Biology (9700) | Passage of information from parents to offspring

🧬 Chapter 16.1: Passage of Information from Parents to Offspring (Meiosis and Variation)

Welcome to the fascinating world of Inheritance! This chapter is all about continuity—how you get the traits from your parents—but also about variation—why you aren't an exact clone of either of them. The mechanism that manages this incredible balancing act is called meiosis, often referred to as the reduction division.

Don't worry if the stages seem complicated. We’ll break down the "chromosome dance" step by step, focusing on the key actions that ensure the next generation is genetically diverse.

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1. Haploid (n) vs. Diploid (2n)

These are two fundamental terms describing the number of chromosome sets in a cell.

Key Definitions:

• Diploid (2n): A cell containing two complete sets of chromosomes. This is the normal state for most body cells (somatic cells). In humans, 2n = 46 (23 pairs).
• Haploid (n): A cell containing one complete set of chromosomes. This is the state of gametes (sex cells: sperm and egg). In humans, n = 23 (single chromosomes).

Analogy: Think of a diploid cell as having two identical sets of LEGO instructions (one from Mom, one from Dad). A haploid cell only has one set of instructions.

2. Homologous Pairs of Chromosomes

In a diploid cell (2n), the chromosomes exist in pairs.

• A homologous pair consists of two chromosomes—one inherited from the mother and one inherited from the father.
• These two chromosomes are the same size and shape and carry the same genes in the same locations (locus).
• However, they may carry different versions (alleles) of those genes.

Quick Review: Before division starts, DNA replication occurs during the S phase of interphase. After replication, each chromosome consists of two identical sister chromatids joined at the centromere. Homologous chromosomes still exist as a pair, but now each member of the pair is duplicated.

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3. Meiosis: The Need for Reduction Division (LO 3)

When sexual reproduction occurs, two gametes fuse in fertilisation.

If the gametes were diploid (2n + 2n), the resulting zygote would be 4n. The next generation would be 8n, and so on. This would lead to an unsustainable doubling of genetic material every generation!

The importance of meiosis is simple: it is a reduction division that halves the chromosome number (2n $\rightarrow$ n). This ensures that when the sperm (n) and egg (n) fuse, the resulting zygote returns to the correct diploid number (2n), maintaining the characteristic chromosome number for the species.

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4. The Behaviour of Chromosomes during Meiosis (LO 4, 5)

Meiosis consists of two successive divisions: Meiosis I (the reduction division, separating homologous pairs) and Meiosis II (separating sister chromatids, similar to mitosis).

A. Meiosis I (Separating Homologous Pairs)

Prophase I

• Chromosomes condense and become visible.
• The nuclear envelope breaks down.
• The spindle microtubules form.
• Crucially, homologous chromosomes pair up (forming bivalents).
• Crossing over occurs (exchange of genetic material between non-sister chromatids). This is a vital source of variation!

Metaphase I

• The bivalents (homologous pairs) line up along the equator of the spindle (the metaphase plate).
• Random orientation occurs here: the orientation of each homologous pair is independent of the others.

Anaphase I

• The homologous pairs separate and are pulled towards opposite poles by shortening spindle fibres.
• Importantly: Sister chromatids remain attached to each other.
• The chromosome number is effectively halved at this stage.

Telophase I

• Chromosomes arrive at the poles.
• The spindle disintegrates.
• The nuclear envelope may reform (or the cell may proceed directly to Meiosis II).
• Cytokinesis (division of the cytoplasm and cell surface membrane) occurs, resulting in two haploid cells (n).

B. Meiosis II (Separating Sister Chromatids)

This stage is essentially a mitotic division of the two cells produced in Meiosis I.

Prophase II

• Chromosomes condense again (if they decondensed in Telophase I).
• A new spindle forms in each of the two haploid cells.
• The nuclear envelope breaks down (if it reformed).

Metaphase II

• The individual chromosomes (still composed of two sister chromatids) line up along the metaphase plate in each cell.

Anaphase II

• The sister chromatids separate and are pulled to opposite poles.
• Once separated, the chromatids are now considered individual chromosomes.

Telophase II

• Chromosomes arrive at the poles.
• The nuclear envelope reforms around the four sets of haploid chromosomes.
• Cytokinesis occurs, resulting in four genetically different haploid gametes.

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5. Mechanisms that Produce Genetic Variation

Meiosis is not just about reducing chromosome number; it's the engine of genetic variation in sexual reproduction. Variation is achieved primarily through three mechanisms (LO 6, 7).

A. Crossing Over (Recombination)

This occurs during Prophase I when homologous chromosomes are paired up (synapsis).

1. Non-sister chromatids wrap around each other and break.
2. They rejoin, exchanging sections of genetic material. The point of exchange is called the chiasma (plural: chiasmata).
3. The resulting chromatids are now recombinant: they contain a mix of alleles from both the mother and the father.

Result: Crossing over breaks up linkage groups, producing gametes with combinations of alleles that were not present in the parent organism.

B. Random Orientation (Independent Assortment)

This occurs during Metaphase I.

1. Homologous pairs line up randomly on the metaphase plate.
2. For any one pair, the paternal chromosome could face either pole, and the maternal chromosome could face the opposite.
3. The orientation of one pair is completely independent of the orientation of all other pairs.

Example: If an organism has 4 pairs of chromosomes (2n=8), there are $2^4 = 16$ possible combinations of chromosomes in the resulting gametes. For humans (n=23), the number of possible unique combinations is $2^{23}$ (over 8 million!) just from this stage alone!

C. Random Fusion of Gametes (LO 7)

Even after meiosis produces millions of unique sperm and eggs, the final step in generating variation is the random fusion during fertilisation.

• Any one of the $2^{23}$ possible sperm gametes can fuse with any one of the $2^{23}$ possible egg gametes.
• The chance fusion of these two genetically unique cells produces a zygote that is highly unique.

Did you know? The chance that any two siblings (excluding identical twins) inherit the exact same combination of chromosomes from both parents is incredibly small (about 1 in 70 trillion!), thanks to random orientation and fusion.