When Does Crossing Over Occur?

Crossing over occurs during Prophase I of meiosis — specifically in the pachytene sub-stage. It is the exchange of genetic material between non-sister chromatids of homologous chromosomes, and it happens at structures called chiasmata.

If you need the one-line exam answer: Prophase I of meiosis I.

Where it sits in the sequence:

1. Interphase — DNA replicates; each chromosome now has two sister chromatids
2. Prophase I — homologous chromosomes pair up and crossing over happens here
3. Metaphase I — homologous pairs line up at the equator
4. Anaphase I — homologous chromosomes separate
5. Telophase I — two haploid cells form
6. Meiosis II — sister chromatids separate, giving four haploid cells

The five sub-stages of Prophase I, since the question is often asked at this level of detail:

A useful memory aid is that pachytene is the "thick" stage — the name comes from the Greek for thick — and crossing over happens when the paired chromosomes are at their most tightly associated.

What actually happens:

1. Homologous chromosomes — one from each parent — align closely in a process called synapsis, forming a bivalent or tetrad of four chromatids.
2. The synaptonemal complex, a protein scaffold, holds them precisely together.
3. Breaks occur in the DNA of non-sister chromatids.
4. The broken segments are exchanged and rejoined.
5. The crossing points, visible under a microscope, are called chiasmata (singular: chiasma).

The exchange is between non-sister chromatids — one from the maternal chromosome and one from the paternal. Sister chromatids are identical, so swapping between them would achieve nothing.

Why it matters. Crossing over is one of the main sources of genetic variation, and this is usually the follow-up exam question:

• It produces recombinant chromatids carrying combinations of alleles that existed in neither parent.
• Together with independent assortment in Metaphase I, it means the number of genetically distinct gametes a person can produce is effectively unlimited.
• That variation is the raw material natural selection acts on.
• It also has a mechanical role: chiasmata physically hold homologous pairs together until anaphase I, and failures here cause nondisjunction, where chromosomes segregate incorrectly.

Does crossing over occur in mitosis? Normally, no. Mitosis produces two genetically identical daughter cells, and homologous chromosomes don't pair up in mitosis, so there's no opportunity for the exchange. There is a rare exception called mitotic recombination, which can occur when DNA damage is repaired, but it isn't part of the normal cell cycle and it isn't what an exam question is asking about.

Meiosis vs. mitosis on this point:

Linkage and gene mapping. Genes physically close together on the same chromosome are less likely to be separated by crossing over, because there's less DNA between them for a break to occur in. Genes far apart are separated more often. Measuring how often two genes are separated — the recombination frequency — lets geneticists work out their relative positions, which is the basis of classical gene mapping. A recombination frequency of 1% is defined as one map unit, or centimorgan.

Terms worth keeping straight:

• Synapsis — the pairing of homologous chromosomes
• Bivalent / tetrad — the paired structure of four chromatids
• Chiasma — the visible point of crossing over
• Recombination — the general term for the resulting new allele combinations
• Synaptonemal complex — the protein structure enabling precise alignment