Cell Cycle (HL) (HL IB Biology)

• Plants and animals originate from a zygote which will divide repeatedly to form an embryo
  • During this stage of development, there is a rapid increase in the number of cells which allows the embryo to grow
    • This rapid increase in cell number is known as proliferation
    • Proliferation for growth occur within early-stage animal embryos, as well as growth regions in plants known as meristems

Plant meristems

• Plant growth occurs in regions known as meristems
  • The cells in these regions are known as meristem cells
• Meristem cells are undifferentiated; they actively divide by mitosis to produce new plant tissue
• Meristems can be found in the growing tips of plant roots and shoots
  • A meristem at the tip of a shoot is a shoot apical meristem
  • A meristem at the tip of a root is a root apical meristem
  • Meristem tissue with the potential to form new side branches from the main plant stem can be found in regions known as axillary buds
• Meristems can also be found parallel to the sides of a stem e.g. within the vascular bundles that contain the xylem vessels and the phloem
  • These are known as lateral meristems and enable plant stems to grow in diameter
  • Lateral means ‘from the side’
• The regions of meristem tissue in the vascular bundles are known as cambium

Diagram to show the location of plant meristems

The growing parts of plants are known as meristems, and can be found in the shoot apex (shoot apical meristem), the root apex (root apical meristem), and the sides of the stem (lateral meristem)

Diagram to show the location of the cambium meristem tissue

Lateral meristem tissue is known as cambium. It can be found between the xylem and phloem in stems and roots.

Early-stage animal embryos

• Following human fertilisation, the newly fertilised ovum divides by mitosis to form two diploid nuclei (i.e. each nucleus contains two sets of chromosomes) and the cytoplasm divides equally to form a two-cell embryo
• Mitosis continues to form a four-cell embryo and this process continues until eventually, the embryo takes the shape of a hollow ball called a blastocyst (with an internal group of cells called blastomeres)
  • Blastomeres will eventually develop into the foetus
  • These cells are undifferentiated and could develop into any type of specialised cell at this point

Cell proliferation for cell replacement and tissue repair

• Cells, such as those that form part of skin and blood, need to be constantly replaced
  • This is achieved by the process of cell proliferation whereby cells divide by mitosis
• Skin cells that form part of the epidermis (outer layer of the skin) are lost on a daily basis and will be replaced multiple times during a lifetime
  • This is achieved by the proliferation of the stem cells found in the basal (bottom) layer of the epidermis
  • These newly formed daughter cells will form new layers in the epidermis to replace the top layers lost through wear and tear
• Tissue repair during wound healing is another example of the importance of cell proliferation
  • Once the wound is sealed by a blood clot, the blood vessels will dilate to increase blood flow to the damaged area
  • This enables fibroblasts and white blood cells called macrophages to reach the wound
    • Macrophages will engulf any pathogen that entered through the broken skin while the fibroblasts will proliferate to facilitate the closure of the wound
    • The fibroblasts achieve this by breaking down the fibrin in the blood clot while producing a new matrix of collagen fibres to help close the wound and support new skin cells

Wound healing diagram

The proliferation of fibroblasts will speed up the process of wound healing

• Mitosis is part of a precisely controlled process known as the cell cycle
  • Cell proliferation is achieved using 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 occurring in the following order:
  • interphase
    • This includes the G₁, S and G₂ phases
  • nuclear division (mitosis)
  • cell division (cytokinesis)
• The length of the cell cycle varies depending on:
  • The 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

Cell cycle diagram

The cell cycle

S = synthesis (of DNA); G = growth; M = mitosis

• Interphase is the longest and most active phase of the cell cycle
• During interphase, the cell:
  • Increases in mass and size
  • Carries out many cellular functions in the nucleus and cytoplasm e.g. synthesising proteins and replicating its DNA ready for mitosis (these only occur during interphase)
  • Increases the number of mitochondria
  • Increases the number of chloroplasts (if they are a plant or algae cell)

The phases of interphase

• Interphase consists of three phases:
  • G₁ phase
  • S phase
  • G₂ phase
• 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
• It is at some point during the G₁ phase a signal is received telling the cell to divide again (although some cells do not receive this signal and will never divide; they enter the G₀ phase)
• After the G₁ phase of interphase the cell enters the next phase of the cell cycle, the S phase – S stands for synthesis (of DNA)
  • The S phase is relatively short
  • The DNA in the nucleus replicates, resulting in each chromosome consisting of two identical sister chromatids
• 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 synthesised 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

• The cell cycle is a sequence of stages including interphase (G₁, S & G₂), mitosis (M) and cytokinesis (C)
• There are three checkpoints in the cell cycle which must be overridden before the next stage can begin
  • These checkpoints are located at G₁, G₂ and M 
• The cycle is controlled by cyclins (a group of proteins) and kinases (enzymes)
• There are four different cyclins (D, E, A & B) whose concentrations rise and fall over the cycle:
  • Each of these will trigger specific events in the cell cycle to occur

Cyclin control of the cell cycle diagram

Cyclins control the cell cycle. The presence of certain cyclins triggers a specific stage of the cell cycle.

• When each of the different cyclins reach a certain concentration (or threshold level) they trigger the next stage of the cell cycle
• This ensures key processes (e.g. DNA replication, organelle multiplication and protein synthesis) occur at the correct time
• When a specific cyclin has reached a certain concentration it will bind with another group of proteins (cyclin-dependent kinases) forming a complex which is activated
• This complex phosphorylates (attaches a phosphate) a target protein which activates it, causing it to trigger specific functions (e.g. DNA replication)
• Once the specific function is complete the phosphate is released, the cyclin breaks down and the cyclin-dependent kinases become inactive

Mechanism of cyclin action diagram

The mechanism for cell cycle control by cyclins