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| DNA in Plants |
DNA is the hereditary or genetic material, present in all cells, that carries information for the structure and function of living things.
In the plant kingdom, DNA, or deoxyribonucleic acid, is contained within the membrane-bound cell structures of the nucleus, mitochondria, and chloroplasts. DNA has several properties that are unique among chemical molecules.
It is universal to all living organisms, having the same structure and function in each. It is capable of reproducing itself in a process known as self-replication. This property allows cell division, and thus continuity, growth, and repair.
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It carries in its structure the genetic code, or set of instructions, for cellular development and maintenance. Finally, it undergoes changes in chemical structure, from both environmental and internal causes, called mutations, which contribute to evolution, diversity, and disease.
Chemical Structure
DNA is a simple molecule, consisting of four nucleotides. Each nucleotide has a five-carbon sugar (deoxyribose), a phosphate, and one of four possible nitrogenous bases: the double-ringed purines of adenine (A) and guanine (G) and the single-ringed pyrimidines of thymine (T) and cytosine (C).
Most of the properties of DNA relate to the unique bonds that form among the nucleotides: The sugarphosphate components align themselves linearly, while the nitrogen rings bond perpendicularly.
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| Chemical Structure |
The nitrogen rings further bond in a very specific fashion:A always pairs with T, and Galways pairs with C. The DNAmolecule thus appears as a ladder, the sides being sugar-phosphate; the rungs, the A-T and G-C pairs.
Further bonding and folding produces a structure shaped like a spiral ladder, known as a double helix. This double helix is compactly packaged in ropelike structures known as chromosomes, which are visible under a light microscope before and during cell division.
During the daily life of the cell, the DNA appears as an indistinguishable dark mass call chromatin (an inclusive term referring to DNA and the proteins that bind to it, located in the nuclei of eukaryotic cells).
The Watson and Crick Model
In the mid-1800’s Gregor Mendel, an Austrian monk, postulated that genetic material existed. He discovered the laws of heredity using the pea and other plants in his garden to study the inheritance of such traits as flower color.
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Nearly seventy years passed before scientists James Watson and Francis Crick, in 1953, proposed the double helix as the most plausible model for each of the unique properties of the molecule. Their model was verified by X-ray diffraction techniques soon afterward.
Several researchers, working at Columbia University and elsewhere in the United States, had led the way prior to Watson and Crick by discovering the chemical composition of this genetic material and the nitrogenous base pairing: The amount of adenine always equaled that of thymine and like-wise with guanine and cytosine.
The Watson and Crick model also suggested that the two sides, or strands, of DNA run in opposite directions: That is, the phosphate sugar of one side points upward, while the other strand points downward.
This property is known as antiparallel bonding. The Watson and Crick model could easily explain how DNA replicates during cell division and how genetic information is encoded in its structure.
Self-Replication
Self-replication, which allows the continuity of generations and the growth and repair of individual organisms, occurs during cell division. DNA must be able to produce exact copies of itself. The molecule is uniquely designed for this: A series of enzyme-mediated steps allows the double helix to unwind or unzip, like a zipper, separating the two strands.
Next, nucleotides from digested food enter first the cell and then the nucleus. They bind to a corresponding nucleotide: A with T and G with C. The process continues until two new double-stranded molecules of DNA have formed, each a copy of the other and each going into the new cells that resulted from cell division.
Protein Synthesis
The information encoded in DNA allows for all the development and maintenance of the cell and the organism. The language of this code lies in a linear reading of adjacent nucleotides on each strand. Every three nucleotides specify or fit a particular amino acid, the individual units of proteins.
A second molecule, ribonucleic acid (RNA), copies the molecular structure of DNA and brings the information outside the nucleus into the surrounding cytoplasm of the cell,where the amino acids are assembled, in specified order, to produce a protein.
Post production modifications of these proteins, such as the addition of sugars, fats, or metals, allow a vast array of functional and structural diversity. Plant DNA codes for a variety of substances that are unique to plants. These products sustain not only the plants themselves but also entire ecological niches, as well as humankind.
Mitochondrial and Chloroplastic DNA
A second, independently functioning set of DNA exists in two organelles outside the cell’s nucleus, the mitochondria and the chloroplast. It is in the mitochondria, the power sources of cells, where carbohydrates, fats, and proteins are broken down to their raw elements with the release of stored chemical bond energy in the form of heat (calories).
The second region in which DNA is housed outside the nucleus is in the chloroplast, a structure unique to plant cells. In chloroplasts, photosynthesis occurs, the process by which plants are able to transform carbon dioxide, water, and solar energy to produce sugars and, later, fats and proteins, with the release of oxygen. This critical process undertaken by plants sustains most life on earth.
Both mitochondrial and chloroplastic DNA replicate separately from nuclear DNA during cell division. It is postulated that these organelles once, billions of years ago, may have been independently living organisms that were incorporated into other cells to form the eukaryotic cells that make up nonbacterial life-forms such as fungi, protists, plants, and animals.
Plant Proteins
A large array of proteins that are unique to plants are encoded on plant DNA. A group that has received much attention are the so-called phytochemicals, substances with powerful health benefits. Well studied classes are few, including the flavonoids, phytosterols, carotenoids, indoles, coumarins, organosulfurs, terpenes, saponins, lignans, and isothiocyanates.
Each group contains specific proteins that are both antioxidants and anticarcinogens protecting animal cells from cancer-causing agents. The carotenoids, such as beta-carotene, found in orange and yellow fruits and vegetables, and lycopenes, found in tomatoes, appear to protect animals against heart disease and stroke as well as cancer.
The phytosterols, like those found in soybeans, are estrogen like compounds that mimic female hormones. These appear to protect female organs from cancers and also appear to lower cholesterol.
Plant Hormones
Large segments of plant DNA are devoted to coding for specialized plant hormones. Hormones are substances that are produced by one group of cells, circulate to another site, and affect the DNA of the target cells. In plants, these hormones control cell division, growth, and differentiation.
There are five well-described classes of plant hormones: the auxins, gibberellins, cytokinins, ethylene, and abscisic acid. Among the auxins’ functions is allowing phototropism, the property that makes plants bend toward the light.
Produced in the roots, auxins travel to stems,making cells elongate on the dark side of plant tissue. Ethylene is a gaseous substance that ripens fruits and causes them to drop from the plant. Abscisic acid contributes to the aging and falling of leaves.
Genetically Modified Plants
Because plants are easy to manipulate, plant DNA is second only to bacterial DNA as a primary experimental subject for bioengineers. The direct modification of DNA by adding or removing a particular segment of genes that code for specific traits is the focus of bioengineering and biotechnology.
Because plants provide the major food source for human and livestock populations, genetically modified foods have been developed that resist insects, bacteria, viruses, and other pests and decrease the need for external pesticides.
Genetically modified plant crops are designed to enhance a variety of characteristics, from looking and tasting good to growing faster or ripening more slowly to having no seeds.
The introduction of genes from other kingdoms, such as the animal kingdom, into plant DNA is allowing scientists to develop future crops that may contain human vaccines, human hormones, and other pharmaceutical products.
A tomato was the first federally approved bioengineered food to be sold in the United States. Today, dozens of produce items and livestock feed are in some way genetically modified.
