Genetics Exam 2 Assigned Practice Problems

In this chapter, we focused on how gene mutations arise and how cells repair RNA damage. At the same time, we found opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter: (a) How do we know that mutations occur spontaneously? (b) How do we know that certain chemicals and wavelengths of radiation induce mutations in DNA? (c) How do we know that DNA repair mechanisms detect and correct the majority of spontaneous and induced mutations?

(a) 1. Fluctuation test 2. Replica plating test (b) agents generating mutation-inducing DNA damage ex) radiation energy in wavelengths shorter than 380 nm (c) light repair, excision repair, mismatch repair; Ames test

Predict the effect on the inducibility of the lac operon of a mutation that disrupts the function of (a) the crp gene, which encodes the CAP protein, and (b) the CAP-binding site within the promoter.

(a) Because 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. (b) Without a CAP binding site there would be a reduction in the inducibility of the lac operon.

Even though the lac Z, Y, and A structural genes are transcribed as a single polycistronic mRNA, each gene contains the initiation and termination signals essential for translation. Predict what will happen when a cell growing in the presence of lactose contains a deletion of one nucleotide (a) early in the Z gene and (b) early in the A gene.

(a) Due to the deletion of a base early in the lac Z gene, there will be "frameshift" of all the reading frames downstream from the deletion, thereby altering many amino acids. It is likely that either premature chain termination of translation will occur or the normal chain termination will be ignored. Regardless, a mutant condition for the Z gene will be likely. If such a cell is placed on a lactose medium, it will be incapable of growth because beta-galactosidase is not available. (b) If the deletion occurs early in the A gene, one might expect impaired function of the A gene product, but it will not influence the use of lactose as a carbon source.

In this chapter, we focused on the translation of mRNA into proteins as well as on protein structure and function. Along the way, we found many opportunities to consider the methods and reasoning by which many of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions: (a) What experimentally derived information led to Holley's proposal of the two-dimensional cloverleaf model of tRNA? (b) What experimental information verifies that certain codons in mRNA specify chain termination during translation? (c)How do we know, based on studies of Neurospora nutritional mutations, that one gene specifies one enzyme? (d) On what basis have we concluded that proteins are the end products of genetic expression? (e) What experimental information directly confirms that the genetic code, as shown in Figure 13-7, is correct? (f) How do we know that the structure of a protein is intimately related to the function of the protein?

(a) The base sequences in tRNA suggested a cloverleaf secondary structure due to within-strand complementary base pairing. Such a model was later supported by X-ray crystallography and denaturation studies. (b) When UAA, UGA, or UAG triplets occur at internal sites in genes, premature translation termination occurs and verifies the chain terminating function of these triplets. (c) Examination of nutritional mutations in Neurospora showed that upsets in metabolic pathways could result from mutant genes that segregated and assorted in typical fashion. In some cases, sufficient information was available to show that defective enzymes caused the metabolic upset. Thus, mutant genes must be responsible for the production of defective enzymes. (d) Because enzymes are proteins, it is reasonable to conclude that genes make proteins. In addition, early work on hemoglobin showed that one gene was responsible for making one of the polypeptide chains in hemoglobin. (e) Degeneracy and the Wobble Hypothesis by Francis Crick (f) because the function cannot ensue without the structure constituting the functional domain based on its make up

One of the most prevalent sexually transmitted diseases is caused by the bacterium Chlamydia trachomatis and leads to blindness if left untreated. Upon infection, metabolically inert cells differentiate, through gene expression, to become metabolically active cells that divide by binary fission. It has been proposed that release from the inert state is dependent on heat-shock proteins that both activate the reproductive cycle and facilitate the binding of chlamydiae to host cells. Researchers made the following observations regarding the heat-shock regulator system in Chlamydia trachomatis: (1) a regulator protein binds to a cis-acting DNA element; (2) R and D function as a repressor-operator pair; (3) R functions as a negative regulator of transcription; (4) D is composed of an inverted-repeat sequence; (5) repression by R is dependent on D being supercoiled. (a) Based on this information, devise a model to explain the heat-dependent regulation of metabolism in Chlamydia trachomatis. (b) Some bacteria, like E. coli, use a heat-shock sigma factor to regulate heat-shock transcription. Are the above findings in Chlamydia compatible with use of a heat-sensitive sigma factor?

