Database results:
    examBoard: Pearson Edexcel
    examType: IGCSE
    lessonTitle: DNA Changes and Phenotype
Biology - Genetics and Inheritance - Variation and Evolution - DNA Changes and Phenotype - BrainyLemons
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Variation and Evolution » DNA Changes and Phenotype

What you'll learn this session

Study time: 30 minutes

  • How DNA mutations occur and their different types
  • The relationship between DNA changes and phenotype
  • How mutations can affect protein structure and function
  • The role of mutations in genetic disorders
  • How mutations contribute to evolution and genetic diversity

Introduction to DNA Changes and Phenotype

Your DNA is like an instruction manual for your body. Sometimes, there can be changes or mistakes in these instructions. These changes in DNA are called mutations and they can affect how you look, function and develop - your phenotype. Let's explore how small changes in your genetic code can have big effects on your body!

Key Definitions:

  • DNA: Deoxyribonucleic acid, the molecule that carries genetic instructions for development and functioning of all living organisms.
  • Mutation: A change in the DNA sequence that can alter genes.
  • Phenotype: The observable characteristics of an organism resulting from the interaction of its genetic makeup with the environment.
  • Genotype: The genetic makeup of an organism.

📈 DNA Structure Reminder

DNA is made up of nucleotides containing:

  • A sugar (deoxyribose)
  • A phosphate group
  • A nitrogenous base (A, T, G, or C)

The bases pair up (A with T, G with C) to form the famous double helix structure. The sequence of these bases forms a code that determines protein production.

🏠 From DNA to Phenotype

Your DNA contains genes that code for proteins. These proteins perform functions in your body that determine your traits. The path from DNA to phenotype is:

DNA → RNA → Protein → Function → Trait

Changes anywhere along this path can affect your phenotype!

Types of DNA Mutations

Mutations can happen in different ways. Think of DNA as a sentence made of letters. Just like typos in a sentence can change its meaning, mutations can change how your DNA works.

🔁 Substitution

One base is swapped for another.

Original: ATGCTA

Mutated: ATGGTA

Like changing "cat" to "cot"

➕ Insertion

An extra base is added.

Original: ATGCTA

Mutated: ATGACTA

Like changing "cat" to "cart"

➖ Deletion

A base is removed.

Original: ATGCTA

Mutated: ATCTA

Like changing "cat" to "ct"

Effects of Mutations on Proteins

DNA provides the code for making proteins through a process called protein synthesis. When DNA changes, the proteins it codes for can also change. Let's see how this works:

📝 The Genetic Code

DNA is read in groups of three bases called codons. Each codon codes for a specific amino acid. For example:

  • ATG codes for methionine
  • TGG codes for tryptophan
  • TAA, TAG and TGA are stop codons (they tell the cell to stop making the protein)

🎯 Mutation Effects

Different mutations can have different effects:

  • Silent mutation: Changes the DNA but doesn't change the amino acid (no effect on phenotype)
  • Missense mutation: Changes one amino acid for another (may affect protein function)
  • Nonsense mutation: Creates a stop codon, resulting in a shortened protein
  • Frameshift mutation: Insertions or deletions that change the reading frame of the DNA, affecting all amino acids that follow

How Mutations Affect Phenotype

Mutations can have various effects on an organism's phenotype, ranging from no effect at all to significant changes in appearance or function.

The Spectrum of Mutation Effects

😐 Neutral

Many mutations have no noticeable effect on phenotype. They might:

  • Occur in non-coding regions of DNA
  • Be silent mutations that don't change the amino acid
  • Affect proteins in ways that don't impact function
👍 Beneficial

Some mutations provide an advantage:

  • Lactose tolerance in adults
  • Resistance to certain diseases
  • Better adaptation to environmental conditions

These can be favoured by natural selection.

👎 Harmful

Many mutations disrupt normal function:

  • Can cause genetic disorders
  • May prevent proteins from working properly
  • Can affect development or metabolism

Case Study Focus: Sickle Cell Anaemia

Sickle cell anaemia is caused by a single base substitution in the gene for haemoglobin (the protein that carries oxygen in red blood cells).

The mutation changes just one amino acid in the protein chain (glutamic acid to valine). This small change causes haemoglobin molecules to stick together when oxygen levels are low, making red blood cells form a sickle shape.

These sickle-shaped cells can block small blood vessels and break down more easily, leading to anaemia and pain.

Interestingly, people with just one copy of the sickle cell gene (carriers) have some protection against malaria. This is why the mutation is more common in regions where malaria is prevalent - it's an example of how a harmful mutation can also be beneficial in certain environments.

Mutations and Genetic Disorders

Many genetic disorders are caused by mutations in specific genes. Understanding these mutations helps scientists develop treatments and genetic counselling strategies.

🔬 Examples of Genetic Disorders

  • Cystic fibrosis: Caused by mutations in the CFTR gene, affecting salt transport across cell membranes
  • Huntington's disease: Caused by an expanded repeat of CAG in the HTT gene
  • Phenylketonuria (PKU): Mutations in the PAH gene prevent the breakdown of phenylalanine

👥 Inheritance Patterns

Genetic disorders can be inherited in different ways:

  • Autosomal dominant: One mutated copy is enough to cause the disorder (e.g., Huntington's)
  • Autosomal recessive: Two mutated copies are needed (e.g., cystic fibrosis)
  • X-linked: Mutations on the X chromosome, often affecting males more than females (e.g., haemophilia)

Mutations and Evolution

Mutations are the raw material for evolution. Without mutations, there would be no genetic variation for natural selection to act upon.

How Mutations Drive Evolution

Evolution works through these key processes:

  1. Mutation: Creates genetic variation
  2. Natural selection: Favours beneficial mutations
  3. Genetic drift: Random changes in allele frequency
  4. Gene flow: Exchange of genes between populations

Over time, these processes can lead to new species forming - a process called speciation.

Case Study Focus: Antibiotic Resistance

Antibiotic resistance in bacteria is a perfect example of mutations and natural selection in action - and it's happening right now!

When bacteria are exposed to antibiotics, most die. However, if a random mutation gives some bacteria resistance to the antibiotic, these bacteria survive and reproduce.

Over time, the resistant bacteria become more common in the population. This is evolution through natural selection, driven by mutations.

This is why doctors are careful about prescribing antibiotics - overuse speeds up the evolution of resistant bacteria, creating "superbugs" that are difficult to treat.

Summary: From DNA Changes to Phenotype

Let's recap what we've learned about DNA changes and phenotype:

💡 Key Points

  • Mutations are changes in DNA sequence that can alter genes
  • Types of mutations include substitutions, insertions and deletions
  • Mutations can affect protein structure and function
  • The effects of mutations range from neutral to beneficial to harmful
  • Mutations provide the genetic variation needed for evolution
  • Many genetic disorders are caused by specific mutations

📖 Further Study

To deepen your understanding of DNA changes and phenotype:

  • Research specific genetic disorders and their molecular basis
  • Explore how genetic testing works to identify mutations
  • Learn about gene therapy approaches for treating genetic disorders
  • Investigate current research on CRISPR and other gene editing technologies
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