Introduction
Every living organism, from the smallest bacterium to the largest whale, contains a remarkable molecule that carries the instructions needed for life. This molecule is called DNA. DNA acts like a biological instruction manual, telling cells how to grow, function, repair themselves, and reproduce.
The DNA you carry today is a combination of genetic information inherited from your parents. It determines many of your inherited characteristics and provides the instructions for making the proteins that keep your body alive. Because of its central role in biology, DNA is often called the molecule of life.
What Is DNA?
DNA stands for deoxyribonucleic acid.
DNA is the molecule that:
- Stores genetic information.
- Carries instructions for building and maintaining an organism.
- Passes inherited information from parents to offspring.
Every living organism contains DNA, although the amount and arrangement differ between species.
Figure 1. DNA is the molecule that stores the genetic instructions needed for life.
Where Is DNA Found?
In most organisms, DNA is found inside the nucleus of cells.
DNA is packaged into structures called chromosomes.
Animal and plant cells contain:
- A nucleus.
- Chromosomes inside the nucleus.
- DNA making up the chromosomes.
Some small amounts of DNA are also found in:
- Mitochondria (in plants and animals).
- Chloroplasts (in plants).
In bacteria, which do not have a nucleus, DNA is found in the cytoplasm.
Figure 2. In most cells, DNA is packaged into chromosomes inside the nucleus.
DNA and Chromosomes
DNA is an extremely long molecule.
To fit inside the nucleus, it is tightly coiled around proteins to form chromosomes.
Each chromosome contains:
- One long DNA molecule.
- Thousands of genes.
Humans normally have:
- 46 chromosomes
- Arranged into 23 pairs
Each parent contributes one chromosome to each pair.
DNA Is the Hereditary Material
DNA is the hereditary material because it is passed from parents to their offspring.
During reproduction:
- Half of a child's DNA comes from the mother.
- Half comes from the father.
This is why children often resemble their parents in characteristics such as:
- Eye colour.
- Hair colour.
- Blood group.
- Height (partly).
- Facial features.
Figure 3. DNA carries inherited information from parents to their offspring.
What Does DNA Do?
DNA contains instructions for making proteins.
Proteins are responsible for many important functions, including:
- Building body tissues.
- Repairing damaged cells.
- Carrying oxygen.
- Digesting food.
- Fighting infections.
- Controlling chemical reactions through enzymes.
Without DNA, cells would not know how to make the proteins they need to survive.
DNA and Genes
A gene is a section of DNA that contains instructions for making a specific protein or controlling a particular characteristic.
Each chromosome contains many genes.
Examples of characteristics influenced by genes include:
- Eye colour.
- Blood group.
- Hair type.
- Some inherited diseases.
Genes are the basic units of heredity.
Figure 4. Genes are sections of DNA found on chromosomes.
DNA and Growth
Every cell in your body contains essentially the same DNA.
Different cells use different genes.
For example:
- Muscle cells produce proteins needed for movement.
- Skin cells produce proteins that protect the body.
- Nerve cells produce proteins that help transmit signals.
DNA controls which proteins each type of cell produces.
This allows specialised cells to perform different functions.
DNA and Development
DNA guides development from a single fertilised egg into a complete organism.
During development, DNA controls:
- Cell division.
- Cell growth.
- Cell specialisation.
- Organ formation.
These instructions ensure that body systems develop correctly.
DNA and Reproduction
Before cells divide, DNA is copied so that each new cell receives a complete set of genetic instructions.
During reproduction:
- DNA is passed to offspring.
- Inherited characteristics are transmitted to the next generation.
This allows species to continue from one generation to the next.
Figure 5. DNA is copied before cell division so each new cell receives the genetic instructions it needs.
Why DNA Is Important
DNA is essential because it:
- Stores genetic information.
- Controls protein production.
- Directs growth and development.
- Enables reproduction.
- Passes inherited characteristics to future generations.
Without DNA, life as we know it would not exist.
Everyday Applications of DNA
Scientists use DNA in many fields.
Examples include:
- Medical diagnosis.
- Genetic testing.
- Crime scene investigations.
- Identifying family relationships.
- Studying evolution.
- Conserving endangered species.
DNA technology has become one of the most powerful tools in modern biology.
Figure 6. DNA is used in medicine, forensic science, conservation, and biological research.
Worked Example
Question
Complete the statements.
- DNA is found mainly inside the __________ of plant and animal cells.
- DNA is organised into structures called __________.
- Sections of DNA that contain instructions for characteristics are called __________.
Solution
- Nucleus
- Chromosomes
- Genes
Real-World Connection
Doctors can use DNA testing to identify inherited genetic disorders and help families understand their risk of passing certain conditions to future generations. DNA analysis is also used in forensic science to identify individuals from tiny biological samples, making it an important tool in modern criminal investigations.
Did You Know?
If you stretched out all the DNA from a single human cell, it would be about 2 metres long. Yet it fits inside a nucleus that is only about 0.000006 metres (6 micrometres) across because the DNA is tightly coiled around specialised proteins and packed into chromosomes.
Key Terms
Chromosome – A tightly coiled structure made of DNA and proteins that contains many genes.
DNA (Deoxyribonucleic acid) – The molecule that stores genetic information in living organisms.
Gene – A section of DNA that contains instructions for making a specific protein or controlling a characteristic.
Genetic information – The inherited instructions stored in DNA.
Heredity – The passing of genetic information from parents to offspring.
Nucleus – The membrane-bound organelle that contains most of a cell's DNA in plants and animals.
Protein – A molecule built from amino acids that performs many structural and functional roles in living organisms.
Key Takeaways
- DNA (deoxyribonucleic acid) is the molecule that stores the genetic information needed for life.
- In most organisms, DNA is found in the nucleus, where it is packaged into chromosomes.
- DNA is the hereditary material passed from parents to offspring during reproduction.
- Genes are sections of DNA that contain instructions for making proteins and controlling inherited characteristics.
- DNA directs growth, development, cell function, and reproduction by controlling protein production.
- Understanding DNA is fundamental to biology, medicine, genetics, and biotechnology.
