In: Biology
The sequence of nitrogenous bases in an RNA and DNA strand is always written in the 5? to 3? direction because __________.
1. each strand of RNA or DNA has an unlinked 3? carbon and an unlinked 5? carbon |
2. DNA and RNA are synthesized in this direction in cells |
3. DNA strands are antiparallel |
4. nucleotides are added to the 5? end of the nucleic acid |
Answer is option 2: DNA and RNA are synthesized in this direction.
Explanation:
DNA and RNA both are polymers of nucleotides. There are four nucleotides (NT) A, G, C and T/U.
A and G are Purines, C and T/U are pyrimidines.
Each NT having 3 components, 1. Nitrogen bases (purines Aand G; Pyramidines C and T/U)
2. Ribose sugar (5 carbon compound)
3. Phosphate (PO4-3)
Ribose and Phosphate groups are acts as backbones. Nitrogen bases can form H-bonds.
First carbon in ribose sugar can form the bond with Nitrogen bases.
Second carbon having free -H and -OH groups in RNA but having only two free -H atoms in DNA.
Third carbon having free -OH group and -H atoms. This free OH group can form the bond with phosphate group of 5' carbon on another NT.
Fourth carbon having only one free -H atom.
Fifth carbon attached to fourth carbon and phosphate group. This 5' phosphate group can form the phospho-diester bond with 3' OH of previous NT.
Conclusion:
In DNA or RNA synthesis, the first NT 5' end having free phosphate group which not involved in any bonding but OH group on 3rd carbon of first NT can form phosphodiester bond with 5' phosphate of another NT. So the synthesis occurred only in 5' to 3' direction.
5' phosphate of first NT and 3' OH of last NT not involved in any bonding. So we represented the ends of DNA and RNA as 5' and 3'.