With respect to DNA, the terms 3' and 5' (pronounced 3-prime and 5-prime, respectively) are
With respect to DNA, the terms 3' and 5' (pronounced 3-prime and 5-prime, respectively) are used in order to refer to one strand or the other. What do these two terms signify?
- A. 3' refers to the unbound hydroxyl group and 5' refers to the unbound carboxyl group at the end of each DNA strand. .
- B. 3' refers to the unbound hydroxyl group and 5' refers to the unbound phosphate group at the end of each DNA strand.
- C. 3' refers to the unbound methyl group and 5' refers to the unbound hydroxyl group at the end of each DNA strand.
- D. 3' refers to the unbound sulfhydral group and 5' refers to the unbound phosphate group at the end of each DNA strand
Correct Answer: B. 3' refers to the unbound hydroxyl group and 5' refers to the unbound phosphate group at the end of each DNA strand.
Explanation
When nucleotides bond together and form DNA strands, the first and last nucleotides in the strand have slightly different structures than the rest of the nucleotides between them. On one end of the strand, the nucleotide has an exposed hydroxyl group bound to the third carbon in the carbon ring: this end of the strand is thus called 3'. On the opposite end of the strand, the nucleotide has a phosphate group attached to the 5' carbon in the carbon ring, and is thus called the 5' end. These two groups are exposed because they are used in the bonding of nucleotides to one another to form the strand, but each strand ends with one nucleotide that only is bound on one side: thus, leaving either the hydroxyl or phosphate group exposed (depending on which end you are observing). These terms are useful because they allow us to discuss the directionality of DNA-related events- if we didn't have terms for directionality the concept would be much more confusing. Example: DNA polymerase synthesizes the new DNA strand in the 5'-3' direction. Without 3'/5' how would we determine which way the reaction occurs?