CIE AS/A Level Biology -16.1 Passage of information from parents to offspring- Study Notes- New Syllabus
CIE AS/A Level Biology -16.1 Passage of information from parents to offspring- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -16.1 Passage of information from parents to offspring- Study Notes- New Syllabus
Key Concepts:
- explain the meanings of the terms haploid (n) and diploid (2n)
- explain what is meant by homologous pairs of chromosomes
- explain the need for a reduction division during meiosis in the production of gametes
- describe the behaviour of chromosomes in plant and animal cells during meiosis and the associated behaviour of the nuclear envelope, the cell surface membrane and the spindle (names of the main stages of meiosis, but not the sub-divisions of prophase I, are expected: prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II and telophase II)
- interpret photomicrographs and diagrams of cells in different stages of meiosis and identify the main stages of meiosis
- explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically different gametes
- explain that the random fusion of gametes at fertilisation produces genetically different individuals
Haploid (n) and Diploid (2n)
🌱 Basic Definitions
Haploid (n):- A haploid cell contains one complete set of chromosomes.
- Represented as n.
- Example: In humans, gametes (sperm and egg) are haploid, with 23 chromosomes.
- Diploid (2n):
- A diploid cell contains two complete sets of chromosomes – one from each parent.
- Represented as 2n.
- Example: In humans, somatic (body) cells are diploid, with 46 chromosomes (23 pairs).
📊 Comparison Table
| Feature | Haploid (n) | Diploid (2n) |
|---|---|---|
| Chromosome sets | 1 set | 2 sets |
| Number in humans | 23 | 46 |
| Cell types | Gametes (sperm, egg) | Body cells (skin, muscle, nerve, etc.) |
| Produced by | Meiosis | Mitosis |
| Role | Sexual reproduction (fertilisation restores diploid state) | Growth, repair, maintenance |
🔬 Key Points
- During fertilisation, two haploid gametes (n + n) combine → form a diploid zygote (2n).
- Meiosis reduces diploid (2n) cells to haploid (n) gametes, ensuring chromosome number remains stable across generations.
🧠 Summary Box:
Haploid (n): one set of chromosomes (e.g., human gametes, 23).
Diploid (2n): two sets of chromosomes (e.g., human somatic cells, 46).
Fertilisation restores diploid number; meiosis maintains chromosome balance.
Haploid (n): one set of chromosomes (e.g., human gametes, 23).
Diploid (2n): two sets of chromosomes (e.g., human somatic cells, 46).
Fertilisation restores diploid number; meiosis maintains chromosome balance.
Homologous Pairs of Chromosomes
🌱 Definition
- Homologous chromosomes are pairs of chromosomes (one from the mother, one from the father) that:
- Are similar in length, shape, and position of the centromere.
- Carry the same genes at the same loci (positions), but may have different alleles (versions of a gene).
🔬 Key Features
- Each somatic (diploid) cell has homologous pairs.
- In humans: 46 chromosomes (2n) → 23 homologous pairs.
- Example: Chromosome 7 from the mother and chromosome 7 from the father = a homologous pair.
- During meiosis I, homologous chromosomes pair up and may exchange DNA in crossing over, increasing genetic variation.
📊 Characteristics of Homologous Chromosomes
| Feature | Homologous Chromosomes |
|---|---|
| Origin | One maternal + one paternal |
| Gene loci | Same position on both chromosomes |
| Alleles | Can be identical or different |
| Appearance | Similar size, shape, centromere position |
| Role in meiosis | Pair up, undergo crossing over, then separate |
🧠 Summary Box:
Homologous pairs = matching maternal and paternal chromosomes.
They carry the same genes, but may have different alleles.
In humans: 23 homologous pairs ensure proper gene inheritance and variation during meiosis.
Homologous pairs = matching maternal and paternal chromosomes.
They carry the same genes, but may have different alleles.
In humans: 23 homologous pairs ensure proper gene inheritance and variation during meiosis.
Need for Reduction Division in Meiosis
🌱 What is Reduction Division?
- Reduction division = the first meiotic division (Meiosis I).
- Chromosome number is halved from diploid (2n) to haploid (n).
- Example in humans:
- Body cells → 46 chromosomes (2n)
- Gametes (sperm/egg) → 23 chromosomes (n)
🔬 Why Reduction Division is Necessary?
- Maintains Chromosome Number Across Generations
- Without reduction, fertilisation would double chromosome number every generation.
- Example: If gametes were diploid (46) → fertilisation would give 92 → unsustainable.
- Produces Haploid Gametes
- Ensures gametes (sperm, egg) have half the number of chromosomes.
