PPT 6

Gene Expression in Eukaryotes

• Focus on Eukaryotes: Understanding gene expression primarily in eukaryotic cells.

• DNA Structure:

  • Genes: Instructions for protein synthesis.

  • Chromatin: DNA in a non-dividing state, loosely wound around histone proteins.

  • Chromosomes: Condensed form of chromatin post-DNA replication.

  • Sister Chromatids: Duplicated chromosomes consisting of two identical halves.

Reasons for DNA Activation

• Gene Expression:

  • Protein production needed for cellular function or export.

  • Essential for creating new organelles and proteins.

• Cell Replacement:

  • Cells age/damage necessitate new cell generation through mitosis.

• Reproduction:

  • Involves formation of gametes via meiosis.

Transcription and Translation Central Dogma

• Transcription: Formation of pre-mRNA from DNA.

  • Uses DNA template in the nucleus.

  • RNA bases: Adenine (A) pairs with Uracil (U) and Cytosine (C) with Guanine (G).

  • RNA polymerase reads the DNA strand from 3’ to 5’ while synthesizing RNA in a 5’ to 3’ direction.

Steps of Transcription

1. Initiation:

   • RNA polymerase binds to the promoter region (TATA Box).

   • Determines which DNA strand will be transcribed.

   • Proteins must bind to the promoter for RNA polymerase to attach.

2. Elongation:

   • RNA polymerase unwinds DNA and elongates the RNA transcript at 60 nucleotides per second.

   • DNA helix re-forms behind the RNA polymerase.

3. Termination:

   • RNA pol detaches upon reaching a termination sequence, releasing the RNA transcript.

RNA Processing in Eukaryotes

• Pre-mRNA Modification:

  • RNA splicing removes introns and joins exons to form mature mRNA.

  • Processed mRNA exits the nucleus via nuclear pores.

Translation Overview

• Ribosome's Role: Reads mRNA to synthesize a polypeptide chain in the cytoplasm.

  • Codons: Series of three nucleotides on mRNA dictate specific amino acids.

  • Transfer RNA (tRNA) and ribosomes work in this phase.

Steps of Translation

1. Initiation:

   • mRNA binds with the small ribosomal subunit.

   • tRNA pairs its anticodon with the start codon (AUG).

   • Large ribosomal subunit then attaches, forming the initiation complex.

2. Elongation Cycle:

   • Continues as tRNA brings amino acids to the growing polypeptide, catalyzing peptide bond formation, and shifting the tRNA to the next codon.

3. Termination:

   • Encountering a stop codon (UAA, UAG, or UGA) ends translation and releases the newly synthesized protein.

Mutations and Their Effects

• Point Mutations: Can lead to single amino acid changes.

  • Silent: No effect on protein synthesis.

  • Missense: Different amino acid substitution (e.g., sickle-cell anemia).

  • Nonsense: Changes codon to a stop signal, truncating the protein.

  • Frameshift Mutations: Due to insertions or deletions altering the reading frame, affecting downstream protein synthesis.

Example: Sickle-Cell Anemia

• Genetic Basis: Mutation in beta-globin gene causing defect in hemoglobin production.

  • Affects oxygen transport, changing red blood cell shape leading to health issues.

Gene Functionality and Structure

• Protein Structures: Vary in shape and function based on amino acid sequence and folding (primary, secondary, tertiary, quaternary structures).

• Efficacy of Normal vs. Abnormal Hemoglobin: Abnormal proteins tend to aggregate and affect cellular function, demonstrating significant differences from normal counterparts.