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Chapter 11: Problem 36

Antibiotics such as chloramphenicol, tetracycline, and erythromycin inhibit protein synthesis in bacteria but have no effect on the synthesis of proteins encoded by eukaryotic nuclear genes (see Section 15.4 in Chapter 15 for a discussion of the effects of antibiotics on protein synthesis). Cycloheximide inhibits the synthesis of proteins encoded by nuclear genes but has no effect on bacterial protein synthesis. How might these compounds be used to determine which proteins are encoded by mitochondrial and chloroplast genomes?

Short Answer

Expert verified
Use cycloheximide to block nuclear protein synthesis; remaining synthesis suggests organelle-encoded proteins. Use antibiotics to verify, as they inhibit organelle-encoded proteins.

Step by step solution

01

Understanding Antibiotic Action on Bacteria

Antibiotics like chloramphenicol, tetracycline, and erythromycin inhibit protein synthesis specifically in bacteria. These antibiotics affect the bacterial ribosomes, which are different from eukaryotic ribosomes, hence they do not affect protein synthesis in eukaryotic nuclear-encoded genes.
02

Understanding Cycloheximide Action

Cycloheximide is known to inhibit the synthesis of proteins encoded by nuclear genes in eukaryotic cells. However, it does not inhibit protein synthesis in bacteria or organelles such as mitochondria and chloroplasts that have similarities to bacterial ribosomes.
03

Applying Antibiotics to Cell Cultures

To determine which proteins are encoded by mitochondrial and chloroplast genomes, cells can be treated with cycloheximide. Since cycloheximide blocks nuclear-encoded protein synthesis, any proteins still being synthesized are likely encoded by mitochondrial and chloroplast genomes.
04

Analyzing the Effects of Organelle Gene Inhibition

Next, treat cell cultures with antibiotics like chloramphenicol, tetracycline, or erythromycin. These antibiotics inhibit bacterial-like protein synthesis, which is similar to that in mitochondria and chloroplasts. Proteins whose synthesis stops upon treatment with these antibiotics are likely encoded by organelle genomes.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Antibiotic Effects on Ribosomes
Antibiotics are molecules that can inhibit the growth or kill microorganisms by targeting specific bacterial functions. One common target is the ribosome, a complex molecular machine in cells responsible for protein synthesis.
An interesting fact about ribosomes is that they differ between bacteria and eukaryotes. Antibiotics like chloramphenicol, tetracycline, and erythromycin take advantage of these differences.
  • These antibiotics are designed to affect only bacterial ribosomes, which are structured differently from those in eukaryotes.
  • This specificity means that while antibiotics halt bacterial protein production, they do not directly affect proteins produced by eukaryotic nuclear genes.

This selective inhibition is crucial in medical treatments to target bacterial infections without harming the host鈥檚 cells.
Organelle Genomes
Within eukaryotic cells, not all genomes reside in the nucleus. Organelles like mitochondria and chloroplasts have their own DNA.
These organelle genomes are remnants of early symbiosis events, allowed by their ancient bacterial ancestors.
  • Mitochondrial DNA (mtDNA) is mostly involved in coding proteins for the cell鈥檚 energy production activities.
  • Chloroplast DNA (cpDNA) is found in plants and is pivotal for photosynthesis.

Both types of organelle genomes share similarities with bacterial genomes, making studies and experimentation on them a unique field.
Understanding these genomes provides insights into evolution and energy processes within cells.
Mitochondrial Protein Synthesis
The synthesis of proteins within mitochondria presents a fascinating blend of eukaryotic and prokaryotic processes.
Mitochondria have their own ribosomes, which resemble those found in bacteria, not the eukaryotic ribosomes seen in the cell鈥檚 cytoplasm.
This similarity allows researchers to use antibiotics like chloramphenicol to determine if a protein is mitochondrial.
  • If a protein continues to be synthesized in the presence of cycloheximide but stops when antibiotics like chloramphenicol are introduced, it is likely an mtDNA-encoded protein.

This approach helps in deciphering the roles of different mitochondrial proteins and their genetic origins.
Chloroplast Protein Synthesis
Chloroplasts, found in plant cells, are akin to the mitochondria in animal cells, having their own genetic material and ribosomes.
Chloroplast ribosomes are also similar to bacterial ribosomes, allowing antibiotics that target bacterial protein synthesis to be useful tools in studying chloroplast functions.
  • When experiments are conducted with cycloheximide, eukaryotic cytoplasmic ribosome activity is halted, but chloroplast activity is not affected initially.
  • Subsequent introduction of antibiotics like tetracycline or erythromycin reveals changes, helping scientists identify proteins synthesized by cpDNA.

This methodology helps elucidate how plant cells manage protein synthesis and how chloroplasts contribute to cellular functions essential for photosynthesis.

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Most popular questions from this chapter

Describe the composition and structure of the nucleosome.

In a study of a muscle disorder, several affected families exhibited vision problems, muscle weakness, and deafness (M. Zeviani et al. 1990. American Journal of Human Genetics \(47: 904-914\) ). Analysis of the mtDNA from affected members of these families revealed that large numbers of their mtDNA molecules possessed deletions of varying lengths. Different members of the same family and even different mitochondria from the same person possessed deletions of different sizes, so the underlying defect appeared to be a tendency for the \(\mathrm{mtDNA}\) of affected persons to have deletions. A pedigree of one of the families studied is shown below (see Chapter 6 for a discussion of how to read and interpret pedigrees). The researchers concluded that this disorder is inherited as an autosomal dominant trait, and they mapped the disease-causing gene to a position on chromosome 10 in the nucleus. a. What characteristics of the pedigree rule out inheritance of a trait encoded by a gene in the \(\mathrm{mtDNA}\) as the cause of this disorder? b. Explain how a mutation in a nuclear gene might lead to deletions in mtDNA.

Describe the different classes of DNA sequence variation that exist in eukaryotes.

What is the difference between euchromatin and heterochromatin?

In 1979 , bones found outside Ekaterinburg, Russia, were shown to be those of Tsar Nicholas and his family, who were executed in 1918 by a Bolshevik firing squad in the Russian Revolution (see the introduction to Chapter 14 ). To prove that the skeletons were those of the royal family, mtDNA was extracted from the bone samples, amplified by PCR, and compared with mtDNA from living relatives of the tsar's family. a. Why was DNA from the mitochondria analyzed instead of nuclear DNA? What are some of the advantages of using mtDNA for this type of study? b. Mitochondrial DNA from which living relatives would provide useful information for verifying that the skeletons were those of the royal family?
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