2.1.6 Cell division

• Cell division:

  • Eukaryotic cells - Enter cell and divide by mitosis.
  • Prokaryotic cells - Replicate by binary fission.
  • Viruses - No cell division because they are not living things.
• The cell cycle:

  • Interphase (G1, S, G2)
  • Nuclear division (mitosis or miosis)
  • Cytokinesis.
• Interphase is the longest stage in the cell cycle;

  G1:

  • Protein synthesis occurs (makes proteins involved in synthesising organelles).
  • Organelles replicate.
  • Cell is checked (correct size, nutrients, growth factors, damage to DNA). If this criteria isn't met the cell will not continue on.

  S:

  • DNA is replicated.

  G2:

  • Cell continues to grow, energy stores increase and newly replicated DNA is checked for any errors.
• Mitosis creates two identical diploid cells and is used for growth, tissue repair and asexual reproduction in plants, animals and fungi.
• Mitosis - Prophase:

  • Chromosomes condense and become visible.
  • Centrioles separate and move to opposite poles of the cell.
  • Centrioles make spindle fibres that will make a spindle apparatus.
• Mitosis - Metaphase:

  • Chromosomes align along equator.
  • Spindle fibres are released from centrioles and attach to centromere and chromatids.
  • Spindle assembly checkpoint occurs to ensure every chromosome has attached to a spindle fibre before anaphase.
• Mitosis - Anaphase:

  • Spindle fibres shorten and move towards centrioles. They pull the centromere and chromatids to the opposite poles with them.
  • This makes the centromere divide into two and the chromatids are pulled to opposite ends of the cell. This requires ATP provided by respiration in the mitochondria.
• Mitosis - Telophase:

  • Chromosomes at each end of the pole become longer and thinner.
  • Spindle fibres disintegrate and nuclear membrane reforms.
• Mitosis - Cytokinesis:

  • Animals - A cleavage furrow forms in the middle of the cell causing the cell membrane to draw inwards until the cell splits into two.
  • Plants - Cell membrane splits into two new cells due to fusing of vesicles from Golgi apparatus. The cell wall forms new sections around the membrane to complete the division of the cells.
• Observing Mitosis - Onion and garlic root tips.

  • Thin slice of root tip is placed onto a microscope slide and broken down with a needle. A stain is added to make chromosomes visible and cover slip is pushed down to squash the tip and achieve a single layer of cells.

  Miotic index = (Number of cells in mitosis / Total number of cells) x 100
• Meiosis is two nuclear divisions resulting in four genetically different haploid daughter cells.

  • Haploid (n) - One copy of each chromosome.
  • Diploid (2n) - Two copies of each chromosome.

  Genetic differences are introduced by two processes; Independent assortment of homologous chromosomes and Crossing over.
• Crossing over:

  • During prophase I, homologous chromosomes pair to form bivalents.
  • It occurs between non-sister chromatids of bivalents.
  • Breaks occur in the genetic material where chromatids are exchanged between the pairs, resulting in new combinations of alleles.
• Independent assortment:

  • During metaphase I the homologous pairs line up on opposite ends of the equator. It is random on which side the paternal and maternal pair aligns.
  • There are 23 different homologous pairs meaning there are 2 to the power of 23 ways they could assort themselves. As a result, each gamete has a different combination of maternal and paternal chromosomes.
• Meiosis - Interphase:

  • DNA is copied.
  • S phase.
  • 2n (due to duplication of DNA).
• Meiosis - Prophase I:

  • Homologous pairs pair up.
  • Crossing over occurs.
  • Nucleus breaks down.
  • Centrioles move to poles.
  • Spindles form.
• Meiosis - Metaphase I:

  • Homologous chromosomes line up along equator.
• Meiosis - Anaphase I:

  • Chromosome pairs are split.
  • Chromosomes pulled to either end of the cell.
• Meiosis - Telophase I:

  • Membrane forms around the nucleus and cell splits into two.

  There are now 2 cells with 23 chromosomes each.
• Meiosis - Prophase II:

  • Centrioles move to poles on either end of the cells.
• Meiosis - Metaphase II:

  • Chromosomes line up along the equator.
• Meiosis - Anaphase II:

  • Chromatids pulled to the spindle fibres.
• Meiosis - Telophase II:

  • Nuclear membrane reforms and the cell is split into a further two.

