Final answer:
The 5S rRNA is transcribed independently by RNA polymerase III from multiple gene copies, which may be spread among many chromosomes. It does not originate from the cleavage of a long single precursor rRNA transcript like the 18S, 28S, and 5.8S rRNAs, which are derived from a 45S precursor and transcribed by RNA polymerase I.
Step-by-step explanation:
The 5S rRNA is a component of ribosomal RNA (rRNA) that originates from a separate gene transcript rather than being cleaved from a single long precursor rRNA transcript like the other eukaryotic rRNAs (18S, 28S, and 5.8S). In eukaryotes, a large precursor known as a 45S pre-rRNA transcript is processed to give rise to the 18S, 28S, and 5.8S rRNAs.
This processing involves cleavage of the precursor molecule. In contrast, the 5S rRNA is independently transcribed by RNA polymerase III, typically from multiple gene copies that may be distributed across different chromosomes. The 5S rRNA genes have a unique feature in that the promoter sequences used by RNA polymerase III for transcription are located within the transcribed part of the genes, which is unusual compared to the other rRNAs transcribed by RNA Polymerase I and mRNAs transcribed by RNA Polymerase II.
After binding to this internal promoter, RNA polymerase III re-positions itself and begins the transcription of the RNA molecule, resulting in a transcript that includes the promoter sequence with minimal posttranscriptional processing. Due to its separate transcription mechanism, 5S rRNA stands apart as the rRNA component originating from an individual gene transcript in eukaryotic cells.