Introduction:

The process of removal of introns and joining of exons in a defined order in a primary transcript is known as splicing. This process is crucial for the generation of mature mRNA molecules, which can be translated into functional proteins. Splicing occurs in eukaryotes, but not in prokaryotes. Let's explore this process in more detail.

Eukaryotes:

In eukaryotes, including plants, animals, and fungi, the primary transcript, also known as pre-mRNA, undergoes splicing to remove introns and join exons. This process is carried out by a large macromolecular complex called the spliceosome. The spliceosome recognizes specific sequences at the boundaries of introns and exons, known as splice sites.

Spliceosome assembly and splicing:

1. Spliceosome assembly: The spliceosome is composed of small nuclear ribonucleoprotein particles (snRNPs) and other proteins. The assembly of the spliceosome begins with the binding of the U1 snRNP to the 5' splice site, followed by the binding of the U2 snRNP to the branch point sequence within the intron.

2. Splicing initiation: The U4/U6 and U5 snRNPs join the spliceosome, forming an active complex. The U1 and U4 snRNPs dissociate, allowing the U6 snRNA to base pair with the 5' splice site. This rearrangement positions the 5' splice site for the subsequent steps of splicing.

3. Splicing catalysis: The spliceosome undergoes a series of conformational changes, leading to the formation of the catalytic center. This center brings the 5' splice site together with the branch point sequence, facilitating the cleavage of the 5' splice site and the formation of a lariat intermediate.

4. Exon ligation: The 5' exon is joined to the 3' exon through a transesterification reaction. The intron is released as a lariat-shaped molecule, which is rapidly degraded.

Prokaryotes:

In prokaryotes, such as bacteria, splicing does not occur. The primary transcripts in prokaryotes are usually polycistronic, meaning they contain multiple genes in a single mRNA molecule. The introns found in eukaryotic genes are absent in prokaryotic genes.

Prokaryotes and Protista:

Protista, which includes a diverse group of eukaryotic microorganisms, also undergo splicing similar to other eukaryotes. Therefore, the correct answer is option d) Prokaryotes and Protista.

In summary, the process of removal of introns and joining of exons in a defined order through splicing occurs in eukaryotes, including Protista, but not in prokaryotes. Splicing is a complex and highly regulated process that ensures the production of mature mRNA molecules for protein synthesis.