Final answer:
Spliceosomes in the nuclei of eukaryotic cells are made of small nuclear ribonucleoproteins (snRNPs), which are essential for the splicing of pre-mRNA into mRNA. The correct answer to the question is option 4, SnRNPs.
Step-by-step explanation:
Spliceosomes are complex structures found in the nuclei of eukaryotic cells that are essential for the processing of pre-messenger RNA (pre-mRNA) into mature mRNA. They are not made of ribosomes, the Golgi apparatus, or the endoplasmic reticulum. Instead, spliceosomes are composed of small nuclear ribonucleoproteins (snRNPs) and other associated proteins. snRNPs are particles that combine RNA and proteins and play a critical role in identifying intron-exon boundaries in pre-mRNA and performing the splicing necessary to remove introns from pre-mRNA molecules.
The correct answer to the question 'What are spliceosomes made of in the nuclei of eukaryotic cells?' is option 4, SnRNPs. This is because spliceosomes are specifically assembled from snRNPs, which are the building blocks that carry out the intricate process of splicing mRNA transcripts within the nucleus.