The Eukaryotic cell cycle and Mitosis
This Factsheet covers the relevant AS syllabus content of the major examining boards. By studying this factsheet the candidate will gain a knowledge and understanding of:
• the different phases of the eukaryotic cell cycle (a eukaryotic cell has membrane bound organelles) • the importances of mitosis
• the process of mitosis
The eukaryotic cell cycle
Fig 2. Quantity of DNA in the cell during different phases
This is illustrated by Fig 1.
DNA quantity/arbitrary units
Fig 1. The eukaryotic cell cycle
The G1, S and G2 phases are termed interphase. Interphase is the stage of the cell cycle between cell divisions. It is not a resting stage, since in an actively dividing and growing cell new DNA and proteins are being synthesised and in a non-dividing mature cell, the G1 or G2 cell is performing all its metabolic cell functions and specific jobs.
The importances of mitosis
Cell division consists of two phases, mitosis and cytokinesis. Mitosis, is the division of the nucleus. Cytokinesis is the division of the cytoplasm which usually, but not always, occurs immediately after nuclear division. Only a small time period of the cell cycle consists of the cell division stages. The other phases, known as G1, S and G2 take up the majority of the time.
Cell division by mitosis is important during growth of eukaryotic organisms and is the way in which eukaryotes increase their cell numbers, either in a population of a single celled organism, such as Amoeba or yeast, or within the body of a multicellular organism. Growth may be allometric meaning that different parts of the organism grow at different rates. This can be due to mitosis occuring at different rates in different organs. Mitosis is also important during repair of damaged tissue.
The total time of the cell cycle varies from around 30 minutes in growing yeast cells, 18 to 24 hours in animal sperm-producing cells, 10 to 30 hours in plant meristematic cells to several weeks in slowly regenerating tissues.
Mitosis produces two ‘daughter’ or offspring nuclei from the original parent nucleus. The chromosome number of each of the offspring nuclei is the same as the parent nucleus and the genome (nature and arrangement of the genes) is kept exactly the same. Thus mitosis maintains the same chromosome number and genotype throughout growth, life and repair. Mitosis ensures that every body cell throughout life, with the exception of gametes, has a genome identical to that of the original zygote. Mitosis of a diploid cell will produce two diploid cells and mitosis of a haploid cell will produce two haploid cells. For example, when the haploid leafy gametophyte stage of a moss or the haploid gametophyte prothallus of a fern produce haploid gametes they do so by mitosis.
For example in a cell with a 24 hour cycle the times of the phases would be around G1 10 hours, S 9 hours, G2 4 hours and mitosis with cytokinesis 1 hour.
Fully differentiated cells generally remain arrested in the G1 stage and will not normally divide again. Some cells remain arrested in the G2 stage, for example, human cardiac muscle cells. The synthesis of DNA occurs in the S phase when the quantity of DNA in the cell is doubled. This is shown in Fig 2.
The eukaryotic cell cycle and mitosis
The process of mitosis
Anaphase: Fig 5 shows the appearance of a cell in mid anaphase.
For easy reference the process of mitosis is divided into four phases.These are prophase, metaphase, anaphase and telophase. Cytokinesis occurs at the end of telophase.
Prophase: The events of prophase are shown in Fig 3.
Fig 5 Appearance of a cell in mid anaphase.
Fig 3. Appearance of the cell in prophase