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Introduction

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Unit Overview

• Unit 6: Gene Expression and Regulation

• Based on the textbook: Biology for the AP® Course, 1st edition

Gene Expression and Translation

Warm-Up Activity

• Translate the mRNA sequence: AUGCUGCGGUAA

  • Codon: sequence of three nucleotides corresponding to a specific amino acid or stop signal.

Concepts of Translation

• Translation: Process whereby mRNA sequence determines the order of amino acids in a protein.

• Ribosomes consist of:

  • Small subunit and Large subunit (composed of RNA and protein).

  • tRNA-binding sites exist in the large subunit, responsible for translation process.

tRNA Structure and Function

tRNA Basics

• tRNAs: Noncoding RNA single strands; shape resembles a cloverleaf due to internal base pairing.

• Contains an anticodon that matches the mRNA codon, carrying a specific amino acid.

• Base pairing occurs:

  • First base of mRNA codon matches the last base of the tRNA anticodon.

Aminoacyl tRNA Synthetases

• Enzymes that link specific amino acids to their corresponding tRNAs.

• There are 20 distinct aminoacyl tRNA synthetases, one for each amino acid.

Genetic Code

• Genetic Code Basics:

  • Defines the relationship between mRNA codons and their corresponding amino acids.

  • Redundant: Many amino acids can be encoded by multiple codons.

Stages of Translation

1. Initiation

• Begins when the AUG codon is recognized, establishing Methionine as the first amino acid.

2. Elongation

• Successive amino acids are sequentially added to the growing polypeptide chain.

3. Termination

• Stops when a stop codon is reached:

  • No corresponding tRNA for stop codons.

  • A release factor binds, releasing the polypeptide from the ribosome.

mRNA and Polypeptides in Eukaryotes vs. Prokaryotes

• In eukaryotes: mRNA codes for a single polypeptide.

• In prokaryotes: mRNA can encode multiple polypeptides.

Transcription Overview

Transcription Process

• RNA Polymerase reads the DNA sequence to produce a complementary RNA strand.

• The process begins at a promoter and concludes at a terminator.

RNA Processing in Eukaryotes

• Involves modifying the primary RNA transcript:

  • Introns removed, exons spliced together.

  • Addition of GTP cap and poly(A) tail.

Regulation of Gene Expression

Transcription Factors in Eukaryotes

• Transcription Factors: Proteins that activate or repress gene expression via binding to DNA sequences (e.g., TATA box).

  • General transcription factors are necessary for RNA polymerase binding at the promoter region.

Regulation in Prokaryotes (Operons)

• Positive Regulation: Gene off by default; turned on by activators binding.

• Negative Regulation: Gene on by default; turned off by repressors binding.

Operons: Mechanism of Gene Regulation

• Operon: Structure including operator, promoter, and controlled genes. Can be:

  • Repressible Operon: Off by default (e.g., tryptophan operon).

  • Inducible Operon: On by default; off in the presence of an inducer (e.g., lac operon).

Lac Operon Example

• Lactose presence initiates the synthesis of lactose processing enzymes when glucose is low.

• Lactose acts as an inducer, modifying the repressor's binding.

Tryptophan Operon Mechanics

• Tryptophan Operon: Usually on; binds to repressor to turn off.

• Operon remains active in low tryptophan conditions.

Hierarchical Levels of Gene Expression Regulation in Eukaryotes

Overall Regulation Levels

• At different levels, including transcription, RNA processing, and translation.

Mechanisms of Epigenetics

DNA Methylation and Gene Suppression

• Methyl groups added at CpG sites can switch genes off; removal of methylation activates genes.

Epigenetic Inheritance

• During cell division, methylation patterns pass to daughter cells, influencing gene expression across generations.

Cell Specialization and Development

Specialization in Multicellular Organisms

• Cells become specialized; from totipotent cells (fertilized egg) to pluripotent and multipotent cells.

Transcription Factors as Master Regulators

• Some transcription factors, like Pax6, play critical roles in development, activating pathways in various organisms.