  4 haploid gametes have now been produced (2 egg, 2 sperm).
• The cell cycle - Diagram

  A) Check - Size, Growth factors, Nutrients and DNA errors

  B) Check DNA has been synthesised correctly

  C) Check any DNA errors have been corrected

  D) Check - Spindle fibres assembly

  
• Specialised cells - Erythrocytes:

  • Biconcave shape to increase SA for diffusion and increase cell flexibility to fit through narrow capillaries.
  • No nucleus means more space to hold haemoglobin and increase oxygen supply.
• Specialised cells - Neutrophils:

  • Lobed nucleus and granular cytoplasm allowing for flexibility to surround pathogens and engulf them.
  • Contain lysosomes filled with hydrolytic enzymes.
  • Created from stem cells in bone marrow.
• Specialised cells - Sperm:

  • Flagellum contains many mitochondria to release energy for locomotion to enable the sperm to move towards the egg.
  • Acrosome in head of cell contains digestive enzymes to digest wall of egg cell for penetration and fertilising the egg.
• Specialised cells - Palisade cells:

  • Located in the mesophyll tissue layer of leaves.
  • Rectangular, tightly packed cell containing many chloroplasts to maximise light energy for photosynthesis.
  • Thin cell wall to reduce diffusion distances of carbon dioxide.
• Specialised cells - Guard cells:

  • Flexible walls on one side to help with bending when turgid to open stomata and closing when flaccid. This helps control water loss by transpiration.
• Specialised tissue - Squamous epithelial cells:

  • Single layer of flat cells in contact with basement membrane of epithelium cells. This allows for short diffusion distances.
  • Located in the lining of the lungs.
• Specialised tissue - Ciliated epithelium cells:

  • Hair like projections to sway substances (eg mucus) out of lungs, or and egg in the oviduct.
  • Goblet cells also located with them. They release the mucus to trap molecules (eg trapping dust in trachea).
• Specialised tissue - Cartilage:

  • Connective tissue that is firm and flexible.
  • Located in the outer ear, nose and end of bones.
  • Provides structural support and prevents bones from rubbing together.
  • Made up of elastin, collagen fibres and chondrocyte cells within an extracellular matrix.
• Specialised tissue - Muscle:

  • Composed of tissues that contract and relax to make movement.
  • Skeletal muscles cause the skeleton to move. It's made up of myofibrils containing the proteins actin and myosin.
  • Smooth muscle is in organs and cardiac muscles is in the heart.
  • Muscles have multiple fibres connecting with connective tissues in between.
• Specialised tissue - Xylem:

  • Make up the vascular bundle in plants, responsible for transporting water and mineral ions.
  • Made up of elongated, hollow dead cells with lignin in the walls to strengthen and waterproof them.
  • Created from stem cells in the meristem.
• Specialised tissue - Phloem:

  • Make up the vascular bundle responsible for transporting organic substances in photosynthesis.
  • Made up of sieve tube elements, which have perforated end walls and lack most organelles (making transport of sugars easier) and companion cells, which contain organelles to provide resources for the sieve tube elements.
  • Created from the stem cells in meristems.
• Stem cells are undifferentiated cells that can continuously divide and become specialised.
• Stem cells:

  • Totipotent - Divide and produce any type of body cell. They translate to only one part of their DNA, resulting in differentiation. They only occur for a limited time in early mammalian embryos.
  • Pluripotent - Become any type of cell, excluding extra embryonic tissue. They are found in the embryo and typically used in research with prospect to treat human disorders, however sometimes treatment doesn't work or the cells continuously divide, creating tumours.
• Stem cells:

  • Multipotent and unipotent are found in mature mammals and can divide to form a limited number of different cell types.

  Multipotent cells, such as in bone marrow, can differentiate into a limited number of cells whereas unipotent can only differentiate into one type of cell.
• Potential uses of stem cells:

  • Repairing damaged tissues.
  • Treatment of neurological conditions (eg Alzheimer's and Parkinson's)
  • Research into developmental biology.