DNA, Genes, and Chromosomes
4. Genome and Genetic Information
Learning outcomes
- I can define a genome as the complete set of genetic information in an organism.
-
I can explain how genetic information is stored within a genome.
- I can compare the genomes of different organisms.
- I can explain why all cells in an organism contain the same genome.
- I can describe how genomes contribute to biological diversity.
Introduction
Every living organism carries a complete set of genetic instructions that allows it to grow, develop, reproduce, and survive. These instructions are stored in its genome. From tiny bacteria to giant blue whales, every organism has a genome that contains all the DNA needed to build and maintain that species.
Although all humans share a very similar genome, small differences in DNA make every individual unique. By studying genomes, scientists can learn about inherited diseases, trace evolutionary relationships, improve crops, and better understand the diversity of life on Earth.
What Is a Genome?
A genome is the complete set of genetic information (DNA) in an organism.
The genome includes:
- Every chromosome.
- Every gene.
- DNA that does not code for proteins.
In other words, the genome is all of an organism's DNA.
Every species has its own unique genome.
Figure 1. A genome is the complete collection of an organism's DNA.
How Is Genetic Information Stored?
Genetic information is stored as a sequence of four chemical bases in DNA.
These bases are:
- A – Adenine
- T – Thymine
- C – Cytosine
- G – Guanine
The order of these bases forms the genetic code.
Different sequences of bases create different genes, and different genes provide instructions for making different proteins.
DNA, Genes, Chromosomes, and the Genome
These genetic structures are related in a hierarchy.
From smallest to largest:
- DNA bases
- Genes
- Chromosomes
- Genome
A simple way to think about this is:
- Genes are sections of DNA.
- Chromosomes contain many genes.
- The genome is the complete collection of chromosomes and all their DNA.
Figure 2. Genes are sections of DNA, chromosomes contain many genes, and the genome includes all the DNA in an organism.
Do All Cells Contain the Same Genome?
Almost every cell in a multicellular organism contains the same genome.
For example:
A skin cell, a muscle cell, and a nerve cell all contain the same DNA.
However, these cells perform different functions because they switch different genes on and off.
Examples:
- Muscle cells produce proteins for movement.
- Nerve cells produce proteins that transmit electrical signals.
- Skin cells produce proteins that protect the body.
The genome is the same, but different genes are active in different cell types.
Why Don't All Cells Look the Same?
Although all cells contain the same genome:
- Different genes are expressed in different cells.
- Different proteins are produced.
- Cells become specialised for different functions.
This process is called gene expression.
Gene expression allows one genome to produce many different types of cells.
Figure 3. Different cell types use different genes from the same genome to perform specialised functions.
Comparing Genomes
Different species have different genomes.
They vary in:
- Genome size.
- Number of chromosomes.
- Number of genes.
- DNA sequence.
For example:
| Organism | Approximate Number of Genes |
|---|---|
| Human | 20,000–21,000 |
| Fruit fly | ~14,000 |
| Rice plant | ~37,000 |
| Baker's yeast. | ~6,000 |
Having more genes does not necessarily mean an organism is more complex.
Similarities Between Genomes
Although organisms are different, many genes are shared.
Examples:
- Humans and chimpanzees share about 98–99% of their DNA.
- Humans and mice share many genes involved in growth and development.
- Even bacteria share some genes responsible for basic cellular functions.
These similarities provide evidence that all living organisms share common evolutionary ancestors.
Figure 4. Different organisms have different genomes, but many genes are shared because of common ancestry.
How Genomes Contribute to Biological Diversity
Small differences in DNA sequences create genetic variation.
These differences lead to variations in:
- Appearance.
- Growth.
- Disease resistance.
- Behaviour.
- Adaptation.
Over many generations:
- Genetic variation allows natural selection to occur.
- Populations evolve.
- New species may eventually form.
Genomes therefore play a central role in creating Earth's biodiversity.
Why Scientists Study Genomes
Genome research helps scientists:
- Understand inherited diseases.
- Develop new medicines.
- Improve crops.
- Breed healthier livestock.
- Study evolution.
- Protect endangered species.
- Investigate biodiversity.
Modern genome sequencing has transformed biology and medicine.
Figure 5. Genome research supports medicine, agriculture, conservation, and evolutionary biology.
The Human Genome Project
One of the greatest scientific achievements in biology was the Human Genome Project.
Completed in 2003, it successfully determined the sequence of nearly all the DNA in the human genome.
The project has helped scientists:
- Identify genes linked to diseases.
- Improve medical research.
- Better understand human evolution.
- Develop personalised medicine.
Today, genome sequencing continues to become faster, cheaper, and more accurate.
Worked Example
Question
Place the following structures in order from smallest to largest.
- Chromosome
- Genome
- Gene
- DNA
Solution
Smallest → Largest
DNA → Gene → Chromosome → Genome
Real-World Connection
Doctors can compare a patient's genome with reference genomes to identify genetic changes associated with inherited disorders or certain types of cancer. This information helps healthcare professionals diagnose diseases, predict how patients may respond to specific medicines, and develop personalised treatment plans based on an individual's genetic information.
Did You Know?
The human genome contains about 3.2 billion DNA base pairs. If all the DNA from a single human cell were printed as text, it would fill thousands of large books. Despite this enormous amount of information, the DNA fits inside a microscopic cell nucleus because it is tightly packaged into chromosomes.
Key Terms
DNA (Deoxyribonucleic acid) – The molecule that stores genetic information.
Gene – A section of DNA that contains instructions for making a protein or controlling a trait.
Gene expression – The process by which a cell uses the information in a gene to produce a protein.
Genome – The complete set of genetic information (DNA) in an organism.
Genetic information – The instructions stored in DNA that control the structure and function of an organism.
Human Genome Project – An international scientific project that determined the sequence of nearly all the DNA in the human genome.
Chromosome – A structure made of DNA and proteins that contains many genes.
Key Takeaways
- A genome is the complete set of DNA found in an organism.
- Genetic information is stored in the sequence of the four DNA bases: A, T, C, and G.
- Genes are sections of DNA, chromosomes contain many genes, and the genome includes all of an organism's chromosomes and DNA.
- Almost all cells in an organism contain the same genome, but different cells express different genes, allowing them to perform specialised functions.
- Genomes vary between species, yet many genes are shared because living organisms evolved from common ancestors.
- Small differences between genomes create genetic variation, which contributes to biological diversity and drives evolution.