(a) The simplest model for the action of R and D in Chlamydia would be to have the repressor element become ineffective in binding the cis-acting element when heat-shocked. This could happen in two ways. Either the supercoiled DNA alters its conformation and becomes ineffective at binding R, or the D-binding efficiency of the R protein is altered by heat. In either case, the genes for infectivity are transcribed in the presence of the heat shock. (b) The most straightforward comparison between the heat shock R and D system in Chlamydia and the heat-shock sigma factor in E. coli would be where the R-D system is inactivated in Chlamydia and a sigma factor is activated by heat in E. coli.

Predict the level of genetic activity of the lac operon as well as the status of the lac repressor and the CAP protein under the cellular conditions listed in the accompanying table.

(a) With no lactose and no glucose, the operon is off because the lac repressor is bound to the operator and although CAP is bound to its binding site, it ill not override the action of the repressor. (b) With lactose added to the medium, the lac repressor is inactivated and the operon is transcribing the structural genes. With no glucose, the CAP is bound to its binding site, thus enhancing transcription. (c) With no lactose present in the medium, the lac repressor is bound to the operator region, and since glucose inhibits adenyl cyclase, the CAP protein will not interact with its binding site. The operon is therefore "off." (d) With lactose present, the lac repressor is inactivated; however, since glucose is also present, CAP will not interact with its binding site. Under this condition, transcription is severely diminished, and the operon can be considered to be "off."

In this chapter, we focused on how eukaryotic genes are regulated at different steps in their expression, from chromatin modifications to control of protein stability. At the same time, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter: (a) How do we know that promoter and enhancer sequences control the initiation of transcription in eukaryotes? (b) How do we know that eukaryotic transcription factors bind to DNA sequences at or near promoter regions? (c) How do we know that double-stranded RNA molecules can control gene expression?

(a) because without them transcription could not occur (b) because it has been tested (c) because they are more likely to change from wild-type to mutant and vice versa

A variety of neutral and muscular disorders are associated with expansions of trinucleotide repeat sequences. The table below lists several disorders, the repeat motifs, their locations, the normal number of repeats, and the number of repeats in the full mutations. (a) Most disorders attributable to trinucleotide repeats result from expansion of the repeats. Two mechanisms are other proposed to explain repeat expansions: (1) unequal synapsis and crossing over and (2) errors in DNA replication where single-stranded, base-paired loops are formed that conflict with linear replication. Present a simple sketch of each mechanism. (b) Notice that some of the repeats occur in areas of the gene that are not translated. How can a mutation occur if the alteration is not reflected in an altered amino acid sequence? (c) In the two cases where the repeat expansions occur in exons, the extent of expansion is considerably less than when the expansion occurs outside exons. Present an explanation for the observation.

(a) part of image (b) the mutation is that they are not translated (c) because introns are not spliced, which is a regulation activity in riding the RNA of potential mutations

In this chapter, we focused on the regulation of gene expression in prokaryotes. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) How do we know that bacteria regulate the expression of certain genes in response to the environment? (b) What evidence established that lactose serves as the inducer of a gene whose product is related to lactose metabolism? (c) What led researchers to conclude that a repressor molecule regulates the lac operon? (d) How do we know that the lac repressor is protein? (e) How do we know that the trp operon is a repressible control system, in contrast to the lac operon, which is an inducible control system?

(a) they produce certain enzymes only when specific chemical substrates are present in order to adapt to the environment their subjected to (b) -Monad 1946 -when lactose added to medium - E. coli respond with production of enzymes involved in lactose metabolism -when lactose absent from medium - the enzyme concentration decreased (c) a mutant of E coli produced large amount of a repressor, where Gibert showed that the repressor binds to DNA containing the operator portion of the lac operon (d) -IPTG which is a radioactive sulfur analog of lactose, binds to lac repressor -I negative did not show binding -determined protein from binding of IPTG (a radioactive sulfur) (e) -lactose is inducible because its presence causes the synthesis of lac-utilizing structural protein -trp is repressible because it's presence represses the transcription of proteins for the synthesis of trp

For the lac genotypes shown in the accompanying table, predict whether the structural genes (Z) are constitutive, permanently repressed, or inducible in the presence of lactose.

*see powerpoint 16&17 notes

Exon shuffling is a proposal that relates exons in DNA to the repositioning of functional domains in proteins. What evidence exists in support of exon shuffling? Two schools of thought have emerged concerning the origin of exons, "intron-early" and "intron-late." Briefly describe both theories and present support for each.

