Final answer:
Different signal-sequence combinations are key to protein targeting and their downstream effects in cells, influencing both mRNA and protein synthesis. Complexity in signaling pathways arises from variables such as phosphorylation and dimerization, which, combined with genetic code translation involving codons, contribute to the diversity and adaptation of protein expression in various organisms.
Step-by-step explanation:
Different signal-sequence combinations determine the downstream proteins targeted, thus playing a crucial role in protein folding, modification, and targeting within cells. A signal sequence, typically at the N-terminal end and spanning 10-30 amino acids, directs proteins to their specific cellular destinations and is subsequently removed. This is critical for the proper functioning and regulation of cellular activities, which includes the completion of mRNA synthesis or influencing protein synthesis. During and after translation, proteins may undergo phosphorylation or glycosylation - biochemical modifications that enhance their functional diversity. Additionally, the translation process can be species-specific, with some organisms having adaptations in their translation apparatus, enabling them to produce unique proteins with novel functions, such as silk in arthropods.
The downstream cellular response is influenced by various factors such as phosphatase activity and dimerization. Signaling pathways can be complex, with a single pathway capable of branching off toward different endpoints based on the conditions within the cell or the interplay between multiple pathways. This versatility ensures that several external demands are met before a cell commits to a specific response. The intricate nature of these pathways highlights the diversity of protein expression across different cell types and the role of signal integration in cellular responses.
Given that the 'mRNA alphabet' consists of four nucleotides while the 'protein alphabet' consists of twenty amino acids, the genetic code uses a three-nucleotide combination, or codon, to specify amino acids during translation. This creates 64 (4 x 4 x 4) possible codons, with some of them coding for the same amino acid. Moreover, certain amino acid sequence combinations are innately challenging for the translation apparatus, suggesting the need for species-specific adaptations to overcome these difficulties and potentially drive speciation.