What is DNA and why is it important in inheritance?

DNA, or deoxyribonucleic acid, is the hereditary material in almost all living organisms. It contains the instructions needed for an organism to develop, survive, and reproduce. In the context of inheritance, DNA is crucial because it carries the genetic information from parents to offspring, determining traits such as eye color, height, and even susceptibility to certain diseases. Understanding DNA is fundamental in the IB MYP Science curriculum as it forms the basis of genetic inheritance.

How is DNA structured and what are its main components?

DNA is structured as a double helix, resembling a twisted ladder. The sides of this ladder are made of sugar and phosphate molecules, while the rungs consist of nitrogenous bases. There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair specifically (A with T and C with G) to form the rungs of the ladder. This structure is essential for the replication and function of DNA, which is a key concept in the IB MYP Science curriculum.

How does DNA replication occur and why is it necessary?

DNA replication is the process by which a cell makes an exact copy of its DNA. This occurs during cell division, ensuring that each new cell receives a complete set of genetic information. The process involves unwinding the double helix and using each strand as a template to synthesize a new complementary strand. This is crucial for growth, repair, and reproduction in living organisms, making it a significant topic in the study of inheritance within the IB MYP Science framework.

What role do genes play in the function of DNA?

Genes are segments of DNA that contain the instructions for building proteins, which perform most of the functions in a cell. Each gene has a specific sequence of bases that encodes the information needed to produce a particular protein. The expression of genes determines an organism's traits and characteristics. In the context of inheritance, genes are passed from parents to offspring, influencing inherited traits. Understanding the role of genes is a fundamental aspect of the IB MYP Science curriculum.

How do mutations in DNA affect genetic inheritance?

Mutations are changes in the DNA sequence that can occur naturally or due to environmental factors. These changes can affect the structure and function of proteins, potentially leading to variations in traits or even genetic disorders. Some mutations may be beneficial, providing an advantage in certain environments, while others can be harmful. In the study of inheritance, understanding mutations is important as they contribute to genetic diversity and evolution, a key topic in the IB MYP Science curriculum.

Why is "Using 'gene' and 'allele' interchangeably." a common mistake in DNA?

Why it happens: Both relate to inheritance. How to avoid it: GENE = the location/instruction (e.g. 'eye colour gene'). ALLELE = one possible version (e.g. 'blue', 'brown'). One gene, multiple possible alleles.

Why is "Writing the complementary strand in the wrong direction." a common mistake in DNA?

Why it happens: Easy slip. How to avoid it: For MYP, the simple reversed-complement is fine — directional details (5' vs 3') come at A level.

InheritanceIB MYP Sciences (Years 1-5)

DNA

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DNA — IB MYP Sciences: the double helix, the four bases and how genes work

DNA carries the genetic information that controls every living cell. This MYP Sciences note covers the double-helix structure, the four bases (A, T, G, C), the idea of genes, and how DNA is copied and read by cells.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Describe the double-helix structure of DNA.
MYP Sciences A — Name the four bases and state the pairing rules.
MYP Sciences A — Define gene, genome, chromosome.
MYP Sciences D — Discuss DNA's role in inheritance and biotechnology.

Structure of DNA

Two strands twisted, joined by base pairs.

DNA is made of two long strands twisted around each other to form a double helix — discovered by Watson and Crick (with critical input from Rosalind Franklin's X-ray data) in 1953.

Each strand is a chain of repeating units called nucleotides:

Sugar (deoxyribose).
Phosphate group.
One of FOUR BASES: Adenine, Thymine, Guanine, Cytosine.

The two strands are held together by hydrogen bonds between the bases. The bases PAIR specifically:

A always pairs with T.
G always pairs with C.

This is called complementary base pairing. So if one strand reads AGCT..., the other reads TCGA....

The double helix is about 2 nm wide. If you stretched out all the DNA in one human cell it would be ~2 metres long — packed into a nucleus only ~10 μm across.

Double helix: two strands twisted.
Backbone: sugar + phosphate.
Four bases: A, T, G, C.
A pairs with T; G pairs with C.

Genes, chromosomes and the genome

Gene = a section of DNA. Chromosome = a packaged DNA molecule. Genome = all the DNA.

