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

1. Step 1: Understand the general characteristics of mycoplasmas. Mycoplasmas are a group of bacteria known for lacking a cell wall and having sterols in their cytoplasmic membranes, which is unusual for bacteria.
2. 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.
3. Step 3: Recall that mycoplasmas contain sterols in their membranes (option b) and use the UGA codon to code for tryptophan instead of a stop signal (option c). Also, their rRNA sequences are closely related to Gram-positive bacteria (option d).
4. Step 4: Consider the presence of cytochromes (option a). Mycoplasmas generally lack cytochromes because they have limited metabolic capabilities and do not perform typical respiratory electron transport chains.
5. Step 5: Conclude that the characteristic not typical of mycoplasmas is the presence of cytochromes (option a), as this distinguishes them from other bacteria.

Key Concepts

Mycoplasma Cell Membrane Composition

Mycoplasmas are unique among bacteria because they lack a cell wall and instead have sterols in their cytoplasmic membranes. These sterols, acquired from the host or environment, provide membrane stability and flexibility, which is essential for their survival and shape.

Genetic Code Variations in Mycoplasmas

Mycoplasmas exhibit a genetic code variation where the UGA codon, typically a stop signal in most organisms, codes for the amino acid tryptophan. This deviation is a distinctive molecular feature important for protein synthesis in these bacteria.

Phylogenetic Relationship and rRNA Sequences

The rRNA nucleotide sequences of mycoplasmas are closely related to those of Gram-positive bacteria, reflecting their evolutionary lineage. This similarity helps classify mycoplasmas within the Firmicutes phylum despite their lack of a typical Gram-positive cell wall.