21.9
Nucleic Acids
Nucleic acids are the most important macromolecules for the continuity of life. They carry the cell's genetic blueprint and carry instructions for its…
Nucleic acids are polymers of nucleotides – molecules composed of a pentose sugar, a nitrogen-containing base, and a phosphate group.
There are two types of nucleic acids: deoxyribonucleic acid, DNA, and ribonucleic acid, RNA.
Their chemical structures differ depending on which pentose sugar and nitrogenous bases they contain.
The pentose sugar in RNA is ribose, which has a hydroxyl group attached to carbon-2. The sugar in DNA is deoxyribose, which has only a hydrogen atom but no oxygen at carbon-2.
The nitrogenous base is bonded to carbon-1 and the phosphate at carbon-5. Both RNA and DNA contain the bases adenine, cytosine, and guanine; but DNA has thymine, while RNA has uracil.
In DNA and RNA, guanine and cytosine form complementary base pairs, linked by three hydrogen bonds. Adenine and thymine form base pairs in DNA while adenine and uracil pair in RNA, both linked together by two hydrogen bonds.
Various DNA or RNA polymerase enzymes catalyze the polymerization of nucleotides.
A phosphodiester bond is formed between a hydroxyl group attached to carbon-3 and the phosphate group attached to carbon-5 of the next nucleotide. This reaction leaves an unattached 5’ end with a free phosphate group and an unattached 3’ end with a free hydroxyl group.
When paired with a complementary strand, the two molecules are antiparallel, meaning the 5’ end of one strand pairs with the 3’ end of the other.
The strands are held together by intermolecular forces, including hydrophobic effects, van der Waals interactions, and the specific hydrogen bonds that form between the nitrogenous bases. DNA is a double helix composed of two polynucleotide chains wound around each other. In contrast, RNA is often found as a single-stranded molecule.
However, RNA can bind to a complementary RNA or DNA. It can also exhibit intra-strand complementary base pairing resulting in different types of RNA secondary structures that have distinct functions within the cell.
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Q1: What are the three main components of a nucleotide?
A nucleotide consists of three components: a pentose sugar (five-carbon sugar), a nitrogenous base, and a phosphate group. The nitrogenous base attaches to carbon-1 of the sugar, while the phosphate group attaches to carbon-5. These three parts combine to form the basic building block of DNA and RNA.
Q2: How do DNA and RNA differ in their sugar and base composition?
DNA contains deoxyribose sugar with a hydrogen atom at carbon-2, while RNA contains ribose sugar with a hydroxyl group at carbon-2. Both share adenine, guanine, and cytosine bases, but DNA contains thymine whereas RNA contains uracil instead. These structural differences affect their stability and function in the cell.
Q3: What is a phosphodiester bond and how does it form?
A phosphodiester bond forms between the hydroxyl group on carbon-3 of one nucleotide's sugar and the phosphate group on carbon-5 of the next nucleotide. DNA and RNA polymerase enzymes catalyze this polymerization reaction, creating a sugar-phosphate backbone. This bonding leaves a free 5' phosphate end and a free 3' hydroxyl end on the nucleic acid strand.
Q4: What are complementary base pairs and how many hydrogen bonds hold them together?
Complementary base pairs follow specific pairing rules: guanine pairs with cytosine through three hydrogen bonds, while adenine pairs with thymine in DNA or uracil in RNA through two hydrogen bonds. These hydrogen bonds hold the strands together and ensure accurate replication and transcription. The specificity of base pairing is fundamental to nucleic acid function.
Q5: Why are DNA strands described as antiparallel?
DNA strands are antiparallel because they run in opposite directions: the 5' end of one strand pairs with the 3' end of the complementary strand. This orientation is maintained by hydrogen bonds between nitrogenous bases and hydrophobic interactions. The antiparallel arrangement is essential for the double helix structure and for accurate DNA replication.
Q6: What is the difference between DNA and RNA structure?
DNA typically exists as a double helix with two polynucleotide chains wound around each other, with the sugar-phosphate backbone on the outside and bases stacked inside. RNA is usually single-stranded but can form secondary structures through intramolecular base pairing between complementary sequences. These structural differences enable their distinct roles in storing genetic information and directing protein synthesis.
Q7: How do the four major types of RNA differ in their functions?
Messenger RNA (mRNA) carries genetic instructions from DNA to ribosomes. Ribosomal RNA (rRNA) is a major ribosome component that aligns mRNA and catalyzes peptide bond formation. Transfer RNA (tRNA) delivers correct amino acids during protein synthesis. MicroRNA (miRNA) regulates gene expression by interfering with mRNA messages. Each RNA type has a distinct structure and role in protein synthesis and gene regulation.