Up until now we have focused primarily on how transcription and DNA replication use similar solutions to generate a copy of a DNA strand. However, you have probably guessed that transcription has some unique aspects that make it different from replication.
- Structure. It is important to note though that unlike DNA, RNA differs from DNA quite a bit in structure. RNA is single-stranded, which means that it can fold into many different structures. These different structures are important for the function of the many different types of RNA, which we will discuss in more detail later.
- Polymerase. RNA polymerases, unlike DNA polymerases, do not need a primer to start the RNA strand.
In addition to the differences we have 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 are looking to copy a particular recipe for apple pie, you will need to know exactly which page to start on. If you open to a random page in the cookbook, you are much more likely to end up with fried chicken. That's the problem transcription faces.
- 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 nucleotide strand created after DNA replication, the nucleotide strand created after DNA transcription is often modified and edited.
How does transcription deal with these obstacles? The answer isn't entirely straightforward. In fact, we are about to tell you that different organisms sometimes deal with these obstacles in different ways. We'll tell you how prokaryotes and eukaryotes approach these issues generally. Overall prokaryotes tend to approach these issues with a straightforward solution, which is one reason that scientists study them. Eukaryotes often use a similar, yet more complicated approach.