A biologist must be able to work with a variety of mixtures. He or she must be able to plan the preparation of mixtures, read formulas for mixtures, describe them, store them properly, dilute them, analyze them for content and/or concentration, pipette them, and handle them safely. This talk will present basic concepts and definitions, and the rationale behind descriptions of types of mixtures.  It is part of a presentation on the methodology related to understanding and preparing solutions.

Many, but not all, mixtures used in a biology laboratory are made by mixing a solid with water.  Many, but not all, of these mixtures are true solutions. Here are some examples of mixtures that a biologist might encounter in a laboratory.  Some are true solutions and some are not.

• physiological saline solutions

• buffers

• cell suspensions

• soil suspensions

• staining solutions

• microbiological media

• chromatography slurries

• dishwater

• milk

• protein solutions

• DNA solutions

• density gradients

The word “mixture” can be defined as a heterogeneous association of substances that cannot be represented by a single chemical formula. This definition does not limit mixtures to solids mixed with liquids. Two or more gases, solids, or liquids can be mixed, and two or more different phases of matter can be combined in a mixture.  Because of the importance of liquid solutions and similar mixtures to biology, this series of talks will focus primarily on mixtures in which the major component is a liquid.

Remember that genetic variation contributes to the evolutionary potential of a population. A mutation, or a random change in an organism's genetic material, can lead to changes in the gene pool if the mutation is heritable (carried by the sperm or eggs). The word "mutation" also refers to the process by which a gene or chromosome is modified. Although they are relatively rare occurrences, mutations can introduce new alleles into a population and, therefore, provide raw material for the evolutionary process.

Because mutation is a random event, new mutations can be either harmful, neutral, or advantageous. Harmful mutations, like the albino alligator depicted in the figure, confer lower fitness (reproductive success) to an individual. Neutral mutations have no net effect on the fitness of an individual. Advantageous mutations increase fitness, providing an advantage to an individual. The frequency of an allele that confers a fitness advantage is likely to increase in a population.

Mutations only have evolutionary consequences if they are passed on to the next generation. Examples of types of mutations include changes in DNA sequence at specific locations (point mutations), sequence changes as a result of recombination, and changes caused by transposable elements (copies of DNA sequences that become inserted into different sites in the genome).

Alternatives to the biological species concept establish different criteria for the definition and identification of species. One alternative is the phylogenetic species concept, which defines species as discreet, irreducible groups of organisms that are "diagnosably different" from other groups and share a common ancestor. Subtle variations of this idea have been proposed, such as the genealogical species concept that defines a species as a group whose members are more closely related to one another than to any organism outside the group. Together, these concepts highlight the role of evolutionary history and genetic divergence in the process of speciation. Phylogenetic relationships are often inferred through the use of quantitative methods. For example, current molecular techniques now permit the direct comparison of genetic information to create groupings or to assign individuals to specific species groups. Before the advent and wide use of DNA, and other sequencing information, phylogenetic relationships were inferred from morphology, geographical distribution, and other characteristics related to phenotype.

While the phylogenetic species concepts are concerned with the identification of historically related groups, a number of alternative species concepts emphasize the origins of the discreet groups seen in nature. For example, the ecological species concept defines species as a group of organisms that has adapted to a particular niche in an environment and evolves independently from all groups outside of its range. Thus, species are identified by the use of a common set of environmental resources. In contrast, the recognition species concept defines a species as a group of "biparental organisms that share a common fertilization system."  According to this species concept, species are identified as a group of organisms that only recognize other members of the group as potential mates.

These, and other species concepts, have been proposed to address some of the limitations of the biological species concept. For example, both the ecological and phenotypic species concepts are used to define species in groups that reproduce asexually, for which the biological species concept is not useful.

Cells are the fundamental structural and functional units within living organisms. All living organisms consist of one or more cells. With the exception of bacteria, all organisms are made of eukaryotic cells, which have a membrane-enclosed nucleus and organelles (e.g., mitochondria, endoplasmic reticulum, and ribosomes). The nucleus within each cell contains the hereditary information for the entire organism, encoded within DNA.

In multi-cellular organisms, cells differentiate and specialize. Specialized cells organize into tissues (e.g., muscle, blood, bone, fat, nerve), which make up organs (e.g., kidneys, heart, stomach, lung), which, in turn, comprise organ systems (e.g., respiratory, digestive, excretory). Genes that do not pertain to the functioning of each individual cell become inactive, or "turn off." For example, a kidney cell uses only the DNA needed to be a kidney cell. The remaining information is "turned off," but it is still present. There are more than 200 different types of cells (nerve cells, muscle cells, epithelial cells, blood cells, bone cells, etc.) among the human body's estimated 100,000,000,000,000 total cells.

