3.9
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 made up of a pentose sugar, a nitrogen-containing base, and a phosphate group.
There are two types of nucleic acids: Deoxyribonucleic acid, or DNA, and ribonucleic acid, or RNA. Their chemical structures differ based on the 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 a hydrogen atom instead of a hydroxyl group at carbon-2. The nitrogenous base is bonded to carbon-1 of the sugar, while the phosphate group is attached to carbon-5.
Both RNA and DNA contain the bases adenine, cytosine, and guanine. But DNA contains thymine, while RNA contains uracil.
These bases are grouped into two categories: purines and pyrimidines. Adenine and guanine are purines with a double-ring structure, while the other bases are pyrimidines with a single-ring structure.
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 in RNA, adenine and uracil pair together. Both pairs are linked by two hydrogen bonds.
DNA and RNA polymerase enzymes catalyze the formation of nucleic acid chains by linking nucleotides together.
A phosphodiester bond forms between the hydroxyl group attached to carbon-3 of one nucleotide and the phosphate group attached to carbon-5 of the next nucleotide. This reaction leaves a free phosphate group at the five-prime end and a free hydroxyl group at the three-prime end.
When paired with a complementary strand, the two molecules are antiparallel, meaning one strand runs five prime to three prime, while the complementary strand runs three prime to five prime.
The strands are held together by several intermolecular forces, including hydrogen bonds between bases, hydrophobic effects, and van der Waals interactions between stacked bases.
DNA is a double helix made up 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 strand. It can also form intra-strand complementary base pairs, resulting in different RNA secondary structures that have distinct functions within the cell.
View the full transcript and gain access to JoVE Core videos
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 holds DNA strands together in the double helix?
DNA strands are held together by multiple intermolecular forces including hydrogen bonds between complementary nitrogenous bases, van der Waals interactions, and hydrophobic effects. Adenine pairs with thymine using two hydrogen bonds, while guanine pairs with cytosine using three hydrogen bonds. These forces maintain the double helix structure while allowing strand separation when needed.
Q4: How are nucleotides linked together to form polynucleotide chains?
Nucleotides are linked by phosphodiester linkages, where the phosphate group attached to carbon-5 of one nucleotide bonds to the hydroxyl group on carbon-3 of the next nucleotide. This creates a sugar-phosphate backbone with an unattached 5' end containing a free phosphate and a 3' end with a free hydroxyl group.
Q5: What is the difference between purines and pyrimidines?
Purines, including adenine and guanine, have a double carbon-nitrogen ring structure. Pyrimidines, including cytosine, thymine, and uracil, have a single carbon-nitrogen ring structure. Both types contain an amino group that makes them basic, and they serve as the nitrogenous bases in nucleotides.
Q6: 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 the complementary base pairing rules, where adenine pairs with thymine and guanine pairs with cytosine, creating a stable double helix structure.
Q7: How does RNA structure differ from DNA structure?
RNA is typically single-stranded, unlike the double-stranded DNA. However, RNA can form secondary structures through intramolecular base pairing between complementary sequences within the same molecule, creating distinct three-dimensional shapes essential for its function. RNA can also bind to complementary RNA or DNA strands when needed.