📘 Lesson 3,

7.4 – Sequence Homology

🧬 What is Sequence Homology?

Let’s begin with the term “sequence”. In biology, a sequence usually refers to the order of building blocks in DNA, RNA, or proteins.

• DNA is made up of four chemical letters: A, T, C, and G.
• RNA is similar but has U instead of T.
• Proteins are made from chains of smaller units called amino acids, and they also have specific sequences.

Now, let’s understand the word “homology”.

Homology means similarity due to shared ancestry. That means two things are homologous if they both came from the same original thing in the past.

So, sequence homology means two DNA, RNA, or protein sequences are similar because they both came from a common ancestor. They may not be 100% identical now, but they still show signs that they were once the same.

🧬 Why Does Sequence Homology Matter?

Imagine two modern cars that look similar. Even though one is a Honda and the other is a Toyota, their shape, engine, and technology may be similar because both companies started from earlier car designs. That’s the idea of shared origin.

In biology, similar DNA or protein sequences across different organisms tell us that those organisms share common ancestry, and often, the genes or proteins still perform similar functions.

🔍 Types of Homologous Sequences

Biologists divide homologous sequences into two categories based on how they evolved:

1️⃣ Orthologs

🧠 Definition:
Orthologs are genes or protein sequences found in different species that come from the same gene in a common ancestor.

📚 How They Evolved:
They formed when one species split into two (a process called speciation) and each new species kept a copy of that same gene.

🔧 Function:
Orthologs usually keep the same function across different species.

📌 Example:
There’s a gene in humans that helps us digest milk (called lactase). A similar gene exists in cows and monkeys. These are orthologs because they all came from a common ancestor and still help digest milk sugar (lactose).

2️⃣ Paralogs

🧠 Definition:
Paralogs are genes within the same species that came from a gene duplication event.

📚 How They Evolved:
Instead of splitting into two species, a single species’ DNA accidentally made a copy of a gene. Over time, each copy might change a little and do something different.

🔧 Function:
Paralogs can evolve new or slightly different functions, even though they came from the same original gene.

📌 Example:
In humans, we have a family of genes called globin genes that make different types of hemoglobin (the protein in red blood cells that carries oxygen). These globin genes are paralogs—they all came from a single ancestral gene but have slightly different jobs, like working in babies or adults.

🧠 Why Is Sequence Homology Important?

Understanding sequence homology helps scientists:

🔗 1. Discover Evolutionary Relationships

• By comparing genes between species, scientists can build evolutionary trees to see how species are related.

🔬 2. Predict Functions of Unknown Genes

• If a scientist finds a gene in a plant that looks 95% similar to a known gene in humans, they can guess it might have the same function.

💊 3. Study and Treat Diseases

• Many diseases in humans are caused by faulty genes. If animals (like mice) have homologous genes, they can be used in research to study disease mechanisms or test treatments.

📌 Example:
If scientists find that a gene causing heart disease in humans is homologous to a gene in mice, they can study the disease in mice to find treatments for humans.