Genes are segments of deoxyribonucleic acid DNA that contain the code for a specific protein that functions in one or more types of cells in the body. Genes are contained in chromosomes, which are mainly in the cell nucleus. A chromosome contains hundreds to thousands of genes.
The frequencies of recombination are approximately proportional more Genes and Enzymes Early genetic studies focused on the identification and chromosomal localization of genes that control readily observable characteristics, such as the eye color of Drosophila.
How these genes lead to the observed phenotypes, however, was unclear. The first insight into the relationship between genes and enzymes came inwhen it was realized that the inherited human disease phenylketonuria see Molecular Medicine in Chapter 2 results from a genetic defect in metabolism of the amino acid phenylalanine.
This defect was hypothesized to result from a deficiency in the enzyme needed to catalyze the relevant metabolic reaction, leading to the general suggestion that genes specify the synthesis of enzymes.
Clearer evidence linking genes with the synthesis of enzymes came from experiments of George Beadle and Edward Tatum, performed in with the fungus Neurospora crassa. In the laboratory, Neurospora can be grown on minimal or rich media similar to those discussed in Chapter 1 for the growth of E.
For Neurospora, minimal media consist only of salts, glucose, and biotin; rich media are supplemented with amino acids, vitamins, purines, and pyrimidines. Beadle and Tatum isolated mutants of Neurospora that grew normally on rich media but could not grow on minimal media.
Each mutant was found to require a specific nutritional supplement, such as a particular amino acidfor growth. Furthermore, the requirement for a specific nutritional supplement correlated with the failure of the mutant to synthesize that particular compound. Thus, each mutation resulted in a deficiency in a specific metabolic pathway.
Since such metabolic pathways were known to be governed by enzymes, the conclusion from these experiments was that each gene specified the structure of a single enzyme—the one gene-one enzyme hypothesis.
Many en-zymes are now known to consist of multiple polypeptides, so the currently accepted statement of this hypothesis is that each gene specifies the structure of a single polypeptide chain. Identification of DNA as the Genetic Material Understanding the chromosomal basis of heredity and the relationship between genes and enzymes did not in itself provide a molecular explanation of the gene.
Chromosomes contain proteins as well as DNAand it was initially thought that genes were proteins. The first evidence leading to the identification of DNA as the genetic material came from studies in bacteria.
These experiments represent a prototype for current approaches to defining the function of genes by introducing new DNA sequences into cells, as discussed later in this chapter.
The experiments that defined the role of DNA were derived from studies of the bacterium that causes pneumonia Pneumococcus. Virulent strains of Pneumococcus are surrounded by a polysaccharide capsule that protects the bacteria from attack by the immune system of the host.
Because the capsule gives bacterial colonies a smooth appearance in culture, encapsulated strains are denoted S.
Mutant strains that have lost the ability to make a capsule denoted R form rough-edged colonies in culture and are no longer lethal when inoculated into mice. In it was observed that mice inoculated with nonencapsulated R bacteria plus heat-killed encapsulated S bacteria developed pneumonia and died.
Importantly, the bacteria that were then isolated from these mice were of the S type. Subsequent experiments showed that a cell-free extract of S bacteria was similarly capable of converting or transforming R bacteria to the S state. Thus, a substance in the S extract called the transforming principle was responsible for inducing the genetic transformation of R to S bacteria.
In Oswald Avery, Colin MacLeod, and Maclyn McCarty established that the transforming principle was DNAboth by purifying it from bacterial extracts and by demonstrating that the activity of the transforming principle is abolished by enzymatic digestion of DNA but not by digestion of proteins Figure 3.
Although these studies did not immediately lead to the acceptance of DNA as the genetic material, they were extended within a few years by experiments with bacterial viruses. In particular, it was shown that, when a bacterial virus infects a cell, the viral DNA rather than the viral protein must enter the cell in order for the virus to replicate.
Moreover, the parental viral DNA but not the protein is transmitted to progeny virus particles. The concurrence of these results with continuing studies of the activity of DNA in bacterial transformation led to acceptance of the idea that DNA is the genetic material.
DNA is extracted from a pathogenic strain of Pneumococcus, which is surrounded by a capsule and forms smooth colonies S. Given the central role of DNA as the genetic material, elucidation of its three-dimensional structure appeared critical to understanding its function.
Analysis of these data revealed that the DNA molecule is a helix that turns every 3.
In addition, the data showed that the distance between adjacent bases is 0. An important finding was that the diameter of the helix is approximately 2 nm, suggesting that it is composed of not one but two DNA chains. The central features of the model are that DNA is a double helix with the sugar-phosphate backbones on the outside of the molecule.
The bases are on the inside, oriented such that hydrogen bonds are formed between purines and pyrimidines on opposite chains. The base pairing is very specific: A always pairs with T and G with C.Choosing Traits b. Selecting for Disability c. “Cadillac” Genome Sequencing d.
Ancestry Testing on chromosomes. Genetic testing looks for variations among genes that are associated with disease or of genetic diseases like cystic fibrosis or Hun-tington’s. If either parent carries the mutation. In the past three decades, scientists have had immense success in identifying genes and their variants that contribute to an array of diseases.
While the identification of such ge. In a groundbreaking historical genetics paper reported results which indicated that a substantial proportion of men in the world are direct line descendants of Genghis Khan.
Study 52 BIOL - Chpt 10 - Patterns of Genetic Inheritance flashcards from Jason S. on StudyBlue.
Role of Chromosomes & Genes in Heredity Heredity plays a key role in the development of traits and is transmitted through genes. The traits we inherit help shape our behavior, are determined by pairs.
The role of chromosomes in heredity. Thomas Hunt Morgan was sceptical of the way that biological traits discover the physiological basis for inheritance.