Ch 17. Gene Expression: From Gene to Protein

Gene Expression: From Gene to Protein

Introduction to Gene Expression

Gene expression is the process by which information encoded in DNA directs the synthesis of proteins, which are essential for cellular structure and function. This process involves two main stages: transcription and translation.

The Flow of Genetic Information

Beadle and Tatum's Experiment

Beadle and Tatum demonstrated that genes act by regulating distinct chemical events. By exposing Neurospora crassa (bread mold) to X-rays, they created mutants that could not survive on minimal medium unless supplemented with specific nutrients. This led to the one gene–one enzyme hypothesis, later refined to one gene–one polypeptide.

• Wild-type cells grow on minimal medium.

• Mutant cells require specific supplements (e.g., arginine, ornithine, citrulline) to grow, indicating a block in a metabolic pathway.

Metabolic Pathway and Genetic Control

Each gene encodes a specific enzyme that catalyzes a step in a metabolic pathway. Mutations in different genes block the pathway at different steps, which can be identified by supplementing the medium with intermediates.

Class	Growth on Minimal Medium	Growth with Ornithine	Growth with Citrulline	Growth with Arginine
Wild type	Yes	Yes	Yes	Yes
Class I (Gene A mutant)	No	Yes	Yes	Yes
Class II (Gene B mutant)	No	No	Yes	Yes
Class III (Gene C mutant)	No	No	No	Yes

Additional info: This table summarizes the growth requirements of different mutant classes, illustrating the genetic control of metabolic pathways.

Basic Principles of Transcription and Translation

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

• Transcription: Synthesis of RNA from a DNA template.

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

• Ribosomes: Sites of protein synthesis.

Additional info: In prokaryotes, transcription and translation are coupled, while in eukaryotes, they are separated by the nuclear envelope.

The Genetic Code

The genetic code is a set of rules by which information encoded in mRNA is translated into proteins. It is based on codons, which are triplets of nucleotides.

• There are 64 codons: 61 code for amino acids, 3 are stop signals.

• The code is redundant but not ambiguous.

• Codons must be read in the correct reading frame.

Transcription: DNA-Directed Synthesis of RNA

Stages of Transcription

Transcription occurs in three main stages: initiation, elongation, and termination.

• Initiation: RNA polymerase binds to the promoter region, aided by transcription factors in eukaryotes.

• Elongation: RNA polymerase synthesizes the RNA strand, complementary to the DNA template.

• Termination: In bacteria, transcription ends at a terminator sequence; in eukaryotes, after the polyadenylation signal.

RNA Processing in Eukaryotes

Primary RNA transcripts (pre-mRNA) undergo several modifications before becoming mature mRNA:

• 5' Cap: Modified guanine nucleotide added to the 5' end.

• Poly-A Tail: 50–250 adenine nucleotides added to the 3' end.

• RNA Splicing: Removal of introns and joining of exons.

Additional info: These modifications protect mRNA, aid in export from the nucleus, and help ribosome binding.

Spliceosomes and Ribozymes

Spliceosomes, composed of proteins and small RNAs, carry out RNA splicing. Some RNA molecules (ribozymes) can catalyze their own splicing.

Alternative RNA Splicing

Alternative splicing allows a single gene to code for multiple proteins by including or excluding different exons.

Translation: RNA-Directed Synthesis of a Polypeptide

Transfer RNA (tRNA) Structure and Function

tRNA molecules bring amino acids to the ribosome and match them to the mRNA codons via their anticodon region.

• Each tRNA is linked to a specific amino acid by aminoacyl-tRNA synthetase.

• tRNA structure includes an anticodon and an amino acid attachment site.

Ribosome Structure and Function

Ribosomes are composed of large and small subunits, each made of rRNA and proteins. They have three binding sites for tRNA:

• P site: Holds the tRNA with the growing polypeptide chain.

• A site: Holds the tRNA with the next amino acid.

• E site: Exit site for discharged tRNAs.

Stages of Translation

Translation occurs in three stages: initiation, elongation, and termination.

• Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA; the large subunit completes the initiation complex.

• Elongation: Amino acids are added one by one to the growing chain through codon recognition, peptide bond formation, and translocation.

• Termination: When a stop codon is reached, a release factor binds, releasing the polypeptide and dissociating the ribosome.

Polyribosomes and Protein Targeting

Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome. Proteins may be targeted to specific cellular locations, such as the endoplasmic reticulum for secretion.

Mutations and Their Effects on Protein Structure and Function

Types of Mutations

Mutations are changes in the genetic material. Point mutations affect a single nucleotide pair and can be classified as:

• Silent mutations: No effect on the amino acid sequence.

• Missense mutations: Change one amino acid to another.

• Nonsense mutations: Change an amino acid codon to a stop codon, truncating the protein.

• Insertions/Deletions: Add or remove nucleotides, potentially causing frameshifts.

Additional info: Mutations can have a range of effects, from benign to causing diseases such as sickle-cell anemia.