-Ford Doolittle proposed that these intervening sequences were part of the genome of the most primitive ancestors of modern-day eukaryotes -"Intron-early" argue that if similarities in intron DNA sequences are found in identical positions within genes shared by distantly related eukaryotes, then they must have also been present in primitive ancestral genomes -Jeffrey Palmer (intron-late) argue that introns first appeared much later in evolution, when they were acquired by a single group of eukaryotes that are ancestral to modern-day eukaryotes but not prokaryotes.

List as many different categories of protein functions as you can. Wherever possible, give an example of each category.

-building muscle and tissues -enzymes -hormone production -energy -immune function

The emergence of antibiotic-resistant strains of Enterococci and transfer of resistant genes to other bacterial pathogens have highlighted the need for new generations of antibiotics to combat serious infections. To grasp the range of potential sites for the action of existing antibiotics, sketch the components of the translation machinery and using a series of numbered pointers, indicate the specific location for the action of the antibiotics shown.

-observe where they affect the parts in initiation of translation

Contrast positive versus negative control gene expression.

-positive: regulatory molecule enhances transcription; requires a molecule to be added for DNA transcription to occur -negative: regulatory molecule interferes with transcription, lactose and tryptophan examples; requires molecule to be removed from DNA for transcription to occur

Assuming that each nucleotide is 0.34 nm long in the mRNA, how many triplet codes can occupy at one time the space in a ribosome that is 20 nm in diameter?

20/ 0.34 = apx. 59 (number of nucleotides) 59/ 3 = apx. 20 (number of triplet codes)

Contrast and compare the mutagenic effects of deaminating agents, alkylating agents, and base analogs.

All three of the agents are mutagenic because they cause base substitutions. Deaminating agents oxidatively deaminate bases such that cytosine is converted to uracil and adenine is converted to hypoxanthine. Uracil pairs with adenine, and hypoxanthine pairs with cytosine. Alkylating agents donate an alkyl group to the amino or keto groups of nucleotides, thus altering base-pairing affinities. 6-ethyl guanine acts like adenine, thereby pairing with thymine. Base analogs such as 5-bromouracil and 2-amino purine are incorporated as thymine and adenine, respectively, yet they base-pair with guanine and cytosine, respectively.

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

An amino acid in the presence of ATP, Mg2+, and a specific aminoacyl synthetase produces an amino acid-AMP enzyme complex. This complex interacts with a specific tRNA to produce the aminoacyl tRNA.

Review the Chapter Concepts list on page 417. These concepts relate to the many different regulatory steps that contribute to gene expression in eukaryotic organisms. The fifth concept summarizes several of the steps in posttranscriptional regulation. Write a short essay describing how the recent data emerging from the ENCODE project influence our views of the relative importance of transcriptional and posttranscriptional gene regulation in humans.

An approximation of the percentage of regulatory elements/transcription products in the human genome, the binding sites of regulatory proteins and the general architecture or regulatory elements within chromatin.

The cancer drug melphalan is an alkylating agent of the mustard gas family. It acts in two ways: by causing alkylation of guanine bases and by cross linking DNA strands together. Describe two ways in which melphalan might kill cancer cells. What are two ways in which cancer cells could repair the DNA-damaging effects of melphalan?

Any agent that inhibits DNA replication, either directly or indirectly, through mutation and/or DNA cross linking, will suppress the cell cycle and may be useful in cancer therapy. Since guanine alkylation often leads to mismatched bases, they can often be repaired by a variety of mismatched repair mechanisms. However, DNA cross linking can be repaired by recombinational mechanisms. Thus, for such agents to be successful in cancer therapy, suppressors of DNA repair systems are often used in conjunction with certain cancer drugs.

Describe the role of attenuation in the regulation of tryptophan biosynthesis.

Attenuation functions to reduce the synthesis of tryptophan when it is in full supply. It does do 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.

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.

Cancer cells often have abnormal patterns of chromatin modifications. In some cancers, the DNA repair genes MLH1 and BRCA1 are hypermethylated on their promoter regions. Explain how this abnormal methylation pattern could contribute to cancer.

Cancer cells often originate under the influence of mutations in tumor-suppressor genes or proto-oncogenes. Should hyper-methylation occur in one of many DNA repair genes, the frequency of mutation would increase because the DNA repair system is compromised. The resulting increase in mutation might occur in tumor-suppressor genes or proto-oncogenes.

Provide a definition of chromatin remodeling, and give two examples of the phenomenon.

Chromatin is remodeled when there are significant changes in chromatin organization. Such remodeling involved changes in DNA methylation and interaction of DNA with histones in nucleosomes. Nucleosome remodeling complexes alter nucleosome structure and position by a number of processes, including histone modification.

