The Directionality of DNA Polymerases
- Reading a book upside down and backward would be confusing and probably really hard.
- DNA polymerases face a similar challenge during replication: they can only work in one direction.
DNA polymerases always add nucleotides to the 3' end of a growing DNA strand.
The 5' and 3' Ends of DNA: What Do They Mean?
- DNA strands have a specific directionality, defined by the structure of their nucleotides.
Nucleotide Structure
- Each nucleotide consists of three parts:
- A phosphate group
- A sugar (deoxyribose in DNA)
- A nitrogenous base (adenine, thymine, cytosine, or guanine)
- The sugar molecule in DNA has carbon atoms numbered 1' to 5'.
- These numbers help define the directionality of the DNA strand.
The 5' and 3' Ends
- The 5' end of a DNA strand is where the phosphate group is attached to the 5' carbon of the sugar.
- The 3' end is where the hydroxyl group (-OH) is attached to the 3' carbon of the sugar.
- Think of a DNA strand as a train track.
- The 5' end is like the starting point of the track, marked by a signal post (the phosphate group).
- The 3' end is the open end of the track, where new sections can be added (the hydroxyl group).
How DNA Polymerases Work
- DNA polymerases are enzymes responsible for synthesizing new DNA strands during replication.
- However, they have a strict rule: they can only add nucleotides to the 3' end of a growing strand.
