Nucleic Acids and DNA Structure

Introduction to Nucleic Acids

Nucleic acids are essential macromolecules that store and transmit genetic information in all living organisms. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Nucleic acids are polymers made up of monomers called nucleotides.

• Each nucleotide consists of three components:

  • Phosphate group

  • Five-carbon sugar (deoxyribose in DNA, ribose in RNA)

  • Nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil replaces thymine in RNA)

• DNA and RNA differ in their sugar and base composition.

Structural Features of DNA and RNA

DNA and RNA polymers have distinct structural features that determine their function and stability.

• DNA contains deoxyribose sugar, which lacks an oxygen atom at the 2' carbon.

• RNA contains ribose sugar, which has a hydroxyl group (-OH) at the 2' carbon, making RNA more reactive and less stable than DNA.

• The nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T). In RNA, uracil (U) replaces thymine.

• DNA is typically double-stranded and forms a double helix, while RNA is usually single-stranded but can form complex secondary structures.

Comparison of DNA and RNA Monomers

The monomers of RNA differ from those of DNA in two key ways:

• Bases: RNA contains uracil instead of thymine.

• Sugar: RNA contains ribose instead of deoxyribose.

• The presence of the –OH group on ribose makes RNA more reactive and less stable than DNA.

Base Pairing and Double Helix Structure

DNA's double helix structure is stabilized by specific base pairing and hydrophobic interactions.

• Base pairing: Adenine pairs with thymine (A-T) via two hydrogen bonds; guanine pairs with cytosine (G-C) via three hydrogen bonds.

• Purines (A, G) always pair with pyrimidines (C, T/U) to maintain a uniform diameter of the helix.

• Hydrophobic interactions between stacked bases create a non-polar core, while the sugar-phosphate backbone interacts with the aqueous environment.

• Major and minor grooves in the helix allow for protein binding, which can be sequence-specific or non-specific.

Table: Comparison of DNA and RNA

DNA Replication and Synthesis

Overview of DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. This ensures that each daughter cell receives an identical set of genetic information.

• Replication is semiconservative: each new DNA molecule consists of one old strand and one newly synthesized strand.

• Replication begins at specific sites called origins of replication (ORI).

• In prokaryotes, there is typically a single origin; in eukaryotes, there are multiple origins along linear chromosomes.

Enzymes and Proteins Involved in DNA Replication

Several enzymes and proteins coordinate the replication process:

• Helicase: Unwinds the DNA double helix.

• Single-strand binding proteins (SSB): Stabilize unwound DNA.

• Primase: Synthesizes RNA primers to initiate DNA synthesis.

• DNA polymerase: Adds nucleotides to the growing DNA strand in the 5' to 3' direction.

• Ligase: Joins Okazaki fragments on the lagging strand.

Leading and Lagging Strand Synthesis

DNA replication occurs differently on the two strands:

• Leading strand: Synthesized continuously toward the replication fork.

• Lagging strand: Synthesized discontinuously away from the fork in short segments called Okazaki fragments.

• All DNA polymerases synthesize new DNA in the 5' to 3' direction.

Polymerase Chain Reaction (PCR)

PCR is a laboratory technique used to amplify specific DNA sequences in vitro. It mimics natural DNA replication but uses heat and specialized enzymes.

• Purpose: To generate millions of copies of a specific DNA segment.

• Key components:

  • DNA template

  • Primers (short DNA sequences that initiate synthesis)

  • dNTPs (deoxynucleotide triphosphates)

  • Taq polymerase (heat-stable DNA polymerase)

• Three main steps per cycle:

  1. Denaturation: DNA strands are separated by heating (~94°C).

  2. Annealing: Primers bind to target sequences (~60°C).

  3. Elongation: Taq polymerase synthesizes new DNA (~72°C).

• These steps are repeated for 20-40 cycles to amplify DNA.

Table: PCR Cycle Steps

Directionality of DNA Synthesis

DNA and RNA synthesis always proceeds in the 5' to 3' direction due to the chemistry of nucleotide addition.

• The 3' hydroxyl group of the growing strand attacks the incoming nucleotide's phosphate group.

• If errors occur at the 3' end, elongation cannot continue.

Equation for DNA polymerization:

Telomeres and Telomerase

Linear eukaryotic chromosomes have ends called telomeres that require special mechanisms for complete replication.

• During replication, the lagging strand cannot fully replicate the very end of the chromosome, leading to gradual shortening.

• Telomerase is an enzyme that extends telomeres, preventing loss of important genetic information.

• Telomerase is active in germ cells and some stem cells, but not in most somatic cells.

Summary Table: DNA Replication in Prokaryotes vs. Eukaryotes

Additional info:

• RNA molecules can form secondary and tertiary structures, such as hairpin loops, and some have catalytic activity (ribozymes).

• Major and minor grooves in DNA are important for protein-DNA interactions, including regulation of gene expression.

• Practice worksheets and review sessions are recommended for mastering these concepts.