Review the Chapter Concepts list on page 396. These all relate to the regulation of gene expression in prokaryotes. Write a brief essay that discusses why you think regulatory systems evolved in bacteria and, in the context of regulation, discuss why genes related to common functions are found together in operons.

Description of the evolutionary advantages of the efficient response to environmental resources and challenges when such resources are present. Having related functions in operons provides for coordinated responses.

Review the Chapter Concepts list on page 367. These concepts relate to how gene mutations occur, their phenotypic effects, and how mutations can be repaired. The first four concepts focus on the effects of gene mutations in diploid organisms. Write a short essay describing how these concepts would apply, or not apply, to a haploid organism such as E. coli.

Differences between diploid and haploid organisms and, with the exception of phenomena such as cell death, disease, and cancer mutational circumstances are attributable to both groups of organisms.

How does an enzyme function? Why are enzymes essential for living organisms on Earth?

Enzymes function 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 posses active sites and/or other domains that are sensitive to the environment. The active site is considered to be a crevice, or pit, which binds reactants, thus enhancing their interaction. The other domains mentioned above may influence the conformation and, therefore, the function of the active site.

Geneticists often use ethyl-methane sulfonate (EMS) to induce mutations in Drosophila. Why is EMS a mutagen of choice for genetic research? What would be the effects of EMS in a strain of Drosophila lacking functional mismatch repair systems?

Ethyl-methane sulfonate (EMS) alkylates the keto groups at the sixth position of guanin and a the fourth position of thymine. In each case, base-pairing affinities are altered and transition mutations result. Altered bases are not readily repaired, and once the transition to normal bases occurs through replication, such mutations avoid repair altogether.

Why is gene regulation more complex in a multicellular eukaryote than in a prokaryote? Why is the study of this phenomenon in eukaryotes more difficult?

Eukaryotic cells contain greater amounts of DNA and this DNA is associated with various proteins. The diversity of cells in a multicellular eukaryote suggests that certain genes are active in some cells but not in others. Eukaryotes have many chromosomes and those chromosomes are enclosed in a nuclear envelope. There are different levels at which regulation can occur in eukaryotes. In eukaryotes, it is difficult to isolate certain molecular species. In eukaryotes, it is difficult to interpret the behavior of isolated molecules in an artificial environment. Gene expression in eukaryotes is tissue specific .

The hormone estrogen converts the estrogen receptor protein from an inactive molecule to an active transcription factor. The ER binds to cis-acting sites that act as enhancers, located near the promoters of a number of genes. In some tissues, the presence of estrogen appears to activate transcription of ER-target genes, whereas in other tissues, it appears to repress transcription of those same genes. Offer an explanation as to how this may occur.

General transcription factors associate with a promoter to stimulate transcription of a specific gene. Some trans-acting element, when bound to enhancers, interact with coactivators to enhance transcription by forming an enhanceosome that stimulates transcription initiation. Transcription can be repressed when certain proteins bind to silencer DNA elements and generate repressive chromatin structures. The same molecule may bind to a different chromosomal regulatory site, depending on the molecular environment of a given tissue type.

One of the most famous cases of an X-linked recessive mutation in humans is that of hemophilia found in the descendants of Britain's Queen Victoria. The pedigree of the royal family indicates that Victoria was heterozygous for the trait; however, her father was not affected, and there is no evidence that her mother was a carrier. What are some possible explanations of how the mutation arose? What types of mutations could lead to the disease?

If a gene is incompletely penetrant, it may be present in a population and only express itself under certain conditions. It is unlikely that the gene for hemophilia behaved in this manner. If a gene's expression is suppressed by another mutation in an individual, it is possible that offspring may inherit a given gene and no inherit its suppressor. Such offspring would have hemophilia. It is possible that the mutation in Queen Victoria's family was new, arising in the father. Lastly, it is possible that the mother was heterozygous, and by chance, no other individuals in her family were unlucky enough to receive the mutant gene.

Most mutations are thought to be deleterious. Why, then, is it reasonable to state that mutations are essential to the evolutionary process?

It is true that most mutations are deleterious however there are a few mutations that are beneficial and provide for differential propagation of the variation which is the basis for evolution.

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 hydrogen bonding could occur between nucleic acids, and specific covalent bonding could occur between an amino acid and a nucleic acid tRNA.

