Nucleic Acid Structures and DNA Replication: Study Notes

Nucleic Acids: Structure and Function

Introduction to Nucleic Acids

Nucleic acids are complex organic molecules essential for the storage, transmission, and expression of genetic information within all living cells. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Nucleic acids store and transmit genetic information.

• Central Dogma: Describes the flow of genetic information: DNA → RNA → Protein.

Types of Nucleic Acids

• DNA (Deoxyribonucleic Acid):

  • Structure: Double helix of two antiparallel strands.

  • Function: Stores genetic information for development, functioning, growth, and reproduction.

  • Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

  • Sugar: Deoxyribose.

• RNA (Ribonucleic Acid):

  • Structure: Typically single-stranded.

  • Function: Transfers genetic information from DNA to protein synthesis machinery; also has regulatory and catalytic roles.

  • Bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G).

  • Sugar: Ribose.

Structure of Nucleic Acids

Nucleotides and Nucleosides

Nucleic acids are polymers made up of monomers called nucleotides. Each nucleotide consists of a nitrogenous base, a five-carbon sugar, and a phosphate group.

• Nucleoside: Nitrogenous base + sugar.

• Nucleotide: Nucleoside + phosphate group.

Nitrogenous Bases

• Purines: Adenine (A), Guanine (G) – double-ring structure.

• Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA) – single-ring structure.

Five-Carbon Sugars

• Deoxyribose: Found in DNA (lacks an -OH at the 2' carbon).

• Ribose: Found in RNA (has an -OH at the 2' carbon).

Phosphate Group

• Phosphate groups are attached to the sugar, forming the backbone of the nucleic acid structure via phosphodiester bonds.

Phosphodiester Bond

The phosphodiester bond links the 3' carbon atom of one sugar molecule to the 5' carbon atom of another, creating the sugar-phosphate backbone of nucleic acids.

$\text{Phosphodiester bond:} \quad \text{3'-OH (sugar)} \xrightarrow{\text{phosphate}} \text{5'-OH (next sugar)}$

N-Glycosidic Bonds

• In purines (A, G): Bond forms between N9 of the base and C1' of the sugar.

• In pyrimidines (C, T, U): Bond forms between N1 of the base and C1' of the sugar.

Nucleotide Base Pairing

Types of Nucleotide Bases

• Adenine (A): Pairs with Thymine (T) in DNA, Uracil (U) in RNA.

• Thymine (T): Pairs with Adenine (A) in DNA; not present in RNA.

• Cytosine (C): Pairs with Guanine (G) in both DNA and RNA.

• Guanine (G): Pairs with Cytosine (C) in both DNA and RNA.

• Uracil (U): Replaces Thymine in RNA; pairs with Adenine (A).

Base Pairing Rules

• A-T (DNA) / A-U (RNA): Two hydrogen bonds.

• C-G: Three hydrogen bonds (provides extra stability).

Purines and Pyrimidines

Purines

• Structure: Double-ring, larger than pyrimidines.

• Types: Adenine (A), Guanine (G).

• Metabolites: Uric acid (clinical significance: high levels can cause gout/kidney stones).

Pyrimidines

• Structure: Single-ring, smaller than purines.

• Types: Cytosine (C), Thymine (T, DNA), Uracil (U, RNA).

• Metabolites: Broken down into β-alanine and β-aminoisobutyric acid (disorders can cause orotic aciduria).

The Double Helix

Definition and Discovery

• The double helix is the three-dimensional structure of DNA, composed of two intertwined strands forming a spiral.

• Discovered by James Watson and Francis Crick (1953), with key X-ray diffraction data from Rosalind Franklin.

Structure of the Double Helix

• Backbone: Sugar-phosphate backbone with negative charges on the outside.

• Base pairs: Located in the center, connected by hydrogen bonds.

Antiparallel Strands

• DNA strands run in opposite directions (5'→3' and 3'→5').

• This orientation is essential for replication and function.

• New nucleotides are always added to the 3' end.

Major and Minor Grooves

• Major Groove: Wider and deeper; accessible for protein binding (e.g., transcription factors).

• Minor Groove: Narrower and shallower; some proteins and small molecules bind here.

• Significance: Grooves allow proteins to recognize specific DNA sequences without unwinding the helix.

Importance of Grooves

• Protein-DNA Interactions: Regulatory proteins, enzymes, and other molecules bind in grooves for replication, transcription, and repair.

• Drug Targeting: Some drugs (e.g., doxorubicin) target grooves to disrupt DNA function.

Nucleosides and Bases

• The name of a base changes when it is attached to a sugar (e.g., adenine → adenosine).

