DNA

DNA (deoxyribonucleic acid) is a double-stranded helix of nucleotides which carries the genetic information of a cell. It encodes the information for the proteins and is able self-replicate.

Nucleotides are nitrogen-containing molecules which link together to form strands of DNA and RNA.

RNA (ribonucleic acid) is an information encoded strand of nucleotides, similar to DNA, but with a slightly different chemical structure. There are three main forms of RNA, each a slightly different function. mRNA (messenger RNA) is the mediating template between DNA and proteins. The information from a particular gene is transferred from a strand of DNA by the construction of a complementary strand of RNA through a process known as transcription. Next three nucleotide segments of RNA, called tRNA (transfer RNA), which are attached to specific amino acids, match up with the template strand of mRNA to order the amino acids correctly. These amino acids are then bonded together to form a protein. This process, called translation occurs in the ribosome, which is composed of proteins and the third kind of RNA, rRNA (ribosomal RNA)

mRNA (messenger RNA) is the mediating template between DNA and proteins. The information from a particular gene is transferred from a strand of DNA by the construction of a complementary strand of RNA through a process known as transcription. Next three nucleotide segments of RNA, called tRNA (transfer RNA), which are attached to specific amino acids, match up with the template strand of mRNA to order the amino acids correctly. These amino acids are then bonded together to form a protein. This process, called translation occurs in the ribosome, which is composed of proteins and the third kind of RNA, rRNA (ribosomal RNA)

cDNA are strong, cloned copies of otherwise fragile mRNA - the essential messenger element of the genes in the DNA which help in the coding of proteins.

Recombinant DNA (rDNA) refers to DNA which has been altered by joining genetic material from two different sources. It usually involves putting a gene from one organism into the genome of a different organism, generally of a different species.

The genome of an organism is its set of chromosomes, containing all of its genes and associated DNA.

A chromosome is a grouping of coiled strands of DNA, containing many genes. Most multicellular organisms have several chromosomes, which together comprise the genome. Sexually reproducing organisms have two copies of each chromosome, one from the each parent.

Chromosome, microscopic structure within cells that carries the molecule deoxyribonucleic acid (DNA)—the hereditary material that influences the development and characteristics of each organism. In bacteria and bacteria-like organisms called archaebacteria, chromosomes consist of simple circles of DNA floating freely in the organism. In all other life forms, collectively called eukaryotes, chromosomes are highly complex structures in which the shape of the DNA molecules is linear, rather than circular. In these organisms, chromosomes are found within a well-defined nucleus.

Chromosomes consist chiefly of proteins and DNA. Tiny chemical subunits called nucleotide bases form the structure of DNA. A sequence of bases along a DNA strand that codes for the production of a protein is known as a gene. Genes occupy precise locations on the chromosome.

Each cell contains enough DNA to form a thread extending about 1.5 m (about 5 ft). Proteins called histones play a key role in packaging DNA within chromosomes. Sections of the DNA molecule wind around clusters of histones to form units called nucleosomes, which resemble spools encircled with thread. Another type of protein, called nonhistone chromosomal protein, further compresses nucleosomes into a compact, narrow coil. Chromosomes become most condensed when a cell is preparing to divide.

The chromosome structure ensures that even when the DNA is highly confined it is free to carry out transcription—the production of messenger ribonucleic acid (mRNA), the molecule that determines the types of proteins a cell will produce. In addition, chromosomes permit DNA to replicate, or reproduce itself, so that as a cell divides to produce two cells, each of these will contain all of the necessary genetic information.

Scientists are learning how DNA loosens its connection with histones in order to replicate itself and participate in the synthesis of mRNA. Evidence suggests that enzymes interact with the tails of histones, which protrude from the nucleosomes. These interactions may temporarily disrupt the nucleosome structure so that the DNA is free to interact with the enzymes that help to generate either mRNA or new copies of DNA.

Centromeres and Telomeres

The chromosomes of nearly all eukaryotic life forms contain two important structures—centromeres and telomeres. During cell division, the centromere—visible through a microscope as a knotlike structure—connects to an apparatus called the spindle. The spindle contains fibers that move the centromeres around, causing the rest of each chromosome to follow. This process ensures that each chromosome moves to its proper place during mitosis, when a cell divides to give rise to two cells, and during meiosis, the process of cell division that gives rise to eggs or sperm.

