Cell continuity

• existing cells divide and produce new cells
• Chromosomes and the genetic material

  All the genetic material in an organism is called its genome. When cells produce new cell cells, it must pass on the right genetic material to each cell. DNA is found in the nucleus and organise into chromosomes. All cells and organisms of the same species have the same number of chromosomes
• Chromosomes.

  • thread like structures in the nucleus
  • Composed of DNA and protein
  • Carry thousands of genes

  In a non-dividing cell, the chromosomes are uncoiled and form a tangled mass called chromatin. When cells are dividing the chromosomes contract and thicken to clearly distinguishable chromosomes
• Haploid cells

  • Contains one of each type of chromosome
  • The letter ‘n’ is used to represent a set of chromosomes
• diploid cells

  • Contains two of each type of chromosome
  • One set of chromosomes comes from the egg and one from the sperm at fertilisation
  • Represented as ‘2n’
• Interphase

  • Cell grows and increases in size
  • It syntheses molecules eg proteins, lipids
  • Organelles are replicated e.g. mitochondria, chloroplast, ribosomes
  • DNA uncoils
  • DNA replicates
  • Each chromosomes make an identical copy of itself
• Division

  • Nucleus divides (mitosis)
  • Cytoplasm divides (cellular division)
• Mitosis and cell division

  During growth and cell multiplication, the number of chromosomes per cell must be kept constant. this is the function of mitosis.
• Mitosis produces two identical daughter cells. (They have the same number and same type of chromosomes as the original parent nucleus)

  haploid and diploid cells can divide by mitosis
• Mitosis is a continuous process, but it may be divided into four stages: prophase, metaphase, anaphase, telophase . during interphase the daughter cells grow and prepare to divide again
• Interphase

  • cell is not dividing.
  • Chromosomes uncoil and are not visible
  • DNA replicates
  • Molecules and new organelles are synthesised
• Prophase

  • Chromosomes are coiled and appear as threads
  • Each chromosome has replicated and appears as two identical chromosomes
  • Spindle fibres are formed
  • Nuclear membrane breaks down
  • chromosomes lineup on the equator
  • Centromere are attached to spindle fibres from each side of the cell

  
• Anaphase

  • Centromeres split
  • Spindle fibres contract
  • Identical chromosomes separate and pulled to the opposite poles of each other

  
• Telophase

  • Chromosomes uncoil and begun to revert to chromatin
  • nuclear membranes form
  • nuclei formed
  • during telophase, division of cytoplasm is taking place Animal cell: cytoplasm is divided by a cleavage furrow forming along the equator, cell pinched in two🔽

  
• telophase 


  Plant cell: cytoplasm divided by a cell plate forming across the middle. Cell membranes and cell walls are formed on each side of this plate

  
• Results of mitosis

  • Two identical daughter cells
  • Chromosome number is constant
  • Identical genetic material and information passes into each new cell
• Role of mitosis

  1. In unicellular organisms e.g. amoeba, mitosis allows the organism to reproduce (asexual reproduction)cells produced genetically identical
  2. in multicellular organisms, mitosis occurs during

  • growth: zugote
  • replacement of cells
  • repair of tissues
• Location of mitosis


  Flowering plants: root tip, shoot tip, meristems, buds

  Animals: skin, bone marrow
• Meiosis

  results

  1. Diploid nucleus divided by meiosis to form 4 daughter cells with half the number of chromosomes present in the parent cells
  2. daughter cells genetically different

  
• Results of meiosis

  1. in sexual reproduction, gamete fuse to form a zygote. Gamete nuclei must have half the number of chromosomes present in parent diploid nuclei I.e. gametes must be haploid.

  
• Results of meiosis


  2. Genetic variation

  During meiosis, genes are separated producing different arrangements every time. daughter nuclei are always different.
• Difference between mitosis and meiosis

  Mitosis

  1. Number of chromosomes stay constant
  2. Daughter cells are identical

  Meiosis

  1. Number of chromosomes is halved
  2. Daughter cells are different
• Cancer


  Manufactures control the cell cycle, when this control is lost cancer cells are formed
• Cancer may be defined as

  Group of disorders of the body in which cells loose normal control of mitosis
• Cancer cells form tumours

  • Benign tumours

  the cancer cells do not move and invade other tissues. These tumours are generally not like threading.

  • Malignant tumours

  The cancer cells spread to other tissues and may be life threatening. The spread of cancer Cells is called metastasis
• Causes of cancer


  Mutations (changes in genetic material) can change normal cells into cancer cells. Cancer is caused by mutagens.

  eg radiation, chemicals