DNA Structure and Replication

Key Terms and Definitions

This section introduces essential vocabulary and concepts related to the structure and replication of DNA, which are foundational topics in General Biology.

• Nucleotide: The basic building block of nucleic acids, consisting of a nitrogenous base, a five-carbon sugar (pentose), and a phosphate group.

• Nitrogenous Bases: Organic molecules (adenine, guanine, cytosine, thymine, uracil) that form the rungs of the DNA and RNA molecules.

• Pyrimidines: Single-ring nitrogenous bases (cytosine, thymine, uracil).

• Purines: Double-ring nitrogenous bases (adenine, guanine).

• Deoxyribose: The five-carbon sugar found in DNA.

• Ribose: The five-carbon sugar found in RNA.

• Base Pairing: The specific hydrogen bonding between purines and pyrimidines (A-T and G-C in DNA).

• Antiparallel: The orientation of the two strands of DNA running in opposite directions (5' to 3' and 3' to 5').

• Hydrogen Bond: A weak bond between complementary nitrogenous bases in DNA.

Structure of Nucleic Acids

DNA and RNA are polymers of nucleotides, each with a specific structure and function.

• DNA: Double-stranded helix, contains deoxyribose, bases are A, T, G, C.

• RNA: Usually single-stranded, contains ribose, bases are A, U, G, C.

• Base Pairing Rules: In DNA, adenine (A) pairs with thymine (T) via two hydrogen bonds, and guanine (G) pairs with cytosine (C) via three hydrogen bonds.

• Pyrimidines: Cytosine, thymine, uracil.

• Purines: Adenine, guanine.

• Phosphodiester Bond: The covalent bond linking nucleotides in a strand.

Polarity of DNA Strands

DNA strands have directionality, referred to as 5' (five prime) and 3' (three prime) ends, based on the carbon atoms in the sugar molecule.

• 5' End: Terminates with a phosphate group attached to the fifth carbon of the sugar.

• 3' End: Terminates with a hydroxyl group attached to the third carbon of the sugar.

• Antiparallel Orientation: The two DNA strands run in opposite directions, which is essential for replication and function.

Hydrogen Bonding in Base Pairs

Hydrogen bonds stabilize the double helix structure of DNA.

• A-T Base Pair: Two hydrogen bonds.

• G-C Base Pair: Three hydrogen bonds (stronger pairing).

DNA Replication: Overview

DNA replication is the process by which a cell duplicates its DNA before cell division. It is semi-conservative, meaning each new DNA molecule consists of one old and one new strand.

• Origin of Replication: Specific sequence where replication begins.

• Replication Fork: The Y-shaped region where the DNA is split into two strands for copying.

• Enzymes Involved: DNA polymerase, helicase, ligase, primase, topoisomerase, exonuclease.

• Single-Stranded Binding Proteins (SSBPs): Stabilize unwound DNA.

• Leading Strand: Synthesized continuously in the 5' to 3' direction.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments.

• Okazaki Fragments: Short DNA fragments synthesized on the lagging strand.

• DNA Ligase: Joins Okazaki fragments together.

Comparison of DNA Replication in Prokaryotes and Eukaryotes

While the basic mechanism of DNA replication is conserved, there are key differences between prokaryotic and eukaryotic cells.

Enzymes and Proteins in DNA Replication

Several enzymes and proteins coordinate the accurate and efficient replication of DNA.

• Helicase: Unwinds the DNA double helix.

• Primase: Synthesizes RNA primers to initiate DNA synthesis.

• DNA Polymerase: Adds nucleotides to the growing DNA strand; also proofreads and corrects errors.

• Ligase: Seals nicks and joins Okazaki fragments on the lagging strand.

• Topoisomerase: Relieves supercoiling ahead of the replication fork.

• Single-Stranded Binding Proteins (SSBPs): Prevent re-annealing of separated DNA strands.

Directionality and Synthesis of DNA Strands

DNA polymerase can only add nucleotides to the 3' end of a growing DNA strand, resulting in continuous synthesis on the leading strand and discontinuous synthesis on the lagging strand.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized away from the replication fork in short segments (Okazaki fragments).

• Discontinuous Synthesis: The lagging strand is synthesized in fragments because DNA polymerase cannot synthesize in the 3' to 5' direction.

Energy for DNA Replication

The energy for DNA synthesis comes from the hydrolysis of high-energy phosphate bonds in deoxynucleoside triphosphates (dNTPs).

• Reaction: Each incoming dNTP loses two phosphate groups (pyrophosphate), releasing energy to form the phosphodiester bond.

Equation:

Summary Table: Key Enzymes in DNA Replication

Key Concepts and Applications

• Replication Fork: The area where the DNA double helix is separated into two single strands for copying.

• Replication Bubble: The region of DNA where the double helix has unwound and replication is occurring.

• Cell Cycle: DNA replication occurs during the S phase of the cell cycle.

• Proofreading: DNA polymerase has proofreading ability to correct errors during replication.

Example: DNA Replication in E. coli

In Escherichia coli, DNA replication begins at a single origin (oriC) and proceeds bidirectionally around the circular chromosome, using many of the enzymes described above.

Additional info:

• Some details, such as the specific names of DNA polymerases in eukaryotes (e.g., DNA polymerase α, δ, ε), can be added for advanced study.

• Replication in eukaryotes is more complex due to chromatin structure and multiple origins of replication.