Telomeres are specialized sequences of DNA that are found at the tips of chromosomes. Telomeres serve as a kind of cap that prevents the ends of chromosomes from attaching to the ends of other chromosomes. Scientists suspect that telomeres may influence the activity of nearby genes and may play a role in determining the life span of a cell.

Chromosome Number

In the cells of most organisms that reproduce sexually, chromosomes occur in pairs: One chromosome is inherited from the female parent, and one is inherited from the male parent. The two chromosomes of each pair contain genes that correspond to the same inherited characteristics. Each pair of chromosomes is different from every other pair of chromosomes in the same cell.

The number of chromosome pairs in an organism varies depending on the species. The number of chromosomes characteristic of a particular organism is known as the diploid number. Dogs, for example, have 38 pairs of chromosomes and a diploid number of 76, while tomato plants have 12 pairs of chromosomes and a diploid number of 24.

Sex cells (eggs or sperm) contain only half the number of chromosomes found in the other cells of an organism. This reduced number of chromosomes in the sex cells is known as the haploid number. During fertilization, an egg and sperm unite to form a cell known as a zygote, the first cell of the offspring. The zygote contains the diploid number of chromosomes characteristic of the species.

Most organisms have complete sets of matching chromosomal pairs, known as autosomes. In mammals, birds, and some other organisms, one pair of chromosomes is not identical. Known as the sex chromosomes, this pair plays a dominant role in determining the sex of an organism. Females have two copies of the X chromosome, while males have one Y chromosome and one X chromosome. Both males and females inherit one sex chromosome from the mother (always an X chromosome) and one sex chromosome from the father (an X in female offspring and a Y in male offspring). The presence of the Y chromosome determines that a zygote will develop into a male.

The Y chromosome is about one-third the size of the X chromosome and contains only a fraction of the number of genes. At one point in evolutionary history, the X and Y chromosomes were equal in size and gene number, but the two chromosomes gradually diverged over the course of 300 million years. These unmatched sex chromosomes produce a pattern of gene inheritance known as sex-linked inheritance, which differs from genes found on autosomes. In males, which carry an X and a Y chromosome, some genes found on the X chromosome may be missing on the Y chromosome. As a result, the organism will usually develop the trait associated with the gene on the X chromosome. In fruit flies, for instance, the gene for eye color is located on the X chromosome. A male fruit fly will inherit the eye color found on the X chromosome, since no gene for eye color is found on the Y chromosome.

Human Chromosomes

Humans have 23 pairs of chromosomes, with a diploid number of 46. Scientists number these chromosome pairs according to their size—the largest is chromosome 1 and the smallest is chromosome 23. In human chromosomes, errors may occur that give rise to embryos with more or less genetic material, sometimes resulting in mental retardation or health problems. In a process called nondisjunction, paired members of chromosomes fail to separate from one another during meiosis. Nondisjunction can lead to a condition known as Down syndrome, in which a person has three copies of a small chromosome designated as chromosome 21. Another condition that may result from nondisjunction is Turner syndrome, a disorder in which a female has only a single X chromosome. Genetic errors occur if part of a chromosome is either missing or duplicated. Chromosomes sometimes undergo changes called translocations, in which part of one chromosome breaks off and attaches to another chromosome. A translocation involving chromosomes 9 and 22 is linked to a type of leukemia called chronic myelogenic leukemia. On the sex chromosomes, problems arise in men when an abnormal gene is present on the X chromosome. With no healthy gene found on the Y chromosome to override the abnormal gene, disease may result. Men who inherit a mutated gene that causes hemophilia from their mother on the X chromosome will develop this bleeding disorder since they are missing a normal version of the gene on their Y chromosome.

