Genetics Midterm 3

the alpha chain of eukaryotic hemoglobin is composed of 141 amino acids. What is the minimum number of nucleotides in an mRNA coding for this polypeptide chain?

423

List and describe the role of all the molecular constituents of a functional polyribosome

A functional polyribosome will contain the following components: mRNA, charged tRNA, large and small ribosomal subunits, elongation and perhaps initiation factors, peptide transferase, GTP MG++, nascent proteins, and possibly GTP dependent release factors

Describe the experimental rational that allowed the lac repressor to be isolated.

A single E coli cell contains very few molecules of the lac repressor. however, the lac Iq mutation causes a 10x increase in repressor protein production, thus facilitating its isolation. With the use of dialysis against a radioactive gratuitous inducer, Gilbert and Muller Hill were able to identify the repressor protein in certain extracts of lac Iq cells. Te material that bound the labeled IPTG was purified and shown to be heat labile and have other characteristics of protein. Extracts of lac I- cells did not bind the labeled IPTG.

Summarize the steps involved in charging tRNAs with their appropriate amino acids.

An amino acid in the presence of ATP, Mg++, and a specific ammoniacal syntheses produces an amino acid-AMPenzyme complex (+PPi). This complex interacts with a specific tRNA to produce the aminoacyl tRNA.

Describe the role of attenuation in the regulation of trypotphan biosynthesis

Attenuation functions to reduce the synthesis of tryptophan when it is in full supply. It does so by reducing transcription of the tryptophan operon. The same phenomenon is observed when tryptophan activates the repressor to shut off transcription of the tryptophan operon.

Predict the effect on the indelibility of the lac operon of a mutation that disrupts the function of the crp gene, which encodes the CAP protein, and the CAP binding site within the promoter

Because the activated CAP is a component of the cooperative binding of RNA polymerase to the lac promoter, absence of a functional crp would compromise the positive control exhibited by CAP Without a CAP binding site there would be a recession in the indelibility of the lac operon.

Phenylketonurics cannot convert phenylalanine to tyrosine Why don't these individuals exhibit a deficiency of tyrosine?

Both phenylalanine and tyrosine can be obtained from the diet. most natural proteins contain these amino acids.

Describe what collinearity means. Of what significance is the concept of collinearity in the study of genetics?

Colinearity refers to the sequential arrangement of subunits, amino acids, and nitrogenous bases in proteins and DNA, respectively. Sequencing of genes and products in MS2 phages and studies on mutations in the A subunit of the tryptophan synthetase gene indicate a collinear relationship.

How does an enzyme function? why are enzymes essential for living organisms on earth?

Enzymes fuction to regulate catabolic and anabolic activities of cells. They influence (lower) the energy of activation, thus allowing chemical reactions to occur under conditions that are compatible with living systems. Enzymes possess active sites and/or other domains that are sensitive to the environment. The active site is considered to be a crevice, or pit, which bins reactants, thus enhancing their interaction. The other domains mentioned above may influence the conformation and, therefore, the function of the active site.

Contrast the role of the repressor in an inducible system and in a repressible system.

In an inducible system, the repressor that normally interacts with the operator to inhibit transcription is inactivated by an inducer, thus permitting transcription, In a repressible system, a normally inactive repressor is activated by a co repressor, thus enabling it (the activated repressor) to bind to the operator to inhibit transcription. Because the interaction of the protein (repressor) has a negative influence on transcription, the systems described here are forms of negative control.

What are isoaccepting tRNAs? Assuming that there are only 20 different ammoniacal tRNA syntheses but 31 different tRNAs, speculate on parameters that might be used to ensure that each charged tRNA has received the correct amino acid.

Isoaccepting tRNAs are those tRNAs that recognize and accept only one type of amino acid. In some way, each of the 20 different ammoniacal tRNA synteases must be able to recognize either the base composition and/or tertiary structure of each of the isoaccepting tRNA species. Otherwise, the fidelity of translation would be severely compromised. The most direct solution to the problem would be to have each synthase recognize each anticodon. Another reasonable consideration might involve the variable up, which, in conjunction with the anticodon, might enable such specificity. In reality, several characteristics of each tRNA are involved: one or more of the anticodon bases, portions of the acceptor rm, and a particular base that lies near the CCA terminus.

