2.1 DNA Replication, Transcription and Translation

Cells and Tissues

Introduction to DNA Replication

• Instructor: Dr. Dimitrios Cakouros

• Contact: dimitrios.cakouros@adelaide.edu.au

Learning Outcomes

1. DNA Replication, Damage, and Repair

   • Processes involved in DNA replication, damage, proofreading, and repair.

2. Transcription and Translation

   • Sequence of events in transcription of DNA to RNA and subsequent translation to proteins.

Nucleus - Information Storage

Components

• Nucleolus: Region within the nucleus responsible for ribosome production.

• Nuclear Envelope: Double membrane enclosing the nucleus.

• Chromosomes: Structures containing DNA that carry genetic information.

• Nuclear Pores: Openings allowing transport in and out of the nucleus.

• Chromatin: Complex of DNA and proteins that forms chromosomes.

Central Dogma: Flow of Information

• DNA → RNA → Protein

  • Replication: Copying of DNA.

  • Transcription: Conversion of DNA to RNA.

  • Translation: Synthesis of proteins from mRNA.

Importance of Studying DNA Replication

Relevance

• Cell Division: Ensures daughter cells receive identical genetic information.

• Embryogenesis and Development: Critical for growth and tissue repair.

• Therapeutic Target for Cancer: Rapidly dividing cells exploit DNA replication, making it a target for treatment with inhibitors.

Mechanisms and Applications

DNA Replication Process

Timeline

• Occurs during S phase of the cell cycle

• Duration: Approximately 8 hours

• Key Feature: Semi-conservative replication; each parent strand serves as a template.

Characteristics of DNA

• Directionality: DNA has a 5' to 3' directionality.

• Replication Fork: The site where the DNA unwinds and replicates.

Enzymatic Activities

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

• RNA Primer Synthesis: DNA polymerases require an RNA primer to initiate synthesis.

• Okazaki Fragments: Lagging strand synthesis occurs in fragments due to the replication fork's direction.

Ligation

• DNA Ligase: Enzyme that joins Okazaki fragments together, forming a continuous DNA strand.

DNA Damage and Repair

Types of Damage

• Caused by:

  • UV light, ionizing radiation, environmental factors, and chemotherapeutic agents.

  • May result in breakages and modifications in DNA structure.

• Types of DNA damage

  • Double/ single stranded breaks

    • Normal cellular

      activity

    • Ionizing radiation (x- rays)

    • chemotherapy

  • Chemical bond between neighbouring nucleotide

    • Ultraviolet light Cross links T-C

  • Nucleotide modification

    • Reactive oxygen species

    • Chemotherapy

    • Chemicals

  • Chemical linkage of two strands

    • Reactive oxygen species

    • Chemotherapy

    • Chemicals

Repair Mechanisms

1. Nucleotide Excision Repair: Removes damaged nucleotides and replaces them.

2. Base Excision Repair: Removes and replaces damaged bases.

3. Mismatch Repair: Corrects wrongly paired bases.

1. Double Strand Break Repair:

   • Non-Homologous End Joining: Direct ligation of broken ends.

   • Homologous Recombination: Uses an undamaged strand as a template for repair.

Transcription: From DNA to RNA

Process Overview

1. Initiation:

   • DNA uncoils and gene sequences are exposed for transcription.

2. Transcription:

   • mRNA strand is built complementary to the DNA template.

   • Enzymes, like RNA polymerase, play a crucial role in transcription.

RNA Types

• Messenger RNA (mRNA): Encodes amino acid sequences.

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

• Ribosomal RNA (rRNA): A key component of ribosomes where protein synthesis occurs.

• The difference between a RNA and a DNA template strand is that the Thymine is replaced by Uracil.

RNA Processing

1. 5' Cap Addition: Protects mRNA from degradation and is vital for translation.

2. Polyadenylation: Addition of a poly(A) tail for stability and export out of the nucleus.

3. Splicing: Removal of introns and joining of exons to produce mature mRNA.

Translation: From RNA to Protein

Process Overview

1. Initiation: mRNA binds to ribosome, tRNA brings specific amino acids.

2. Elongation: Amino acids are linked forming a polypeptide chain.

3. Termination: Completed protein is released and may undergo modification.

• A codon consists of three nucleotides on the mRNA that correspond to a specific amino acid, which is recognized by the tRNA during translation.

The Genetic Code

• Composition: 20 amino acids and 4 nucleotide bases (A, C, G, U).

• Combinations: 64 possible codons, making the genetic code universal and redundant.

Conclusion

• Significance of DNA and Protein Synthesis: These processes are fundamental to life, regulating cellular functions and ensuring survival.