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Ordinary Level Revision Notes
Leaving Cert Ordinary Level Biology
Chapter 11: DNA, RNA and Genetics
Inheritance, Variation & Biotechnology
Subtopics Covered
- Learning Outcomes
- Big Picture: From DNA to Inheritance
- DNA and RNA: Nucleic Acids
- DNA Base Pairing and Double Helix
- DNA Replication
- Protein Synthesis: OL Awareness Only
What This Pack Includes
- Structured chapter notes formatted for ExamsLogic website reading
- Exam-focused diagrams, definitions, and worked examples
- Interactive practice sections carried over from the source notes
- Independent study guidance based on the official curriculum
- Print-friendly layout for future PDF export when needed
This publication is an independent educational resource developed by ExamsLogic and compiled by experienced educators. It is based on publicly available official curricula, including Cambridge, Pearson Edexcel, IB, and the Irish Leaving Certificate. This product is not endorsed by, affiliated with, or sponsored by any examination board or governing authority. All registered trademarks remain the property of their respective owners.
1. Learning Outcomes
- Describe the basic structure of DNA and RNA nucleotides.
- Name the sugar, phosphate and nitrogenous bases found in DNA and RNA.
- Explain complementary base pairing: A with T, C with G, and A with U in RNA.
- Outline DNA replication as unzipping and copying genetic templates.
- Use key inheritance terms correctly: gene, allele, genotype, phenotype, dominant and recessive.
- Complete simple monohybrid crosses using Punnett squares.
- Understand codominance and sex linkage at Ordinary Level standard.
- Describe mutation, variation and the role of mutation in natural selection.
- Explain Darwin and Wallace’s theory of natural selection.
- Outline genetic engineering stages and applications in medicine and agriculture.
HL-only awareness: Detailed protein synthesis, dihybrid crosses, Mendel’s laws in detail, independent assortment and gene linkage are Higher Level topics. This OL chapter gives only brief awareness where useful.
2. Big Picture: From DNA to Inheritance
DNA
DNA stores genetic instructions in the sequence of bases along chromosomes.
Genes
A gene is a section of DNA that influences a characteristic or codes for a product.
Inheritance
Alleles are passed from parents to offspring through gametes during sexual reproduction.
Biology link: Genetics explains why offspring resemble parents, why variation exists, and how living things can change over generations.
3. DNA and RNA: Nucleic Acids
DNA and RNA are nucleic acids. They are made from small units called nucleotides.
| Nucleotide part | DNA | RNA |
|---|---|---|
| Phosphate | Present | Present |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, U, C, G |
| Full name | Deoxyribonucleic acid | Ribonucleic acid |
| General structure | Usually double-stranded | Usually single-stranded |
Exam Tip: DNA has thymine (T), while RNA has uracil (U). This is one of the easiest comparison marks to earn.
4. DNA Base Pairing and Double Helix
DNA is usually a double-stranded molecule. The bases pair in a specific way:
A pairs with T | C pairs with G
| Base | Full name | Pairs with |
|---|---|---|
| A | Adenine | Thymine in DNA |
| T | Thymine | Adenine |
| C | Cytosine | Guanine |
| G | Guanine | Cytosine |
| U | Uracil | Adenine in RNA |
Common Mistake: Pairing A with C or T with G. Correct DNA pairs are A-T and C-G.
5. DNA Replication
DNA replication is the copying of DNA before cell division. It ensures that new cells receive the same genetic information.
DNA unzips→
Bases exposed→
New bases pair→
Two identical DNA molecules
| Step | What happens? | Ordinary Level explanation |
|---|---|---|
| 1. Unzipping | The two DNA strands separate. | Base pairs are separated. |
| 2. Template use | Each old strand acts as a template. | The exposed bases guide the new strand. |
| 3. Complementary pairing | New nucleotides join using A-T and C-G pairing. | This copies the base sequence accurately. |
| 4. Result | Two identical DNA molecules form. | Each new DNA molecule has one old strand and one new strand. |
Exam Tip: In replication questions, use the words “unzips”, “template”, “complementary bases” and “identical copies”.
6. Protein Synthesis: OL Awareness Only
At Ordinary Level, students should understand that genes carry instructions for making proteins, but the detailed steps are Higher Level.
| Idea | Simple OL meaning | HL detail not required here |
|---|---|---|
| Gene | A section of DNA that influences a characteristic or codes for a product. | Detailed transcription and translation. |
| mRNA | A messenger molecule that carries copied genetic information. | Codons and detailed mRNA formation. |
| Ribosome | Site where proteins are made. | tRNA anticodon matching and amino acid chain formation. |
HL Only: Detailed transcription in the nucleus and translation at ribosomes are Higher Level. For OL, keep the idea simple: DNA instructions help cells make proteins.
