Click image to open full size
Cell division allows organisms to grow, repair tissues, and produce new cells. Mitosis and meiosis are the two main ways eukaryotic cells divide, but they serve very different purposes. Mitosis makes genetically similar body cells for growth and maintenance. Meiosis makes sex cells and creates genetic variation that is essential for sexual reproduction.
Both processes begin after DNA has been copied, so each chromosome consists of two sister chromatids. In mitosis, one division separates sister chromatids and produces two diploid cells with the same chromosome number as the parent cell. In meiosis, two divisions occur, and homologous chromosomes separate first, then sister chromatids separate, producing four haploid cells. Crossing over and independent assortment during meiosis shuffle genes into new combinations, which increases variation in offspring.
Key Facts
- Mitosis: 1 cell division produces 2 daughter cells.
- Meiosis: 2 cell divisions produce 4 daughter cells.
- Diploid cells have two sets of chromosomes: 2n.
- Haploid cells have one set of chromosomes: n.
- DNA replication occurs before mitosis and before meiosis I during interphase.
- In humans, mitosis keeps chromosome number at 46, while meiosis reduces 46 to 23.
Vocabulary
- Homologous chromosomes
- A pair of chromosomes with the same genes in the same order, one inherited from each parent.
- Sister chromatids
- Two identical copies of a chromosome that are joined together after DNA replication.
- Diploid
- A cell state with two complete sets of chromosomes, written as 2n.
- Haploid
- A cell state with one complete set of chromosomes, written as n.
- Crossing over
- The exchange of DNA segments between homologous chromosomes during meiosis I that creates new gene combinations.
Common Mistakes to Avoid
- Saying mitosis makes gametes, which is wrong because mitosis produces body cells used for growth and repair, not sex cells.
- Thinking chromosome number doubles after mitosis, which is wrong because mitosis preserves the original chromosome number in the daughter cells.
- Confusing homologous chromosomes with sister chromatids, which is wrong because homologous chromosomes are similar pairs from different parents, while sister chromatids are identical copies of one chromosome.
- Forgetting that genetic variation in meiosis starts before the cells separate, which is wrong because crossing over and independent assortment occur during meiosis I.
Practice Questions
- 1 A diploid cell in a species has 2n = 12. After mitosis, how many daughter cells form and how many chromosomes are in each daughter cell?
- 2 A diploid parent cell has 2n = 16. After meiosis is complete, how many cells are produced and how many chromosomes are in each cell?
- 3 Explain why meiosis creates more genetic variation than mitosis, using the ideas of crossing over and independent assortment.