Ch. 9 - The Molecular Biology of Translation

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook

Sanders 3rd Edition

Ch. 9 - The Molecular Biology of Translation

Problem B.5

Chapter 9, Problem B.5

Describe the gene and protein defects in phenylketonuria (PKU). How are these defects connected to disease symptoms?

Verified step by step guidance

Step 1: Understand that phenylketonuria (PKU) is caused by mutations in the gene encoding the enzyme phenylalanine hydroxylase (PAH), which is responsible for converting the amino acid phenylalanine into tyrosine.

Step 2: Recognize that the gene defect leads to a deficient or nonfunctional PAH enzyme, resulting in an inability to metabolize phenylalanine properly.

Step 3: Explain that the protein defect (nonfunctional or deficient PAH enzyme) causes phenylalanine to accumulate in the blood and tissues, leading to toxic levels.

Step 4: Connect the accumulation of phenylalanine to the disease symptoms, such as intellectual disability, developmental delays, and neurological problems, due to its toxic effects on brain development and function.

Step 5: Summarize that the gene mutation causes a defective enzyme, which disrupts normal metabolism of phenylalanine, and this metabolic block leads directly to the clinical manifestations of PKU.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Phenylketonuria (PKU) and its Genetic Basis

PKU is an inherited metabolic disorder caused by mutations in the PAH gene, which encodes the enzyme phenylalanine hydroxylase. This enzyme is responsible for converting phenylalanine to tyrosine. Defects in the PAH gene lead to reduced or absent enzyme activity, causing phenylalanine accumulation.

Role of Phenylalanine Hydroxylase Enzyme

Phenylalanine hydroxylase catalyzes the hydroxylation of phenylalanine to tyrosine, a precursor for neurotransmitters. When this enzyme is defective, phenylalanine builds up to toxic levels, disrupting normal brain development and function, which underlies the neurological symptoms of PKU.

Connection Between Molecular Defects and Disease Symptoms

The accumulation of phenylalanine and its metabolites in PKU leads to neurotoxicity, causing intellectual disability, seizures, and behavioral problems. The lack of tyrosine also impairs neurotransmitter synthesis, further contributing to cognitive and developmental issues seen in untreated PKU patients.

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Textbook Question

Several lines of experimental evidence pointed to a triplet genetic code. Identify three pieces of information that supported the triplet hypothesis of genetic code structure.

Textbook Question

Outline the events that occur during initiation of translation in E. coli.

Textbook Question

A portion of a DNA template strand has the base sequence

5′-...ACGCGATGCGTGATGTATAGAGCT...-3′

Assume the mRNA is written in the correct reading frame. Determine the amino acid sequence encoded by this fragment. Identify the N- and C-terminal directions of the polypeptide.

Textbook Question

In the experiments that deciphered the genetic code, many different synthetic mRNA sequences were tested.

Predict the results of experiments examining GCUA repeats.

Textbook Question

A portion of a DNA template strand has the base sequence

5′-...ACGCGATGCGTGATGTATAGAGCT...-3′

Which is the third amino acid added to the polypeptide chain?

Textbook Question

A portion of a DNA template strand has the base sequence

5′-...ACGCGATGCGTGATGTATAGAGCT...-3′

Identify the sequence and polarity of the mRNA transcribed from this fragmentary template-strand sequence.