- When they fuse at fertilisation → restore diploid number (2n) in the zygote.
- Genetic Stability
- Keeps species-specific chromosome number constant.
- Prevents abnormalities due to uncontrolled chromosome multiplication.
- Generates Genetic Variation (extra benefit of meiosis)
- Crossing over and independent assortment during reduction division increase variation → essential for evolution and adaptation.
📊 Summary Table
| Purpose of Reduction Division | Explanation |
|---|---|
| Prevents chromosome doubling | Keeps chromosome number constant across generations |
| Produces haploid gametes | So fertilisation restores diploid state |
| Maintains genetic stability | Ensures normal development and survival |
| Promotes variation | Through crossing over & independent assortment |
🧠 Key Recap:
Reduction division (Meiosis I) → Diploid (2n) → Haploid (n).
Prevents doubling of chromosomes in each generation.
Ensures stable chromosome number and allows variation + fertilisation success.
Reduction division (Meiosis I) → Diploid (2n) → Haploid (n).
Prevents doubling of chromosomes in each generation.
Ensures stable chromosome number and allows variation + fertilisation success.
Behaviour of Chromosomes During Meiosis (Plants & Animals)
🌱 Overview
- Meiosis = 2 successive divisions producing 4 haploid gametes from one diploid cell.
- Involves:
- Chromosomes (pairing, crossing over, separation)
- Nuclear envelope (breaks down/reforms)
- Spindle fibres (attach & pull chromosomes)
- Cell surface membrane (divides cytoplasm by cytokinesis)
🔬 Stages of Meiosis and Behaviour
- 1. Prophase I
- Chromosomes: condense, become visible as homologous pairs; crossing over occurs.
- Nuclear envelope: breaks down.
- Spindle: forms and attaches to centromeres.
- Cell surface membrane: intact, but preparing for division.
- 2. Metaphase I
- Chromosomes: homologous pairs (bivalents) align on equator, attached to spindle fibres.
- Nuclear envelope: already broken down.
- Spindle: fibres extend from poles to centromeres.
- Membrane: still intact around the whole cell.
- 3. Anaphase I
- Chromosomes: homologous pairs separate; whole chromosomes (not chromatids) move to opposite poles.
- Nuclear envelope: absent.
- Spindle: pulls chromosomes apart.
- 4. Telophase I
- Chromosomes: reach poles, may partially decondense.
- Nuclear envelope: reforms around chromosomes at each pole.
- Spindle: disassembles.
- Cell surface membrane: cytokinesis occurs → 2 haploid daughter cells formed.
- 5. Prophase II
- Chromosomes: re-condense (if they had decondensed).
- Nuclear envelope: breaks down again.
- Spindle: reforms in each haploid cell.
- 6. Metaphase II
- Chromosomes: line up individually along equator (not in pairs).
- Nuclear envelope: absent.
- Spindle: attaches to centromeres.
- 7. Anaphase II
- Chromosomes: sister chromatids separate and are pulled to opposite poles.
- Spindle: fibres shorten, pulling chromatids apart.
- 8. Telophase II
- Chromosomes: decondense into chromatin.
- Nuclear envelope: reforms around each set of chromatids.
- Spindle: disappears.
- Cell surface membrane: cytokinesis produces 4 haploid gametes (n).
📊 Comparison Table: Key Events in Meiosis
| Stage | Chromosomes | Nuclear Envelope | Spindle | Membrane |
|---|---|---|---|---|
| Prophase I | Pair, crossing over | Breaks down | Forms | Intact |
| Metaphase I | Homologous pairs on equator | Absent | Fully formed | Intact |
| Anaphase I | Homologs separate | Absent | Pulls chromosomes | Intact |
| Telophase I | Chromosomes at poles | Reforms | Disappears | Cytokinesis → 2 cells |
| Prophase II | Re-condense | Breaks down | Forms | Intact |
| Metaphase II | Chromosomes line up singly | Absent | Fully formed | Intact |
| Anaphase II | Sister chromatids separate | Absent | Pulls chromatids | Intact |
| Telophase II | Chromatids at poles → chromatin | Reforms | Disappears | Cytokinesis → 4 haploid cells |
🧠 Key Recap:
Meiosis I = reduction division (homologous pairs separate).
Meiosis II = separation of sister chromatids (like mitosis).
Nuclear envelope breaks down in prophase, reforms in telophase.
Spindle ensures accurate chromosome movement.
Cell surface membrane divides cytoplasm at end of each division.
Meiosis I = reduction division (homologous pairs separate).
Meiosis II = separation of sister chromatids (like mitosis).