Scientists called cytogeneticists look at a person's chromosomes in the laboratory to determine whether the individual has the usual number of chromosomes and whether these chromosomes have missing or extra segments. To examine chromosomes, cytogeneticists grow samples of a person's blood cells in the laboratory and expose the cells to a chemical called colchicine, which disrupts the spindle apparatus that is normally present in dividing cells. This disruption immobilizes the chromosomes during cell division, when they are most condensed and visible. Chromosomes are then stained with various dyes, which produce a pattern of vertical bands. Cytogeneticists take photographs of the banded chromosomes through a microscope to create images called karyotypes, in which the members of each chromosome pair are arranged next to each other for easy comparison. The analysis of karyotypes reveals whether a person has extra or missing chromosomes, as well as whether large segments of chromosomes are absent, rearranged, or duplicated.

Experiments involving artificial chromosomes—chromosomes that are synthesized in the laboratory—are providing new insights into the structure and function of chromosomes. The first artificial chromosomes, produced in the 1980s, were chromosomes of yeast cells. The first artificial human chromosomes were created in 1997.

Identifying the disease-causing genes associated with chromosomes will help researchers devise new diagnostic tools to determine a person's risk for disease as well as new therapies to replace or repair faulty genes. As part of the Human Genome Project, an international collaboration of scientists to decode all of the genes in the human body, researchers successfully decoded chromosome 22 in 1999. This chromosome is the second smallest of the 23 chromosome pairs in humans. More than half of the genes identified on chromosome 22 were previously unknown in humans.

Protein, any of a large number of organic compounds that make up living organisms and are essential to their functioning. First discovered in 1838, proteins are now recognized as the predominant ingredients of cells, making up more than 50 percent of the dry weight of animals. The word protein is coined from the Greek proteios, or "primary."

Protein molecules range from the long, insoluble fibers that make up connective tissue and hair to the compact, soluble globules that can pass through cell membranes and set off metabolic reactions. They are all large molecules, ranging in molecular weight from a few thousand to more than a million, and they are specific for each species and for each organ of each species. Humans have an estimated 30,000 different proteins, of which only about 2 percent have been adequately described. Proteins in the diet serve primarily to build and maintain cells, but their chemical breakdown also provides energy, yielding close to the same 4 calories per gram as do carbohydrates.

Besides their function in growth and cell maintenance, proteins are also responsible for muscle contraction. The digestive enzymes are proteins, as are insulin and most other hormones. The antibodies of the immune system are proteins, and proteins such as hemoglobin carry vital substances throughout the body.

Nutrition

Whether found in humans or in single-celled bacteria, proteins are composed of units of about 20 different amino acids, which, in turn, are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. In a protein molecule these acids form peptide bonds—bonds between amino and carboxyl (COOH) groups—in long strands (polypeptide chains). The almost numberless combinations in which the acids line up, and the helical and globular shapes into which the strands coil, help to explain the great diversity of tasks that proteins perform in living matter.

To synthesize its life-essential proteins, each species needs given proportions of the 20 main amino acids. Although plants can manufacture all their amino acids from nitrogen, carbon dioxide, and other chemicals through photosynthesis, most other organisms can manufacture only some of them. The remaining ones, called essential amino acids, must be derived from food. Eight essential amino acids are needed to maintain health in humans: leucine, isoleucine, lysine, methionine, phenylalanine, theonine, tryptophan, and valine. All of these are available in proteins produced in the seeds of plants, but because plant sources are often weak in lysine and tryptophan, nutrition experts advise supplementing the diet with animal protein from meat, eggs, and milk, which contain all the essential acids.

Most diets—especially in the United States, where animal protein is eaten to excess—contain all the essential amino acids. (Kwashiorkor, a wasting disease among children in tropical Africa, is due to an amino acid deficiency.) For adults, the Recommended Dietary Allowance (RDA) for protein is 0.79 g per kg (0.36 g per lb) of body weight each day. For children and infants this RDA is doubled and tripled, respectively, because of their rapid growth.

Structure of Proteins

The most basic level of protein structure, called the primary structure, is the linear sequence of amino acids. Different sequences of the acids along a chain, however, affect the structure of a protein molecule in different ways. Forces such as hydrogen bonds, disulfide bridges, attractions between positive and negative charges, and hydrophobic ("water-fearing") and hydrophilic ("water-loving") linkages cause a protein molecule to coil or fold into a secondary structure, examples of which are the so-called alpha helix and the beta pleated sheet. When forces cause the molecule to become even more compact, as in globular proteins, a tertiary protein structure is formed. When a protein is made up of more than one polypeptide chain, as in hemoglobin and some enzymes, it is said to have a quaternary structure.