Base	Nucleoside
Adenine (A)	Adenosine
Guanine (G)	Guanosine
Cytosine (C)	Cytidine
Thymine (T)	Thymidine
Uracil (U)	Uridine

DNA vs RNA

• Both alkali and heat can denature DNA (separate strands), but do not break phosphodiester bonds in DNA.

• Phosphodiester bonds in RNA are cleaved by alkali, making RNA less stable and more prone to self-cleavage.

• RNA is generally less stable than DNA due to the presence of the 2'-OH group in ribose.

Structure of Chromosomes

Prokaryotes vs Eukaryotes

• Prokaryotes: Simple, unicellular (bacteria, archaea); DNA is circular and found in the nucleoid.

• Eukaryotes: Complex, multicellular (plants, animals, fungi); DNA is linear and found in the nucleus.

Feature	Prokaryotic DNA	Eukaryotic DNA
Structure	Circular	Linear
Location	Nucleoid (cytoplasm)	Nucleus
Genome Size	Smaller	Larger
Number of Chromosomes	Single	Multiple
Histones	Absent	Present (chromatin)
Plasmids	Often present	Rarely present
Replication Origin	Single origin	Multiple origins
Packing	Less structured	Highly structured
Complexity	Simpler	More complex (introns and exons)

Chromatin and Nucleosomes

• DNA is negatively charged due to the phosphate backbone.

• DNA wraps around positively charged histone proteins to form nucleosomes, aiding in compaction and regulation.

Haploid and Diploid Cells

Haploid Cells

• Contain a single set of chromosomes (n).

• Examples: Gametes (sperm and egg cells in humans).

• Humans: n = 23 chromosomes.

Diploid Cells

• Contain two sets of chromosomes (2n), one from each parent.

• Examples: Somatic (body) cells.

• Humans: 2n = 46 chromosomes.

General Features of RNA

Structure of RNA

• Typically single-stranded.

• Contains ribose sugar (with 2'-OH group).

• Bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G).

Types of RNA

• mRNA (Messenger RNA): Carries genetic information from DNA to ribosome.

• rRNA (Ribosomal RNA): Forms ribosome structure and catalyzes protein synthesis.

• tRNA (Transfer RNA): Brings amino acids to ribosome during translation.

• Other types: snRNA (small nuclear RNA), miRNA (microRNA).

DNA Replication

Overview

• Definition: The process by which a cell duplicates its DNA, producing two identical copies.

• Importance: Essential for cell division and genetic inheritance.

Semiconservative Replication

• Each daughter chromosome contains one parental and one newly synthesized strand.

• Ensures genetic continuity across generations.

Cell Cycle and Replication

• In eukaryotes, DNA replication occurs during the S phase of the cell cycle, followed by G2 and M phases.

• Each daughter cell receives an exact copy of the parent DNA.

Steps of DNA Replication

1. Initiation

2. Elongation

3. Termination

Initiation

• Origin of Replication: Specific DNA sequences where replication begins.

• Helicase: Unwinds the DNA double helix by breaking hydrogen bonds.

• Replication Fork: Y-shaped structure formed by unwinding.

• Topoisomerase: Relieves supercoiling ahead of the fork by making temporary cuts.

• SSBs (Single-Strand Binding Proteins): Stabilize single-stranded DNA.

Elongation

• Primase: Synthesizes a short RNA primer complementary to the DNA template.

• DNA Polymerase: Adds nucleotides to the 3' end of the primer, synthesizing new DNA.

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

• Lagging Strand: Synthesized discontinuously as Okazaki fragments.

• Sliding Clamp: Holds DNA polymerase in place during strand extension.

RNA Primer Requirement

• DNA polymerase requires a free 3'-OH group to function.

• RNA primer (oligonucleotide) is synthesized by primase in the 5'→3' direction.

• DNA polymerase adds deoxyribonucleotides to the 3'-OH of the primer.

Termination

• Exonuclease: Removes RNA primers (DNA polymerase I and RNase H).

• DNA Ligase: Fills the gaps left by RNA primers, joining Okazaki fragments into a continuous strand.

Key Equations and Concepts

• Base Pairing: $\text{A} = \text{T}$ (in DNA), $\text{C} = \text{G}$

• Directionality: DNA and RNA are synthesized in the 5'→3' direction.

• Phosphodiester Bond Formation: $\text{(3'-OH)} + \text{(5'-phosphate)} \rightarrow \text{phosphodiester bond} + \text{H}_2\text{O}$

Additional info: These notes provide a comprehensive overview of nucleic acid structure and DNA replication, suitable for undergraduate biochemistry students preparing for exams.