A young man was brought to the emergency room after being hit by a car. His leg bone was broken in half, and the bone was protruding through the skin. He was recovering well after the orthopedic surgery. After several days he developed wound infection. The wound culture revealed a new bacterial organism that was resistant to all available antibiotics. The drugs acting as antibiotics specifically inhibit bacterial proliferation by inhibiting any of the processes such as replication, transcription or translation. Which of the following statements is exclusively true of prokaryotes but not of eukaryotes?
A. Most of the prokaryotic cells are diploid
B. DNA is complexed with histones
C. Genes contain introns
D. Transcription and translation occur at the same time
E. Each gene has its own promoter.
The correct answer is- D.
In prokaryotic organisms, the primary transcripts of mRNA-encoding genes begin to serve as translation templates even before their transcription has been completed (figure-1).This is because the site of transcription is not compartmentalized into a nucleus as it is in eukaryotic organisms. Thus, transcription and translation are coupled in prokaryotic cells. Consequently, prokaryotic mRNAs are subjected to little processing prior to carrying out their intended function in protein synthesis.
Figure-1- In prokaryotes the processes of transcription and translation are coupled.
Nearly all eukaryotic RNA primary transcripts undergo extensive processing between the time they are synthesized and the time at which they serve their ultimate function, whether it be as mRNA or as a component of the translation machinery such as rRNA, 5S RNA, or tRNA or RNA processing machinery, snRNAs. Processing occurs primarily within the nucleus and includes nucleolytic cleavage to smaller molecules and coupled nucleolytic and ligation reactions (splicing of exons). However, the processes of transcription, RNA processing and even RNA transport from the nucleus are highly coordinated.
Figure-2- In eukaryotes transcription and translation occur at different locations. Transcription occurs in the cell’s nucleus, mRNA then moves to the cytoplasm for translation.
As regards other options:
A) “Most of the prokaryotic cells are diploid”, is an incorrect statement- A single circular DNA is present in the cell. There is no nuclear boundary around the prokaryotic DNA.
B. “DNA is complexed with histones”, is also not correct. Prokaryotes lack histones. Histones are nucleoproteins, rich in basic amino acids. Much of the DNA in eukaryotes is associated with histone proteins to form a structure called the nucleosome. Nucleosomes and higher-order structures formed from them serve to compact the DNA.
C. “Genes contain introns” in prokaryotes is also wrong. Intervening sequences (introns) exist within most but not all mRNA encoding genes of higher eukaryotes but they are not present in prokaryotes.
E. ‘Each gene has its own promoter”, is also an incorrect statement for prokaryotes. The mRNA in prokaryotes is polycistronic, it carries the information of multiple genes, and each gene in prokaryotes thus does not require its own separate promoter.Please help Biochemistry for Medics by "CLICKING ON THE ADVERTISEMENTS" every time you visit us. Thank you!