How are the nucleic acids DNA and RNA structured as polymers of nucleotides?
How are the nucleic acids DNA and RNA structured as polymers of nucleotides?
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Nucleic acids, which include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are essential biomolecules that serve as the building blocks of genetic information in living organisms. Both DNA and RNA are structured as polymers of nucleotides, which are the fundamental units that make up these macromolecules.
Structure of Nucleotides
Each nucleotide consists of three main components:
- A nitrogenous base: This can be a purine (adenine [A] or guanine [G]) or a pyrimidine (cytosine [C], thymine [T] in DNA, or uracil [U] in RNA).
- A pentose sugar: In DNA, this sugar is 2-deoxyribose, while in RNA, it is ribose. The difference lies in the presence of a hydroxyl group (-OH) on the 2′ carbon of ribose, which is absent in deoxyribose.
- A phosphate group: This group is attached to the 5′ carbon of the sugar and contributes to the acidic nature of nucleic acids.
These components combine to form a nucleotide, which links together to create long chains known as polynucleotides.
Polymerization and Backbone Formation
Nucleotides polymerize through phosphodiester bonds, where the phosphate group of one nucleotide covalently bonds to the hydroxyl group on the 3′ carbon of another nucleotide’s sugar. This process results in a sugar-phosphate backbone that alternates between sugar and phosphate groups, forming the structural framework of both DNA and RNA.
Directionality
The resulting polynucleotide chains have directionality, typically described as 5′ to 3′. This means that one end of the chain has a free phosphate group (5′ end), while the other end has a free hydroxyl group (3′ end).
Structural Differences Between DNA and RNA
DNA Structure
- Double Helix: DNA typically exists as a double-stranded helix, where two polynucleotide strands wind around each other. The strands are anti-parallel, meaning one runs in the 5′ to 3′ direction while the other runs from 3′ to 5.
- Base Pairing: The nitrogenous bases pair specifically (adenine with thymine via two hydrogen bonds; guanine with cytosine via three hydrogen bonds), forming the rungs of the helical structure. This complementary base pairing is crucial for DNA replication and function.
RNA Structure
- Single-Stranded: RNA usually exists as a single strand but can fold into complex three-dimensional shapes due to intramolecular base pairing.
- Base Composition: In RNA, thymine is replaced by uracil. Thus, RNA contains adenine, guanine, cytosine, and uracil
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