Some mutations in the tra gene of Drosophila cause XX females to appear as males. In contrast, other mutations in tra cause XY male flies to appear as normal females. Which of these phenotypes would you expect if a mutation in a tra resulted in a null allele? Which would you expect if the mutation produced a constitutively active tra gene product?

Mutations in the tra gene of Drosophila can dramatically alter development such that a normal female is produced if the TRA protein is present and a male develops when the TRA protein is absent. A null tra allele would produce males because of male-specific splicing while a constitutively active tra gene would produce females.

In 1962, F. Chapeville and others reported an experiment in which they isolated radioactive 14C-cysteinyl-tRNA. They then removed the sulfur group from the cysteine, creating alanyl-tRNA. When alanyl-tRNA was added to a synthetic mRNA calling for cysteine, but not alanine, a polypeptide chain was synthesized containing alanine. What can you conclude from this experiment?

One can conclude that the amino acid is not involved in recognition of the codon.

A series of mutations in the bacterium Salmonella typhimurium results in the requirement of either tryptophan or some related molecule in order for growth to occur. From the data, suggest a biosynthetic pathway for tryptophan.

Precursor -> (insert trp-8) -> AA -> (insert trp-2) -> IGP -> (insert trp-3) -> I -> (insert trp-1) -> TRY

Define and compare the four levels of protein organization.

Primary: the linear arrangement or sequence of amino acids; this sequence determines 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 binds between cysteine residues, interactions of hydrophilic side chains with water, and interactions of hydrophobic side chains with each other. Quaternary: the association of two (dimer) or more polypeptide chains; called oligometric, such a protein is made up of more than one protein chain.

Present an overview of RNA-induced gene silencing achieved through RNA interference (RNAi). How do the silencing processes begin, and what major components participate?

RNA interference begins with a double-stranded RNA being processed by a protein called Dicer that, in combination with RISC, generates short interfering RNA (siRNA). Unwinding of siRNA produces an antisense strand that combines with a protein to cleave mRNA complementary sequences. Short RNAs called microRNAs pair with the 3'-untranslated regions of mRNAs and block their translation.

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?

Since there are three nucleotide that code for each amino acid, there would be 423 code letters (nucelotides), 426 including a termination codon. This assumes that other features, such as the poly-A tail, the 5' cap, and noncoding leader sequences, are omitted.

What is a spontaneous mutation, and why are spontaneous mutations rare?

Spontaneous mutations arise from a variety of sources, including errors in DNA replication, spontaneous lesions, and transposable genetic elements. It is very rare, it occurs 1 in 10,000 and 1 in a trillion. Low rate is due to cellular repair mechanisms.

If one spontaneous mutation occurs with a human egg cell genome, and this mutation changes an A to a T, what is the most likely effect of the mutation on the phenotype of an offspring that develops from this mutated egg?

The phenotypic influence of any base changes is dependent on a number of factors, including its location in coding or noncoding regions, its potential in dominance or recessive-ness, and its interaction with other base sequences in the genome. If a base change is located in a noncoding region, there may be no influence on the phenotype. However, some noncoding regions in a traditional sense may influence other genes and/or gene products. If a mutation that occurs in a coding region acts as a full recessive, there should be no influence on the phenotype. If mutant gene acts as a dominant, then there would be an influence on the phenotype. Some genes interact with other gene in a variety of ways that would be difficult to predict without additional information.

During translation, what molecule bears the codon? the anticodon?

The sequence of base triplets in mRNA constitutes the sequence of codons. A three-base portion of the tRNA constitutes the anticodon.

Discuss the potential difficulties of designing a diet to alleviate the symptoms of phenylketonuria.

Too little phenylalanine will restrict protein synthesis Too much of phenylalanine and its derivatives cause PKU in phenylketoneurics.

Contrast the roles of tRNA and mRNA during translation and list all enzymes that participate in the transcription and translation process.

Transfer RNAs are "adaptor" molecules in that they provide a way for amino acids to interact with sequences of bases in nucleic acids. Amino acids are specifically and individually attached to the 3' end of tRNAs that possess a three-base sequence (the anticodon) to base-pair with three bases of mRNA. Messenger RNA, on the other hand, contains a copy of the triplet codes that are stored in DNA> the sequences of bases in mRNA interact three at a time, with the anticodons of tRNAs. Enzymes involved in transcription include the following: -RNA polymerase (e.coli) -RNA polymerase I,II,III (Eukaryotes) Those involved in translation include the following: -Aminoacyl tRNA synthetases -Peptidyl transferase -GTP-dependent release factors

It is estimated that about 0.2 percent of human mutations are due to transposon insertions, and a much higher degree of mutational damage is known to occur in some other organisms. In what way might transposons contribute positively to evolution?

