AP Syllabus focus:
‘During prophase I, non-sister chromatids exchange genetic material by crossing over, generating new allele combinations.’
Crossing over is a precisely timed, enzyme-driven DNA exchange during prophase I of meiosis. It reshuffles alleles between homologous chromosomes, producing recombinant chromatids that help explain genetic variation among gametes.
Core idea: recombination during prophase I
In prophase I, each chromosome has already been replicated (two sister chromatids).
This diagram shows homologous chromosomes paired in synapsis and highlights that DNA exchange occurs between non-sister chromatids. The labeled crossover point illustrates how a reciprocal swap produces chromatids that now contain mixed maternal and paternal segments. Use it to visually distinguish sister chromatids (replicated copies) from the non-sister chromatids that actually recombine. Source
Homologous maternal and paternal chromosomes pair closely so that DNA segments can be exchanged between non-sister chromatids, creating new allele combinations.
Key terms and what “crossing over” changes
DEFINITION
Term: Crossing over: reciprocal exchange of corresponding DNA segments between non-sister chromatids of a homologous chromosome pair during prophase I, producing recombinant chromatids.
Crossing over changes the combination of alleles along a chromatid, not the number of chromosomes.
This figure depicts a homologous chromosome pair undergoing a crossover between non-sister chromatids, producing two recombinant chromatids and two non-recombinant chromatids. The before/after comparison makes it clear that the outcome is an exchange of corresponding segments at matching loci. It’s a concise visual for explaining how recombination generates new allele combinations without changing chromosome count. Source
The exchanged segments are aligned at matching loci, so the two chromatids remain homologous but become genetically “mixed.”
Chiasma: the visible, physical site where homologous chromatids remain connected after crossing over; it reflects underlying DNA exchanges.
Chiasmata are important because they reflect stable links between homologs after synapsis begins to dissolve.
How crossing over happens (mechanistic overview)
Crossing over occurs after homologs align during synapsis, when homologous chromosomes are held together closely enough to allow DNA repair machinery to swap corresponding regions.
Stepwise process (high-utility AP level)
Homolog alignment
Homologous chromosomes pair gene-by-gene, positioning matching DNA sequences near each other.
DNA breakage and strand invasion
Enzymes introduce controlled double-strand breaks in one chromatid.
A broken DNA end invades the non-sister chromatid of the homolog, pairing with a complementary sequence.
DNA synthesis and junction formation
DNA polymerases extend the invading strand using the homolog as a template.
Interconnected DNA structures form, physically linking the chromatids at the exchange region.
Resolution
The junctions are cut and re-ligated.
Depending on how the cuts are made, chromatids can emerge with a crossover (segments swapped) that yields recombinant chromatids.
Biological outcomes of crossing over
Crossing over is a form of genetic recombination that creates variation by rearranging existing alleles into new combinations along a chromosome.
What students should be able to state clearly
Crossing over occurs between non-sister chromatids, not sister chromatids.
It happens during prophase I after homologs have paired.
It produces recombinant chromatids, which can carry allele combinations not found in either parent chromosome.
Why it matters for heredity
New allele combinations can change which trait-associated alleles travel together into gametes.
The same pair of homologs can experience multiple crossovers, increasing the number of possible recombinant outcomes.
Chiasmata help maintain homolog association long enough for accurate meiotic progression, linking the physical chromosome behavior to genetic outcomes.
Common misconceptions to avoid
Crossing over does not create new alleles; it reshuffles existing alleles into new combinations.
Crossing over is not random DNA swapping; it occurs at aligned, corresponding regions between homologs.
Only some chromatids in a tetrad become recombinant; others can remain parental, depending on where crossovers occur.
FAQ
Some repair outcomes restore DNA without exchanging flanking regions (non-crossover outcomes).
These can still alter sequence information locally (e.g., via biased repair), but they do not create the same large-scale allele reshuffling as crossovers.
Crossovers are more likely in hotspots—genomic regions with higher break/repair activity.
Hotspots can concentrate recombination into particular intervals, making allele reshuffling uneven across the genome.
Most sexually reproducing eukaryotes show crossing over, but frequency varies widely among species, sexes, and chromosome regions.
Some lineages exhibit reduced recombination in specific chromosomes or developmental contexts.
A chiasma reflects crossover resolution that physically links chromatids.
If repair resolves without exchanging flanking DNA, it may not produce a stable chiasma even though recombination machinery acted.
Large inversions or rearrangements can disrupt alignment between homologs.
Misalignment can reduce successful crossovers or create abnormal recombinant products, potentially impacting gamete viability.
Practice Questions
State where and between which chromatids crossing over occurs, and give one genetic consequence. (2 marks)
Occurs in prophase I (1)
Between non-sister chromatids of homologous chromosomes (1)
Consequence: recombinant chromatids/new allele combinations (1) (credit any one consequence; max 2 total)
Describe how crossing over during prophase I generates recombinant chromatids. Include key structural features formed during the process. (5 marks)
Homologous chromosomes pair closely during synapsis (1)
Exchange occurs between non-sister chromatids (1)
Controlled DNA breakage and rejoining (or strand invasion/repair-based exchange) (1)
Formation of chiasmata as visible sites of crossover/physical linkage (1)
Produces recombinant chromatids with new allele combinations (1)
