DNA

DNA
Lesson 1 - Page 1

BACKGROUND:

The blueprint for the structure and functioning of our bodies is contained in the genetic material found in the nucleus. The genetic material (chromatin) is composed of DNA (Deoxyribonucleic acid) and protein. During certain times in a cell's life the chromatin will condense and form x-shaped structures called chromosomes. Chromosomes are found in the nucleus of cells at some time in their life spans. Human beings have 46 chromosomes, arranged in 23 pairs. Heredity is encoded in DNA within the chromosomes. A gene is a very small cluster of chemical units which group up to form the DNA molecule. RNA (ribonucleic acid) is the messenger of DNA within the cell. Forms of RNA direct the cell to manufacture specific enzymes and other proteins. Modern biological research is developing a much more complicated picture than what is described above. Students in the fifth grade should learn the general outline and parts, but to really understand what is going on is not really completely known.

DNA functions by carrying the template or "map" of chemical compounds, amino acids, that are used to build proteins. DNA directs the production of proteins by providing the sequence of amino acids necessary to produce specific proteins. Amazingly, there are only 20 amino acids that are utilized to produce every protein in the human body. Of course, certain "modified" amino acids exist, but these still require the starting 20 amino acids. In order to gain an understanding of DNA, we must start at the heart of the matter, the cell.

This summary mainly is concerned with human cells but the basic theory holds for most organisms. Most human cells have a distinguishable central structure called the nucleus. The nucleus is the "storage area" for the cell's genetic material. The nucleus houses DNA and its corresponding helper molecule, RNA (ribonucleic acid). The nucleus is often referred to as being the "control center" of the cell and, the nucleus does in fact regulate cell activity. This regulation is brought about under the influence of the genetic information that is housed in the cell. This genetic information informs the cell of what activities it is to undertake and what it is to accomplish. The cell carries out these "orders" by performing specific biochemical reactions, often times under the influence of enzymes (proteins that are catalysts) that are produced in the cell. The nucleus is the depository for nucleic acids (DNA and RNA). It is in the nucleus where DNA replication occurs.

The sugar-phosphate backbone consists of deoxyribose sugar groups connected together by phosphate groups. The sugar groups are, in turn, connected to the four bases: adenine, guanine, cytosine, and thymine. Adenine and guanine are called purines while cytosine and thymine are called pyrimidines. The bases are often abbreviated as A, G, C, and T. A purine base can only bond to a pyrimidine base, and a pyrimidine only to a purine. Adenine will only bond with thymine and guanine will only bond with cytosine. The nucleotide base pairs are held together by hydrogen bonds. It follows then that the number of adenine bases will equal the number of thymine bases and the number of guanine bases will equal the number of cytosine bases. This presumption led the way for the formulation of Chargaff's rules which was the basis of DNA investigation prior to the discovery of the double helical nature of DNA.

DNA carries a template that determines amino acid sequences which are then used to produce proteins at the ribosomes. Since proteins cannot produce other proteins DNA serves as a "storage" center for the amino acid sequences of all the proteins produced by an organism. A particular amino acid sequence that codes for a specific protein is called a gene. The genes of an organism are stored in structures called chromosomes, which are the familiar x-shaped structures often seen in biology books. An organism's DNA is not always organized into chromosomes, rather these structures appear at particular times during the life cycle of a cell. A chromosome consists of DNA associated with a group of proteins known as the histone proteins. DNA is a long, linear, polymer (poly=many, mer=unit) that if stretched out could be up to several or even thousands of meters long. One chromosome consists of a single molecule of DNA.

Humans reproduce sexually through the successful union of a sperm and egg cell. The sperm and egg, referred to as gametes, contain one half the number of chromosomes found in other human cells. When a cell contains one half the number of chromosomes it is referred to as being haploid. Conversely, when a cell contains the full number of chromosomes it is referred to as being diploid. Human sperm and egg cells are haploid and contain 23 chromosomes. The diploid number for human cells is 46 chromosomes. When an egg and sperm unite each cell contributes 23 chromosomes and the resulting fertilized egg has 46 chromosomes. Through this mechanism genetic variability and heredity is expressed. Therefore a child will have received one half of his DNA from his mother and one half from his father. The traits that are then expressed in the child are a function of which DNA (ie, the father's or mother's) was expressed. The study of how traits are inherited and passed on through generations is referred to as genetics.

