Gene Expression: From Gene to Protein (Chapter 17 Study Notes)

Gene Expression: From Gene to Protein

Introduction

Gene expression is the process by which the information encoded in a gene is used to direct the synthesis of a protein, ultimately determining an organism's traits. This chapter covers the central dogma of molecular biology, the mechanisms of transcription and translation, eukaryotic RNA modifications, and the impact of mutations on protein structure and function.

The Central Dogma of Molecular Biology

Overview

The central dogma describes the flow of genetic information within a biological system. It states that DNA is transcribed into RNA, which is then translated into protein.

• Central Dogma: DNA → RNA → Protein

• Transcription: The process of copying a gene's DNA sequence into messenger RNA (mRNA).

• Translation: The process by which mRNA is decoded to build a polypeptide (protein).

• Proteins: Serve as the functional links between genotype (genetic makeup) and phenotype (observable traits).

Example: A change in DNA sequence can alter the protein produced, leading to visible changes in phenotype, such as pigment differences in animals.

What is a Gene?

Definition and Function

A gene is a fundamental unit of heredity, consisting of a sequence of DNA that encodes information for building a specific protein or functional RNA.

• Gene: A segment of DNA that contains instructions for making a particular protein or RNA molecule.

• Expression: A gene is "expressed" or "active" when its information is used to produce RNA and, usually, protein.

• One Gene-One Enzyme Hypothesis: Proposed by Beadle and Tatum, stating that each gene encodes a specific enzyme.

Example: The gene for pigment production encodes an enzyme required for pigment synthesis. If the gene is mutated, the enzyme may not function, resulting in a lack of pigment.

Transcription: DNA to RNA

Mechanism

Transcription is the process by which RNA is synthesized from a DNA template. This occurs in the nucleus of eukaryotic cells.

• RNA Polymerase: The enzyme that synthesizes RNA by reading the DNA template strand.

• Promoter: A DNA sequence where RNA polymerase binds to initiate transcription.

• Template Strand: The DNA strand that is transcribed into RNA.

• Coding Strand: The non-template DNA strand, which has the same sequence as the RNA (except T is replaced by U).

• Base Pairing: In RNA, uracil (U) replaces thymine (T).

Stages of Transcription:

• Initiation: RNA polymerase binds to the promoter and begins RNA synthesis.

• Elongation: RNA polymerase moves along the DNA, adding nucleotides to the growing RNA strand.

• Termination: Transcription ends when RNA polymerase reaches a terminator sequence.

Equation:

$\text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA}$

RNA Processing in Eukaryotes

Modifications and Splicing

In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA.

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

• Poly-A Tail: Addition of a string of adenine nucleotides to the 3' end.

• Splicing: Removal of noncoding regions (introns) and joining of coding regions (exons).

• Alternative Splicing: Different combinations of exons can be joined to produce multiple proteins from one gene.

• Spliceosome: A complex of proteins and small RNAs that catalyzes splicing.

• Ribozymes: Catalytic RNA molecules that can perform splicing reactions.

Example: Alternative splicing allows a single gene to code for different proteins in different cell types.

The Genetic Code

Codons and Universality

The genetic code is a set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein.

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

• Redundancy: Most amino acids are encoded by more than one codon.

• Universality: The genetic code is nearly universal among all organisms.

Equation:

$\text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}$

Example: Recombinant DNA technology uses the universality of the genetic code to express genes from one species in another.

Translation: RNA to Protein

Mechanism

Translation is the process by which ribosomes synthesize proteins using the information in mRNA.

• tRNA: Transfer RNA molecules bring amino acids to the ribosome and match them to the mRNA codons via their anticodon.

• Ribosome: The molecular machine that assembles amino acids into a polypeptide chain.

• Reading Frame: The way nucleotides are grouped into codons; shifting the frame changes the protein produced.

• Initiation: Translation begins at the start codon (AUG).

• Elongation: Amino acids are added one by one to the growing polypeptide.

• Termination: Translation ends at a stop codon.

• Polyribosomes: Multiple ribosomes can translate a single mRNA simultaneously.

Example: Human cells can produce thousands of proteins per second due to polyribosome activity.

Protein Targeting and Localization

Signal Peptides and Ribosome Populations

Proteins are directed to specific cellular locations by signal peptides. Ribosomes can be free in the cytosol or bound to the endoplasmic reticulum (ER).

• Signal Peptide: A short amino acid sequence that directs the ribosome to the ER.

• SRP (Signal Recognition Particle): Binds to the signal peptide and helps dock the ribosome to the ER.

• Free Ribosomes: Synthesize proteins for use in the cytosol.

• Bound Ribosomes: Synthesize proteins for secretion or for use in the endomembrane system.

Example: Insulin is synthesized on bound ribosomes and secreted from pancreatic cells.

Mutations and Their Effects

Types and Consequences

Mutations are changes in the DNA sequence that can affect protein structure and function. They can be caused by errors in replication or by environmental factors such as UV light and chemicals.

• Base Pair Substitution: Replacement of one nucleotide with another.

• Silent Mutation: Does not change the amino acid sequence due to code redundancy.

• Missense Mutation: Changes one amino acid in the protein, potentially altering function.

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

• Frameshift Mutation: Insertion or deletion of nucleotides shifts the reading frame, usually with severe effects.

Example: Sickle-cell anemia is caused by a missense mutation in the hemoglobin gene.

Functional Products of Genes

Gene Products

The final product of gene expression can be a protein or a functional RNA molecule.

• Protein: Most genes encode proteins.

• Functional RNA: Some genes encode RNAs such as tRNA, rRNA, snRNA, or microRNA.

Example: rRNA is a structural and catalytic component of ribosomes.

Applications of Recombinant DNA Technology

Uses in Medicine and Agriculture

Recombinant DNA technology allows scientists to manipulate genes for practical purposes.

• Protein Production: Synthesis of insulin and clotting factors in bacteria.

• Genetically Modified Crops: Crops resistant to drought or pests.

• Vaccines: Improved vaccine design and effectiveness.

Example: Human insulin produced in bacteria for diabetes treatment.

Summary Table: Types of Mutations and Their Effects

Mutation Type	Description	Effect on Protein
Silent	Base substitution; no change in amino acid	No effect
Missense	Base substitution; changes one amino acid	Variable effect
Nonsense	Base substitution; creates stop codon	Truncated protein
Frameshift	Insertion/deletion; shifts reading frame	Severely altered protein

Additional info:

• Exons often code for different functional domains in proteins, allowing for complex protein structures and functions.

• Epigenetic marks and regulatory regions also play roles in gene expression, though not all are covered in detail here.