AP Biology 6.7 Mutations Study Notes - New Syllabus Effective 2025
AP Biology 6.7 Mutations Study Notes – New syllabus
AP Biology 6.7 Mutations Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the various types of mutation.
Key Concepts:
- Mutations
6.7.A Describe the Various Types of Mutation
Mutations are changes in the DNA sequence that may affect how a gene is expressed. These changes can occur naturally or be triggered by damage from external factors like radiation or chemicals. Not all mutations are harmful; some are neutral or even beneficial.
📌 What Causes a Mutation?
DNA Damage
- Caused by radiation (e.g., UV, X-rays) → breaks DNA strands
- Caused by chemicals → cross-linking with DNA or forming abnormal bonds
- If DNA damage is not repaired correctly, permanent mutations occur.
Errors During Replication/Transcription
- DNA Polymerase errors → more serious, because DNA is permanent and inherited
- RNA Polymerase errors → less serious, RNA is temporary and not inherited
🔧 DNA polymerase has proofreading ability, but RNA polymerase does not
🧬 Types of Gene Mutations
1️⃣ Base Substitution (Point Mutations)
A single nucleotide is replaced with another.
| Mutation Type | What Happens | Effect on Protein |
|---|---|---|
| Nonsense | Codon becomes a STOP codon prematurely | Protein is truncated or broken |
| Missense | Codon codes for wrong amino acid | Protein may malfunction |
| Silent | Codon changes, but same amino acid made | No effect (neutral mutation) |
2️⃣ Frameshift Mutations
Caused by:
- Insertion: Extra base(s) added
- Deletion: Base(s) removed
🧬 Not in multiples of 3 → shifts the reading frame → totally changes codons → protein is incorrect
⚠️ Can be severe and damaging
🧩 Gene Rearrangement Mutations (Chromosomal)
These affect larger DNA segments or entire genes:
| Type | What Happens | Impact |
|---|---|---|
| Deletion | Segment lost | Gene function may be lost |
| Duplication | Segment copied again | Can lead to altered gene expression |
| Inversion | Segment flips orientation | May disrupt regulatory regions |
| Translocation | DNA moves between chromosomes | Can disrupt or mis regulate genes |
🌱 Transposons (Jumping Genes)
- DNA segments that move around the genome
- Can interrupt genes or disrupt gene expression
- Act like natural mutagens
🧪 Mutation Outcomes: Neutral, Harmful, or Beneficial
| Type | Example | Effect |
|---|---|---|
| Neutral | Silent mutation | No change to protein |
| Harmful | Sickle-cell mutation | Dysfunctional protein |
| Beneficial | Mutation giving antibiotic resistance | Survival advantage |
🧬 Mutations & Heredity
- DNA mutations are passed from cell to cell (mitosis) or to offspring (meiosis)
- That’s why mutations in DNA are long-term
- RNA mutations don’t last, since RNA is temporary
6.7.A.1 Effects of DNA Mutations on Proteins and Phenotypes
Mutations are changes in the DNA sequence. They can affect the type or amount of protein produced, which may change the phenotype of an organism.
🔍 Key Points:
- Mutations can be:
- Beneficial – may give a trait that helps survival
- Detrimental – may cause disease or reduce function
- Neutral – no noticeable effect on the protein or trait
🧬 Mutation Sources:
- DNA replication errors
- Radiation (X-rays, UV)
- Chemical mutagens
- Spontaneous (random changes)
🔹 Types of Mutations You Need to Know:
| Mutation Type | Description | Effect on Protein/Phenotype |
|---|---|---|
| Point Mutation | One nucleotide is replaced by another | May change one amino acid |
| Frameshift Mutation | Caused by insertion or deletion of nucleotides (not in multiples of 3) | Changes entire reading frame – major effect |
| Nonsense Mutation | A point mutation that changes a codon into a STOP codon prematurely | Creates a truncated (short) protein |
| Silent Mutation | A change in the DNA that doesn’t change the amino acid | No effect on the protein (neutral) |
✅ Quick Summary:
Mutations can change protein structure or expression, which may affect traits. Some mutations cause no change, while others can disrupt or improve protein function.
6.7.B How Changes in Genotype Affect Phenotype
📌 Big Idea:
A genotype is the DNA code (genes) an organism has. A phenotype is the physical expression of those genes like eye color, enzyme function, or disease presence.
➡️ Mutations (changes in DNA) can change the instructions for making proteins.
➡️ This can lead to a change in phenotype, because proteins control most traits.
