How Does Meiosis Create Gametes With Half the Usual DNA?
- Meiosis and mitosis both involve cell division, but they solve different biological problems.
- Mitosis is used for growth and repair, so it makes identical copies of cells with the same chromosome number.
- Meiosis is used for sexual reproduction, so it must:
- halve the chromosome number (haploid gametes)
- generate genetic variation
- Because of this, meiosis includes two divisions and has extra steps such as homologous pairing and crossing over that never occur in mitosis.
Meiosis
A type of cell division that produces four genetically different haploid cells, each containing half the chromosome number of the parent cell.
- It's common to think meiosis is simply “double mitosis” even though it's not.
- The key differences are:
- Two divisions instead of one
- Homologous pairing and crossing over
- Formation of haploid cells
- Four genetically different outcomes
Why Does Meiosis Need Two Divisions?
- Human body cells are diploid.
- Gametes must be haploid so that fertilisation restores the original diploid number.
- One division cannot achieve this reduction accurately, which is why meiosis splits chromosomes in two steps:
- Meiosis I halves the chromosome number
- Meiosis II separates chromatids to avoid duplication
- This structure ensures every gamete carries one complete set of chromosomes, but in a new combination.
- A simple way to track the logic:
- Meiosis I reduces
- Meiosis II refines
Meiosis I: Reducing Chromosome Number
- Meiosis I is where the main difference from mitosis happens.
- Instead of splitting sister chromatids, the cell splits homologous chromosome pairs.
- Meiosis I also includes the two events that create most genetic variation: crossing over and independent assortment.
- Once Meiosis I is complete, the cell is haploid, even though chromatids are still attached.
Homologous chromosomes
Chromosome pairs (one from each parent) that carry the same types of genes.
Prophase I
- Chromosomes condense so they can be moved without breaking.
- Homologous chromosomes pair up, which is essential because meiosis separates parental versions into different gametes.
- Crossing over occurs.
- Sections of DNA swap between homologues.
- This produces new genetic combinations, increasing variation before the cell even divides.
- Nuclear membrane breaks down and spindle fibres form.
Crossing over happens between homologous chromosomes, not sister chromatids.
Metaphase I
Independent assortment
Random orientation of homologous pairs during metaphase, creating varied chromosome combinations.
- Homologous pairs randomly line up side-by-side along the centre.
- This randomness generates independent assortment, which increases variation.
- Spindle fibres attach to each homolog.
Anaphase I
Reduction division
The point where chromosome number is halved.
- Homologous chromosomes are pulled apart to opposite poles.
- Sister chromatids remain together.
- This is the exact moment the cell becomes haploid.
Telophase I and Cytokinesis
Haploid cells
Cells that contain one set of chromosomes.
- Chromosomes will reach opposite ends, and the nuclear membranes may briefly reform.
- The cell divides into two haploid cells, with each chromosome still containing two chromatids, which will separate in Meiosis II.
Meiosis II: Separating Sister Chromatids
- Meiosis II works like a mitosis-style division, but it starts from haploid cells, not diploid ones.
- Because the chromatids stayed together during Meiosis I, they must now be separated so each gamete ends up with one chromatid per chromosome.
- This step does not change chromosome number. It simply refines the distribution to prevent duplication errors.
- Once Meiosis II finishes, the cell has completed the process of forming four unique haploid gametes.
- Meiosis I changes the number of chromosome sets.
- Meiosis II changes the structure of those sets by separating chromatids.
Prophase II
- Chromosomes re-condense if they had loosened.
- Nuclear membranes break down again.
- New spindle fibres form.
- This division ensures chromatids are split evenly.
Metaphase II
- Chromosomes line up single file at the centre.
- Each still consists of two sister chromatids.
- This stage looks similar to mitosis, but the starting cells are haploid, not diploid.
- Metaphase I aligns pairs.
- Metaphase II aligns individual chromosomes.
Anaphase II
- Sister chromatids finally separate.
- They move to opposite poles.
- Each chromatid becomes an independent chromosome.
- This ensures each gamete receives one copy of each chromosome.
- Chromatids do not separate in Anaphase I.
- They separate here in Anaphase II.
Telophase II and Cytokinesis
- Nuclear membranes reform around each set of chromatids.
- The cells split.
- Four genetically different haploid gametes are produced.
- These differences arise from crossing over and independent assortment.
Why Does Meiosis Creates Variation?
- Crossing over shuffles alleles within chromosomes.
- Independent assortment shuffles whole chromosomes.
- These processes operate before fertilisation even happens.
- When a gamete meets another gamete, variation increases further through random fertilisation.
- This continuous reshuffling helps populations adapt.
- Think of shuffling two decks of cards then dealing four hands.
- The hands will differ every time even though the same cards exist.
- In the MYP eAssessment of N21, Question 2b, the question assesses the ability to distinguish and compare mitosis and meiosis using outcomes rather than definitions.
- Statements about the number of daughter cells, chromosome number, and genetic similarity must be placed precisely.
- Careful reading is essential, as some statements apply to both processes, while others apply to only one. Misplacing a single item can result in losing all marks for that section.
- Why must gametes be haploid?
- What key events in Prophase I increase variation?
- What separates in Anaphase I vs Anaphase II?
- Why is Metaphase I essential for genetic diversity?
- How many cells does meiosis produce and why are they different?