Gene Structure, Regulation, and Mutations: Study Notes

Gene Structure and Information Flow

Basic Components of a Gene

Genes are segments of DNA that contain the instructions for synthesizing proteins or functional RNA molecules. The structure of a gene includes several key regions, each with a specific function in the process of gene expression.

• Promoter: A DNA sequence upstream of the gene where RNA polymerase binds to initiate transcription.

• Transcription Start Site (TSS): The location where RNA synthesis begins.

• Transcription Stop Site: The point where RNA polymerase stops transcribing the DNA into RNA.

• Translation Start Site: The codon (usually AUG) where ribosomes begin translating mRNA into protein.

• Translation Stop Site: The codon (UAA, UAG, or UGA) where translation ends.

• Region of Transcribed mRNA: The portion of DNA that is copied into RNA.

• Gene: The entire DNA sequence required to produce a functional product, including regulatory and coding regions.

Example: In a diagram of a gene, the promoter is located just before the transcription start site, followed by the coding region (which includes the translation start and stop sites), and ending at the transcription stop site.

Transcription and mRNA Processing

Transcription Overview

Transcription is the process by which RNA polymerase synthesizes a complementary RNA strand from a DNA template. The resulting RNA is called the primary transcript or pre-mRNA in eukaryotes.

• RNA Polymerase: The enzyme responsible for synthesizing RNA from the DNA template.

• Directionality: RNA is synthesized in the 5' to 3' direction.

• Labeling mRNA Ends: The 5' end of the mRNA corresponds to the start of transcription, and the 3' end corresponds to the end of transcription.

Example: In a diagram, RNA polymerase moves along the DNA, synthesizing mRNA, which grows from the 5' to 3' end.

Gene Regulation: Enhancers and Control Elements

Regulatory Elements and Cell-Specific Expression

Gene expression is regulated by DNA sequences called enhancers and control elements, which interact with transcription factors to increase or decrease transcription rates. These elements determine when and where a gene is expressed.

• Enhancer: A DNA sequence that can increase transcription from a distance, often in a cell-type-specific manner.

• Control Elements: Short DNA sequences within promoters or enhancers that bind specific transcription factors.

• Transcription Factors: Proteins that bind to control elements to regulate gene expression.

Example: The same gene may be "on" in muscle cells and "off" in eye cells due to the presence or absence of specific activators binding to enhancer regions.

Experimental Analysis of Gene Regulation

Reporter Gene Assays

Reporter gene assays are used to study the function of enhancers and control elements. By linking a reporter gene (such as luciferase or GFP) to different regulatory sequences, researchers can measure the effect of these sequences on gene expression.

• Reporter Gene: A gene whose product is easy to detect and measure.

• Relative Expression: The level of reporter mRNA indicates the activity of the regulatory elements tested.

Example: A bar graph may show higher reporter gene expression when an enhancer is present, and lower expression when the enhancer is deleted or mutated.

Mutations and Their Effects

Types of Mutations

Mutations are changes in the DNA sequence that can affect gene function in various ways. The consequences depend on the type and location of the mutation.

• Nonsense Mutation: A change that introduces a premature stop codon, leading to a truncated, usually nonfunctional protein.

• Missense Mutation: A change that results in a different amino acid being incorporated into the protein, which may alter protein function.

• Deletion of Enhancer Sequence: Removal of regulatory DNA can reduce or eliminate gene expression without altering the coding sequence.

Example: In the WRN gene, nonsense mutations in the coding region result in loss of detectable protein, as shown by immunohistochemistry images.

Codon Table and Genetic Code

The genetic code is a set of rules by which the sequence of nucleotides in mRNA is translated into the sequence of amino acids in a protein. Each codon (three nucleotides) specifies a particular amino acid or a stop signal.

• Start Codon: AUG (codes for methionine)

• Stop Codons: UAA, UAG, UGA

Example: A codon table is used to determine the amino acid sequence encoded by a given mRNA sequence.

Summary Table: Types of Mutations and Their Effects

Mutation Type	Effect on Protein	Example
Nonsense	Premature stop codon; truncated protein	WRN mutant 1: UCA AAA UAA
Missense	Single amino acid change; possible altered function	WRN mutant 2: UCA AAA UAA
Enhancer Deletion	Reduced or abolished gene expression	Reporter gene assay with deleted enhancer

Key Equations and Concepts

• Transcription: $\text{DNA} \xrightarrow{\text{RNA polymerase}} \text{RNA}$

• Translation: $\text{mRNA} \xrightarrow{\text{ribosome}} \text{Protein}$

Important Summary

• All DNA instructions (genes and their control elements) are present in every cell.

• What differs between cells is which genes are "on" or "off" due to regulatory elements and transcription factors.