Transposons cause changes in DNA in a variety of ways, including massive chromosomal alterations. In most cases, changes in DNA are harmful to organisms, while rare cases, an evolutionary advantage occurs because the new genetic variation confers a selective advantage.

Early detection and adherence to a strict dietary regime have prevented much of the mental retardation that used to occur in those afflicted with phenylketonuria (PKU). Affected individuals now often lead normal lives and have families. For various reasons, such individuals adhere less rigorously to their diet as they get older. 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 phenylalanine is toxic, many newborns are severely and irreversibly retarded at birth. Expectant mothers should return to a low-phenylalanine intake during pregnancy.

Why would a mutation in a somatic cell of a multicellular organism escape detection?

When conducting genetic screens, one assumes that all the cells of an organism are genetically identical. Therefore, the organism responds to the screen enabling detection of the mutation. Since a somatic mutation first appears in a single cell, it is highly unlikely that the organism will be sufficiently altered to respond to a screen because none of the other cells in that organism will have the same mutation.

Review the Chapter Concepts list on page 337. These all relate to the translation of genetic information stored in mRNA into proteins and how chemical information in proteins impart function to those molecules. Write a brief essay that discusses the role of ribosomes in the process of translation as it relates to these concepts.

When involved in protein synthesis, a ribosome will contain the following components: mRNA, charged tRNA, large and small ribosomal subunits, elongation, and perhaps, intiation factors, peptidyl transferase, GTP, Mg2+, nascent proteins, and possibly GTP-dependent release factors. Together, these components order and form peptide bonds between adjacent amino acids, thereby forming proteins.

HbS results from the substitution of valine for glutamic acid at the number 6 position in the beta chain of human hemoglobin. HbC is the result of a change at the same position in the beta chain, but in this case, lysine replaces glutamic acid. Return to the genetic code table and determine whether single-nucleotide changes can account for these mutations. Then view Figure 14-6 and examine the R groups in the amino acids glutamic acid, valine, and lysine. Describe the chemical differences between the three amino acids. Predict how the changes might alter the structure of the molecule and lead to altered hemoglobin function.

With the codes for valine being GUU, GUC, GUA, and GUG, single base changes from glutamic acid's GAA and GAG can cause the glutamic acid and valine switch. The normal glutamic acid is a negatively charged amino acid, whereas valine carries no charge and lysine is positively charged. Given these significant charge changes, one would predict some, if not considerable, influence on protein structure and function. Such changes could stem from internal changes in folding or interactions with other molecules in the RBC, especially other hemoglobin molecules.

Why are X rays more potent mutagens than UV radiation?

X rays are of higher energy and shorter wavelength than UV light. They have greater penetrating ability and can create more disruption of DNA.

Phenylketonurics are often more lightly pigmented than are normal individuals. Can you suggest a reason why this is so?

because they lack melanin

In the hypothetical extraterrestrial organism, the Quagarre, the immunoglobulin genes undergo rearrangements similar to those in humans, with two main differences. One difference is that the k light-chain V regions are located on chromosome 4 and the other k light-chain regions are located on chromosome 10. Another difference is that the Quagarre immunoglobulin gene do not rearrange until the creature is 13 years old. By examining a mitotic spread of chromosomes from a Quagarre B cell, how could you estimate the creature's age?

by observing the differences in the chromosomes

Mammography is an accurate screening technique for the early detection of breast cancer in humans. Because this technique uses X rays diagnostically, it has been highly controversial. Can you explain why? What reasons justify the use of X rays for such a medical screening technique?

it is highly controversial because x-rays are known to induce mutations, specifically cancer-causing agents. the usage of x-rays, even at such a small dosage, is essential in order to detect these things. otherwise, we do not know and cannot fight it or it becomes too late

DNA methylation is commonly associated with a reduction of transcription. The following data come from a study of the impact of the location and extent of DNA methylation on gene activity in human cells. A bacterial gene, luciferase, was cloned next to eukaryotic promoter fragments that were methylated to various degrees, in vitro. The chimeric plasmids were then introduced into tissue culture cells, and the luciferase activity were assayed. These data compare the degree of expression of luciferase with differences in the location of DNA methylation. What general conclusions can be drawn from these data?

outside and inside have similar results aside from the relative luciferase expression rates