Gene Expression and the Central Dogma: Study Notes for General Biology

Gene Expression and the Central Dogma

Introduction

The central dogma of molecular biology describes the flow of genetic information within a cell, from DNA to RNA to protein. This process is fundamental to all living organisms and underlies how genes direct cellular activities.

The Central Dogma

Overview

• Central Dogma: The framework explaining the one-way flow of genetic information: DNA → RNA → Protein.

• Transcription: Genetic information in DNA is copied to RNA.

• Translation: RNA is used to synthesize proteins, which perform cellular functions.

• This process is universal for most species.

Gene and Gene Expression

Definition and Types

• Gene: A specific segment of DNA that codes for a particular protein or functional RNA.

• Constitutive Genes: Genes essential for basic cellular maintenance, expressed at all times in all cells.

• Regulated Genes: Genes expressed only under certain conditions or in specific cell types.

Parts of a Gene

Structure

• Promoter: Region where transcription machinery binds to initiate transcription.

• General Coding Region: Contains the sequence that is transcribed into RNA.

• Terminator: Sequence signaling the end of transcription.

Promoter Region of a Gene

Function and Features

• Recognition Sites: Specific short nucleotide sequences where transcription factors and RNA polymerase bind.

• TATA Box: A common promoter element in eukaryotes, located on the coding strand.

Types of RNA

Shapes and Functions

• mRNA (messenger RNA): Carries genetic information from DNA to ribosomes for protein synthesis.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation; has anticodon regions complementary to mRNA codons.

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Transcription

Steps of Transcription

• Initiation: Transcription factors and RNA polymerase assemble at the promoter region, forming the transcription initiation complex.

• Elongation: RNA polymerase unwinds DNA and synthesizes mRNA by adding nucleotides complementary to the template strand (5' to 3' direction).

• Termination: RNA polymerase encounters a terminator sequence, releases the mRNA transcript, and detaches from DNA.

Strand Selection

• Template Strand: The DNA strand read by RNA polymerase to build mRNA.

• Coding Strand: The non-template strand; its sequence matches the mRNA (except T is replaced by U).

Base Pairing Rules

• RNA uses Uracil (U) instead of Thymine (T).

• Base pairing: A-U, T-A, C-G, G-C.

RNA Processing (Eukaryotes)

Modifications

• 5' Cap: Addition of a modified guanine nucleotide to the 5' end; protects mRNA and aids in translation initiation.

• Poly-A Tail: Addition of 100-250 adenine nucleotides to the 3' end; increases mRNA stability and export from nucleus.

• Splicing: Removal of introns (non-coding regions) and joining of exons (coding regions) to form mature mRNA.

• Alternative Splicing: Allows a single gene to code for multiple proteins by including or excluding different exons.

Translation

Steps of Translation

• Initiation: Ribosome assembles on mRNA and begins protein synthesis at the start codon (AUG).

• Elongation: tRNAs bring amino acids to the ribosome, matching codons with anticodons, and the polypeptide chain grows.

• Termination: Translation ends at a stop codon; the completed protein is released.

Codons and Anticodons

Definitions

• Codon: A sequence of three mRNA nucleotides that specifies an amino acid.

• Anticodon: A sequence of three tRNA nucleotides complementary to the mRNA codon.

Gene Expression in Prokaryotes vs. Eukaryotes

Comparison

Feature	Prokaryotes	Eukaryotes
Location of Transcription	Cytoplasm	Nucleus
RNA Processing	None	5' cap, poly-A tail, splicing
Gene Organization	Operons (multiple genes)	Single genes
Timing	Transcription and translation are coupled	Transcription and translation are separated

Mutations and Their Effects

Types of Mutations

• Point Mutations: Change a single nucleotide; can be silent (no effect), missense (change amino acid), or nonsense (introduce stop codon).

• Frameshift Mutations: Insertions or deletions that shift the reading frame, often resulting in nonfunctional proteins.

Protein-Level Effects

• Silent Mutation: No change in protein sequence.

• Missense Mutation: Alters one amino acid in the protein.

• Nonsense Mutation: Creates a premature stop codon, truncating the protein.

• Frameshift Mutation: Changes all downstream amino acids, usually severely affecting protein function.

Summary Table: Key Terms

Term	Definition
Gene	Segment of DNA coding for a protein or RNA
Promoter	DNA region where transcription starts
Exon	Coding region of a gene
Intron	Non-coding region removed during splicing
mRNA	Messenger RNA, template for protein synthesis
tRNA	Transfer RNA, brings amino acids to ribosome
rRNA	Ribosomal RNA, forms ribosome structure

Key Equations

• Transcription direction: $5' ightarrow 3'$

• Base pairing: $A-U$, $T-A$, $C-G$, $G-C$

Example

If the template DNA strand is 3'-ATTCGAGGCTTACGT-5', the mRNA sequence will be 5'-UAAGCUCCGAAUGCA-3'.

Additional info: These notes expand on the original slides and text to provide a comprehensive, self-contained study guide suitable for exam preparation in a General Biology course.