Nucleic Acid Synthesis: Replication and Transcription (GOB Chemistry Study Guide)

Nucleic Acid Synthesis: Replication and Transcription

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information in cells: DNA is replicated, transcribed into RNA, and then translated into protein. Each step is essential for the storage, transfer, and expression of genetic information.

• Replication: DNA makes a copy of itself.

• Transcription: DNA is used as a template to synthesize RNA.

• Translation: RNA is used to synthesize proteins.

• Each process has three phases: Initiation, Elongation, and Termination.

Structure and Function of Nucleic Acids

Nucleic acids (DNA and RNA) are polymers of nucleotides. Their structure enables the storage and transfer of genetic information through complementary base pairing.

• DNA: Double-stranded helix, bases are C, G, A, T, sugar is 2'-deoxyribose.

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

• Complementarity: Enables accurate replication and transcription.

Energy of Nucleic Acid Synthesis

Phosphoanhydride Hydrolysis

The energy required for nucleic acid synthesis comes from the hydrolysis of phosphoanhydride bonds in nucleoside triphosphates (NTPs for RNA, dNTPs for DNA). This reaction is exergonic and drives the polymerization process.

• General reaction: NTP + H2O → NDP + Pi

• ATP hydrolysis: ATP + H2O → ADP + Pi (1 ATP equivalent)

• ATP + H2O → AMP + PPi: (2 ATP equivalents)

• PPi hydrolysis: PPi + H2O → 2 Pi (irreversible)

DNA Polymerase Reaction

DNA polymerase catalyzes the addition of nucleotides to the growing DNA strand during replication. The reaction uses two ATP equivalents per nucleotide added.

• Reaction: DNAn-OH (3') + dNTP → DNAn+1 + PPi

• PPi + H2O → 2 Pi

• DNA polymerase only adds nucleotides to the 3' OH of the primer strand.

• Template strand determines which base is added via complementarity.

Replication Initiation

Proteins and Structural Features

Several proteins are required to initiate DNA replication at the replication fork:

• Helicase: Unwinds the DNA double helix using ATP hydrolysis.

• Topoisomerase: Relieves strain from overwound DNA ahead of the fork.

• Single-stranded binding protein: Stabilizes single-stranded DNA to prevent reannealing.

• Primase: Synthesizes a short RNA primer for DNA polymerase to extend.

Replication Fork and Origin of Replication

Replication begins at origins of replication, which are rich in A-T base pairs due to weaker hydrogen bonding. Each replication bubble has two forks, and DNA polymerase can only extend from an existing 3' OH.

Replication Elongation and Termination

Elongation: Leading and Lagging Strands

DNA polymerase synthesizes both strands simultaneously, but only in the 5'→3' direction:

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized discontinuously in Okazaki fragments (1000–2000 bp).

• RNA primers are pushed aside and removed by nucleases.

Processivity and Proofreading

DNA polymerase is highly processive due to the DNA clamp, which allows it to add thousands of nucleotides before dissociating. It also has 3'→5' exonuclease activity for proofreading, ensuring high fidelity (1 error per 1 billion base pairs).

Replication Termination and Telomeres

At the end of replication, RNA primers are removed, and Okazaki fragments are joined by DNA ligase. Linear chromosomes have telomeres, repetitive DNA sequences extended by telomerase to allow complete replication.

• DNA ligase reaction: DNA ligase + ATP → DNA ligase-AMP + PPi

• Telomerase is active in dividing cells; telomere length decreases with cell age.

Gene Structure and Transcription Initiation

Types of RNA

Three main types of RNA are involved in translation:

• Messenger RNA (mRNA): Carries genetic information for protein synthesis.

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

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

DNA vs. RNA Structure

• DNA: C, G, A, T; 2'-deoxyribose; double-stranded.

• RNA: C, G, A, U; ribose; single-stranded.

Transcription Initiation and Gene Structure

• Gene: DNA sequence coding for a protein.

• Genome: All genetic material of an organism.

• Promoter: Region upstream of the gene (often AT-rich, e.g., TATA box) where transcription factors bind to initiate transcription.

• Transcription bubble: DNA is unwound for RNA polymerase to access the template strand.

Transcription Elongation

RNA Polymerase Function

RNA polymerase synthesizes RNA from a DNA template, adding nucleotides to the 3' end. The reaction is driven by hydrolysis of 2 ATP equivalents per nucleotide added.

• Reaction: RNAn-OH (3') + NTP → RNAn+1 + PPi

• PPi + H2O → 2 Pi

• RNA polymerase is processive and has proofreading ability, though less accurate than DNA polymerase.

Transcription Termination and mRNA Processing

Termination Mechanisms

• RNA-dependent: Short helix forms in RNA, disrupting RNA polymerase.

• Protein-dependent: Rho protein binds RNA and disrupts RNA polymerase.

mRNA Processing

• End modifications: 5' cap and 3' poly-A tail added.

• Splicing: Introns are removed, exons are retained and translated.

• All types of RNA are processed before translation.

Summary Table: Key Proteins and Functions in Replication

Protein	Function
Helicase	Unwinds DNA double helix
Topoisomerase	Relieves strain from overwound DNA
Single-stranded binding protein	Stabilizes single-stranded DNA
Primase	Synthesizes RNA primer
DNA polymerase	Synthesizes new DNA strand
DNA clamp	Promotes processivity of DNA polymerase
Exonuclease	Proofreads and removes incorrect bases
DNA ligase	Joins Okazaki fragments
Telomerase	Extends telomeres

Key Equations

• $\text{NTP} + \text{H}_2\text{O} \rightarrow \text{NDP} + \text{P}_i$

• $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$

• $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{AMP} + \text{PP}_i$

• $\text{PP}_i + \text{H}_2\text{O} \rightarrow 2 \text{P}_i$

• $\text{DNAn-OH (3')} + \text{dNTP} \rightarrow \text{DNAn+1} + \text{PP}_i$

• $\text{RNAn-OH (3')} + \text{NTP} \rightarrow \text{RNAn+1} + \text{PP}_i$

Additional info:

• Telomeres are essential for maintaining chromosome integrity during replication of linear chromosomes.

• DNA replication is semiconservative: each new DNA molecule contains one old strand and one new strand.