Gene: a section of DNA that codes for one specific protein (and so for one specific trait, like eye colour or blood type).

Chromosome: a single long DNA molecule wrapped around protein 'scaffolding' for compact storage in the nucleus. Humans have 46 chromosomes (23 pairs).

Genome: ALL the DNA in an organism. The human genome has ~3 billion base pairs and ~20 000 genes.

DNA's two main jobs:

Copy itself (replication) before cell division — so each new cell gets the full set of instructions.
Be READ to make proteins: the sequence of bases in a gene specifies the sequence of amino acids in a protein. This happens in two steps — transcription (DNA → RNA) and translation (RNA → protein).

Each protein does a specific job in the cell — enzymes catalyse reactions, structural proteins (collagen, keratin) provide shape, transport proteins (haemoglobin) carry molecules, etc.

MYP criterion D classic: 'Discuss the ethical implications of genome editing.' Strong answers cover potential benefits (curing genetic diseases) vs risks (off-target effects, designer babies, inequality of access).

Gene: section of DNA coding for one protein.
Chromosome: a single DNA molecule wrapped up.
Genome: all the DNA in an organism.
DNA replicates before cell division and is read to make proteins.

How it’s examined

Criterion A tests structure, base pairing and definitions. Criterion D often asks about applications like genetic engineering or DNA forensics.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — DNA

Step-by-step solutions to past-paper-style questions on dna, written exactly the way a tutor would explain them at the board.

1Base pairing
Getting started• bases
Question
One strand of DNA reads ATTGCAT. Write the complementary strand.
Step-by-step solution
1. Step 1
  A↔T, G↔C.
2. Step 2
  ATTGCAT → TAACGTA.
Answer
TAACGTA.
2Gene vs chromosome vs genome
Building confidence• vocabulary
Question
Place these from SMALLEST to LARGEST: chromosome, gene, genome.
Step-by-step solution
1. Step 1
  Gene is a section of a DNA molecule.
2. Step 2
  Chromosome is one whole DNA molecule (contains many genes).
3. Step 3
  Genome is ALL the DNA (all chromosomes together).
Answer
Gene < chromosome < genome.
3Uses of DNA knowledge
Stretch• applications
Question
Give three uses of understanding DNA in medicine or science.
Step-by-step solution
1. Step 1
  Genetic testing for inherited diseases (e.g. cystic fibrosis carriers).
2. Step 2
  DNA fingerprinting in forensics (matching crime-scene DNA to suspects).
3. Step 3
  Genetic engineering (e.g. making insulin in bacteria).
Answer
Disease testing, forensics, genetic engineering (e.g. human insulin from GM bacteria).

Key Definitions and Keywords — DNA

Definitions to memorise and the exact keywords mark schemes credit for dna answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

DNA
Examiner keyword
Deoxyribonucleic acid — the double-helix molecule that stores genetic information.
Base pair
Examiner keyword
Two complementary bases on opposite DNA strands (A-T or G-C).
Gene
Examiner keyword
A section of DNA that codes for one specific protein.
Chromosome
Examiner keyword
A single DNA molecule packaged with proteins. Humans have 46 (23 pairs).
Genome
Examiner keyword
All the DNA in an organism.

Common Mistakes and Misconceptions — DNA

The traps other students keep falling into on dna questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Using 'gene' and 'allele' interchangeably.
Why it happens
Both relate to inheritance.
How to avoid it
GENE = the location/instruction (e.g. 'eye colour gene'). ALLELE = one possible version (e.g. 'blue', 'brown'). One gene, multiple possible alleles.
✕Writing the complementary strand in the wrong direction.
Why it happens
Easy slip.
How to avoid it
For MYP, the simple reversed-complement is fine — directional details (5' vs 3') come at A level.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

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DNA — frequently asked questions

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DNA

Short Study Notes in the form of Slides

Short Study Notes — DNA

Detailed Notes

What you’ll learn

How it’s examined

1Base pairing

2Gene vs chromosome vs genome

3Uses of DNA knowledge

DNA

Base pair

Gene

Chromosome

Genome

✕Using 'gene' and 'allele' interchangeably.

✕Writing the complementary strand in the wrong direction.

Practice questions

Past paper style quiz

Assess your understanding

DNA — frequently asked questions