DNA Replication

Overview of DNA Replication

DNA replication is the process by which a cell copies its DNA, ensuring that each daughter cell receives an identical genetic blueprint. This process is semiconservative, meaning each new DNA molecule contains one original (parent) strand and one newly synthesized (daughter) strand.

• Semiconservative replication: Each daughter DNA molecule consists of one parental and one new strand.

• Purpose: To ensure genetic continuity during cell division.

Key Enzymes Involved in DNA Replication

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

• Topoisomerase: Relieves supercoiling and twisting stress ahead of the replication fork.

• Primase: Synthesizes short RNA primers to provide a starting point for DNA polymerase.

• DNA Polymerase III: Adds nucleotides to the growing DNA strand in the 5′ → 3′ direction.

• DNA Polymerase I: Removes RNA primers and replaces them with DNA nucleotides.

• DNA Ligase: Joins Okazaki fragments on the lagging strand to create a continuous DNA strand.

• Single-strand binding proteins (SSBs): Stabilize unwound DNA strands and prevent them from reannealing.

Steps of DNA Replication

• Initiation: Begins at origins of replication where initiator proteins open the DNA helix, forming a replication fork.

• Elongation:

  • Leading strand: Synthesized continuously in the direction of the replication fork.

  • Lagging strand: Synthesized discontinuously in short segments called Okazaki fragments, which are later joined by DNA ligase.

• Termination: RNA primers are removed, replaced with DNA, and DNA ligase seals nicks in the sugar-phosphate backbone.

• Proofreading: DNA polymerases check and correct errors to ensure high fidelity of replication.

Central Dogma: Transcription and Translation

Transcription

Transcription is the process by which a gene's DNA sequence is copied into messenger RNA (mRNA). This is the first step in gene expression.

• Location: In eukaryotes, transcription occurs in the nucleus; in prokaryotes, it occurs in the cytoplasm.

• Key enzyme: RNA polymerase, which binds to the promoter region, unwinds DNA, and synthesizes RNA using ribonucleotides (A, U, C, G).

• Stages:

  • Initiation: RNA polymerase binds to the promoter and begins RNA synthesis.

  • Elongation: RNA polymerase moves along the template strand, synthesizing RNA.

  • Termination: RNA polymerase reaches a stop signal and releases the completed mRNA.

RNA Processing in Eukaryotes

• Splicing: Removal of non-coding introns from pre-mRNA.

• 5′ Cap: Addition of a modified guanine nucleotide to the 5′ end for stability and ribosome recognition.

• Poly-A Tail: Addition of a string of adenine nucleotides to the 3′ end for stability and export from the nucleus.

Translation

Translation is the process by which the information in mRNA is decoded to build a protein, a chain of amino acids. This occurs at the ribosome in the cytoplasm.

• mRNA: Carries genetic instructions in codons (three-nucleotide sequences).

• tRNA: Brings the correct amino acid to the ribosome by matching its anticodon to the mRNA codon.

• Ribosome: The molecular machine that reads mRNA and links amino acids together.

• Stages of Translation:

  • Initiation: Ribosome assembles at the start codon (AUG) on mRNA; the first tRNA brings methionine.

  • Elongation: Ribosome moves along mRNA, tRNAs bring amino acids, and the polypeptide chain grows.

  • Termination: Ribosome reaches a stop codon (UAA, UAG, UGA), releasing the completed protein.

Protein Folding, Modification, and Degradation

Protein Folding

After translation, proteins must fold into their correct three-dimensional shapes to function properly. Molecular chaperones assist in this process and prevent misfolding, which can lead to diseases such as Alzheimer's and cystic fibrosis.

Protein Modification

• Phosphorylation: Addition of phosphate groups to regulate activity.

• Glycosylation: Addition of sugars for stability and recognition.

• Lipidation: Attachment of lipids for membrane association.

• Cleavage: Cutting precursor proteins into active forms.

Protein Degradation

• Ubiquitin–Proteasome System: Tags proteins for degradation and breaks them into peptides.

• Autophagy: Degrades larger protein aggregates or damaged organelles via lysosomes.

Microbial Mutation and DNA Repair

Types of Mutations

• Spontaneous mutations: Occur naturally during DNA replication or due to chemical changes.

• Induced mutations: Caused by mutagens such as UV light, X-rays, or chemicals.

• Common types:

  • Point mutations: Single nucleotide changes (silent, missense, nonsense).

  • Frameshift mutations: Insertions or deletions that shift the reading frame.

  • Chromosomal rearrangements: Large-scale changes like duplications or inversions.

DNA Repair Mechanisms

DNA repair systems are essential for maintaining genome stability and cell survival. Without repair, mutations can accumulate, leading to loss of function or cell death.

• Base Excision Repair (BER): Fixes small, non-helix-distorting base lesions.

• Nucleotide Excision Repair (NER): Removes bulky DNA lesions, such as thymine dimers.

• Mismatch Repair (MMR): Corrects errors introduced during DNA replication.

Bacterial Gene Transfer and Evolution

Horizontal Gene Transfer (HGT)

Bacteria can acquire new genetic traits rapidly through horizontal gene transfer, which is the movement of genetic material between organisms other than by descent from parent to offspring. This process is crucial for bacterial adaptation and evolution.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA (usually plasmids) between bacteria via a pilus.

Mobile Genetic Elements (MGEs)

MGEs are DNA segments that can move within or between genomes, facilitating horizontal gene transfer and rapid acquisition of new traits such as antibiotic resistance.

• Plasmids: Small, circular DNA molecules that replicate independently and often carry beneficial genes.

• Transposable Elements (Transposons): DNA sequences that can change position within the genome, sometimes carrying antibiotic resistance genes.

• Insertion Sequences (IS): Simple transposable elements containing only the gene for transposase.

• Integrons: Genetic elements that can capture and express genes, often leading to multidrug resistance.

Bacteriophages (Phages)

Bacteriophages are viruses that infect bacteria. Their DNA can integrate into the bacterial chromosome as a prophage, sometimes conferring new traits such as toxin production (lysogenic conversion).

Significance of Horizontal Gene Transfer

• Antibiotic resistance: Rapid spread of resistance genes among bacterial populations.

• New metabolic abilities: Acquisition of genes for novel metabolic pathways.

• Virulence factors: Gaining genes that enhance pathogenicity.