Which of the following is not characteristic of mycoplasmas?
a. Cytochromes
b. Sterols in cytoplasmic membranes
c. Use of UGA codon for tryptophan
d. rRNA nucleotide sequences similar to those of Gram-positive bacteria

Verified step by step guidance

Step 1: Understand the general characteristics of mycoplasmas. Mycoplasmas are a group of bacteria that lack a cell wall and have unique features compared to other bacteria.

Step 2: Review the options given: (a) Cytochromes, (b) Sterols in cytoplasmic membranes, (c) Use of UGA codon for tryptophan, and (d) rRNA nucleotide sequences similar to those of Gram-positive bacteria.

Step 3: Recall that mycoplasmas contain sterols in their cytoplasmic membranes, which is unusual for bacteria, and they use the UGA codon to code for tryptophan instead of a stop signal.

Step 4: Remember that mycoplasmas have rRNA sequences closely related to Gram-positive bacteria, supporting their classification within this group.

Step 5: Identify that cytochromes are typically involved in electron transport chains and are not characteristic of mycoplasmas, which have limited metabolic capabilities and often lack cytochromes.

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Key Concepts

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

Mycoplasma Cell Membrane Composition

Mycoplasmas are unique among bacteria because they lack a cell wall and incorporate sterols into their cytoplasmic membranes to provide structural stability. This feature distinguishes them from most other bacteria, which do not have sterols in their membranes.

Genetic Code Variations in Mycoplasmas

Mycoplasmas use a slightly altered genetic code where the UGA codon, normally a stop signal in most organisms, codes for the amino acid tryptophan. This deviation is a key molecular characteristic important for identifying and understanding mycoplasma genetics.

Phylogenetic Relationship Based on rRNA Sequences

The rRNA nucleotide sequences of mycoplasmas show close similarity to those of Gram-positive bacteria, indicating their evolutionary relationship. This molecular evidence helps classify mycoplasmas within the bacterial domain despite their unique features.

Which of the following is not characteristic of mycoplasmas?a. Cytochromesb. Sterols in cytoplasmic membranesc. Use of UGA codon for tryptophand. rRNA nucleotide sequences similar to those of Gram-positive bacteria