The Cell Cycle in Eukaryotes (College Board AP Biology)

• Mitosis is part of a precisely controlled process known as the cell cycle
• The cell cycle is the regulated sequence of events that occurs between one cell division and the next
• The cell cycle has three phases:
  • interphase (G1, S and G2)
  • nuclear division (mitosis)
  • cell division (cytokinesis)
• The length of the cell cycle is very variable depending on environmental conditions, the cell type and the organism
  • For example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly)
• The movement from one phase to another is triggered by chemical signals called cyclins
• Not all cells are part of the cell cycle at all times; a cell can enter phase G₀ where it no longer divides
  • Although it can re-enter the cell cycle on receipt of chemical signals

The Cell Cycle Diagram

The cell cycle. S = synthesis (of DNA); G = growth; M = mitosis

Interphase

• During Interphase the cell increases in mass and size and carries out its normal cellular functions (eg. synthesizing proteins and replicating its DNA ready for mitosis)
• Interphase consists of three phases:
  • G₁ phase
  • S phase
  • G₂ phase
• It is at some point during the G₁ phase a signal is received telling the cell to divide again
• The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids)
• This phase of the interphase stage of the cell cycle is called the S phase – S stands for synthesis (of DNA)
  • The S phase is relatively short
• The gap between the previous cell division and the S phase is called the G₁ phase – G stands for growth
  • Cells make the RNA, enzymes and other proteins required for growth during the G₁ phase
• Between the S phase and next cell division event the G₂ phase occurs
  • During the G₂ phase, the cell continues to grow and the new DNA that has been synthesized is checked and any errors are usually repaired
  • Other preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle)
• Interphase = G₁ + S + G₂

Nuclear division (mitosis)

• Follows interphase
• Referred to as the M phase – M stands for mitosis
• Cell growth stops during the M phase

Cytokinesis

• Follows M phase
• Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells
• In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed

• Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)
• Significance of mitosis: mitosis occurs whenever the production of genetically identical nuclei is required in eukaryotic cells
  • Eg. during embryonic development, growth, tissue repair and asexual reproduction

Phases of Mitosis

• Although mitosis is, in reality, one continuous process, it can be divided into four main stages or phases

• These stages are:
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Most organisms contain many chromosomes in the nuclei of their cells (eg. humans have 46) but the diagrams below show mitosis of an animal cell with only four chromosomes, for simplicity
• The different colours of the chromosomes are just to show that half are from the female parent and half from the male parent

Prophase

• Chromosomes condense and are now visible when stained
• The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere
• The two centrosomes (replicated in the G₂ phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
• Spindle fibers (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)
• The nuclear envelope (nuclear membrane) breaks down into small vesicles
• The nucleolus disappears

Prophase of Mitosis Diagram

Prophase

Metaphase

• Centrosomes reach opposite poles
• Spindle fibers (protein microtubules) continue to extend from centrosomes
• Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
• Spindle fibers (protein microtubules) reach the chromosomes and attach to the centromeres
  • This attachment involves specific proteins called kinetochores
• Each sister chromatid is attached to a spindle fiber originating from opposite poles

Metaphase Diagram

Metaphase

Anaphase

• The sister chromatids separate at the centromere (the centromere divides in two)
• Spindle fibers (protein microtubules) begin to shorten
• The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibers (protein microtubules)

Anaphase Diagram

Anaphase

Telophase

• Chromosomes arrive at opposite poles and begin to decondense
• Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
• The spindle fibers break down
• New nucleoli form within each nucleus

Telophase Diagram

Telophase