Interaction with Other Proteins

Polypeptide chains are sequenced and coiled in such a way that the hydrophobic amino acids usually face inward, giving the molecule stability, and the hydrophilic amino acids face outward, where they are free to interact with other compounds and especially other proteins. Globular proteins, in particular, can join with a specific compound such as a vitamin derivative and form a coenzyme, or join with a specific protein and form an assembly of proteins needed for cell chemistry or structure.

Fibrous Proteins

The major fibrous proteins, described below, are collagen, keratin, fibrinogen, and muscle proteins.

Collagen

Collagen, which makes up bone, skin, tendons, and cartilage, is the most abundant protein found in vertebrates. The molecule usually contains three very long polypeptide chains, each with about 1000 amino acids, that twist into a regularly repeating triple helix and give tendons and skin their great tensile strength. When long collagen fibrils are denatured by boiling, their chains are shortened to form gelatin.

Keratin

Keratin, which makes up the outermost layer of skin and the hair, scales, hooves, nails, and feathers of animals, twists into a regularly repeating coil called an alpha helix. Serving to protect the body against the environment, keratin is completely insoluble in water. Its many disulfide bonds make it an extremely stable protein, able to resist the action of proteolytic (protein-hydrolyzing) enzymes. In beauty treatments, human hair is set under a reducing agent, such as thioglycol, to reduce the number of disulfide bonds, which are then restored when the hair is exposed to oxygen.

Fibrinogen

Fibrinogen is a blood plasma protein responsible for blood clotting. With the catalytic action of thrombin, fibrinogen is converted into molecules of the insoluble protein fibrin, which link together to form clots.

Muscle Proteins

Myosin, the protein chiefly responsible for muscle contraction, combines with actin, another muscle protein, forming actomyosin, the different filaments of which shorten, causing the contracting action.

Amino Acids

Amino Acids, important class of organic compounds that contain both the amino (__NH₂) and carboxyl (__COOH) groups. Of these acids, 20 serve as the building blocks of proteins. Known as the standard, or alpha, amino acids, they comprise alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. All 20 are constructed according to a general formula:

As the formula shows, the amino and carboxyl groups are both attached to a single carbon atom, which is called the alpha carbon atom. Attached to the carbon atom is a variable group (R); it is in their R groups that the molecules of the 20 standard amino acids differ from one another. In the simplest of the acids, glycine, the R consists of a single hydrogen atom. Other amino acids have more complex R groups that contain carbon as well as hydrogen and may include oxygen, nitrogen, or sulfur, as well.

When a living cell makes protein, the carboxyl group of one amino acid is linked to the amino group of another to form a peptide bond. The carboxyl group of the second amino acid is similarly linked to the amino group of a third, and so on, until a long chain is produced. This chainlike molecule, which may contain from 50 to several hundred amino acid subunits, is called a polypeptide. A protein may be formed of a single polypeptide chain, or it may consist of several such chains held together by weak molecular bonds. Each protein is formed according to a precise set of instructions contained within the nucleic acid, which is the genetic material of the cell. These instructions determine which of the 20 standard amino acids are to be incorporated into the protein, and in what sequence. The R groups of the amino acid subunits determine the final shape of the protein and its chemical properties; an extraordinary variety of proteins can be produced from the same 20 subunits.

The standard amino acids serve as raw materials for the manufacture of many other cellular products, including hormones and pigments. In addition, several of these amino acids are key intermediates in cellular metabolism.

Most plants and microorganisms are able to use inorganic compounds to make all the amino acids they require for normal growth. Animals, however, must obtain some of the standard amino acids from their diet in order to survive; these particular amino acids are called essential. Essential amino acids for humans include lysine, tryptophan, valine, histidine, leucine, isoleucine, phenylalanine, threonine, methionine, and arginine. They are found in adequate amounts in protein-rich foods from animal sources or in carefully chosen combinations of plant proteins.

In addition to the amino acids that form proteins, more than 150 other amino acids have been found in nature, including some that have the carboxyl and amino groups attached to separate carbon atoms. These unusually structured amino acids are most often found in fungi and higher plants.