Hemoglobins from two individuals are compared by electrophoresis and by fingerprinting. Electrophoresis reveals no difference in migration, but fingerprinting shows an amino acid difference. How is this possible?

It is possible for an amino acid to change without changing the electrophoretic mobility of a protein under standard conditions. If the amino acid is substituted with an amino acid of like charge and similar structure, there is a chance that factors that influence electrophoretic mobility (primarily net charge) will not be altered. Other techniques such as chromatography of digested peptides may detect subtle amino acid differences.

Francis Crick proposed the "adaptor hypothesis" for the function of tRNA. Why did he choose that description?

It was reasoned that there would not be sufficient affinity between amino acids and nucleic acids to account for protein synthesis. For example, acidic amino acids would not be attracted to nucleic acids. With an adaptor molecule, specific H bonding could occur between nucleic acids, and specific covalent bonding could occur between an amino acid and a nucleic acid tRNA.

Define and compare the four levels of protein organization.

Primary: The linear arrangement or sequence of amino acids. This sequence determine the higher level structures. Secondary: alpha helix and beta pleated sheet structures generated by hydrogen bonds between components of the peptide bond. Tertiary: Folding that occurs as a result of interactions between the amino acid side chains. These interactions include, but are not limited to, the following: covalent disulfide bonds between cysteine residues, interactions of hydrophilic side coins with water, and interactions of hydrophobic side chains with each other. quaternary: the association of two (dimer) or more polypeptide chains. Called oligomeric, such a protein is made up of more than one protein chain.

Using sickle cell anemia as an example, describe what is meant by a molecular or genetic disease. What are the similarities and dissimilarities between this type of a disorder and a disease caused by an invading microorganism?

Sickle cell anemia is a molecular disease because it is well understood at the molecular level; there is a base change in DNA, which leads to an amino acid change in the beta chain of hemoglobin It is a genetic disease in that it is inherited from one generation to the next. It is not contagious as might be the case of a disease caused y a microorganism. Diseases caused by microorganisms may not necessarily follow family blood lines, whereas genetic disease do.

Why is an alteration of electrophoretic mobility interpreted as a change in the primary structure of the protein under study?

The electrophoretic mobility of a protein is based on a variety of factors, primarily the net charge of the protein and to some extent, the conformation in the electrophoretic environment. Both are based on the type and sequence (primary structure) of the component amino acids of a protein. The interactions (hydrogen bonds) of the components of the peptide bonds, hydrophobic, hydrophilic, and covalent interactions (as well as others) are all dependent on the original sequence of amino acids and take part in determining the final conformation of a protein. A change in the electrophoretic mobility of a protein would therefore indicate that the amino acid sequence has been changed.

Contrast the need for the enzymes involved in lactose and tryptophan metabolism in bacteria when lactose and tryptophan, respectively, are absent and present.

The enzymes of the lactose operon are needed to break down and use lactose as an energy source. If lactose is the sole carbon source, the enzymes are synthesized to use that carbon source. with no lactose present, there is no "need" for the enzymes. The tryptophan operon contains structural genes for the synthesis of tryptophan. If there is little or no tryptophan in the medium, the tryptophan operon is turned on to manufacture tryptophan. If tryptophan is abundant in the medium, then there is no "need" for the operon to be manufacturing tryptophan syntheses.

Early detection and adherence to a strict dietary regime has prevented much of the mental retardation that used to occur in those afflicted with PKU. Predict the effect that mothers with PKU who neglect their diets might have on newborns

When an expectant mother returns to consumption of phenylalanine in her diet, she subjects her baby to higher than normal levels of phenylalanine throughout its development. Since increased phenyl aline is toxic, many (~90%) newborns are severely and irreversibly retarded at birth. Expectant mothers (who are genetically phenylketonurics) should return to a low phenylalanine intake during pregnancy.

Assume that the structural genes of the lac operon have been fused, through recombinant DNA techniques, to the regulatory apparatus of the area operon. If arabinose is provided in a minimal medium to E. coli carrying this gene fusion, would you expect beta galactosidase to be produced at induced levels? Explain.

When arabinose is present in the medium, the structural genes for the arabinose operon are transcribed. If the structural genes for the lac operon replaced the structural genes for the area operon, then in the presence of arabinose, the lac structural genes would be transcribed and beta galactosidase would be produced at induced levels.