Source: Genetics Allele Frequencies and Basic Biology lecture notes
Difficulty: Introductory | Prerequisites: Basic understanding of cell structure, DNA structure, and the cell cycle (mitosis in particular). If you are not confident with mitosis, review that first.
Meiosis is the type of cell division that produces gametes (eggs and sperm). Where mitosis copies a cell, meiosis halves the chromosome number, so that when two gametes fuse at fertilisation the full set is restored. This topic sits at the heart of genetics: without meiosis there is no genetic variation, and without genetic variation there is no evolution by natural selection. You should already be comfortable with chromosome structure, the difference between haploid and diploid, and the basic mechanics of mitosis before working through these notes.
Meiosis takes one diploid cell (2n) and produces four haploid cells (n), each with unreplicated chromosomes. It does this through two consecutive divisions: meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids (just like mitosis, but in a haploid cell). Crossing over during meiosis I shuffles maternal and paternal DNA, generating genetic diversity.
Meiosis
A type of cell division that reduces chromosome number by half, producing four haploid daughter cells from one diploid parent cell. Think of it as the division that makes sex cells.
Homologous chromosomes (homologs)
A pair of chromosomes, one inherited from each parent, that carry genes for the same traits at the same loci. In simple terms, these are matching chromosomes, one from mum and one from dad.
Sister chromatids
The two identical copies of a single chromosome that are joined at the centromere after DNA replication. Think of them as photocopies of the same chromosome, stapled together.
Diploid (2n)
A cell containing two complete sets of chromosomes, one set from each parent. This is the normal state of most body cells.
Haploid (n)
A cell containing only one complete set of chromosomes. Gametes (eggs and sperm) are haploid.
Chromatid
One half of a replicated chromosome. The term is only used to describe parts of a replicated chromosome, never an unreplicated one.
Crossing over (recombination)
The reciprocal exchange of DNA segments between non-sister chromatids of homologous chromosomes during meiosis I. In simple terms, homologs swap chunks of DNA, mixing maternal and paternal genetic information.
Zygote
The diploid cell formed when two haploid gametes fuse during fertilisation. It is the first cell of a new individual.
Gametogenesis
The process by which haploid daughter cells produced by meiosis develop into mature egg cells or sperm cells.
Kinetochore
A protein structure on the centromere of a chromosome where spindle microtubules attach. Think of it as the handle the cell grabs to pull chromosomes apart.
Reduction division
Another name for meiosis I, because it is the step that reduces chromosome number from diploid to haploid.
Chromosomes replicate during S phase of the cell cycle, before meiosis starts.
An unreplicated eukaryotic chromosome is a single, long DNA double helix wrapped around histone proteins.
After replication, each chromosome consists of two sister chromatids joined at the centromere.
Both sister chromatids contain identical genetic information.
A replicated chromosome is still counted as one chromosome, not two, even though it has two chromatids.
Meiosis I separates homologous chromosomes. One homolog goes to one daughter cell, the other homolog goes to the other.
At the end of meiosis I, each daughter cell has half the chromosome number of the parent cell.
The parent cell is diploid (2n); the two daughter cells are haploid (n).
Chromosomes are still in their replicated form at the end of meiosis I. Each chromosome still consists of two joined sister chromatids.
Key distinction: in meiosis I, sister chromatids stay together. This is what sets meiosis I apart from both meiosis II and mitosis.
Meiosis II separates sister chromatids, with each chromatid becoming an individual daughter chromosome.
At the end of meiosis II, each daughter cell contains one copy of each chromosome in unreplicated form (a single DNA double helix per chromosome).
Meiosis II is functionally equivalent to mitosis occurring in a haploid cell.
The result: four haploid cells, each with unreplicated chromosomes.
One diploid cell with replicated chromosomes enters meiosis.
After meiosis I: two haploid cells, each with replicated chromosomes.
After meiosis II: four haploid cells, each with unreplicated chromosomes.
No additional DNA replication occurs between meiosis I and meiosis II.
During meiosis I, homologous chromosomes pair up and exchange segments of DNA in a process called crossing over.
The result is chromatids that contain both paternal and maternal genetic material.
Crossing over is a fundamental source of genetic variation.
Chromosome movement during both meiosis I and II depends on microtubules of the spindle apparatus.
Spindle microtubules attach to kinetochores located at the centromere of each chromosome.
In animals, some or all of the four haploid cells produced by meiosis go on to form egg cells or sperm cells (gametogenesis).
In many organisms, haploid cells may also divide by mitosis to produce a haploid multicellular organism.
When two haploid gametes fuse during fertilisation, the full diploid chromosome complement is restored.