In the simplest terms, cloning is the creation of a genetically identical copy of an original organism. Plants are relatively easy to clone. Most people are familiar with the use of cuttings or stem segments (such as the "eyes" of potatoes) to create new plants. Technically, the new plants are clones of the original, because they are genetically identical to the parent plant. In the wild, many different kinds of plants and animals use forms of reproduction that copy an exact genotype. This type of reproduction (which is seen in grasses, strawberry plants, sponges and flatworms, for example) often is referred to as asexual reproduction. Even fraternal twins can be thought of as clones, because they have identical sets of DNA.

Molecular biologists use the term "cloning" to refer to a variety of processes that involve making identical copies of part or all of a DNA molecule, a single cell type, or an entire organism. DNA cloning technology, also referred to as molecular cloning, recombinant DNA, or gene cloning, is a common practice in molecular laboratories today. A DNA fragment from one organism is introduced into a self-replicating element (host) such as a bacterial plasmid. Molecular biologists use DNA cloning to create many identical copies of a DNA molecule or to isolate a particular stretch DNA (which involves making identical copies of the DNA of interest).

In 1997, scientists used a somatic cell (a cell that is not an egg or sperm cell) from an adult sheep to produce a reproductive clone via a process called somatic cell nuclear transfer (SCNT). With this technique, scientists transferred the nucleus from a somatic cell of an adult sheep into an egg from which the nucleus had been removed. This type of cloning, called reproductive cloning, still is very rare and difficult to achieve for vertebrate animals.

SCNT also is used in therapeutic cloning to produce many copies of stem cells. Stem cells are undifferentiated cells that can be used as replacement cells to treat a variety of diseases and disorders. The purpose of this type of cloning is not to produce another organism, but to generate copies of cells in sufficient quantities for research and medical treatments.

To clone DNA, scientists use restriction enzymes to cut out the specific DNA segment to be replicated (copied). The segment then is inserted into a bacterial plasmid for replication. Bacterial plasmids are circular DNA molecules distinct from the normal bacterial genome and are capable of replicating separately. Once inserted, the recombinant DNA is replicated, along with the host cell's DNA. Plasmids can carry up to 20,000 base pairs of foreign DNA.

Human insulin often is produced by recombinant DNA technology. The human insulin gene is inserted into a bacterial plasmid and can be induced to produce vast quantities of insulin for the treatment of diabetes. Other specific applications of recombinant DNA technology include the production of human growth hormone, erythropoietin for kidney dialysis patients, clotting factor for hemophiliacs, and hepatitis B vaccine. Although viruses, bacterial artificial chromosomes (BACs), and yeast artificial chromosomes (YACs) also may be used for replicating DNA, bacterial plasmids are most commonly used in this technology.

The concept of vertebrate animal cloning was introduced seriously in 1938. Early stages of cloning research incorporated a process called "twinning." Twinning takes a fertilized egg (after a sperm has naturally fertilized the egg) and waits until it divides into two identical cells. These cells then are separated and each is implanted in a mother. The resulting offspring are genetic twins or identical clones.

Early attempts at animal cloning always used embryonic cells. Remember, as an organism develops, cells differentiate and specialize (nerve, kidney, etc.). Once cells specialize, some of their DNA  "turns off" and becomes inaccessible. Using an embryonic cell for cloning bypasses this hurdle, since all of the DNA still is accessible.
 
In the 1980s, researchers selected embryos at early stages of development and used individual cells to create clones. For example, scientists might have taken a four-cell embryo, broken it up and isolated each of the four cells. Each of these cells then could produce a whole organism on its own, and each new organism would be identical to (or a clone of) the others. However, it was not until 1997 that researchers were able to take a somatic cell from an adult organism and use it to produce a cloned embryo that developed into an adult organism, genetically identical to the donor organism.

To understand how somatic cell nuclear transfer (SCNT) works, we must first understand a few things about cells. SCNT uses two principle types of cells-a somatic cell and an egg cell. A somatic cell is an adult cell or any nucleated cell in the body that is not an egg or sperm cell (gamete or germ cell). Egg cells are female reproductive cells (gametes) that can be fertilized by a sperm cell (also called gametes) to create an embryo.

Remember that the cell's nucleus has DNA, which contains the blueprint of the entire organism. Using SCNT technology, researchers remove the nucleus from a somatic cell and from an egg cell. The nucleus from the somatic cell is injected into the enucleated egg cell, thus replacing the nucleus of the egg cell with a nucleus from the somatic cell. Using electric current or chemicals to stimulate cell division, the cell is reactivated. The egg cell begins dividing, producing two, four, eight... cells. Even though the entire cell nucleus has been replaced, some of the clone's genetic material is contributed from mitochondrial DNA found in the cytoplasm of the donor egg cell. Therefore, the resulting organism will not be an exact copy if the donor of the egg cell and the somatic cell are from different donors.

SCNT was first described in 1983 in amphibians and was later demonstrated in work with sheep and mouse embryos in 1986. Accumulation of technology and data led to the first successful mammal clone, using SCNT, in 1997, when Scottish scientists at Roslin Institute created a sheep named Dolly.