Cell Cycle

Eukaryotic cells follow cell cycle 

Cell content built up during interphase. Three phases G1 S G2

Interphase followed by M phase (mitosis)

Interphase 

G1 is the first growth stage- cell makes new proteins and copies of the organelles 

S phase is when DNA replication occurs.

G2 is second period of cell growth- again, the cell makes more proteins and copies the organelles in preparation for mitosis 

Mitosis and Cytokinesis- M phase 

Prophase- the chromosomes condense (coil up) and become visible as 2 joined chromatids. Spindle fibres attach to the chromosomes at centromere. Nucleus membrane disintegrates.Metaphase- spindle fibres move the chromosomes so that they line up on the metaphase plate at the equator of the cell. Anaphase- the spindle fibres pull the sister chromatids apart. Once they are seperated, the chromatids are called chromosomes in their own right. Telophase- the seperated chromosomes are pulled by the spindle fibres to opposite poles to form daughter nuclei. The chromosomes start to uncoil and a nuclear membrane is made again.Cytokinesis- the membrane is pulled in by part of the cytoskeleton to divide the cytoplasm to form 2 daughter cells. Plant cells have to form a middle lamella and cell wall before the membrane is made. 

Mitotic Index:

Control of Cell Cycle 

G1 Checkpoint- near end of G1. The cell size is monitored. There must be sufficient cell mass to form 2 daughter cells. Controls entry into S phase.G2 Checkpoint- at the end of G2. Checks the success of DNA replication to make sure each daughter cell can receive a complete copy of DNA. Controls entry into mitosis using a protein called mitosis-promoting factor (MPF). Mitosis is triggered if sufficient MPF is present M Checkpoint- during metaphase. Monitors the chromosome alignment to ensure each daughter cell receives one chromatid from each chromosome. Controls entry to anaphase. Triggers exit from mitosis and the start of cytokinesis 

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