BIOL1020 Chap.12 Genome Evolution and CRISPR
Genome Evolution and CRISPR
Genomes evolve through more than point mutation. This chapter compares viral, prokaryotic and eukaryotic architecture, explains why genome size does not rank organism complexity, and separates introns, exons and transposable elements. Local sequence change, regulatory change, horizontal transfer, gene duplication, whole-genome duplication and mobile DNA can all generate novelty at different scales.
CRISPR then shows how bacterial antiviral machinery can be repurposed for programmable DNA targeting, cutting or gene regulation.
What this chapter covers
- 01
Genome architecture across organisms
- 02
C-value paradox and non-coding DNA
- 03
Introns, exons and transposons
- 04
Gene and genome duplication
- 05
Horizontal transfer and gene families
- 06
CRISPR targeting and regulation
Worked example · free
Choosing between editing and reversible repression
- +1Both systems can use guide-RNA complementarity to bind a chosen genomic sequence.
- +1Active nuclease cuts DNA, and cellular repair may create a permanent sequence change.
- +1Inactive Cas can recruit a repressive regulator without cutting the target.
- +1Removing the regulatory system can allow expression to recover, matching the temporary goal better.
Key terms
- Genome
- An organism's complete genetic material, including genes and other information needed for development and reproduction.
- C-Value Paradox
- The lack of a simple relationship between eukaryotic genome size and organismal complexity.
- Gene Duplication
- Creation of an additional gene copy that can retain, divide or change ancestral function.
- Polyploidy
- Possession of additional complete chromosome sets, often created through whole-genome duplication.
- Transposable Element
- A DNA sequence that moves or copies itself to new genomic positions.
- Guide RNA
- An RNA molecule that directs a CRISPR-associated protein to a complementary nucleic-acid target.
Genome Evolution and CRISPR FAQ
Why does genome size not equal organism complexity?
Eukaryotic genomes contain different amounts of non-coding, repetitive and mobile DNA, so size can expand without a proportional increase in protein-coding genes. Regulatory organisation, gene interaction and expression patterns also influence phenotype, which is why the relationship is not a simple ranking.
How can gene duplication create new functions?
An extra copy can preserve the original role while the other copy accumulates changes with less risk of losing the ancestral function. Copies may divide the original job, specialise in different tissues or acquire new activity, producing a related gene family over time.
How does CRISPR target a specific sequence?
A guide RNA base-pairs with a complementary target next to the recognition motif required by the Cas protein. The attached Cas activity then cuts DNA or recruits another regulator. Complementarity supplies targeting, while nuclease or regulatory domains determine the outcome.
Why does genome size not measure complexity?
A genome includes protein-coding genes plus introns, regulatory regions, repeats and transposable elements. These components vary extensively among lineages, so additional DNA does not translate directly into additional organismal complexity. Compare genome architecture and functional organisation before interpreting raw base-pair totals.
How can gene duplication create a new function?
Duplication leaves two related copies where one existed before. If one copy retains the ancestral function, mutations in the other may alter expression or protein activity without immediately removing the original role. Selection, drift and later loss can then shape a diverging gene family.
How do active and inactive Cas systems differ?
An active nuclease can cut targeted DNA, after which cellular repair determines the sequence outcome. Catalytically inactive Cas preserves guide-directed binding but does not make the break. Attached regulatory domains can therefore alter transcription without necessarily changing the underlying target sequence.
Why must CRISPR off-target effects be tested?
A guide can sometimes tolerate partial matching at another genomic site, particularly when an appropriate recognition motif is nearby. Unintended binding or cutting could alter a different gene or regulatory region. Specificity is therefore measured across relevant sites and cells rather than assumed from the intended sequence.
How can a transposable element alter gene expression?
Insertion can disrupt a coding sequence or land near regulatory DNA, changing when and how strongly a neighbouring gene is transcribed. Mobile elements can also promote unequal recombination or carry sequence to a new location. The outcome depends on insertion site and genomic context, so movement does not imply one universal phenotype.
Exam move
Sort genome-change mechanisms by scale: base sequence, regulatory region, exon arrangement, gene copy number, chromosome set, genomic position or lineage transfer. For a scenario, identify what has increased, moved or changed before naming the mechanism. Draw the CRISPR system as target recognition followed by a separate outcome step, then compare active nuclease, inactive binding and regulatory fusions.
Avoid treating all non-coding DNA as functionless. Practise explaining how duplication can preserve one ancestral copy while allowing another to diverge. Finish by asking whether an engineered change is somatic, germline or capable of spreading through a population. Keep targeting evidence separate from repair evidence, because recognising a site does not predetermine the resulting edit.
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