🧠 How It Works: DNA ➝ Protein ➝ Trait
- Gene (DNA) gets transcribed into mRNA
- mRNA is translated into a protein
- Protein carries out a function in the cell/body
- That function shows up as a trait (phenotype)
🔄 When Genotype Changes…
A mutation in the gene can:
- Change the amino acid sequence of the protein
- Make the protein too short (nonsense)
- Shift the reading frame (frameshift)
- Or not change the protein at all (silent)
If the protein is changed:
- It may not work the same way (loss of function)
- It may work better or differently (gain of function)
- It may no longer be made
⚙️ Examples of Genotype ➝ Phenotype Changes
| Genotype Change | Protein Effect | Phenotype Effect |
|---|---|---|
| Missense mutation | One amino acid changed | Protein may misfold → disease or new trait |
| Nonsense mutation | Protein stops early | Short, nonfunctional protein |
| Silent mutation | No change in amino acid | No change in phenotype |
| Frameshift mutation | Whole protein changes | Often severe phenotype change |
🧬 Phenotypic Outcomes Can Be:
- Beneficial (rare): e.g., mutation gives resistance to a disease
- Harmful: e.g., mutation causes sickle cell disease or cancer
- Neutral: e.g., mutation doesn’t affect the protein
👨🔬 Real Life Example:
🧪 Sickle Cell Anemia
- Mutation in hemoglobin gene (genotype)
- Glutamic acid (Glu) ➝ Valine (Val)
- Hemoglobin protein changes shape
- Red blood cells become sickle-shaped (phenotype)
- This affects oxygen transport in the blood
🧬 Same example also shows how a single DNA change can have a big phenotypic effect.
📝 Remember this:
Mutation → altered protein → altered function → changed trait (phenotype)
6.7.B.1 Sources of Mutations & Their Effects
📌 Big Idea:
Mutations are random changes in DNA that can be caused by:
- Internal errors (like during DNA replication)
- External factors (like radiation or chemicals)
These mutations can lead to changes in phenotype and are also a major source of genetic variation in populations.
🧪Where Do Mutations Come From?
A. Errors During DNA Replication
- Happens when the cell copies DNA before division (S phase).
- DNA polymerase can sometimes insert the wrong base.
- Normally, proofreading fixes this but mistakes can escape repair.
B. Faulty DNA Repair Mechanisms
- Cells have repair systems for damaged DNA.
- If these fail or work incorrectly → mutations can be left behind.
C. Environmental (External) Factors
- Also called mutagens things that cause mutations:
- Radiation (UV light, X-rays)
- Chemicals (e.g., tobacco smoke, reactive oxygen species)
- Viruses can sometimes insert their DNA into ours
🔄 How Mutations Affect Organisms
Mutations may be:
| Type | Meaning | Example Effect |
|---|---|---|
| Beneficial | Helps survival or function | Mutation gives resistance to malaria |
| Neutral | Has no noticeable effect | Silent mutation → same protein |
| Harmful | Disrupts function or causes disease | Mutation causes cancer or disorders |
➡️ The environment decides whether a mutation is helpful, harmful, or neutral.
🌱 3. Why Are Mutations Important?
Mutations = Source of Genetic Variation 🧬
This variation is the raw material for evolution. Natural selection acts on this diversity.
For example:
- A mutation might help an organism survive better in its habitat.
- Over generations, helpful mutations can spread in a population.
⚠️ Recap:
| Source of Mutation | Example |
|---|---|
| DNA replication error | Wrong base added during S phase |
| Faulty DNA repair | Damaged DNA not properly fixed |
| Radiation | UV light causing thymine dimers |
| Chemical mutagens | Benzene, tobacco smoke |
🧠 Quick Summary:
- Mutations happen randomly, but some are caused by the environment.
- Some are repaired, but others stick.
- Whether a mutation helps or harms depends on context.
- They are crucial for creating genetic diversity.
6.7.B.2 Chromosomal Errors and Their Effects on Phenotype
📌 Big Idea:
- Mistakes during mitosis or meiosis can cause major genetic changes not just in one gene, but in entire chromosomes.
- These changes can alter an organism’s phenotype, often in significant ways, sometimes causing developmental disorders or producing new traits.
🧩 1. Errors in Mitosis & Meiosis
⚠️ What Can Go Wrong? 
