Question

Which of the following statements is (are) true? (a) Bacterial ribosomes consist of \(40 \mathrm{S}\) and \(60 \mathrm{S}\) subunits. (b) Prokaryotic DNA is normally complexed with histones. (c) Prokaryotic DNA normally exists as a closed circle. (d) Circular DNA is supercoiled.

Short answer

Statements (c) and (d) are true.

Step-by-step solution

Analyze Statement (a)

Assess the accuracy of the statement about bacterial ribosomes. Bacterial ribosomes actually consist of 30S and 50S subunits, not 40S and 60S which are eukaryotic ribosomal subunits. Therefore, statement (a) is false.

Analyze Statement (b)

Evaluate if prokaryotic DNA is complexed with histones. Prokaryotic DNA is not typically associated with histones; this characteristic is more common in eukaryotic DNA. Thus, statement (b) is false.

Analyze Statement (c)

Verify the structural form of prokaryotic DNA. Prokaryotic DNA usually exists as a closed circle rather than linear. Hence, statement (c) is true.

Analyze Statement (d)

Determine if circular DNA is supercoiled. Circular DNA, such as in prokaryotes, is often supercoiled to compact the DNA molecule. Therefore, statement (d) is true.

Key concepts

bacterial ribosomes

Ribosomes are essential components of all living cells, where they facilitate protein synthesis by translating mRNA into proteins. In bacteria, ribosomes are smaller than those found in eukaryotic cells. Bacterial ribosomes are composed of two subunits: 30S (small subunit) and 50S (large subunit). Together, they form the 70S ribosome.
Understanding these differences is crucial because some antibiotics, like tetracycline, specifically attack bacterial ribosomes, disrupting protein synthesis without harming eukaryotic cells.
This selective targeting enables antibiotics to treat bacterial infections effectively without damaging human cells.

closed circular DNA

Prokaryotic DNA, such as in bacteria, typically exists as a closed circular DNA molecule. Unlike the linear DNA found in eukaryotes, this circular structure is crucial for the cell's functions.
One key advantage of a closed circular DNA structure is its stability. There are no free ends, which reduces the risks of DNA degradation and the complexities of end repair.
Such a structure allows bacteria to maintain genetic continuity and integrity with fewer challenges compared to organisms with linear chromosomes.

DNA supercoiling

DNA supercoiling refers to the additional twisting of a DNA molecule beyond its regular helical structure. In bacteria, circular DNA often becomes supercoiled to compact the large DNA molecule into the limited space of the cell.
Supercoiling is achieved by enzymes known as topoisomerases, which introduce or remove supercoils. This compaction is vital because it enables efficient packaging of the genome while ensuring that specific genes remain accessible for transcription.
Supercoiling also plays a role in DNA replication and repair, illustrating its multifaceted importance in bacterial cell biology.

histones in DNA

Histones are proteins that help package and order DNA into structural units called nucleosomes. They are primarily associated with eukaryotic DNA, aiding in the organization and regulation of genetic material.
In prokaryotes, however, DNA packaging differs significantly. Instead of histones, bacteria use other proteins, such as HU and IHF, to aid in the compaction and organization of their DNA.
This distinction is essential because it highlights the differences in genetic regulation and expression between prokaryotic and eukaryotic organisms.