Up until now, we've focused primarily on how transcription and DNA replication use similar mechanisms to generate a copy of a DNA strand. However, you've probably guessed that transcription has some unique aspects that make it different from replication.

• Structure. It's important to note that RNA differs from DNA quite a bit in structure. RNA is single-stranded, which means that it can fold up onto itself into many different structures. These different structures are important for the function of the many different types of RNA, which we'll discuss in more detail later.
• Polymerase. RNA polymerases, unlike DNA polymerases, don't need a primase to start transcribing the RNA strand.

In addition to the differences we've mentioned, transcription has its own special problems to deal with.

• Where to Begin? With DNA replication, the question of where to start is a simpler one, because the entire DNA sequence must be duplicated. With transcription, though, things are a lot trickier. Think about the genome in terms of a giant recipe book. If you're looking to copy a particular recipe for apple pie, you'll need to know exactly which page to start on. If you open to a random page in the cookbook, you're much more likely to end up with fried chicken. And that's bad.
• Where to End? Just like beginnings, transcription needs to know where to stop. We don't need to tell you how disastrous it could be if we used our recipe analogy above and didn't know where the recipe ended. Tuna fish apple pie, anyone?
• Processing the RNA. Unlike the new DNA strand that's created after replication, the transcribed RNA nucleotide strand often needs a bit of sprucing up and gets modified and edited.

How does transcription deal with these obstacles? The answer isn't entirely straightforward, and different organisms deal with these obstacles in different ways. Overall, prokaryotes tend to approach these issues with a straightforward solution, which is one reason that scientists study them. Eukaryotes are more complicated, and have complicated solutions. But no worries. We'll explain everything eventually.