7. Genetic Inheritance Terms
| Term | Meaning | Example / Exam use |
|---|---|---|
| Gene | A section of DNA controlling a characteristic or coding for a product. | Gene for flower colour. |
| Allele | Different form of a gene. | Purple allele or white allele. |
| Dominant | Allele expressed when present. | Represented by capital letter, e.g. B. |
| Recessive | Allele expressed only when no dominant allele is present. | Represented by lower-case letter, e.g. b. |
| Homozygous | Two identical alleles. | BB or bb. |
| Heterozygous | Two different alleles. | Bb. |
| Genotype | The genetic makeup or allele combination. | Bb. |
| Phenotype | The visible or expressed characteristic. | Brown eyes. |
| Locus | The position of a gene on a chromosome. | Gene location. |
Common Mistake: Genotype is the letters. Phenotype is the appearance or expressed trait.
8. Monohybrid Crosses
A monohybrid cross follows the inheritance of one characteristic.
Example: Let B = brown eyes and b = blue eyes. Brown is dominant.
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
| Genotype | Meaning | Phenotype |
|---|---|---|
| BB | Homozygous dominant | Brown eyes |
| Bb | Heterozygous | Brown eyes |
| bb | Homozygous recessive | Blue eyes |
Genotype ratio: 1 BB : 2 Bb : 1 bb
Phenotype ratio: 3 brown : 1 blue
Phenotype ratio: 3 brown : 1 blue
Exam Tip: Always write the parents, gametes, Punnett square, genotype ratio and phenotype ratio if the question has enough marks.
9. Codominance
Codominance occurs when both alleles are expressed in the phenotype.
| Example | Explanation |
|---|---|
| Blood group AB | Allele A and allele B are both expressed. |
| Red and white flower alleles producing mixed expression | Both allele effects can appear together depending on the example used. |
Examiner Secret: In codominance, there is not one fully dominant allele hiding the other. Both alleles affect the phenotype.
10. Sex Linkage
Sex-linked inheritance involves genes located on sex chromosomes, especially the X chromosome.
| Sex chromosomes | Meaning |
|---|---|
| XX | Usually female. |
| XY | Usually male. |
Some conditions are more common in males because males have only one X chromosome. If a recessive allele is present on the X chromosome, there may be no second matching allele to mask it.
Examiner Trap: Do not say sex-linked genes are only found in males. They can be carried by females too, especially if located on the X chromosome.
11. HL Crosses Not Required for OL
| Topic | Status | What OL students should know |
|---|---|---|
| Dihybrid crosses | HL Only | Not required for detailed OL problem solving. |
| Mendel’s Law of Segregation | HL detailed wording | OL students can understand that alleles separate into gametes. |
| Law of Independent Assortment | HL Only | Not required in detail. |
| Gene linkage | HL Only | Not required in detail. |
Quality control note: This chapter keeps the premium look while avoiding overloading Ordinary Level students with Higher Level genetics.
12. Variation and Mutation
Variation means differences between individuals of the same species.
A mutation is a change in genetic material. Mutations can occur in a gene or in a chromosome.
| Type | Meaning | Example / effect |
|---|---|---|
| Gene mutation | A change in the base sequence of a gene. | May alter a protein or characteristic. |
| Chromosome mutation | A change in chromosome number or structure. | Down syndrome is linked with an extra chromosome 21. |
| Helpful mutation | Mutation that gives an advantage. | May help survival in a certain environment. |
| Harmful mutation | Mutation that damages function. | May cause disease or reduce survival. |
| Neutral mutation | Mutation with no obvious effect. | May not affect survival. |
Exam Tip: Mutation creates new genetic variation. Natural selection can act on this variation.
13. Mutagens and Causes of Mutation
A mutagen is something that increases the chance of mutation.
| Mutagen | Example | How it may act |
|---|---|---|
| Radiation | UV light, X-rays, gamma rays | Can damage DNA. |
| Chemicals | Some chemicals in tobacco smoke | Can alter DNA or interfere with copying. |
| Viruses | Some viral infections | May affect genetic control in cells. |
Common Mistake: Confusing mutagens and carcinogens. A mutagen causes mutations; a carcinogen increases cancer risk. Some substances can be both.
14. Evolution by Natural Selection
Evolution is change in inherited characteristics of a population over generations.
Darwin and Wallace proposed the theory of natural selection.
Variation exists→
Competition→
Best adapted survive→
Reproduce→
Useful alleles become common
| Stage | Explanation |
|---|---|
| Variation | Individuals in a population differ. |
| Competition | Organisms compete for resources such as food, space and mates. |
| Survival advantage | Individuals with useful traits are more likely to survive. |
| Reproduction | Survivors pass useful alleles to offspring. |
| Change over time | Useful characteristics become more common in the population. |
Examiner Secret: Natural selection acts on phenotype, but inherited alleles are passed to the next generation.