The resulting diploid cell is called a zygote, the first cell of a new individual.
In meiosis, homologous chromosomes pair up. In mitosis, they do not.
Meiosis involves two divisions; mitosis involves one.
Meiosis produces four haploid cells; mitosis produces two diploid cells.
Meiosis I is the reduction division (the step that halves chromosome number). Meiosis II and mitosis are mechanically similar.
When revising, sketch out this sequence and label each stage with its ploidy and chromatid state:
Start: One diploid cell (2n), chromosomes replicated (each = two sister chromatids joined at centromere).
After meiosis I: Two haploid cells (n), chromosomes still replicated (still two sister chromatids each).
After meiosis II: Four haploid cells (n), chromosomes unreplicated (one DNA double helix each).
Remember: DNA replication happens once (before meiosis I), but the cell divides twice.
Meiosis is the reason siblings from the same parents look different from one another. The combination of independent assortment of homologs and crossing over during meiosis I means that each gamete is genetically unique. This is also why genetic counsellors can estimate the probability of inheriting a condition, but can never guarantee it: the shuffle is real.
Errors in meiosis (nondisjunction) are the cause of conditions such as Down syndrome (trisomy 21), where an extra chromosome ends up in a gamete. Understanding meiosis is the foundation for understanding inheritance patterns, genetic disorders, and the mechanics of evolution.
Students often think a replicated chromosome counts as two chromosomes. It does not. A replicated chromosome (two sister chromatids joined at the centromere) is still one chromosome.
Students often confuse homologous chromosomes with sister chromatids. Homologs are the maternal and paternal versions of the same chromosome. Sister chromatids are identical copies of a single chromosome produced by DNA replication.
Students often think meiosis II reduces ploidy further. It does not. The reduction from diploid to haploid happens in meiosis I. Meiosis II simply separates sister chromatids, like mitosis in a haploid cell.
Students often forget that crossing over occurs during meiosis I, not meiosis II. By the time meiosis II begins, homologs have already been separated into different cells.
Be able to trace the ploidy and chromatid state at every stage: before meiosis, after meiosis I, after meiosis II.
Know what separates during meiosis I (homologs) versus meiosis II (sister chromatids).
Be prepared to compare and contrast meiosis and mitosis. This is a classic exam question.
Understand why meiosis I is called a "reduction division" and why meiosis II is not.
Be able to explain how crossing over produces genetic variation.
Know where kinetochores are and what role they play in chromosome movement.
True or false: A replicated chromosome consists of two chromosomes. (False. It is one chromosome made up of two sister chromatids.)
Fill in the blank: Meiosis I separates ______, while meiosis II separates ______. (homologous chromosomes; sister chromatids)
True or false: DNA replication occurs between meiosis I and meiosis II. (False. Replication occurs only before meiosis I.)
Fill in the blank: The diploid cell that results from fertilisation is called a ______. (zygote)
True or false: Meiosis II is functionally equivalent to mitosis in a haploid cell. (True.)
Q: A diploid organism has 2n = 46 chromosomes. How many chromosomes are in each cell at the end of meiosis I? At the end of meiosis II?
A: At the end of meiosis I, each cell has 23 chromosomes (each still consisting of two sister chromatids). At the end of meiosis II, each cell has 23 chromosomes, now unreplicated.
Q: What is the key event that makes meiosis I a "reduction division"?
A: The separation of homologous chromosomes into different daughter cells, which halves the chromosome number from diploid (2n) to haploid (n).
Q: How does crossing over contribute to genetic diversity?
A: Crossing over exchanges segments of DNA between non-sister chromatids of homologous chromosomes, producing chromatids with new combinations of maternal and paternal alleles. Each gamete therefore carries a unique mix of genetic information.
Q: Explain why meiosis II is described as being similar to mitosis.
A: In both meiosis II and mitosis, sister chromatids are separated into individual daughter chromosomes. The mechanical process is the same. The difference is that meiosis II occurs in a haploid cell, while mitosis occurs in a diploid cell.
Q: A student claims that sister chromatids carry different genetic information. Is this correct? Explain.
A: Under normal circumstances, no. Sister chromatids are produced by DNA replication and carry identical genetic information. The exception is after crossing over, which may introduce small differences between them.
This material connects directly to Mendelian genetics: the law of segregation describes what happens to alleles during meiosis I (homologs separate), and the law of independent assortment describes how different chromosome pairs sort independently during meiosis I.
Crossing over links to the concept of genetic linkage and recombination frequency, which is used to map genes on chromosomes.
Nondisjunction errors during meiosis connect to chromosome abnormalities and aneuploidy, topics that appear in medical genetics and developmental biology.
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