Unit 1.1.3: Cell Division, Cell Diversity and Cellular Organisation

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The Cell Cycle

The Cell Cycle

Cell Cycle. Describes the events that take place as one parent cell divides to produce two new daughter cells which then each grows to full size.

  • The daughter cells produced are genetically identical to each other and the parent cell.

The Cell Cycle is divided into stages: 

  • Interphase - this is split into the G1, S and G2 stages.
  • Mitosis - the nucleus divides and chromatids separate. 
  • Cytokinesis - the cytoplasm divides.
  • Growth Phase - each new cell grows to full size.

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Mitosis only occupies a small proportion of the cell cycle and the remaining larger portion includes copying and checking genetic information on the DNA and other processes associated with growth.

Mitosis. Refers to the process of nuclear division where two genetically identical nuclei are formed from one parent cell nucleus. 

Mitosis is used for: 

  • Growth. 
  • Repair. 
  • Asexual Reproduction. 
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Mitosis consists of the following 4 stages:

  • Prophase. The nuclear envelope breaks down and disappears. Chromosomes shorten and thicken. Centriole breaks into two, moves to opposite poles of the cell to form the spindle.
  • Metaphase. Chromosomes move to the equator of the spindle and becomes attached to the spindle fibres by their centromeres.
  • Anaphase. The replicated sister chromatids are separated from each other once the centromere splits. The spindle fibres shorten, pulling the sister chromatids further from each other, towards the poles. Each of the sister chromatids become an individual chromosome, each one genetically identical to the original chromosome in the parent cell. 
  • Telophase. A nuclear envelope forms around each set of chromosomes and two nuclei and nuclear membranes are formed. The spindle breaks down and disappears. The chromosomes uncoil, so they can no longer be seen under a light microscope. 

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Mitosis Stages Diagram

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Stem Cells and Differentiation

Stem Cells. Undifferentiated genetically identical daughter cells are carrying a full set of genetic information and as a result, being capable of developing into any one of the different cell types found in a fully grown organism. 


Cytokinesis is different in plant and animal cells: 

  • In Plants, it starts with the formation of a cell plate. The cell then layds down a new membrane and cell wall material along this plate.
  • In Animal Cells, cytokinesis starts from the outside - 'nipping in' the cell membrane and cytoplasm along the cleavage furrow. 

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  • Meiosis. Produces gametes containing only one set of chromosomes (23-haploid).
  • Fusion of the male and female gametes results in a zygote containing two sets of chromosomes (diploid). 
  • The daughter cells of meiosis are not genetically identical to each other. 
  • This is because each pair of homologous chromosomes (containing the same genes, but not the same alleles - separate into haploid cells. 


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Differentiation. Refers to the changes occurring in cells of a multicellular organism so that each different type of cell becomes specialised to perform a specific function.

  • Erythrocytes and Neutrophils play different roles but were both produces from stem cells in the bone marrow.
  • Differentiation in plants can be referred to the xylem and phloem. But come from dividing meristem cells such as cambium. 
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How cells are specialized for their functions

How cells are specialized for their functions 


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Tissues, Organs and Organ Systems

Tissues. A collection of cells that are similar to each other and perform a common function. E.g. Xylem & Phloem: Epithelial & Nervous Tissues. 

Organs. A collection of tissues working together to perform a particular function. E.g. Liver & Leaves.

Organ System. A number of organs working together to perform an overall life function.

The leaf is adapted to meet requirements by:

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Organisation of Tissue

  • Epithelial tissue: layers and linings. 
  • Connective tissues: hold structures together and provide support (E.g. Cartilage and Bone). 
  • Muscle tissue: specialised to contract and move parts of the body. 
  • Nervous tissue: converts stimuli to electrical impulses and conducts those impulses.
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