A. Nondisjunction — Failure of chromosomes to separate properly
- Happens during anaphase (I or II in meiosis or mitosis)
- Causes abnormal chromosome numbers in the daughter cells
📉 2. Changes in Chromosome Number
| Type of Error | What It Means | Result |
|---|---|---|
| Aneuploidy | Extra or missing individual chromosome | e.g., 45 or 47 chromosomes instead of 46 |
| Triploidy | 3 complete sets of chromosomes (3n) | Extremely rare in humans, usually not viable |
🧬 New phenotypes may arise from:
- Extra copies of genes being expressed
- Missing genes leading to loss of function
⚠️ 3. Phenotypic Consequences
Mutations involving whole chromosomes can lead to:
i. Visible or Functional Changes
- Unusual traits
- Developmental changes (body structure, cognition, etc.)
ii. Developmental Disorders
- Big chromosomal mistakes usually lead to noticeable developmental effects.
🔧 4. Structural Chromosome Mutations
Even if the number of chromosomes stays the same, their structure can be altered, such as:
| Type of Structural Change | Description |
|---|---|
| Deletion | Part of a chromosome is missing |
| Duplication | A section is repeated |
| Inversion | A segment is flipped in place |
| Translocation | Part of one chromosome attaches to another |
➡️ These can disrupt gene function and cause genetic disorders.
🧠 Summary:
- Mistakes in mitosis/meiosis = major chromosomal changes
- Nondisjunction → abnormal chromosome numbers (aneuploidy, triploidy)
- Structural mutations = changed gene order, function, or dosage
- These often change phenotypes and can cause disorders
6.7.C How DNA Mutations Create Variation for Natural Selection
🧬 What’s the Big Idea?
Changes in DNA sequences (mutations) create genetic variation — the raw material on which natural selection acts.
🔄 How Mutations Lead to Variation:
- Mutations = Changes in the DNA sequence.
- They can occur spontaneously (randomly during DNA replication) or be caused by external factors like radiation or chemicals.
- Some mutations change proteins that affect how organisms look or function — this leads to different traits (phenotypes).
🎯 Natural Selection Needs Variation:
- Natural selection works when some traits help organisms survive and reproduce better in a given environment.
- If a mutation gives an organism a survival advantage, it’s more likely to pass that trait to the next generation.
- Over time, this leads to evolutionary change in the population.
🧪 Mutation Types That Contribute to Variation:
| Mutation Type | Effect on Variation |
|---|---|
| Point Mutation | May slightly change a protein or be silent |
| Missense/Nonsense | Can alter or stop protein function |
| Frameshift | Changes the whole protein downstream |
| Chromosomal Changes | Add or remove large parts of the genome |
Most mutations are neutral or harmful, but some are beneficial — and those are the ones that natural selection favors.
🧠 Bottom Line:
- Mutations introduce new traits.
- Natural selection filters them based on whether they help or hurt survival and reproduction.
6.7.C.1 – Genotype Changes + Genetic Variation for Natural Selection
🔬 Genotype → Phenotype → Natural Selection
- Genotype = the DNA sequence
- Phenotype = the trait or appearance caused by genotype
- A mutation or change in genotype can:
- Do nothing (neutral)
- Harm the organism (detrimental)
- Help it survive & reproduce (beneficial)
If beneficial, the trait may be passed on more frequently → evolution over generations!
🌱 1. Horizontal Gene Transfer in Prokaryotes 🦠
Prokaryotes don’t reproduce sexually, but they have 4 special ways to increase genetic diversity:
| Mechanism | What Happens |
|---|---|
| Transformation | Bacteria absorb foreign DNA from their environment |
| Transduction | Viruses accidentally move DNA between bacterial cells |
| Conjugation | One bacterium directly transfers DNA to another via a pilus |
| Transposition | DNA segments (transposons) move within or between DNA molecules |
➡ These processes create new gene combos → more variation → more material for natural selection.
🦠 2. Viral Recombination Increases Variation Too
- If related viruses infect the same host cell, their genetic material can mix and recombine.
- This creates new viral variants with different traits (e.g., new surface proteins).
- Viruses evolve fast because of this!
🧬 3. Sexual Reproduction Conserves Variation Across Life
- Eukaryotes (like plants, animals, fungi) use meiosis + fertilization to shuffle genes:
- Crossing over (Prophase I)
- Independent assortment
- Random fertilization
- These processes are shared across many species and help maintain high genetic diversity — which is key for evolution!
✅ Final Takeaway:
- Genetic variation comes from both mutations and gene exchange.
- Natural selection acts on these differences to shape populations.
- These mechanisms are essential for evolution and are found in both prokaryotes & eukaryotes.