Meiosis crossing-over

Crossing over The breaking of one maternal and one paternal chromosome during meiosis with exchange of corresponding sections of DNA, and rejoining of the chromosomes. The process leads to an exchange of alleles between chromosomes and is the fundamental principle of human evolution (see also recombination). 

Figure 1-4-3. Large Segment Deletion During Crossing-Over In Meiosis...

Meiosis is cell division that produces two daughter cells with only half the number of chromosomes of the parent cell these are haploid cells. It occurs during the formation of spermatozoa and ova. The normal number of chromosomes is then achieved at fertilisation of an ovum by a spermatozoon. Meiosis also results in crossing over , which is the physical reciprocal exchange of parts... 

During the prophase of the first meiotic division (meiosis I) two homologous pairs of partially "condensed" chromosomes must find each other and pair with appropriate orientation. A protein in the telomeres of the chromosomes seems to be involved.269 277 The key structure in meiotic crossing-over is the ribbonlike synaptonemal complex formed by the pairs of homologous chroma fids.271/278 2791 This complex, in which a proteinaceous core or axial element separates the greatly extended chromatid pairs (Fig. 26-13), is fully formed in the pachytene stage of meiosis. Formation of the synaptonemal complex is preceded by development of a few double-stranded breaks in... 

Crossing over—In meiosis, a process in which adjacent chromosomes exchange pieces of genetic information. 

A FIGURE 9-45 Recombination during meiosis. (a) Crossing over can occur between chromatids of homologous chromosomes before the first meiotic division (see Figure 9-3). 

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