DNA has an analogous helper molecule called RNA (ribonucleic acid.) RNA's structure is similar to DNA's except in the following manners:

1. RNA contains the sugar ribose, whereas DNA contains the sugar deoxyribose. Deoxyribose has one less oxygen than ribose, hence the name deoxy-.

2. RNA contains the base uracil instead of thymine.

3. RNA is usually single stranded.

Genetics is the study of heredity or the passing of traits from parents to offspring. Offspring can inherit dominant traits or recessive traits. A dominant trait is one that prevents another trait from appearing. A recessive trait is one that does not appear when a dominant trait is present. A pure trait (homozygous) is one that is made up of all dominant traits or all recessive traits. A hybrid trait (heterozygous) is a trait that is made up of a combination of dominant and recessive traits.

A gene is the unit of inheritance which is passed from parents to offspring. Genes occur in pairs in chromosomes inside the nucleus of a cell. There are dominant genes for dominant traits and recessive genes for recessive traits. Dominant genes mask recessive genes when paired. Gregor Mendel's experiments on hybridizing garden peas was the first recorded experiment on plant breeding. Mendel successfully studied the inheritance of unit characters. He also kept accurate records of how the characters reappeared in the offspring of selected parents. Mendel was also the first to control pollination techniques. These procedures are understood today, but in 1865 when Mendel's work was done, this was revolutionary. Mendel's work laid unnoticed for 35 years until European plant breeders rediscovered his work.

Mendel selected garden peas for his experiment. He took pollen from a dwarf growing pea and dusted it on a tall growing variety. The seeds resulting from this cross pollination were collected and planted the following season. All the plants that grew from these seeds were tall. Mendel reversed the situation and dusted the dwarf peas with pollen from the tall variety, and this resulted in offspring that were all tall.

Mendel then allowed the tall plants to self pollinate and found that 3/4 were tall plants and 1/4 were dwarf. From this result Mendel concluded that the expression of a given character was dominant, while the other character was recessive.

A phenotype is a visible, noticeable, and recognizable trait. An organism's genotype is his genetic make-up which is located in the nuclei of his cells. For example, the physical traits of a tall pea may be made of a dominant tall pea gene and a recessive dwarf pea gene. The phenotype of this plant will be tall, whereas the genotype will be heterozygous (i.e., a dominant and a recessive gene).

When a cross is performed upon parents that differ in only one single character (i.e., tall, short, etc.) it is termed a monohybrid cross. However, in many cases genes have more than one trait. When two sets of hereditary traits are considered it is called a dihybrid. Mendel continued his experiments with a pure (homozygous) tall, red flowering pea plant (TTRR) and crossed it with a pure dwarf, white flowering pea plant (ttrr). All the offspring will be hybrid (heterozygous) tall, red flowering (TtRr). Since the genes for height are on a different pair of chromosomes from the genes for flower color, the genes assort independently. This allows for four different types of "gametes" to form: TR, Tr, tR, and tr. There can be 16 different combinations that the offspring can have, with any one of them just as likely to occur. However, there are only nine different combinations (different genotypes) and four different phenotypes possible.

Most genes in a uniform population tend to be similar, however there always seems to be a rare individual that differs from his neighbors. The cause of this different individual in a natural population is due to a change in one of the unit genes and is called a mutation.

Some mutations may be economically desirable or advantageous to individuals, but most mutations result in phenotypes less adapted for survival than the parent phenotype. Natural mutations are random. Radiation from some sources like alpha and beta particles, gamma rays, mustard gas, and x-rays are known to cause mutations but the exact reasons are very complicated. Some mutations in plants are desirable commercially, even through they may have no survival value to a plant. For instance, seedless grapes are missing their reproductive seeds. If humans don't vegetatively propagate the grape plant it will not survive. Other desirable mutations occur in nectarines, many ornamental flowers, peanuts and other cash crops.

Plant breeders deliberately increase hereditary variation by hybridization and stimulate mutations by radiation treatments and other techniques. Breeders will then select the most desirable phenotypes and genotypes for propagation. In general, the traits that plant breeders are looking for are: vigorous plant types, high yields, quality, and disease resistance.

Genetic engineering is a field that works with controlling "mutations," and genetic combinations resulting in superior or helpful organisms. Genetic engineering is different from the hybridization principles that were studied in this lab. Genetic engineers "design" genetic combinations that will result in specific and directed genetic combinations. Hybridization relies on random combinations that are driven by probability. Hybridization is more of a "trial and error" type of situation. Breeders try to combine different genetic combinations with hopes of producing a better phenotype and genotype and unlike genetic engineers, they do not "design" the genetic combinations themselves.

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