3. Chromosomes

Introduction

Every cell in your body contains an enormous amount of DNA. If all the DNA from a single human cell were stretched out, it would measure about 2 metres long. Yet this DNA fits inside a tiny nucleus that is only about 6 micrometres across.

This is possible because DNA is carefully wound around special proteins and packed into compact structures called chromosomes. Chromosomes protect DNA, organise genetic information, and ensure that each new cell receives the correct genetic instructions during cell division.

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What Are Chromosomes?

A chromosome is a thread-like structure made of DNA and proteins that carries genetic information.

Chromosomes contain:

• One long DNA molecule.
• Many thousands of genes.
• Special proteins called histones that help package DNA.

Chromosomes are found inside the nucleus of most plant and animal cells.

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Figure 1. Chromosomes are tightly coiled structures made of DNA wrapped around proteins.

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Why Are Chromosomes Important?

Chromosomes organise DNA so that it:

• Fits inside the nucleus.
• Is protected from damage.
• Can be copied accurately.
• Can be passed to new cells during cell division.

Without chromosomes, DNA would become tangled and could not be distributed properly when cells divide.

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DNA, Genes, and Chromosomes

DNA, genes, and chromosomes are closely related.

The relationship is:

• DNA is the genetic material.
• A gene is a section of DNA containing instructions for a protein or trait.
• A chromosome is a long DNA molecule containing many genes.

This relationship can be summarised as:

Chromosome → DNA → Gene

Or, thinking about how they fit together:

• A chromosome is made of DNA.
• DNA contains many genes.

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Figure 2. Chromosomes are made of DNA, and DNA contains many genes.

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How Chromosomes Organise Genetic Information

Each chromosome carries genes in a specific order.

Every gene has a particular location, called a locus, on a chromosome.

Organising genes in this way helps cells:

• Read genetic instructions efficiently.
• Copy DNA accurately.
• Pass the correct genes to new cells.

This organisation is essential for normal growth and development.

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Human Chromosomes

Most human body cells contain:

• 46 chromosomes
• Arranged into 23 pairs

Each pair consists of:

• One chromosome inherited from the mother.
• One chromosome inherited from the father.

The first 22 pairs are called autosomes.

The final pair are the sex chromosomes, which determine biological sex:

• XX (typically female)
• XY (typically male)

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Figure 3. Human body cells normally contain 46 chromosomes arranged in 23 pairs.

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Chromosomes During Cell Division

Before a cell divides:

• DNA is copied.
• Each chromosome is duplicated.

The duplicated chromosomes are then separated so that each new cell receives a complete set of genetic information.

This ensures that new cells contain the same DNA as the original cell.

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Chromosome Numbers in Different Organisms

Different species have different numbers of chromosomes.

The number of chromosomes does not indicate how complex or advanced an organism is.

Examples include:

Every species has its own characteristic chromosome number.

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Figure 4. Different organisms have different numbers of chromosomes.

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Why Do Organisms Have Different Numbers of Chromosomes?

The chromosome number of a species is the result of its evolutionary history.

Having more chromosomes does not mean:

• More genes.
• Greater intelligence.
• Greater complexity.

Instead, chromosome number simply reflects how each species' DNA has been organised over millions of years of evolution.

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Homologous Chromosomes

In organisms that reproduce sexually, chromosomes occur in homologous pairs.

Homologous chromosomes:

• Are similar in size and shape.
• Carry the same genes.
• May contain different versions of those genes, called alleles.

One chromosome in each pair comes from each parent.

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Figure 5. Homologous chromosomes carry the same genes but may contain different alleles.

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Why Chromosomes Matter

Chromosomes allow organisms to:

• Store genetic information safely.
• Pass DNA to offspring.
• Grow by cell division.
• Repair damaged tissues.
• Produce specialised cells.

Without chromosomes, the genetic information needed for life could not be organised or inherited correctly.

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Worked Example

Question

Complete the statements.

1. Chromosomes are made mainly of __________.
2. A chromosome contains many __________.
3. Human body cells normally contain __________ chromosomes arranged into __________ pairs.

Solution

1. DNA
2. Genes
3. 46, 23

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Real-World Connection

Doctors often examine a person's chromosomes using a technique called a karyotype, which displays all the chromosomes in matching pairs. A karyotype can help identify chromosomal conditions, such as Down syndrome, which is caused by an extra copy of chromosome 21. Studying chromosomes allows healthcare professionals to diagnose certain genetic disorders and better understand human development.

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Did You Know?

Although humans have 46 chromosomes, some other organisms have many more. For example, dogs have 78 chromosomes, while a small fruit fly has only 8. Despite these differences, many of the genes that control basic life processes are remarkably similar across these species because they share common evolutionary ancestors.

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Key Terms

Autosomes – The first 22 pairs of chromosomes in humans that are not involved in determining biological sex.

Chromosome – A thread-like structure made of DNA and proteins that carries many genes.

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.

Homologous chromosomes – A matching pair of chromosomes, one inherited from each parent, carrying the same genes.

Karyotype – An organised display of an individual's chromosomes arranged in pairs.

Nucleus – The organelle in plant and animal cells that contains most of the cell's DNA.

Sex chromosomes – The pair of chromosomes that determine biological sex (XX or XY in humans).

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Key Takeaways

• Chromosomes are thread-like structures made of DNA and proteins that contain genetic information.
• Chromosomes organise and protect DNA, allowing it to fit inside the cell nucleus and be copied accurately during cell division.
• A gene is a section of DNA, and each chromosome contains many genes.
• Human body cells normally contain 46 chromosomes arranged into 23 pairs, including 22 pairs of autosomes and 1 pair of sex chromosomes.
• Different organisms have different chromosome numbers, but a larger number of chromosomes does not mean an organism is more complex.
• Chromosomes are essential for growth, development, reproduction, and the inheritance of genetic information.