By Levi Clancy for Student Reader on
Growth 1 (G1)
Two types of proteins control the eukaryotic cell cycle: CDKs and ubiquitin-protein ligases. In a diploid eukaryotic cell, there are two versions of each chromosome, one from the mother and another from the father. The two corresponding chromosomes are called homologous chromosomes. Homologous chromosomes need not be genetically identical.
During growth 1 (G1), an interphase, phase is the normal growth phase. Chromosomes are highly decondensed in most regions, allowing access of regulatory proteins to the DNA. Within the nucleus, individual chromosomes are found within diffuse but non-overlapping domains.
Synthesis Phase (S)
DNA synthesis occurs. Results in 4n chromosomes in diploid organisms (like vertebrates); 2n chromosomes in haploid organisms (like yeast). n = number of distinct types of chromosomes. During the S phase of the cell cycle chromosomes are replicated to produce two complete copies of each. DNA replication results in an identical copy of each chromosome. These copies are called sister chromatids.
Together, these chromatids are considered one chromosome. When separated, though, each sister chromatid is a chromosome. The 2 copies of the original chromosomes are called sister chromosomes.
Growth 2 (G2)
During Growth 2 (G2), another interphase, the cell doubles in size. Centromeres (Microtubule Organizing Centers--MTOCs) form.
When conditions are good, the cell will replicate.
Mitosis vs meiosis
Mitosis and meiosis have many similarities, but their differences are emphasized below.
The second cell division of meiosis is similar to mitotic cell division (sister chromatid segregation), but the first division is different (homologous chromosome segregation). Crossing over occurs during prophase I of meiosis, and it contributes to genetic variation.
|Cell division||One division||Two divisions.|
|Chromosome replication||One replication||One replication|
|Chromosome number||2n → 4n → 2n||2n → 4n → 2n → 1n|
Below is a table with the number of chromosomes and chromatids present in a human cell during various stages of mitosis and meiosis.
|Meiosis metaphase I||46||92|
|Meiosis telophase I||23||46|
|Meiosis telophase II||23||23|