The Cell Cycle

Cell cycle = a highly ordered sequence of events that takes place in a cell, resulting in division of the cell and producing genetically identical daughter cells

Phases of Cell cycle

in eukaryotic cells, cell cycle has 2 main phases.

Interphase → Stage where the cell grows, does its normal functions such as producing enzymes etc, and copies its DNA before dividing. Cell spends majority of time in this phase - actively preparing for cell division

During interphase :

  • DNA is replicated

  • Protein Synthesis

  • mitochondria grow and divide

  • Chloroplasts grow and divide - in plant cells

  • metabolic processes of cells occur

3 Stages of Interphase

  1. G1 → 1st growth phase: Proteins from which organelles are synthesised are produced and organelles replicate - increasing cell size

  2. S → synthesis stage: DNA replicated in the nucleus

  3. G2 → 2nd growth phase: Cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors.

Mitotic Phase → The period of cell division. Involves 2 stages

  • Mitosis → Nucleus Divides

  • Cytokinesis → Cytoplasm divides and 2 cells are produced

G0

name given to the phase when cell temporarily/permanently leaves the cycle. Number of reasons for this including:

  • Differentiation → A cell that becomes specialised to carry out a particular function can no longer divide.

  • DNA of cell may be damaged → No longer able to divide and enters period of ‘permanent cell arrest’. Normal cells can only divide a set amount of times and eventually becomes ‘senescent’

  • Senescent cells are old, damaged, or aged cells that no longer divide but remain metabolically active. An increased number of these have been linked to many age related diseases.

A few cells that enter G0 can be stimulated to go back into cell cycle and divide again (lymphocytes - in an immune response).

Control of cell cycle

It’s vital to ensure a cell only divides when all the conditions are precise (cell size, replicated DNA is error free, chromosomes are in correct positions etc). This is to ensure cell division is carried out immaculately.

  • Checkpoints are control mechanisms for cell cycle. They monitor and verify whether processes at each phase of the cell have been accurately completed before cell is allowed to progress any further.

Checkpoints occur at various stages

  • G1 checkpoint - at the end of the G1 phase. If not met, it enters a resting state (G0)

  • G2 checkpoint - end of the G2 phase before the mitotic phase. Has to check whether DNA has been replicated without error. If passes, cell initiates molecular processes that signal mitosis.

  • Spindle assembly checkpoint (also called metaphase checkpoint) → At the point in mitosis where all chromosomes all chromosomes are attached to spindles and have aligned . Mitosis can’t proceed until checkpoint has passed;.