Rapid Review

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 of chromatids per chromosome
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.