Rucete ✏ AP Biology In a Nutshell
15. Regulation and Mutations
This chapter explains how gene expression is regulated in prokaryotes and eukaryotes, how it contributes to cell specialization, and how different types of mutations affect gene function, phenotype, and evolution.
Regulation of Gene Expression in Prokaryotes
• Prokaryotes regulate genes using operons—clusters of genes controlled by a shared promoter and regulatory elements.
• Operons contain: – Promoter: RNA polymerase binding site – Operator: binding site for repressor protein – Structural genes: code for functional proteins
• Two types of operons:
Inducible operons (e.g., lac operon): – Off by default; turned on in the presence of the inducer (e.g., lactose). – Repressor binds to operator unless inducer is present. – Lactose binds to repressor → repressor inactivated → transcription begins.
Repressible operons (e.g., trp operon): – On by default; turned off when the product is abundant. – Tryptophan (corepressor) binds to repressor → complex binds to operator → transcription stops.
• Positive regulation: – When glucose is low, cAMP binds to CAP → CAP binds to DNA → enhances RNA polymerase binding at promoter → increases transcription of the lac operon.
Regulation of Gene Expression in Eukaryotes
• Eukaryotes use multiple regulatory sequences and proteins to control transcription levels.
• Key components:
– Promoters: where RNA polymerase binds
– Regulatory switches: enhancers (activator binding sites), silencers (repressor binding sites)
– Regulatory proteins: • Activators: upregulate expression • Repressors: downregulate expression • Transcription factors: help RNA polymerase bind • Mediators: coordinate protein interactions
Epigenetic Regulation
• Changes to DNA or histone proteins that alter gene expression without changing DNA sequence.
– DNA methylation: decreases transcription.
– Histone acetylation: loosens chromatin → increases transcription.
– Euchromatin: loosely packed → active transcription.
– Heterochromatin: tightly packed → low expression.
• siRNA (small interfering RNA): binds to mRNA, forming double-stranded RNA → degraded → blocks translation.
Differential Gene Expression and Cell Specialization
• Different cells express different sets of genes → leads to cell differentiation.
• Expression levels influence phenotype (e.g., more melanin = darker skin).
• Timing of transcription factor expression during development is critical (e.g., Hox genes determine body plan).
Mutations and Genetic Variation
• Mutations are heritable changes in the DNA sequence.
• They can be spontaneous (random) or induced (e.g., radiation, chemicals).
• Mutations increase genetic diversity and drive evolution.
Types of Mutations
Point mutations (single nucleotide changes):
– Silent: no change in amino acid due to redundancy in the genetic code.
– Missense: one amino acid is changed → can be harmful, beneficial, or neutral.
– Nonsense: codon becomes a stop codon → prematurely ends translation → usually harmful.
Frameshift mutations:
– Caused by insertions or deletions (indels) not in multiples of 3.
– Shifts the reading frame → changes all downstream amino acids.
– Often leads to nonfunctional proteins.
Chromosomal Mutations
• Larger scale changes:
– Duplication: segment repeated → can increase gene dosage.
– Deletion: segment lost → may eliminate essential genes.
– Inversion: segment reversed → may disrupt gene expression.
– Translocation: segment moves to a different chromosome → can cause cancer (e.g., leukemia).
Horizontal Gene Transfer in Prokaryotes
• Prokaryotes can acquire new genes without reproduction:
– Transformation: uptake of free DNA from the environment.
– Transduction: DNA transferred via bacteriophages (viruses).
– Conjugation: plasmids transferred through a pilus between bacteria.
• Horizontal gene transfer increases genetic diversity in bacterial populations.
In a Nutshell
Gene expression is regulated through operons, transcription factors, epigenetics, and small RNAs. These regulatory systems guide development and maintain homeostasis. Mutations introduce genetic variation, and while some are harmful, others drive evolution. In bacteria, horizontal gene transfer adds further diversity beyond vertical inheritance.