Final answer:
A primary RNA transcript becomes a mature mRNA through the process of splicing, wherein introns are removed and exons are joined, followed by the addition of a poly-A tail. A 5' cap is also added for mRNA stability and recognition by the protein synthesis machinery.
Step-by-step explanation:
After a primary RNA transcript of the light chain has been made (with 3` cap and poly-A tail), it becomes a mature mRNA through splicing and polyadenylation. During RNA processing, the pre-mRNA undergoes several modifications. A 5' cap is added to the RNA, and introns are spliced out while exons are joined together. Finally, a poly-A tail is added to the 3' end. This results in a mature mRNA that can then be translated into protein. The process of splicing involves the removal of non-coding regions (introns) from the pre-mRNA and the rejoining of coding regions (exons) to produce the continuous coding sequence that will specify the protein product.
RNA splicing, capping, and polyadenylation are all essential steps in the post-transcriptional modification of the primary RNA transcript, which occurs prior to mRNA translation where the actual protein synthesis takes place. Therefore, the correct answer to how the primary RNA transcript becomes a mature mRNA is a) Splicing and polyadenylation.