15. Evidence for Evolution
| Evidence | How it supports evolution |
|---|---|
| Fossils | Show organisms that lived in the past and how forms changed over time. |
| Comparative anatomy | Similar structures in different species suggest common ancestry. |
| Embryology / molecular evidence | Similarities in development or DNA can show relatedness. |
Examiner Trap: Individuals do not evolve during their lifetime. Populations evolve over generations.
16. Genetic Engineering
Genetic engineering is the deliberate alteration of genetic material, often by transferring a useful gene from one organism to another.
Isolate gene→
Cut DNA→
Insert gene→
Transform cell→
Expression
| Stage | Meaning | Simple example |
|---|---|---|
| Isolation | The useful gene is identified and obtained. | Gene for human insulin. |
| Cutting | DNA is cut using enzymes. | Restriction enzymes cut DNA. |
| Insertion | The useful gene is inserted into a vector, such as a plasmid. | Gene placed into bacterial plasmid. |
| Transformation | The vector is placed into a host cell. | Bacterium receives plasmid. |
| Expression | The host uses the gene to make a useful product. | Bacteria produce insulin. |
Exam Tip: For genetic engineering, learn the sequence: isolate, cut, insert, transform, express.
17. Applications of Genetic Engineering
Medicine
- Production of human insulin by bacteria.
- Production of vaccines or useful proteins.
- Research into gene therapy.
Agriculture
- Crops with pest resistance.
- Crops with improved nutritional value.
- Crops with tolerance to difficult growing conditions.
Examiner Trap: Genetic engineering is not the same as selective breeding. Genetic engineering directly changes or transfers genes; selective breeding chooses parents with desired traits.
18. Interactive Simulator: DNA Base Pair Builder
Choose the Complementary Base
DNA template base: A
Feedback
A pairs with ?
For DNA, remember A-T and C-G. U is used in RNA, not DNA.
19. Interactive Simulator: Genetics Term Detective
Identify the Term
Clue: The visible expression of a characteristic.
Feedback
Ready
Use the definitions carefully. Many marks are lost through weak vocabulary.
20. High-Value Exam Guidance
Exam Tip 1: DNA bases: A pairs with T, C pairs with G.
Exam Tip 2: RNA has uracil instead of thymine.
Exam Tip 3: In Punnett squares, show gametes clearly at the top and side.
Exam Tip 4: For natural selection, mention variation, survival, reproduction and inheritance.
Examiner Trap 1: Dominant does not mean “more common”. It means expressed when present.
Examiner Trap 2: Mutation is random; natural selection is not random because the environment selects useful traits.
Examiner Trap 3: Do not write that organisms choose to adapt. Individuals with useful inherited traits survive more often.
Common Mistake 1: Confusing allele and gene. A gene is a DNA section; an allele is a form of a gene.
Common Mistake 2: Writing phenotype as letters. Letters like Bb are genotype.
Examiner Secret 1: “Heterozygous” always means two different alleles.
Examiner Secret 2: Genetic engineering questions often reward the correct order of stages.
Examiner Secret 3: For evolution, the population changes over generations, not one individual changing by effort.
21. MCQs with Instant Answers
1. Which base is found in RNA but not DNA?
Answer: B. RNA contains uracil instead of thymine.
2. In DNA, adenine pairs with:
Answer: C. In DNA, A pairs with T.
3. A gene is:
Answer: A. A gene is a section of DNA that influences a characteristic or codes for a product.
4. The genotype Bb is:
Answer: C. Bb has two different alleles, so it is heterozygous.
5. A phenotype is:
Answer: A. Phenotype is the expressed characteristic.
6. A mutation is:
Answer: A. A mutation is a change in genetic material.
7. Natural selection requires:
Answer: B. Variation allows some individuals to survive and reproduce more successfully.
8. Genetic engineering involves:
Answer: B. Genetic engineering deliberately changes or transfers genetic material.
9. Which is an application of genetic engineering?
Answer: A. Bacteria can be genetically engineered to produce human insulin.
10. Down syndrome is linked with:
Answer: A. Down syndrome is associated with an extra chromosome 21.
22. Structured Exam Questions
- Describe the structure of a nucleotide. [3]
- Compare DNA and RNA under three headings. [6]
- Outline DNA replication. [5]
- Define the terms gene, allele, genotype and phenotype. [8]
- In a monohybrid cross, B is dominant and b is recessive. Cross Bb with Bb and give the genotype and phenotype ratios. [8]
- Explain codominance using one example. [4]
- Explain why some sex-linked conditions are more common in males. [4]
- Define mutation and distinguish between gene mutation and chromosome mutation. [6]
- Describe natural selection. [6]
- Give two types of evidence for evolution. [4]
- Outline the stages of genetic engineering. [6]
- Give two applications of genetic engineering. [4]
23. Detailed Mark Scheme
Q1. Nucleotide contains phosphate [1], sugar [1], nitrogenous base [1].
