs, either on a gel (Northern blotting) or in a whole organism (in situ hybridization).

DNA Technology Allows us to study genes

• Also, RNA can be translated into cDNA by reverse transcriptase and the cDNA amplified by PCR with specific primers (RT-PCR). DNA microarrays allow researchers to compare the expression of many genes at once in different tissues, at different times, under different conditions.

DNA Technology Allows us to Study Genes

• For a gene of unknown function, experimental inactivation of the gene and observation of the resulting phenotypic effects can provide clues to its function. In humans, genome wide association studies use single nucleotide polymorphisms (SNPs) as genetic markers for alleles that are associated with particular conditions.

Cloning organisms may lead to production of stem cells

Studies showing genomic equivalence (that an organisms cells have the same genome) provided the first examples of organismal cloning.Single differentiated cells from mature plants are often totipotent: capable of generating all the tissues of a complete new plant

Cloning organisms may lead to production of stem cells

• Transplantation of the nucleus from a differentiated animal cell into an enucleated egg can sometimes give rise to a new animal

Cloning organisms may lead to production of stem cells

• Certain embryonic stem (ES) or adult stem cells from animal embryos or adult tissues can reproduce and differentiate in vitro as well as in vivo, offering the potential for medical use. ES cells are pluripotent but difficult to acquire. Induced pluripotent (iPS) cells resemble ES cells in their capacity to differentiate; they can be generated by reprogramming differentiated cells. iPS cells hold promise for medical research and regenerative medicine.

The practical applications of DNA technology affect our lives

• DNA technology, including the analysis of genetic markers such as SNPs, is increasingly being used in the diagnosis of genetic and other diseases and offers potential for potential for better treatment of genetic disorders (or even permanent cures through gene therapy), as well as more informed cancer therapies.

The practical applications of DNA technology

• Large scale production of protein hormones and other proteins with therapeutic uses is possible with DNA technology. Some therapeutic proteins are produced in transgenic “pharm” animals.

The practical applications of DNA technology

• Analysis of genetic markers such as short tandem repeats (STRs) in DNA isolated from tissue or body fluids found at crime scenes leads to a genetic profile that can provide definitive evidence that a suspect is innocent or strong evidence of guilt. Such analysis is also useful in parenthood disputes and in identifying the remains of crime victims.

The practical applications of DNA technology

• . Genetically modified microorganimsms can be used to extract minerals from the environment or degrade various types of toxic waste materials. The aims of developing transgenic plants and animals are to improve agricultural productivity and food quality.

The practical applications of DNA technology

• The potential benefits of genetic engineering must be carefully weighed against the potential for harm to humans or the environment.