DNA, Genes, and Chromosomes
| Site: | Young Education |
| Cursus: | Genetics and Inheritance |
| Boek: | DNA, Genes, and Chromosomes |
| Afgedrukt door: | ゲストユーザ |
| Datum: | maandag, 5 oktober 2026, 03:04 |
1. What Is DNA?
Learning outcomes
- I can describe DNA as the molecule that stores genetic information.
- I can explain where DNA is found within cells.
- I can identify DNA as the hereditary material passed from parents to offspring.
- I can describe the basic role of DNA in living organisms.
- I can explain why DNA is important for growth, development, and reproduction.
2. Genes and Their Functions
Learning outcomes
-
I can define a gene as a section of DNA that contains instructions for a trait.
- I can explain how genes influence the characteristics of organisms.
- I can describe the relationship between genes and proteins.
- I can identify examples of inherited traits controlled by genes.
- I can explain why different organisms share many genes.
3. Chromosomes
Learning outcomes
-
I can describe chromosomes as structures that contain DNA.
- I can explain how chromosomes help organize genetic information.
- I can identify the number of chromosomes found in human body cells.
- I can explain the relationship between chromosomes, genes, and DNA.
- I can compare chromosomes in different organisms.
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.
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.
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.
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.
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.
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.
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.
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:
| Organism | Number of Chromosomes (Body Cells) |
|---|---|
| Human | 46 |
| Dog | 78 |
| Cat | 38 |
| Horse | 64 |
| Fruit fly | 8 |
| Garden pea. | 14 |
Every species has its own characteristic chromosome number.
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Figure 4. Different organisms have different numbers of chromosomes.
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.
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.
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.
Worked Example
Question
Complete the statements.
- Chromosomes are made mainly of __________.
- A chromosome contains many __________.
- Human body cells normally contain __________ chromosomes arranged into __________ pairs.
Solution
- DNA
- Genes
- 46, 23
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.
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.
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).
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.
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.
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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.
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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.
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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.
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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.
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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.
5. DNA Structure and Replication
Learning outcomes
-
I can describe the double-helix structure of DNA.
- I can identify the four nitrogenous bases found in DNA.
- I can explain complementary base pairing.
- I can describe the process of DNA replication.
- I can explain why DNA replication is important before cell division.
Introduction
Every time a cell divides, it must produce an exact copy of its DNA so that each new cell receives the same genetic instructions. This remarkable process happens billions of times every day in the human body, allowing us to grow, repair damaged tissues, and replace worn-out cells.
DNA is perfectly designed for this task. Its famous double-helix structure allows the molecule to store enormous amounts of genetic information while also making it possible to copy that information with remarkable accuracy. Understanding the structure of DNA helps explain how genetic information is passed from one cell to another and from one generation to the next.
The Structure of DNA
DNA (deoxyribonucleic acid) is a long molecule made of repeating building blocks called nucleotides.
Each nucleotide contains:
- A sugar molecule (deoxyribose).
- A phosphate group.
- A nitrogenous base.
The nucleotides join together to form two long strands that twist around each other to create a double helix.
The double helix looks like a twisted ladder.
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Figure 1. DNA consists of two strands that twist together to form a double helix.
The Four Nitrogenous Bases
DNA contains four nitrogenous bases.
These are:
- Adenine (A)
- Thymine (T)
- Cytosine (C)
- Guanine (G)
The order of these bases stores the genetic information used to build proteins and control cell activities.
Complementary Base Pairing
The bases pair in a very specific way.
- Adenine (A) always pairs with Thymine (T).
- Cytosine (C) always pairs with Guanine (G).
This is called complementary base pairing.
The base pairs are held together by hydrogen bonds, which are strong enough to hold the DNA strands together but weak enough to separate during DNA replication.
A simple way to remember the pairing rules is:
- A ↔ T
- C ↔ G
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Figure 2. Complementary base pairing ensures that DNA can be copied accurately.
The Double Helix
The DNA molecule has two main parts.
Sugar-Phosphate Backbone
The sides of the DNA ladder are made of alternating:
- Sugar molecules.
- Phosphate groups.
This forms the backbone of the DNA molecule.
Base Pairs
The "rungs" of the ladder consist of pairs of nitrogenous bases.
These paired bases carry the genetic information.
When the ladder twists, it forms the characteristic double-helix shape.
What Is DNA Replication?
DNA replication is the process by which a cell copies its DNA before cell division.
The result is:
- Two identical DNA molecules.
- Each containing the same genetic information as the original.
