Final answer:
Eukaryotic chromosomes have multiple origins of replication because their genomes are much larger and more complex than those of prokaryotes, necessitating more replication sites to efficiently copy all the genetic material during cell division. Option a is the correct answer.
Step-by-step explanation:
The reason why eukaryotic chromosomes have multiple origins of replication, while prokaryotic chromosomes typically have only one, can be traced back to the size and complexity of eukaryotic genomes. Eukaryotic genomes are much larger and more complex compared to prokaryotic genomes. Eukaryotes have multiple linear chromosomes, each with multiple replication origins, to facilitate the replication of their larger amount of DNA during cell division.
In contrast, prokaryotes have a single, circular chromosome that is able to be replicated quickly from a single origin due to their smaller genome size. Moreover, the rate of replication in eukaryotes is relatively slow, at approximately 100 nucleotides per second, which highlights the necessity for multiple origins of replication to complete the process in a reasonable time frame. The human genome itself can have up to 50,000 origins of replication, illustrating the scale at which eukaryotic cells operate.
Therefore, the correct answer to the question is: 'a. Eukaryotic chromosomes are very big.' This multiple replication origins system allows eukaryotic cells to efficiently replicate their extensive and complex genomes during the cell cycle.