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

Nucleic Acid Synthesis: Replication and Transcription

Energy of Synthesis

Nucleic acid synthesis, including both DNA replication and RNA transcription, requires energy to form new phosphodiester bonds. This energy is provided by the hydrolysis of phosphoanhydride bonds in nucleotide triphosphates (NTPs for RNA, dNTPs for DNA). Hydrolysis is spontaneous and exergonic, driving the polymerization reactions.

• Phosphoanhydride hydrolysis: The reaction $\text{NTP} + \text{H}_2\text{O} \rightarrow \text{NDP} + \text{P}_i$ releases energy.

• ATP hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$ (1 ATP equivalent); $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{AMP} + \text{PP}_i$ (2 ATP equivalents).

• PPi hydrolysis: $\text{PP}_i + \text{H}_2\text{O} \rightarrow 2\text{P}_i$ is considered irreversible.

• DNA polymerase reaction: $\text{DNA}_n\text{-OH (3')} + \text{dNTP} \rightarrow \text{DNA}_{n+1} + \text{PP}_i$; $\text{PP}_i + \text{H}_2\text{O} \rightarrow 2\text{P}_i$

• Energy cost: 2 ATP equivalents are used per base added by DNA polymerase.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA is replicated, transcribed into RNA, and translated into protein. Each process has three phases: initiation, elongation, and termination. Structure and complementarity are key to function.

• Replication: DNA → DNA

• Transcription: DNA → RNA

• Translation: RNA → Protein

DNA Replication: Initiation

Replication begins at specific sites called origins of replication, which are rich in AT base pairs. Several proteins are required to initiate replication and form 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 from.

• Replication fork: The structure where DNA is unwound and new strands are synthesized.

DNA Replication: Elongation and Termination

DNA polymerase synthesizes new DNA in the 5' to 3' direction. The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously in Okazaki fragments. DNA clamp increases processivity, and proofreading ensures high fidelity.

• Leading strand: Synthesized continuously.

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

• DNA clamp: Keeps polymerase attached to template for rapid synthesis.

• Proofreading: 3'→5' exonuclease activity removes incorrect bases.

• Termination: RNA primers are removed by endonucleases; DNA ligase joins fragments using ATP hydrolysis.

• Telomeres: Repetitive sequences at chromosome ends, extended by telomerase.

Gene Structure and Transcription Initiation

Genes are DNA sequences that code for proteins. Transcription initiates at promoter regions, often rich in AT base pairs (TATA box), and is regulated by flanking regions. Three types of RNA are involved in translation: mRNA, tRNA, and rRNA.

• mRNA: Carries genetic information for protein synthesis.

• tRNA: Brings amino acids to the ribosome.

• rRNA: Forms the ribosome and catalyzes protein synthesis.

• Promoter: DNA sequence where transcription factors bind to initiate transcription.

• Transcription bubble: Region where DNA is unwound for RNA synthesis.

Transcription Elongation

Transcription elongation begins when RNA polymerase is phosphorylated, disrupting its interaction with transcription factors. RNA polymerase is processive and has proofreading ability, though less accurate than DNA polymerase. Multiple RNA polymerases can transcribe the same gene simultaneously.

• RNA polymerase: Catalyzes RNA synthesis from DNA template.

• Hybrid helix: Temporary DNA-RNA hybrid forms during transcription.

• Energy: 2 ATP equivalents per base added (NTP → NMP).

Transcription Termination and Processing

Transcription termination occurs when a signal in the mRNA disrupts RNA polymerase, either by forming a short helix (RNA-dependent) or by protein binding (protein-dependent). Eukaryotic mRNAs are processed after transcription: modifications are added to the 5' and 3' ends, and introns are removed by splicing.

• Termination: RNA-dependent or protein-dependent mechanisms.

• mRNA processing: 5' cap, 3' poly-A tail, and splicing (removal of introns, retention of exons).

• Only exons: are translated into protein.

Summary Table: Key Proteins and Functions in Replication

Protein	Function
Helicase	Unwinds DNA double helix
Topoisomerase	Relieves strain from supercoiling
Single-stranded binding protein	Stabilizes single-stranded DNA
Primase	Synthesizes RNA primer
DNA polymerase	Synthesizes new DNA, proofreads
DNA clamp	Increases processivity of polymerase
DNA ligase	Joins Okazaki fragments
Telomerase	Extends telomeres

Summary Table: Types of RNA and Functions

RNA Type	Function
mRNA	Template for protein synthesis
tRNA	Transfers amino acids to ribosome
rRNA	Forms ribosome, catalyzes translation