2.1.6 cell division, cell diversity, cellular organisation

• how do cells divide

  • Eukaryotic cells enter the cell cycle and divide by mitosis or meiosis
  • Prokaryotic cells replicate by binary fission
  • Viruses do not undergo cell division as they are non-living
• what are the 3 components of the cell cycle

  1. Interphase (G1,S,G2)

  2. Nuclear division (mitosis/meiosis)

  3. Cytokinesis
• Interphase

  the longest stage of the cell cycle

  G1 - protein synthesis occurs to make proteins involved in synthesising organelles, The organelles replicate, cell is checked that it is the correct size and has the correct nutrients, growth factors and that there is no damaged DNA. If a cell doesn't pass these checks replication will not continue

  S phase - DNA is replicated

  G2- cell continues to grow, energy stores increase and the newly replicated DNA is checked for copying errors
• what is mitosis


  mitosis creates 2 identical diploid cells and is used for growth, tissue repair and asexual reproduction in plants, animals and fungi
• what are the key stages of mitosis

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• prophase

  • The chromosomes condense and become visible. In aminal cells, the centrioles separate and move to opposite poles of the cell
  • The centrioles create spindle fibres which are released from both poles to create a spindle apparatus - these will attach to the centromere and chromatids on the chromosome in later stages
  • Plants have a spindle apparatus but lack centrioles
• metaphase

  • the chromosomes align along the equator of the cell
  • The spindle fibres are released from the centrioles and attach to the centromere and chromatids
  • The spindle assembly checkpoint occurs in this stage. There is a check to ensure every chromosome has attached to a spindle fibre before mitosis can proceed into anaphase
• anaphase

  • The spindle fibres start to shorten and move towards the centrioles and pull the centromere and sister chromatids to the opposite ends of the cell
  • This causes the centromere to divide into two and the individual chromatids and pulled to each opposite pole
  • This stage requires energy in the form of ATP which is provided by respiration in the mitochondria
• telophase

  • The chromosomes are now at each pole of the cell and become longer and thinner
  • The spindle fibres disintegrate and the nuclear membrane reforms
• cytokinesis

  • the cytoplasm splits into 2 genetically identical cells
  • In animals a cleavage furrow forms in the middle of the cell and the cytoskeleton causes the cell membrane to draw inwards until the cell splits in two
  • In plant cells, the cell membrane splits into 2 new cells due to the fusing of vesicles from the Golgi apparatus. The cell wall forms new sections around the membrane to complete the division into 2 cells
• observing mitosis

  • The stages of mitosis are visible under a light microscope in onion and garlic root tips
  • A thin slice of the root tip is placed on a microscope slide and broken down with a needle
  • A stain is added to make the chromosomes visible and the coverslip is pushed down. This is to squish the tip to achieve a single layer of cells so light can pass through
• mitotic index


  Mitotic index = the number of cells in mitosis/ the total number of cells x 100
• what is meiosis


  Two nuclear divisions in this process result in four genetically different haploid daughter cells

  The 2 rounds of division are meiosis I and meiosis II and both of these contain stages including prophase, metaphase, anaphase, telophase and cytokinesis each
• haploid and diploid


  Haploid has one copy of each chromosome and diploid cells have 2 copies of each chromosome
• what are the 2 key processes in genetic differences in mitosis


  1. Independent assortment of homologous chromosomes

  2. Crossing over
• crossing over

  • during prophase I the homologous chromosomes pair to form bivalents
  • Crossing over genetic material can occur between non-sister chromatids of bivalents
  • Breaks can occur in the genetic material where the chromatids cross over and parts of the chromatids are exchanged between the homologous pairs
  • This results in new combinations of alleles in the resulting gamete
• independent assortment

  • during metaphase I the homologous pairs of chromosomes line up opposite each other on either side of the equator
  • It is random on which side of the equator the paternal and maternal chromosome of each pair aligns
  • As there are 23 different homologous pairs this means there are 2^23 different ways the pairs could assort themselves
• Multicellular organisms are organised

