AP Syllabus focus:
‘Meiosis produces haploid gametes, transmitting chromosomes from diploid parents to offspring in sexually reproducing organisms.’
Meiosis is the cell division process that underpins sexual life cycles by halving chromosome number to form gametes. Understanding what is transmitted, how chromosome number is conserved across generations, and why meiosis is distinct from mitosis is essential.
Core purpose: chromosome transmission in sexual reproduction
Sexually reproducing organisms must balance two requirements:
Transmit genetic information (DNA packaged as chromosomes) from parents to offspring.
Maintain a stable chromosome number across generations.
Meiosis solves this by producing haploid gametes from diploid parent cells, so that fertilization can restore diploidy in the next generation.
Key terms for chromosome number
Chromosome number is commonly described using ploidy, the number of complete chromosome sets.
Diploid (2n): Having two sets of chromosomes, typically one set inherited from each parent.
In most animals, the diploid stage dominates the life cycle; meiosis occurs in specialized tissues to generate gametes.
Haploid (n): Having one set of chromosomes.
Haploid cells carry a single allele for each gene on each chromosome, which is crucial for how allele combinations are formed in offspring.
Gamete: A haploid reproductive cell (e.g., sperm or egg) that can fuse with another gamete during fertilization.
What meiosis produces and why it matters
Meiosis produces daughter cells that differ from the parent cell in two essential ways:
Chromosome number is reduced: diploid → haploid.
Genetic content is not identical: gametes carry different combinations of parental chromosomes.
This aligns directly with the syllabus focus: “Meiosis produces haploid gametes, transmitting chromosomes from diploid parents to offspring in sexually reproducing organisms.”
Reduction in chromosome number
Meiosis includes two successive nuclear divisions after one round of DNA replication, allowing chromosome number to be halved.
= Diploid chromosome set number in the parent cell
= Haploid chromosome set number in each gamete
The reduction is essential because fertilization combines two haploid gametes; without reduction, chromosome number would double every generation.
Diagram of an animal sexual life cycle showing meiosis producing haploid gametes (sperm and egg, 1n) and fertilization forming a diploid zygote (2n). It clarifies how chromosome number is conserved across generations by alternating ploidy reduction (meiosis) and restoration (fertilization). Source
Restoration of diploidy by fertilization (context for transmission)
Although fertilization is a separate event from meiosis, it explains the transmission logic:
Each parent contributes one haploid set of chromosomes in a gamete.
The zygote receives two sets total, restoring diploidy.
Offspring therefore inherit chromosomes from both diploid parents via their gametes.
Chromosomes, homologs, and what is transmitted
A diploid cell contains pairs of chromosomes that carry the same genes in the same order but may have different alleles.
Homologous chromosomes: A pair of chromosomes (one maternal, one paternal) with the same genes at corresponding loci, potentially carrying different alleles.
Meiosis ensures that gametes receive:
One chromosome from each homologous pair (one member of each pair, not both)
Therefore, one allele per gene per locus (for genes on those chromosomes)
This is the physical basis of how parents transmit chromosomes—and thus alleles—to offspring.
How meiosis differs from mitosis (high-level)
Meiosis and mitosis both distribute chromosomes to daughter cells, but their roles and outcomes differ.
Side-by-side schematic comparing mitosis and meiosis, emphasizing that chromosomes replicate once in both processes but meiosis includes two divisions. It highlights the different outcomes: two genetically identical diploid cells from mitosis versus four genetically distinct haploid cells from meiosis. Source
Meiosis (sexual reproduction)
Occurs in germline cells (cells that give rise to gametes)
Produces haploid cells
Produces cells for reproduction
Involves two divisions, yielding (typically) four haploid cells
Mitosis (growth and maintenance)
Occurs in somatic cells (non-reproductive body cells)
Maintains diploid chromosome number
Produces cells for growth, repair, and asexual reproduction in some organisms
Involves one division, yielding two genetically identical daughter cells (in chromosome content)
Why chromosome transmission must be accurate
The central goal of meiosis is faithful transmission of chromosome sets while reducing ploidy. Accurate transmission supports:
Genome stability: correct chromosome number in gametes and offspring
Proper development: most organisms require the expected dosage of genes across chromosomes
Reproductive success: viable gametes and embryos depend on correct chromosome complements
At the overview level, meiosis is best understood as a chromosome-number management system for sexual life cycles: it packages and transmits one complete set of chromosomes per parent to the next generation.
FAQ
Cells establish a segregation system so that each chromosome type is represented once in a gamete.
Key idea:
Each chromosome has a distinct identity (size, centromere position, DNA sequence features) that allows it to behave as a unit during division.
At a practical level, the process relies on robust chromosome attachment to cellular machinery and checkpoint controls that delay division if key attachments are incorrect.
Haploidy means there is only one allele per locus in a gamete.
Consequences:
Each gamete delivers a single “choice” of allele for each gene to the offspring.
When two gametes fuse, the offspring’s genotype is formed by combining one allele from each parent at most loci.
This single-allele state is what makes patterns of inheritance traceable across generations.
Meiosis occurs at different points depending on the dominant life stage.
Examples of placement:
Diplontic life cycle (most animals): meiosis produces gametes directly from diploid cells.
Alternation of generations (many plants): meiosis produces spores that develop into haploid multicellular stages; gametes may be produced later by mitosis in the haploid stage.
In all cases, meiosis is the step that reduces ploidy from $2n$ to $n$.
Meiosis preserves a species’ chromosome number from generation to generation, but different species have evolved different baseline values of $n$ and $2n$.
Reasons chromosome number differs:
Chromosome fusions and fissions over evolutionary time
Large-scale rearrangements that change how DNA is packaged into chromosomes
Meiosis then reliably transmits the established chromosome set number for that species.
Chromosome number counts centromeres/chromosomes, while DNA amount reflects how much DNA is present.
Key distinction:
A cell can have the same chromosome number but different DNA amounts depending on whether DNA has been replicated.
Before division, DNA replication increases DNA content without increasing chromosome number (chromosomes become duplicated structures).
This distinction helps explain why “halving chromosome number” and “reducing DNA content” are related but not identical ideas.
Practice Questions
Explain why meiosis is necessary in sexually reproducing organisms to maintain chromosome number across generations. (2 marks)
Meiosis produces haploid gametes / halves chromosome number (1)
Fertilisation restores diploid number when two haploid gametes fuse, preventing chromosome number doubling each generation (1)
Describe how meiosis enables diploid parents to transmit chromosomes to offspring, including the meaning of diploid and haploid and what a gamete contains. (5 marks)
Defines diploid as , two sets of chromosomes (1)
Defines haploid as , one set of chromosomes (1)
States meiosis produces haploid gametes from diploid cells (1)
States a gamete contains one chromosome from each homologous pair / one set of chromosomes (1)
Explains that fertilisation combines two haploid gametes so offspring receive one set from each parent and return to diploid (1)
