Mitosis & meiosis

• Mitosis


  Cell nucleus divides to produce 2 daughter nuclei containing sets of chromosomes to parent cell
• Mitosis


  • Produces cells that are exact copy of parent cell
  • Daughter cells have same number of chromosomes & are genetically identical to parent cell
• Chromosome


  • Have a characteristic shape, occur in pairs & carry hereditary material of cell
  • Only visible as discrete structures- long thin threads around 50um long
  • Made up of 2 stands- chromatids- joined at a point called centromere
• Chromatid


  Product of chromosome replication
• DNA


  Held in position by proteins called histones- together form a complex called chromatin
• Humans


  Diploid organisms- chromosomes arranged in homologous pairs- same genes, sequence along their length, not identical- alleles for genes may differ
• Diploid


  • Genetic variation increased- each individual has a mixture of characteristics from each parent
  • If a allele of 1 chromosome of a pair is faulty- 2nd allele may provide back up
• Cell cycle


  1. Interphase- occupies most of cell cycle- resting phase- no division takes place

  2. G1- production of structural & functional proteins, cell metabolic rate high, synthesis of organelles e.g mitochondria, ER- cell growth occurs

  3. Checkpoint- ensures: cell has reached critical size- enough nutrients available for next stage to take place- necessary growth factors in place- no DNA damage- ensure damage will not be replicated in S phase & not passed onto daughter cells

  4. S phase- DNA replication occurs- each chromosome has become 2 chromatids & histones form

  5. G2- spindle proteins synthesised- chromosomes checked for error & repaired

  6. Checkpoint- end of G2- ensures: DNA replication has taken place- no DNA damage (must be repaired before proceeding)
• If requirements are met cell enters S phase, if not met cell enters resting phase- G0 - can remain in G0 for short time, very long time or permanent
• Many WBC- lymphocytes only move out of G0 back into cell cycle when they are triggered to do so by presence of specific antigens
• If cells damaged beyond repair or too old they will not return to cell cycle
• Mitosis


  Nuclear division when nucleus divides into 2 genetically identical daughter cells
• Mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Prophase


  • Chromatin condenses (greater strength to prevent damage during mitosis)
  • Chromosomes become visible
  • Nucleus disappears
  • In animal cells, centrioles move to opposite poles & spindle starts to form
  • Each chromosome visible as 2 chromatids held together by centromere
• Metaphase


  • Nuclear membrane has broken down
  • Spindle formed
  • Centromeres attach to spindle
  • Chromosomes line up along equator of cell
• Anaphase


  • Centromere split
  • Contraction of spindle fibres pulls chromatids apart
  • Chromatids pulled to opposite poles of cell
• Checkpoint at end of metaphase


  • Ensures all chromosomes have attached properly to spindle fibres
  • If requirements met, cell completes mitosis
• Telophase


  • Sister chromatids end up at opposite poles of cell & are referred to as chromosomes
  • Chromosomes decondense
  • Nucleolus reappears
  • Nuclear membrane reforms
  • Spindle disappears
• Centromere


  Attaches chromosomes to spindle fibres & as spindle fibres contract, centromere pulls chromatids to opposite poles
• Cell cycle regulation & cancer

  Caused by uncontrolled cell division
• Uncontrolled cell division

  1. Mutation in genes coding for checkpoint proteins

  2. Cell will continue through cell cycle unchecked

  3. No longer any control mechanisms in place
• Cancer cells

  • Cannot enter G0
  • Involves break down in ability of checkpoints in cell cycle to regulate cell division
• Types of tumour


  • Benign tumour
  • Malignant tumour
• Benign tumour


  • Encapsulated in fibrous sheath
  • Usually stop growing
  • Do not travel to other locations in the body
  • Damaged caused due to pressure on surrounding tissue
• Malignant tumour


  • Grows unchecked
  • Cells from primary tumour can spread via circulatory/lymphatic system
  • Form secondary tumours at other locations around the body
• Drug treatment for cancer


  1. Distrust cell cycle

  2. Stop mitosis
• Chemotherapy

  Use of drugs to treat cancer
• Antimetabolites

  • Acts as S phase inhibitors
  • Preventing DNA synthesis
  • 5-FU inhibits an enzyme involved in making nucleotide that contains base thymine
  • Therefore prevent DNA synthesis
• Vincristine


  • Prevents formation of microtubules
  • Binds to tubulin in microtubules spindle fibre
  • Prevents them from functioning properly
  • Stops chromatids pulling apart
  • Preventing anaphase occurring
  • Drugs described as mitosis poisons
• Other drugs work by preventing DNA to unzip
• • Anti-cancer/chemotherapy drugs can stop ordinary cells dividing
  • Unpleasant side effects
• Cytokinesis

  1. Division of cytoplasm

  2. Follows telophase

  3. Leads to G1
• Preparation for division


  1. Cell organelles become evenly distributed towards 2 poles of telophase cells

  2. Along with chromosomes
• Cytokinesis in plant cells


  1. Vesicles produced by Golgi collect at equator

  2. Vesicles contain carbs

  3. Vesicles fuse together to form cell plate

  4. Eventually stretches across cell

  5. Forms middle lamella

  6. Cellulose builds up on each side of lamella to form cell wall of 2 new plants
• Cytokinesis in plant cells


  1. Spindle disintegrates except at equator where fibres increase in number

  2. Region is called phragmoplast

  3. Organelles move to region

  4. Golgi streets vesicles which line up

  5. Vesicles fuse forming cell plate

  6. Contents form middle lamella made of calcium pectate & new cell wall

  7. Plasmodesmata result where vesicles do not fuse & ER runs through

  8. Plasmodesmata form links of continuous cytoplasm & ER between adjacent cells
• Cytokinesis in animal cells


  1. Microfilaments cause constriction in region previously occupied by equator of spindle

  2. Cleavage furrow forms

  3. Membranes meet & fuse producing 2 cells with evenly distributed organelles
• Differences between cytokinesis in plant & animal cells


  • Plant have region called phragmoplast
  • Plant- Golgi vesicles fuse together to form cell plate
  • Animal- Form cleavage furrow
• Cytokinesis in plants


  1. No centrioles

  2. Spindle form independently - no asters

  3. Cell plate forms due to fusion of Golgi vesicles

  4. No cleavage furrow

  5. Membrane does not move

  6. Location in region of growth
• Cytokinesis in animals


  1. 2 pairs of centrioles during division to start spindle formation

  2. 9 triplet microtubules

  3. Asters present

  4. No cell plate as no cell wall

  5. Cleavage furrow present as it is caused by ring of microfibrils

  6. Located in all tissue
• Functions of mitosis


  • Genetic stability - daughter cells genetically identical to parent cells
  • Growth - increased cell number - basis for growth in most multicellular organisms
  • Cell replacement - replacement of dead skin cells
  • Asexual reproduction - cuttings, runners - 1 parent produced genetically identical offspring