  • cells
  • Tissue
  • Organ
  • Organ system
  • Entire organism
• erythrocytes


  Biconcave shape to increase the surface area for diffusion and to increase the cell flexibility for it to fit through narrow capillaries. These cells have no nucleus so there is more space to hold haemoglobin to increase the transport of oxygen
• neutrophils


  Has a lobed nucleus and granular cytoplasm. The cells are flexible to enable them to surround pathogens and engulf them. They contain lysosomes filled with lysozyme. Neutrophils are made from stem cells in bone marrow
• sperm cells


  The flagellum contains many mitochondria to release energy for locomotion to enable the sperm cell to move towards the egg cell. The head of the sperm cell contains digestive enzymes to digest the wall of the egg cell so the sperm can penetrate and fertilise the egg cell
• palisade cells


  Located in the mesophyll tissue layer of leaves. They are rectangular, tightly packed cells that contain many chloroplasts to absorb and maximise light energy for photosynthesis. They have thin cell walls to reduce the diffuse distance of carbon dioxide
• guard cells


  These pair of cells have flexible walls more so on one side which results in the cells bending when turgid to open the stomata and closing when flaccid and this helps control water loss by transpiration
• squamous epithelial cells


  Usually only a single layer of flat cells in contact with the basement membrane of the epithelium providing a short diffusion distance e.g lining of lungs
• ciliated epithelial cells


  These cells have hair life projections that sway to move substances such as mucus out of the lungs or an egg in the oviduct. Goblet cells are also located within the epithelium and these cells release mucus to trap molecules such as trapping dust in the trachea
• cartilage


  A connective tissue that is firm and flexible is located in the outer ear, nose and the end of bones. It provides structural support and it prevents the bones from rubbing together which would damage them. It is made up of elastin and collagen fibres within an extracellular matrix
• muscle


  Composed of tissues that can contract and relax to create movement

  Skeletal muscles cause the skeleton to move and are made up of myofibrils containing the proteins actin and myosin. Smooth muscle is located within organs and cardiac muscle is within the heart. Muscles have multiple fibres connecting with connective tissues in between
• xylem


  The cells that make up the part of the vascular bundle in plants are responsible for transporting water and mineral ions. The tissue is made up of elongated, hollow dead cells, with lignin in the walls to strengthen and waterproof the walls. Xylem tissues are made from the stem cells in the meristem
• phloem


  The cells that make up the part of the vascular bundle responsible for transporting organic substances made in photosynthesis. It is made of sieve tube element cells which have perforated end walls and lack most organelles to make the transport of sugars easier and companion cells which contain organelles to provide resources to the sieve tube elements. Phloem sieve tubes are made from the stem cells in the meristem
• what are stem cells

  Stem cells are undifferentiated cells that can self-renew and become specialised
• what are the 4 types of stem cells

  • Totipotent
  • Pluripotent
  • Multipotent
  • Unipotent
• totipotent cells


  Totipotent cells can divide and produce any type of body cell. During development, totipotent cells translate only part of their DNA resulting in cell specialisation. Totipotent cells occur only for a limited time in early mammalian embryos
• pluripotent cells


  Pluripotent cells are found in embryos and can become almost any type of cell. For this reason, they are used in research however there are some issues as sometimes the treatment does not work or the stem cells continually divide creating tumours
• multipotent and unipotent cells

  Multipotent and unipotent cells are found in mature mammals and can divide to form a limited number of cells

  Multipotent cells such as in the bone marrow can differentiate into a limited number of cells whereas unipotent cells can only differentiate into one type of cell
• potential uses of stem cells

  Stem cells can be used in both research and medicine

  • repairing damaged tissues
  • Treatments of neurological diseases e.g Alzheimers and Parkinsons
  • Research into developmental biology