Slide 1
Meiosis reduces chromosome sets before fertilization combines them
Meiosis reduces chromosome-set number during sexual reproduction. This allows fertilization to combine parental contributions without doubling the number every generation.
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Humans typically have 23 chromosome pairs in most nucleated body cells. Gametes normally carry one chromosome from each pair, so combining egg and sperm restores the paired set.
Slide 2
Matching chromosomes pair despite different gene versions
Before the first meiotic division, homologous chromosomes pair. They carry corresponding genes, although their versions of those genes can differ.
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A gene version is called an allele. Homologous chromosomes are not the same as the newly copied sister chromatids of one chromosome, a distinction that matters when following the two divisions.
Slide 3
Crossing over creates new combinations of existing variants
Crossing over exchanges deoxyribonucleic acid (DNA) between homologous chromosomes. It can create new combinations of inherited alleles, different versions of genes.
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Genes close together are less likely to be separated by a crossover than genes farther apart. This relationship underlies genetic linkage mapping, although recombination rates vary across the genome.
Slide 4
Two divisions separate chromosomes and then their copies
The first meiotic division separates homologous chromosomes. The second separates sister chromatids, the copied halves of each chromosome.
Both divisions follow one preceding round of DNA replication.
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The orientation of each homologous pair contributes another source of assortment. Crossing over and chromosome assortment therefore reshuffle inheritance in different ways, rather than representing the same event twice.
Slide 5
Reshuffling existing variants differs from creating mutations
Meiosis mostly recombines existing genetic variants. New mutations add another source of variation, separate from this reshuffling.
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Chromosome separation can occasionally go wrong, producing cells with extra or missing chromosomes. That possibility shows why accurate pairing and segregation are as central to meiosis as the generation of diversity.
Sources
- Meiosis
National Human Genome Research Institute
- Crossing Over
National Human Genome Research Institute