Cell Cycle
Abbreviation | Meaning | |
Interphase | G1 | 1st growth phase |
S | Synthesis of DNA (chromosome replication) | |
G2 | 2nd growth phase | |
Cell division | M | Mitosis |
C | Cytokinesis |
Mitosis
Phase | Main events |
Prophase | Chromosomes condense |
Metaphase | Chromosomes line up on metaphase plate |
Anaphase | Separation of chromatids |
Telophase | Daughter nuclei form |
Meiosis
- Like mitosis, meiosis is preceded by an interphase consisting also of G1, S, and G2 phases. In the S phase, the chromosomes replicate.
Stage | Main events | Chromosome number | Number ofchromatids perchromosome |
Prophase I | Chromosomes condense; homologous chromosomes come together to form tetrads – crossing-over occurs | 2n | 2 |
Metaphase I | Chromosomes line up on metaphase plate | 2n | 2 |
Anaphase I | Separation of homologous chromosomes | 2n | 2 |
Telophase I | Two daughter cells form. Cytokinesis. | n | 2 |
Prophase II | New mitotic spindle forms | n | 2 |
Metaphase II | Chromosomes line up on metaphase plate | n | 2 |
Anaphase II | Separation of chromatids | n | 2 |
Telophase II | Four daughter cells result from original parent cell. Cytokinesis. | n | 1 |
Note that the chromosome number refers to the number of different chromosomes present. When chromosomes replicate in the S phase, the chromosome number remains the same since the number of different chromosomes has not changed.
Major Differences between Mitosis and Meiosis
Mitosis | Meiosis | |
Importance in animals | Production of cells for growth and repair | Production of gametes; introduces genetic variability in gametes |
Number of divisions | One | Two: there is no DNA replication between the two divisions |
Number of daughter cells | Two | Four |
Genetic composition of daughter cells | 2n (diploid) | n (haploid) |
Other | In prophase I, homologous chromosomes do not synapse | In prophase I, homologous chromosomes synapse, forming tetrads. Subsequently, crossing-over occurs. |
Terminology Used in the Study of Cell Division
- Centriole - One of two structures in the centrosome consisting of microtubules; help organize the spindle.
Centromere - The centralized region joining two sister chromatids.
Centrosome - Material present in the cytoplasm of eukaryotic cells and important in cell division.
Chiasma - Location where crossing-over has occured.
Crossing-over - The exchange of genetic material between non-sister chromatids during prophase I of meiosis.
Homologous chromosomes - Chromosome pairs that possess genes for the same traits at corresponding loci.
Kinetochore - Region on the centromere that links to the mitotic spindle.
Telomere - The structure at the end of an arm of a chromosome.
Tetrad - Group of two homologous chromosomes that are synapsed (joined along their lengths). Since each chromosome consists of two sister chromatids, a tetrad is comprised of four chromatids.