Meiosis I versus mitosis: homologous pairs or sister chromatids
Meiosis I pulls apart homologous pairs (maternal and paternal copies of one chromosome), whereas mitosis pulls apart sister chromatids (the two identical copies of a replicated chromosome).
Remember
In meiosis I the homologous pairs are pulled apart while the sister chromatids stay joined, which halves the chromosome number; in mitosis the sister chromatids themselves separate, so the two cells that result are identical.
Before mitosis or meiosis, the DNA is copied once. Each chromosome is then made of two sister chromatids, which are identical and joined at the centromere (the region that links them). The key difference between mitosis and meiosis I is which of these structures is pulled apart in anaphase (the stage when chromosomes are pulled towards opposite ends of the cell).
In mitosis the centromere splits at anaphase, so the sister chromatids are pulled to opposite ends of the cell. Each chromatid is now a chromosome in its own right, and both daughter cells (the two cells that result) receive the same 46 chromosomes with identical DNA.
In meiosis I a maternal and a paternal copy of the same chromosome line up together as a homologous pair, and in anaphase I these whole chromosomes are pulled apart. The centromeres do not split, so the sister chromatids stay joined. Each cell is left with 23 chromosomes, and each of these is still made of two chromatids, which gives 46 chromatids. Because the chromosome number falls from 46 to 23, meiosis I is called a reduction division.
Each pair faces a random way round, known as independent assortment, so each cell ends up with a different mix of maternal and paternal chromosomes. In meiosis II the sister chromatids are split, as they are in mitosis, and the resulting cells are haploid (they carry one set of chromosomes).
Key facts
- Mitosis, anaphase
- The centromere splits and the sister chromatids are pulled to opposite ends of the cell, so the two daughter cells have identical DNA.
- Meiosis I, anaphase I
- The homologous pairs are pulled apart while the centromeres stay intact and the sister chromatids stay joined.
- Chromosome count
- Meiosis I takes each cell from 46 chromosomes to 23, and each of the 23 is still made of two chromatids.
- Meiosis II
- The sister chromatids are split, as in a mitotic division, leaving each daughter cell with 23 chromosomes.
- DNA replication
- The DNA is copied once, before mitosis and before meiosis, so replication is not the key difference between them.
- Variation
- Independent assortment gives each daughter cell a random mix of maternal and paternal chromosomes.
Exam traps
- Splitting of sister chromatids happens in mitosis and in meiosis II. In meiosis I the sister chromatids stay joined, so it cannot be what distinguishes meiosis I.
- Division of the centromere belongs to mitosis and meiosis II. The centromere stays intact in meiosis I, which is why the sister chromatids stay together.
- DNA replication does not set meiosis I apart, because the DNA is copied once before either mitosis or meiosis begins.
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