Nuclear envelope breaks down in prophase, reforms in telophase.
Spindle ensures accurate chromosome movement.
Cell surface membrane divides cytoplasm at end of each division.
Interpretation of Photomicrographs & Diagrams of Meiosis
🌱 Overview
- Photomicrographs and diagrams can be used to identify stages of meiosis based on chromosome appearance, position, and cell structures.
- Key features to look for: chromosome condensation, alignment, separation, and presence/absence of nuclear envelope.
🔬 How to Identify Stages
| Stage | Key Features in Photomicrographs/Diagrams |
| Prophase I | Chromosomes condense, visible as homologous pairs (bivalents). Crossing over may be seen as chiasmata. Nuclear envelope breaks down. |
| Metaphase I | Bivalents (homologous pairs) aligned along the equator. Spindle fibres attached to centromeres. |
| Anaphase I | Homologous chromosomes separate and move to opposite poles (sister chromatids still joined). |
| Telophase I | Chromosomes reach poles, nuclear envelopes may reform. Cytokinesis begins → 2 haploid cells. |
| Prophase II | In each haploid cell, chromosomes re-condense, nuclear envelope breaks down again. |
| Metaphase II | Individual chromosomes align along equator (not pairs). |
| Anaphase II | Sister chromatids separate at centromeres and move to opposite poles. |
| Telophase II | Chromatids reach poles, decondense into chromatin. Nuclear envelopes reform, cytokinesis → 4 haploid gametes. |
📌 Tips for Interpretation
- Condensed paired chromosomes → Prophase I.
- Pairs at equator → Metaphase I.
- Whole chromosomes moving apart → Anaphase I.
- Sister chromatids separating → Anaphase II.
- 4 cells visible → End of Telophase II.
Genetic Variation in Meiosis
🌱 Key Idea
- Meiosis does not just reduce the chromosome number – it also introduces genetic variation in gametes.
- Two main processes cause this variation: crossing over and independent assortment (random orientation).
🔀 Crossing Over (Prophase I)
- During Prophase I, homologous chromosomes pair up to form bivalents.
- Non-sister chromatids can exchange segments of DNA at points called chiasmata.
- This process is called crossing over.
- Result → chromatids contain a new combination of alleles (genetic recombination).
- Outcome → gametes carry different allele combinations compared to the parents.
Example:
If one chromosome carries alleles A B and the other a b, crossing over could produce new combinations like A b or a B.
🎲 Random Orientation / Independent Assortment
- Happens during Metaphase I and Metaphase II.
- In Metaphase I: homologous pairs line up at the equator randomly.
- Each pair’s orientation is independent of the others.
- E.g., maternal chromosome can face one pole, paternal to the other — completely random.
- In Metaphase II: sister chromatids also orient randomly.
- Result → different possible combinations of maternal and paternal chromosomes in gametes.
Example:
For humans (23 pairs of chromosomes), independent assortment alone gives 2²³ possible gamete combinations (~8.4 million).
🌟 Why It Matters
- Crossing over = introduces new allele combinations.
- Independent assortment = ensures different chromosome sets in gametes.
- Together → meiosis produces genetically unique gametes → essential for variation in offspring after fertilisation.
✅ Summary Box
- Crossing over (Prophase I): swapping of DNA between non-sister chromatids → new allele combinations.
- Random orientation (Metaphase I & II): random arrangement of chromosomes → different gamete combinations.
- These processes ensure genetic diversity in sexually reproducing organisms.
Genetic Variation by Random Fertilisation
🌱 Key Idea
- Fertilisation = the fusion of a haploid sperm (n) and a haploid egg (n) to form a diploid zygote (2n).
- This process is random → any sperm can fuse with any egg.
🎲 Random Fusion of Gametes
- Each gamete is already genetically unique due to crossing over and independent assortment in meiosis.
- Since fertilisation is random:
- One of millions of possible sperm could fuse with one of many possible eggs.
- This multiplies the potential combinations enormously.
Example:
In humans:
Independent assortment gives ~8.4 million possible gametes per parent.
Fertilisation combines sperm and egg randomly → 8.4 million × 8.4 million ≈ 70 trillion possible zygote combinations (without even considering crossing over).
🌟 Why It Matters
- Random fertilisation ensures that no two individuals (except identical twins) are genetically the same.
- It contributes to genetic diversity in a population → which is vital for evolution and survival in changing environments.
✅ Summary Box
- Gametes are unique due to meiosis.
- Random fertilisation = any sperm can fertilise any egg.
- Produces genetically different offspring, ensuring variation in populations.