Gene Expression: From Gene to Protein

Gene Expression: From Gene to Protein

Overview

This chapter explores the molecular mechanisms by which genetic information encoded in DNA is expressed as functional proteins. The process involves transcription and translation, as well as regulatory and modification steps, particularly in eukaryotes. Understanding these processes is fundamental to modern biology and medicine.

The Central Dogma of Molecular Biology

Definition and Flow of Genetic Information

• Central Dogma: The concept that genetic information flows from DNA to RNA to protein.

• Transcription: The process by which a segment of DNA is copied into RNA.

• Translation: The process by which the information in messenger RNA (mRNA) is used to synthesize a polypeptide (protein).

Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

• Genotype: The genetic makeup of an organism (DNA sequence).

• Phenotype: The observable traits of an organism, often resulting from protein activity.

• Proteins link genotype to phenotype by carrying out cellular functions.

What is a Gene?

Historical Perspective and Modern Definition

• The term gene was first used by Wilhelm Johannsen in 1909 to describe inherited factors.

• Beadle and Tatum's "One Gene-One Enzyme" hypothesis established that genes encode specific enzymes (now expanded to proteins in general).

• Modern definition: A gene is a sequence of DNA that codes for a functional product, which can be a protein or functional RNA (e.g., tRNA, rRNA).

Gene Expression in Eukaryotes

Cellular Differentiation and Gene Regulation

• All somatic cells in an organism contain the same DNA, but different cell types express different subsets of genes.

• Gene expression: A gene is "expressed" or "active" when its RNA is being transcribed and, often, translated into protein.

• Regulation of gene expression allows for cellular differentiation and specialized functions.

Structure and Types of RNA

RNA Structure and Function

• RNA is composed of ribonucleotides (A, U, C, G) with ribose sugar.

• Uracil (U) replaces thymine (T) found in DNA.

• Three main types of RNA involved in protein synthesis:

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

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

  • tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

Transcription

Process and Mechanism

• Transcription is the synthesis of RNA from a DNA template by RNA polymerase.

• Only one DNA strand (the template strand) is transcribed.

• Base pairing rules: A-U, T-A, C-G, G-C.

• Stages of transcription:

  1. Initiation: RNA polymerase binds to the promoter region.

  2. Elongation: RNA polymerase adds nucleotides to the growing RNA strand.

  3. Termination: RNA polymerase reaches a terminator sequence and releases the RNA transcript.

• In eukaryotes, transcription factors are required for initiation.

RNA Processing in Eukaryotes

Modifications and Splicing

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

  • Addition of a 5' guanine cap.

  • Addition of a 3' poly-A tail.

  • Removal of non-coding sequences (introns) via RNA splicing; coding regions (exons) are joined together.

  • Splicing is performed by the spliceosome, a complex of proteins and small RNAs.

  • Some RNA molecules (ribozymes) can catalyze their own splicing.

  • Alternative splicing allows a single gene to code for multiple proteins.

The Genetic Code

Codons and Universality

• The genetic code is read in triplets called codons, each specifying an amino acid.

• The code is redundant (multiple codons for most amino acids) and universal (shared by almost all organisms).

• Start codon: AUG (codes for methionine).

• Stop codons: UAA, UAG, UGA (signal termination of translation).

Translation

Process and Mechanism

• Translation is the synthesis of a polypeptide using the information in mRNA.

• Occurs in three stages:

  1. Initiation: Ribosome assembles at the start codon (AUG) on mRNA.

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

  3. Termination: Ribosome reaches a stop codon; the completed polypeptide is released.

• Multiple ribosomes can translate a single mRNA simultaneously (polyribosomes).

• Proteins may be targeted to specific cellular locations based on signal peptides.

Mutations and Their Effects

Types of Mutations

• Point mutations: Changes in a single nucleotide pair.

  • Silent mutation: No change in amino acid sequence.

  • Missense mutation: Changes one amino acid to another.

  • Nonsense mutation: Changes an amino acid codon to a stop codon, truncating the protein.

• Frameshift mutations: Insertions or deletions that alter the reading frame, usually resulting in nonfunctional proteins.

• Mutations can be caused by errors in DNA replication, UV light, radiation, or chemicals.

• While some mutations are harmful, others contribute to genetic diversity and evolution.

Applications of Recombinant DNA Technology

Genetic Engineering and Biotechnology

• Genes can be transferred between species due to the universality of the genetic code.

• Applications include:

  • Production of therapeutic proteins (e.g., insulin, clotting factors).

  • Genetically modified crops with improved traits.

  • Development of more effective vaccines.

Summary Table: Types of Mutations and Their Effects

Type of Mutation	Description	Effect on Protein
Silent	Change in nucleotide does not alter amino acid	No effect
Missense	Change in nucleotide alters one amino acid	Variable effect (may be minor or severe)
Nonsense	Change creates a stop codon	Premature termination; usually nonfunctional protein
Frameshift	Insertion or deletion shifts reading frame	Usually nonfunctional protein; drastic effect

Key Terms

• Gene expression

• Transcription

• Translation

• mRNA, tRNA, rRNA

• Codon

• Mutation

• Spliceosome

• Alternative splicing

• Signal peptide

• Polyribosome