Similarities Between Transcription and DNA Replication
NucleotidesJust like DNA, RNA is a polymer of nucleic acids, each of which is held together by phosphodiester bonds, or bonds that have a carbon double bonded to an oxygen, C=O.
Okay, okay…we know we said we would only talk about similarities, but we lied.
There are two main differences between the nucleotides used in DNA and those in RNA and we'd better mention them now before you start thinking biology is actually a piece of cake. Although, if you have one of those right now, please count us in. Chocolate is our favorite.
1. The sugar backbone of DNA is deoxyribose, while the sugar in RNA is ribose. In fact this is how RNA gets its name (Ribo-Nucleic-Acid)
2. RNA uses the nucleotide base uracil instead of thymine. Like thymine, uracil can base pair with adenine. Therefore, the standard base-pairing rules that apply for DNA also apply for RNA. Score!
Not too hard, right?
PolymeraseDuring DNA replication, a DNA polymerase, or the molecule response for making a DNA polymer, uses one of the DNA strands to make a complementary strand using the base pairing rules. Similarly, transcription also relays on the base pairing rules to make a corresponding RNA with a complementary sequence.
The polymerase responsible for transcription is an RNA polymerase. It adds nucleotides to the chain one at a time. After a base is added, the DNA helix reforms, and the RNA chain is separated from the DNA template strand. Similar to a DNA polymerase, a RNA polymerase catalyzes the formation of the phosphodiester bonds between bases, but these bases are ribonucleic acids rather than deoxyribonucleic acids. They polymerize the new strand in the 5' terminus to 3' direction, like DNA polymerase. Therefore, the RNA polymerase reads the DNA sequence from the 3' to 5' end. The 5' and 3' notations refer to the structure of the nucleic acid at the end of the chain. The reason that this orientation is important is that the polymerase can only add the new nucleotide to the 3' hydroxyl end (-OH).
Bacteria contain only one RNA polymerase, but eukaryotes have three types. These different polymerases are better able to adapt to the higher order chromatin structure of eukaryotic chromosomes, including the incorporation of DNA into nucleosomes. These eukaryotic RNA polymerases also specialize in producing specific types of RNA, which we will describe in more detail in the next section.
RNA polymerase doesn't always transcribe the new RNA strand at a constant rate. Other proteins, called elongation factors, help ensure that the RNA polymerase continues producing the RNA strand in a steady fashion.
And finally, the opening act…
Opening Up the DNA HelixJust like during DNA replication, during transcription the DNA helix must be unwound, revealing a template strand of DNA.