Mutations Used to Determine an Anabolic Pathway Between 1937 and 1941, George W. Beadle and Edward L. Tatum isolated fungal mutants that were unable to synthesize various vitamins and amino acids. They isolated these mutants to try to understand howgenes control specific reactions in various metabolic (synthetic) pathways. The mutants would not grow on a simple medium unless supplied with the nutrient they were unable to synthesize. Beadle and Tatum induced large numbers of mutations in the fungus they were studying and isolated hundreds of mutants. Seven mutants were isolated, each carrying a defective form of a gene involved in the synthesis of the amino acid arginine. Using these mutants, Beadle and Tatum were able to order the chemical reactions by feeding the fungal mutants different metabolic intermediates. If the fungal mutants grew when a particular metabolic intermediate was provided, then the gene mutation affected a step leading to the synthesis of the intermediate. If the mutant did not grow, however, then the gene mutation affected a step that converted the intermediate into arginine. They concluded from their experiments that each gene controlled a different chemical reaction. They confirmed that a specific chemical reaction fails to take place in diploid organisms if both representatives of a given gene were defective. Beadle and Tatum's research strengthened the idea that genes specify enzymes. Their idea became known as the "one gene-one enzyme" hypothesis.
Antlers Antlers are horns which are shed each year and then grow again. In most antlered species only males have antlers. Like true horns, antlers grow out of the bones of the skull. They arise from permanent frontal bone structures called pedicles. At first the antlers have soft, velvetlike coverings of skin over their bone. Instead of hardening, as in true horns, this covering dies off and is rubbed away by the animal. The rubbing is believed to be due to itching caused by the dead velvet. Many zoologists propose that an advantage associated with rubbing off dead velvet is the development of good spatial perception of a stag's antler size and shape that helps to keep him from entanglement in underbrush and forests. Antlers have the same functions as true horns: protection, and use or display to develop ascendancy in a herd. Antlers begin their growth in early summer, on the skull's frontal bone. They begin as small nubs that increase in length over time. The growing antler, sensitive to the touch, is covered with velvet, soft, smooth, and full of blood vessels. This gives it an abundant blood supply, carrying to growing antlers all the nutrients required for the two to three months of the growing period. At first, antlers are made of connective tissue. As time goes on, this tissue calcifies and becomes solid bone. Antler growth is greatest in the late summer.At that time, hard tissue grows around the base of each antler, cutting off its blood supply. This kills the velvet, which loosens and is rubbed off by the stag. Antlers reach their full glory in the mating season, when they can be used to impress potential mates and to vanquish other males. Zoologists observe that the blood running through antler velvet cools down and this cool blood makes stags and bulls more comfortable on hot days. Antlers are shed between January and February, just following the mating season, when they are no longer useful. The first growth of antlers occurs when a stag or bull elk is one to two years old. At first, the antlers are small, straight, and spikelike. At that time young males are often called spikehorns by hunters. As an antlered male grows older its yearly antler crop branches out, having more prongs (points). In old, antlered males these horns mayspread to widths of six feet and be several feet tall. Antler growth is regulated by hormone secretions from the pituitary gland and the testes. The growth begins outside of the breeding season, when the testes are inactive.Whenthe testes begin to make androgens, in preparation for breeding, both antler calcification and velvet tissue death occur. Decreased androgen production in winter leads to shedding of antlers.
