Biology: cell division and cellular organisation


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  • cell cycle consists of interphase, mitosis and cytokinesis. mitosis occupies only about 5% of the time while the rest is used for the duplication and checking of DNA
  • in interphase, DNA replicates so that each chromosome is made up of 2 identical chromatids joined at the centromere
  • during mitosis, nuclear envelope breaks down, spindles form, attach themselves to the centromeres of the condensed chromosomes, and move them to ewautor of the cell. the centromeres then break the skindle fibres pull the chromatids apart to opposite ends of the cell. new nuclear envelopes form around each set of chromosomes. the phases of mitosis are prophase, metaphase, anaphase and telophase
  • during cytokinesis, the cytoplasm splits and 2 new cells are formed
  • mitosis produces 2 new daughter cells that are genetically identical to eachother and to the parent cell 
  • mitosis is used for growth, repair and asexual reproduction
  • multicellular organsims usually contain many different types of cells which have become differentiated to perform different functions. often grouped into tissues cntaining cells that have the same function - eg squamous and ciliated epithelium in animals, xylem and phloem in plants. tissues are grouped into organs, and organs into organ systems
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  • when an animal cell has become differentiated, it is normally unable to become any other type of cell. some cells, however, called stem cells retain the ability to differentiate and divide. stem cells in a young embryo are able to differentiate into any kind of cell; in an adult, stem cells appear to have a limited range of specialised cells that they can form. eg. stem cells in bone marrow produce enthrocytes and leucocytes, cambium produce xylem vessels and phloem sieve tubes. these cells are highly adapted for their functions
  •  cells may divide by meiosis which produces genetically different cells with only half the number of chromosomes of the parent cell. body cells are diploid meaning they have 2 complete sets of chromosomes. matching, or homologous, chromosomes pair up in meiosis and are then shared out into the 4 daughter cells. these are haploid.
  • meiosis is used for production of gametes for sexual reproduction
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Specialised Cells

enthrocytes - red blood cells

  • no nucleus - disintegrates
  • bocincave shape - to increase surface area to volume ratio
  • lots of haemoglobin
  • change shape - flexible to fit around capillaries (squeeze through)
  • very small (less than 1 micrometre)

leucocytes - white blood cells

  • large cytoplasm, lobed nucleus that can change shape with the cell
  • lots of mitochondria to allow endocytis
  • lots of lysosomes or breaking down (neutrophils)

sperm cells

  • acrosome head to break down protection around egg
  • lots of mitochondria for movement
  • flagellum tail for swimming
  • streamlined
  • haploid no. of chromosomes
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Specialised Cells

palisade cells

  • lots of chloroplasts and are mobile (move around)

guard cells

  • thicker wall on one side than other
  • inflates when there is plenty of water - bends into banana shape, opens stoma (hole between 2 guard cells) - become turgid
  • when water is scarce, stoma closes to prevent water loss, guard cells become limp 
  • found mostly on underside of leaves to reduce water loss

all cambium are meristematic but not all meristematic are cambium

vascular cambium tissue can turn into xylem (inside ring) and phloem (outside ring)

xylem - tansport water from root to plant (hollow tubes, rigid walls, non lving, no nucleus)

phloem - uses ATP to carry food (glucose) around the plant (tube, living, 2-way flow, thin layer of plasma membrane)

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covers whole surface of body (internal and external), mad eup of highly packed cells, very small with hardly any gaps but intercellular spaces, provides structural support made up of collagen and glycoprotein. sits on basement membrane (secreted by underlying cells) to bind them to next layer of tissue

squamous epithelia 

  • alveoli, cheek, ventricles, blood vessels
  • not a structured
  • smooth, flat, thin, low friction surface
  • in alveoli, thinness allows rapid diffusion of gases between alveoli and blood

ciliated epithelial cells

  • trachea, bronchi, bronchioles, oviducts
  • causes movement
  • each cilium 3-4 micrometres long, contains microtubules which can slide past eachother causing cilium to bend 
  • sometimes cuboidal, somtimes columnar (tall and narrow)
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