Gene Expression: From Gene to Protein (Chapter 14 Study Notes)

Gene Expression: From Gene to Protein

Overview of Gene Expression

Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically a protein. This process involves two main stages: transcription and translation.

• One Gene–One Enzyme Hypothesis: Each gene encodes a specific enzyme (now expanded to proteins in general).

• One Gene–One Polypeptide Hypothesis: Each gene encodes a single polypeptide chain, which may function independently or as part of a protein complex.

• Gene: A region of DNA that can be expressed to produce a functional product (RNA or protein).

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information in a living cell:

• DNA → RNA → Protein

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a polypeptide using the information in mRNA.

Gene Expression in Prokaryotes vs. Eukaryotes

• Prokaryotes: Transcription and translation occur in the cytoplasm, often simultaneously.

• Eukaryotes: Transcription occurs in the nucleus; translation occurs in the cytoplasm. mRNA processing (capping, polyadenylation, splicing) occurs before translation.

Table: Comparison of Gene Expression in Prokaryotes and Eukaryotes

Feature	Prokaryotes	Eukaryotes
Location of Transcription	Cytoplasm	Nucleus
Location of Translation	Cytoplasm	Cytoplasm
RNA Processing	Absent	Present (capping, poly-A tail, splicing)
Coupling of Transcription & Translation	Yes	No

The Genetic Code

The genetic code is the set of rules by which information encoded in DNA or RNA sequences is translated into proteins by living cells.

• Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid.

• There are 64 possible codons (43 combinations of A, U, G, C).

• The code is redundant (more than one codon may specify the same amino acid) but not ambiguous (each codon specifies only one amino acid).

• Start codon: AUG (methionine)

• Stop codons: UAA, UAG, UGA

Table: Properties of the Genetic Code

Property	Description
Redundant	More than one codon can code for the same amino acid
Unambiguous	Each codon codes for only one amino acid
Universal	Shared by almost all organisms
Non-overlapping	Codons are read one after another, without overlap

Types of RNA and Their Functions

Type of RNA	Function
mRNA (messenger RNA)	Encodes the amino acid sequence of a polypeptide
tRNA (transfer RNA)	Brings amino acids to the ribosome during translation
rRNA (ribosomal RNA)	Forms the core of the ribosome and catalyzes peptide bond formation
snRNA, miRNA, etc.	Various regulatory and structural roles

Transcription (DNA → RNA)

Transcription is the synthesis of RNA using DNA as a template. It is catalyzed by RNA polymerase.

• Initiation: RNA polymerase binds to the promoter region (with help from transcription factors in eukaryotes).

• Elongation: RNA polymerase moves along the DNA, synthesizing RNA in the 5' to 3' direction.

• Termination: RNA polymerase detaches from the DNA when it reaches a terminator sequence.

RNA Processing in Eukaryotes

Primary RNA transcripts (pre-mRNA) are modified before leaving the nucleus:

• 5' Capping: Addition of a modified guanine nucleotide to the 5' end.

• 3' Polyadenylation: Addition of a poly-A tail to the 3' end.

• Splicing: Removal of non-coding introns and joining of exons.

Translation (RNA → Protein)

Translation is the process of assembling a polypeptide based on the information in mRNA. It occurs in the cytoplasm at the ribosome.

• Initiation: The small ribosomal subunit binds to mRNA; the initiator tRNA binds to the start codon (AUG); the large subunit joins to form the initiation complex.

• Elongation: tRNAs bring amino acids to the ribosome; peptide bonds form between amino acids; the ribosome moves along the mRNA.

• Termination: When a stop codon is reached, the polypeptide is released, and the ribosome disassembles.

Table: Steps of Translation

Step	Description
Initiation	Assembly of ribosome, mRNA, and initiator tRNA at start codon
Elongation	Sequential addition of amino acids to growing polypeptide chain
Termination	Release of polypeptide upon reaching stop codon

Mutations

Mutations are changes in the genetic material that can affect gene expression and protein function.

• Large Scale Mutations: Affect entire genes or chromosomes (deletion, duplication, inversion, translocation).

• Small Scale Mutations: Affect one or a few nucleotides (point mutations, insertions, deletions).

• Point Mutation: Change in a single nucleotide pair.

• Silent Mutation: No effect on amino acid sequence.

• Missense Mutation: Changes one amino acid to another.

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

• Frameshift Mutation: Insertion or deletion of nucleotides that alters the reading frame.

Table: Types of Small Scale Mutations

Type	Description
Silent	No change in amino acid sequence
Missense	One amino acid is changed
Nonsense	Codon changed to stop codon
Frameshift	Insertion or deletion shifts reading frame

Summary

• Gene expression involves transcription and translation.

• The genetic code is universal, redundant, and read in triplets (codons).

• Mutations can alter gene expression and protein function, with varying effects.

Additional info: These notes are based on standard college-level biology content for gene expression and protein synthesis, including key differences between prokaryotic and eukaryotic systems, and the impact of mutations on gene products.