DNA Replication - Transcription - Translation

The Nature of Genetic Material

  • Genome: The complete genetic material of an organism.

    • Most of the genome is found in chromosomes, some in plasmids or eukaryotic organelles.

    • Cells' genomes are exclusively DNA; viruses may contain either DNA or RNA.

Chromosome Structure

  • Chromosomes: Distinct structures made of packaged DNA.

    • Eukaryotic Chromosomes:

      • Composed of DNA wrapped around histone proteins.

      • Located in the nucleus; can be diploid (pairs) or haploid (single).

      • Generally linear in structure.

    • Bacterial Chromosomes:

      • DNA is compacted with histone-like proteins.

      • Typically one or two chromosomes.

Gene Categories

  • Three types of genes:

    • Structural Genes: Code for proteins.

    • RNA Genes: Code for RNA machinery in protein production.

    • Regulatory Genes: Control gene expression.

  • Genotype: The complete set of genes in an organism, representing its genetic makeup.

  • Phenotype: The observable traits resulting from the expression of the genotype.

DNA Structure

  • Nucleotide: Basic unit of DNA structure.

    • Composed of phosphate, deoxyribose sugar, and a nitrogenous base.

    • Nucleotides link via a sugar-phosphate backbone.

Nitrogenous Bases

  • Attached along each DNA strand by covalent bonds.

    • Base Pairing Rules:

      • Adenine (A) pairs with Thymine (T)

      • Guanine (G) pairs with Cytosine (C)

  • Bases join through weak hydrogen bonds, allowing the DNA to unzip for accessibility.

Double Helix Structure

  • Antiparallel Arrangement: Strands run in opposite directions (5′ to 3′ and 3′ to 5′).

    • Important for DNA synthesis and protein production.

DNA Replication

  • Replication is semiconservative, with parental strands serving as templates for new strands.

  • Involves about 30 different enzymes to complete the process.

Key Enzymes in DNA Replication

  • Helicase: Unzips the DNA helix.

  • Gyrase: Untangles supercoiled DNA.

  • Primase: Synthesizes RNA primers.

  • DNA Polymerase III: Adds nucleotides and checks for errors.

  • DNA Polymerase I: Removes RNA primers and fills gaps.

  • Ligase: Joins DNA fragments.

Steps of DNA Replication

  1. Origin of Replication: Rich in adenine and thymine, allowing easier strand separation.

  2. Unzipping: Helicase breaks hydrogen bonds, creating two strands.

  3. Binding: Single-strand binding proteins maintain strand separation.

  4. Nucleotide Addition: DNA polymerase III synthesizes new strands based on the templates.

Leading vs. Lagging Strand

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

  • Lagging Strand: Synthesized in fragments (Okazaki fragments) in segments moving away from the replication fork due to its 3′ to 5′ orientation.

  • DNA ligase: Joins Okazaki fragments.

Elongation and Termination of Daughter Molecules

  • Speed of Replication: Bacteria can add nucleotides at a rate of 750 bases per second.

  • Mistakes may occur, termed spontaneous mutations.

Protein Synthesis

  • DNA as Blueprint: Codes for protein synthesis through genes.

    • A segment of DNA that encodes a polypeptide is a gene.

  • Genotype versus Phenotype: Sequence of DNA vs. its expression

RNA Overview

  • RNA: Links DNA to proteins.

    • Differences from DNA:

      • Uracil replaces Thymine; ribose replaces deoxyribose.

    • Types of RNA:

      • mRNA (Messenger RNA): Carries code from DNA.

      • rRNA (Ribosomal RNA): Forms part of the ribosome.

      • tRNA (Transfer RNA): Carries amino acids to ribosomes for protein synthesis.

Transcription Process

  • Converts DNA code into mRNA by RNA polymerase, proceeds in three phases:

    • Initiation: RNA polymerase binds to the promoter; DNA strands unwind.

    • Elongation: RNA polymerase synthesizes mRNA by adding complementary nucleotides.

    • Termination: Stops at a termination signal.

Processing of mRNA

  • Newly formed mRNA (pre-mRNA) undergoes editing where introns are removed by spliceosomes:

    • This results in a mature mRNA composed only of exons.

Translation Process

  • Sequence of events where the mRNA code is translated into a polypeptide chain:

    • Initiation, Elongation, Termination.

  • Translation occurs in ribosomes where mRNA codons match tRNA anticodons for amino acid assembly.

Genetic Code

  • Each three-base sequence (codon) encodes a specific amino acid; redundancy exists to protect against errors.

  • Stop Codons: Signal termination of protein synthesis.

Mutations

  • Any change to the nucleotide sequence can lead to mutations, affecting the phenotype.

  • Categories:

    • Point Mutations: Involve a single base change; can be missense or nonsense mutations.

    • Frameshift Mutations: Insertions or deletions that alter the reading frame.

Repair Mechanisms

  • Cells possess proofreading and repair systems for DNA replication errors, including:

    • Excision Repair: Fixes UV damage and other errors.

Proteins


  • Composed of chains of amino acids connected by peptide bonds, proteins are crucial for function.