This ensures that every new cell receives a complete copy of the genome.
The Steps of DNA Replication
DNA replication occurs in several stages.
Step 1 – The DNA Unzips
The hydrogen bonds between the base pairs break.
The two DNA strands separate.
This creates two template strands.
Step 2 – Complementary Bases Pair
Free nucleotides in the nucleus move into position.
Each exposed base pairs with its complementary partner.
- A pairs with T.
- C pairs with G.
Step 3 – New DNA Strands Form
The new nucleotides are joined together to form complete DNA strands.
Two identical DNA molecules are produced.
Each new DNA molecule contains:
- One original strand.
- One newly made strand.
This is called semi-conservative replication.
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Figure 3. During DNA replication, each original DNA strand acts as a template for a new complementary strand.
Why Is DNA Replication So Accurate?
Complementary base pairing ensures that each base is matched correctly.
For example:
Original strand:
A – T – C – G
New strand:
T – A – G – C
Because the pairing rules are fixed, cells can copy DNA with very few errors.
Special enzymes also help detect and repair mistakes during replication.
Why DNA Replication Is Important
DNA replication is essential because it allows:
- Growth.
- Tissue repair.
- Replacement of damaged cells.
- Reproduction.
- Maintenance of genetic information.
Without DNA replication, new cells would not receive the instructions needed to survive and function.
DNA Replication and Cell Division
Before a cell divides:
- Its DNA is copied.
- Each chromosome is duplicated.
After division:
- Each daughter cell receives one complete copy of every chromosome.
This allows new cells to function normally.
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Figure 4. DNA replication occurs before cell division so each daughter cell receives a complete genome.
DNA Replication and Heredity
DNA replication also plays an important role in reproduction.
By accurately copying DNA:
- Parents pass genetic information to offspring.
- Species maintain their inherited characteristics.
- Genetic information is preserved from one generation to the next.
Occasionally, small copying errors called mutations occur. These mutations introduce new genetic variation that can contribute to evolution.
Why DNA Structure Is Important
The double-helix structure allows DNA to:
- Store large amounts of information.
- Protect genetic information.
- Be copied accurately.
- Pass information between cells and generations.
The structure of DNA is perfectly suited to its role as the molecule of heredity.
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Figure 5. The structure of DNA allows it to store information and replicate accurately.
Worked Example
Question
Complete the table.
| Base | Complementary Base |
|---|---|
| Adenine | ? |
| Cytosine | ? |
| Thymine. | ? |
| Guanine | ? |
Solution
| Base | Complementary Base |
|---|---|
| Adenine | Thymine |
| Cytosine | Guanine |
| Thymine | Adenine |
| Guanine | Cytosine |
Real-World Connection
Scientists use their understanding of DNA replication in many areas of medicine and biotechnology. For example, the polymerase chain reaction (PCR) copies tiny amounts of DNA millions of times, allowing doctors to detect infectious diseases, identify genetic disorders, and analyse DNA collected from crime scenes.
Did You Know?
A single human cell copies about 3.2 billion DNA base pairs every time it divides. Despite this enormous task, DNA replication is incredibly accurate. Thanks to proofreading enzymes, cells make only a tiny number of mistakes, helping to preserve the genetic information needed for healthy growth and development.
Key Terms
Complementary base pairing – The specific pairing of DNA bases: adenine with thymine, and cytosine with guanine.
DNA (Deoxyribonucleic acid) – The molecule that stores genetic information.
DNA replication – The process of producing an identical copy of DNA before cell division.
Double helix – The twisted ladder-shaped structure of DNA.
Hydrogen bond – A weak bond that holds complementary DNA bases together.
Nitrogenous base – One of the four chemical bases in DNA: adenine, thymine, cytosine, or guanine.
Nucleotide – The basic building block of DNA, consisting of a sugar, a phosphate group, and a nitrogenous base.
Semi-conservative replication – A method of DNA replication in which each new DNA molecule contains one original strand and one newly synthesised strand.
Key Takeaways
- DNA has a double-helix structure made of two strands of nucleotides.
- The four nitrogenous bases are adenine (A), thymine (T), cytosine (C), and guanine (G).
- Complementary base pairing ensures that A pairs with T and C pairs with G.
- During DNA replication, the two strands separate, and each serves as a template for building a new complementary strand.
- DNA replication occurs before cell division, ensuring that each new cell receives a complete and nearly identical copy of the genome.
- The structure of DNA allows genetic information to be stored accurately and passed from one cell and one generation to the next.