3.2.2 All cells arise from other cells

• multicellular organisms not all cells keep their ability to


  divide — the ones that do follow a process called the cell cycle

  
• The cell cycle


  ● Starts when a cell has been produced by cell division ● Ends with the cell dividing to produce two identical cells

  
• The cell cycle consists of a

  ● a period of cell growth and DNA replication — called interphase and ● a period of cell division — called mitosis

  
• Interphase (cell growth) is


  ● subdivided into three separate growth stages — G1— S — G2

  
• Stage 1 : The Cell Cycle — Mitosis


  the cycle starts and ends here

  
• Stage 2 : The Cell Cycle — G1 (Gap Phase 1)


  cell grows and new organelles and proteins are made

  
• Stage 3 : The Cell Cycle — S (Synthesis)


  cell replicates its DNA — ready to divide by mitosis

  
• Stage 4 :The Cell Cycle — G2 (gap phase 2)


  cell keeps growing and proteins needed for cell division are made

  
• During Interphase


  the cell carries out normal functions but also prepares to divide

  
• During interphase the


  ● cell's DNA is unravelled and replicated — to double its genetic content ● The organelles are also replicated — so it has spare ones● cells ATP content is increased

  
• ATP provides

  the energy needed for cell division
• Two types of cell division

  ● mitosis ● meiosis
• Mitosis the form of cell division that occurs during

  the cell cycle

  
• In mitosis


  a parent cell divides to produce two genetically identical daughter cells — they contain an exact copy of the DNA of the parent cell
• Mitosis needed for

  ● the growth of multicellular organisms ● repairing damaged tissues
• The cells in our bodies

  grow and divide
• Mitosis is

  ● really one continuous process● described as a series of division stages— prophase — metaphase — anaphase — telophase
• The chromosomes are made of

  two strands joined in the middle by a centromere ● The separate strands are — called chromatids ● Two strands on the same chromosome are — called sister chromatids

  
• Chromatids


  the separate strands

  
• Sister Chromatids

  Two strands on the same chromosome

  
• Cytokenisis

  in animal cells

  
• Cytokenis

  in plant cells

  
• A plant root tip is constantly growing so you'd expect a high mitotic index


  ● you'd expect a high mitotic index — i.e. lots of cells in mitosis● In other tissue samples , a high mitotic index could mean that tissue repair is taking place or that there is cancerous growth in the tissue
• Stage 1 : Mitosis — Prophase


  ● Centrioles move to opposite ends of the cell● Nuclear envelope starts to break down

  
• Stage 1 : Mitosis — Prophase


  ● The chromosomes condense — getting shorter and fatter ● centrioles start moving to opposite ends of the cell — forming a network of protein fibres across called the spindle● The nuclear envelope breaks down and chromosomes lie free in the cytoplasm

  
• Nuclear envelope


  the membrane around the nucleus
• Centrioles

  Tiny bundles of protein
• There are two strands because


  each chromosome has already made an identical copy of itself during interphase — When mitosis over , the chromatids end up as one strand chromosomes in the new daughter cells

  
• Stage 2 : Mitosis — Metaphase


  ● The chromosomes — each with two chromatids ● line up along the middle of the cell and become attached to the spindle by their centromere

  
• Stage 3 : Mitosis — Anaphase


  ● The centromeres divide — separating each pair of sister chromatids ● The spindles contract — pulling chromatids to opposite poles (ends) of the spindle centromere — this makes the chromatids appear v shaped

  
• Stage 4 : Mitosis — Telophase


  ● The chromatids reach the opposite poles on the spindle — They uncoil and become long and thin again— They're now called chromosomes again● nuclear envelope forms around each group of chromosomes — are now two nuclei

  
• Stage 5 : Mitosis — Cytokinesis

  ● starts in anaphase and finishes at telophase ● There are now two daughter cells that are genetically identical to the original cell and to each other● Mitosis is finished and each daughter cell starts the interphase part of the cell cycle — to get ready for the next round of mitosis

  
• Cytokenisis

  Division of the cytoplasm
• Phases of mitosis

  ● prophase ● metaphase● anaphase ● telophase
• The time taken for each stage of mitosis varies depending on

  the cell type and the environmental conditions
• Example : How long does each stage of mitosis take ?

  
• Mitosis and the cell cycle are controlled by


  genes
• Normally , when cells have divided enough times (to make enough new cells)

  they stop
• If there's a mutation in a gene that controls cell division, the cells can


  grow out of control— cells keep on dividing to make more and more cells , which form a tumour
• Cancer is a

  tumour — that invades surrounding tissue