Q2. DNA has deoxyribose/RNA has ribose [2]. DNA has thymine/RNA has uracil [2]. DNA is usually double-stranded/RNA usually single-stranded [2].
Q3. DNA unzips [1]. Strands separate [1]. Each strand acts as a template [1]. Complementary bases pair [1]. Two identical DNA molecules form [1].
Q4. Gene: section of DNA controlling/coding for characteristic/product [2]. Allele: form of a gene [2]. Genotype: genetic makeup/allele combination [2]. Phenotype: expressed/visible characteristic [2].
Q5. Parent genotypes Bb and Bb [1]. Gametes B and b from each parent [1]. Punnett square correct [3]. Genotype ratio 1 BB : 2 Bb : 1 bb [1]. Phenotype ratio 3 dominant : 1 recessive [2].
Q6. Codominance occurs when both alleles are expressed [2]. Example such as AB blood group [1]. Both A and B alleles expressed [1].
Q7. Many sex-linked genes are on the X chromosome [1]. Males have XY [1]. They have only one X chromosome [1]. A recessive allele on the X may be expressed because there is no second X allele to mask it [1].
Q8. Mutation is a change in genetic material [2]. Gene mutation changes the base sequence of a gene [2]. Chromosome mutation changes chromosome number or structure [2].
Q9. Variation exists in a population [1]. Organisms compete [1]. Some have advantageous inherited traits [1]. These survive better [1]. They reproduce more [1]. Useful alleles become more common over generations [1].
Q10. Fossils show organisms from the past/change over time [2]. Comparative anatomy shows similar structures/common ancestry [2]. Accept DNA/molecular evidence if explained.
Q11. Isolate useful gene [1]. Cut DNA [1]. Insert gene into vector/plasmid [1]. Transform host cell [1]. Host expresses gene [1]. Useful product made [1].
Q12. Medicine: insulin production/vaccines/gene therapy research [2]. Agriculture: pest-resistant crops/improved nutrition/tolerant crops [2].
Q2. DNA has deoxyribose/RNA has ribose [2]. DNA has thymine/RNA has uracil [2]. DNA is usually double-stranded/RNA usually single-stranded [2].
Q3. DNA unzips [1]. Strands separate [1]. Each strand acts as a template [1]. Complementary bases pair [1]. Two identical DNA molecules form [1].
Q4. Gene: section of DNA controlling/coding for characteristic/product [2]. Allele: form of a gene [2]. Genotype: genetic makeup/allele combination [2]. Phenotype: expressed/visible characteristic [2].
Q5. Parent genotypes Bb and Bb [1]. Gametes B and b from each parent [1]. Punnett square correct [3]. Genotype ratio 1 BB : 2 Bb : 1 bb [1]. Phenotype ratio 3 dominant : 1 recessive [2].
Q6. Codominance occurs when both alleles are expressed [2]. Example such as AB blood group [1]. Both A and B alleles expressed [1].
Q7. Many sex-linked genes are on the X chromosome [1]. Males have XY [1]. They have only one X chromosome [1]. A recessive allele on the X may be expressed because there is no second X allele to mask it [1].
Q8. Mutation is a change in genetic material [2]. Gene mutation changes the base sequence of a gene [2]. Chromosome mutation changes chromosome number or structure [2].
Q9. Variation exists in a population [1]. Organisms compete [1]. Some have advantageous inherited traits [1]. These survive better [1]. They reproduce more [1]. Useful alleles become more common over generations [1].
Q10. Fossils show organisms from the past/change over time [2]. Comparative anatomy shows similar structures/common ancestry [2]. Accept DNA/molecular evidence if explained.
Q11. Isolate useful gene [1]. Cut DNA [1]. Insert gene into vector/plasmid [1]. Transform host cell [1]. Host expresses gene [1]. Useful product made [1].
Q12. Medicine: insulin production/vaccines/gene therapy research [2]. Agriculture: pest-resistant crops/improved nutrition/tolerant crops [2].
24. Mastery Checklist
- I can name the three parts of a nucleotide.
- I can compare DNA and RNA.
- I can state DNA base-pairing rules.
- I can outline DNA replication using correct terms.
- I can define gene, allele, genotype and phenotype.
- I can distinguish homozygous and heterozygous.
- I can complete a simple monohybrid cross.
- I can explain codominance at OL standard.
- I can describe sex linkage simply.
- I can define mutation and give examples.
- I can explain how mutation contributes to variation.
- I can describe natural selection using clear steps.
- I can give evidence for evolution.
- I can outline genetic engineering stages in order.
- I can give applications of genetic engineering in medicine and agriculture.