Meiosis is the cell division process that makes gametes, such as sperm and egg cells, with half the chromosome number of the original cell. This cheat sheet helps students follow each phase in order and connect chromosome movement to genetic results. A detailed meiosis diagram is useful because many mistakes happen when students confuse homologous chromosomes, sister chromatids, and diploid versus haploid cells.
The main stages are Meiosis I, Meiosis II, and the genetic variation processes that make gametes unique. Meiosis I separates homologous chromosome pairs, changing the cell from diploid to haploid. Meiosis II separates sister chromatids, similar to mitosis, producing four haploid cells.
Crossing over in Prophase I and independent assortment in Metaphase I are major sources of genetic variation.
Key Facts
- Meiosis starts with one diploid cell and produces four haploid cells, so the overall chromosome number changes from 2n to n.
- Before meiosis begins, DNA is copied during S phase, so each chromosome consists of two sister chromatids joined at a centromere.
- In Prophase I, homologous chromosomes pair to form tetrads, and crossing over can exchange DNA between non-sister chromatids.
- In Metaphase I, homologous chromosome pairs line up at the cell equator, and their random orientation causes independent assortment.
- In Anaphase I, homologous chromosomes separate, but sister chromatids stay attached at their centromeres.
- In Telophase I and cytokinesis, two haploid cells form, but each chromosome is still made of two sister chromatids.
- In Anaphase II, sister chromatids separate, and each chromatid becomes an individual chromosome moving toward opposite poles.
- The final result of meiosis is four genetically different haploid gametes with n chromosomes each.
Vocabulary
- Meiosis
- Meiosis is a two-part cell division process that produces haploid gametes from a diploid cell.
- Homologous chromosomes
- Homologous chromosomes are a matching pair of chromosomes, one inherited from each parent, that carry the same types of genes.
- Sister chromatids
- Sister chromatids are identical DNA copies of a chromosome that are joined together at the centromere after replication.
- Crossing over
- Crossing over is the exchange of DNA between non-sister chromatids during Prophase I.
- Independent assortment
- Independent assortment is the random alignment of homologous chromosome pairs during Metaphase I, creating different chromosome combinations in gametes.
- Haploid
- A haploid cell has one set of chromosomes, represented as n, such as a human gamete with 23 chromosomes.
Common Mistakes to Avoid
- Confusing homologous chromosomes with sister chromatids is wrong because homologous chromosomes are a maternal and paternal pair, while sister chromatids are identical copies of one chromosome.
- Saying sister chromatids separate in Meiosis I is wrong because Anaphase I separates homologous chromosomes, while sister chromatids separate in Anaphase II.
- Forgetting that DNA replication happens before meiosis is wrong because chromosomes must duplicate before Prophase I begins.
- Thinking crossing over happens in Metaphase I is wrong because crossing over occurs in Prophase I when homologous chromosomes are paired as tetrads.
- Calling the final products diploid is wrong because meiosis reduces chromosome number, so the four final cells are haploid.
Practice Questions
- 1 A diploid cell has 12 chromosomes before meiosis. How many chromosomes will each gamete have after meiosis is complete?
- 2 If a species has a haploid number of n = 8, how many chromosomes are found in one diploid body cell?
- 3 A cell begins meiosis with 6 duplicated chromosomes. How many cells are produced at the end of Meiosis II, and how many chromosomes are in each cell?
- 4 Explain why crossing over and independent assortment make the gametes from one parent genetically different from each other.