Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 17.5 Mutations of One or a Few Nucleotides Can Affect Protein Structure and Function
Mutations—changes in the genetic information of a cell—can significantly impact protein structure and function. Small-scale mutations involving single or few nucleotide pairs can cause a variety of effects, ranging from silent to devastating.
Point Mutations: Overview
- Point mutations affect only one nucleotide pair.
- If present in gametes, mutations can be inherited, potentially causing genetic disorders.
- Example: Sickle-cell disease, caused by a single nucleotide substitution leading to abnormal hemoglobin.
Types of Small-Scale Mutations
1. Nucleotide-Pair Substitution
- Silent mutations: No effect on the protein due to redundancy in the genetic code.
- Missense mutations: One amino acid replaced by another; effects range from minor to severe.
- Nonsense mutations: A codon for an amino acid becomes a stop codon; results in a prematurely terminated, usually nonfunctional protein.
2. Insertions and Deletions
- Insertions or deletions: Addition or loss of nucleotide pairs.
- Can cause frameshift mutations if not in multiples of three, drastically altering the amino acid sequence.
- Frameshifts typically lead to extensive missense or premature stop codons, resulting in nonfunctional proteins.
Mutagens and Spontaneous Mutations
- Spontaneous mutations: Errors occurring during DNA replication or recombination, usually corrected by proofreading mechanisms.
- Mutagens: Agents causing mutations, including:
- Physical mutagens: X-rays, UV radiation.
- Chemical mutagens: Nucleotide analogs, chemicals altering base-pairing properties, and substances distorting DNA structure.
- Many mutagens are carcinogenic (cancer-causing).
CRISPR-Cas9 Gene Editing
- CRISPR-Cas9 is a powerful, targeted gene-editing technology derived from bacterial defense mechanisms.
- It utilizes the Cas9 protein guided by engineered RNA to cut DNA precisely at a specified location.
- Scientists can disable genes ("knock out") or correct mutations by providing a normal gene template.
- CRISPR-Cas9 has promising therapeutic potential, including correcting mutations like those causing sickle-cell disease.
Defining a Gene
- The definition of a gene has evolved, reflecting different contexts:
- A discrete unit of heredity affecting phenotype.
- A DNA sequence coding for a polypeptide or functional RNA molecule.
- Modern definition: A gene is a region of DNA expressed to produce a functional product (polypeptide or RNA).
In a Nutshell
Small-scale mutations can dramatically affect gene expression, protein structure, and function. While some mutations are silent or harmless, others lead to diseases. Mutagens increase mutation rates, often causing harmful genetic changes. Advanced technologies like CRISPR-Cas9 hold significant promise for treating genetic disorders through precise gene editing.