Cell Division

• Cell Cycle


  Sequence of events that results in the division of the cell, forming 2 genetically identical daughter cells

  
• 3 key stages of the cell cycle

  - Interphase (G1, S, G2)

  - Metaphase (mitosis or meiosis)

  - Cytokinesis

  
• Interphase


  The cell does not divide continuously, but is an active phase of the cell cycle. It includes DNA replication and checking for errors, protein synthesis in the cytoplasm

  It is the longest phase of the cell cycle
• G1 (Growth 1) Phase


  Protein synthesis occurs

  Organelles replicate

  Organelles increase in size

  Cell is checked for correct nutrients, no damaged DNA. If the cell doesn't pass these checks, replication will not continue
• S (Synthesis) Phase


  DNA is replicated
• G2 (Growth 2) Phase

  Cell continues to grow in size

  Energy store increases

  Replicated DNA is checked for copying errors
• G0 Phase


  Cell leaves the cell cycle temporarily or permanently.

  - DNA may be damaged and can no longer divide so enters a period of permanent cell arrest

  - Cell may become specialised to carry out a specific function and is no longer able to divide

  - Growing number of senescent cells because as you age the number of cells in your body increases
• Checkpoints in the Cell Cycle

  Checkpoints are control mechanisms. They monitor the processes at each phase have been accurately completely before going into the next phase. For example, checking there are no errors with DNA replication
• Spindle Assembly Checkpoint


  Checks if every chromosome is attached to a spindle fibre and have aligned
• Mitosis - Significance


  Creates 2 identical diploid cells. It is used for growth, tissue repair and asexual reproduction in plants, animals and fungi
• 4 Key Stages of Mitosis


  PMAT

  - Prophase

  - Metaphase

  - Anaphase

  - Telophase
• Prophase


  - Chromosomes condense and become visible

  - In animal cells, centrioles separate to opposite poles of the cell

  - Centrioles create spindle fibres

  - At the end of prophase, the nuclear envelope has disappeared

  - Plants have spindle fibres, but no centrioles

  
• Metaphase


  - Chromosomes align at the equator of the cell (forming a metaphase plate)

  - Centrioles release spindle fibres and attach to centromere and chromatids

  - Spindle assembly checkpoint happens here to check if every chromosome is attached to a spindle fibre before mitosis can continue to anaphase

  
• Anaphase


  - Spindle fibres start to shorten and move towards the centrioles, pulling the centromere and centrioles to opposite poles.

  - The individual chromatids are pulled to opposite poles

  - THIS STAGE REQUIRES ENERGY IN THE FORM OF ATP (RESPIRATION)

  
• Telophase


  - Chromosomes are now at each pole and become longer and thinner

  - Spindle fibre disintegrates

  - Nuclear envelope reforms
• Cytokinesis


  - The cytoplasm splits into 2 genetically identical cells

  In animals: Cleavage furrow forms in the middle of the cell and the cytoskeleton causing the cell membrane to draw inwards until the cell splits in two

  In plants: Cell membrane splits into 2 new cells due to the fusing of vesicles from the Golgi apparatus. The cell wall forms new sections around the membrane
• Meiosis


  2 nuclear divisions occur in meiosis which results in 4, genetically different haploid daughter cells. It contains half the number of chromosomes of the parent cell

  
• Prophase I


  - Chromosomes condense and become visible

  - Crossing over takes place between non-sister chromatids and alleles are exchanged between the homologous chromosomes

  - Centrioles release spindle fibres and attach to centromere and chromatids

  - The nuclear envelope disintegrates

  
• Metaphase I


  - The homologous pairs line up on the equator (forming a metaphase plate)

  - Centrioles release spindle fibres and attach to centromere and chromatids

  - The homologous pairs are arranged randomly (this is independent assortment)

  
• Anaphase I


  - The spindle fibres shorten and move towards the centrioles, pulling the centromere and centrioles to opposite poles.

  - The chiasmata between the homologous chromosomes break and they are pulled to opposite poles of the cell

  
• Telophase I + Cytokinesis


  - Chromosomes are now at each pole and become longer and thinner

  - Spindle fibre disintegrates

  - Nuclear envelope reforms

  - The cytoplasm splits

  
• Prophase II


  - Chromosomes condense and become visible again

  - Centrioles release spindle fibres and attach to centromere and chromatids

  
• Metaphase II


  Chromosomes align at the equator of the cell (forming a metaphase plate)

  
• Anaphase II


  Spindle fibres start to shorten and move towards the centrioles, pulling the centromere and centrioles to opposite poles.

  - The individual chromatids are pulled to opposite poles

  
• Telophase II + Cytokinesis


  - Chromosomes are now at each pole and become longer and thinner

  - Spindle fibre disintegrates

  - Nuclear envelope reforms

  The cytoplasm splits and 4 genetically different cells are formed
• Significance of Meiosis


  - Increases genetic variation by the process of fertilisation

  - Haploid cells fuse to form a zygote which contains the diploid number of chromosomes
• Crossing Over


  - During Prophase I, the homologous chromosomes pair to form bivalents (crossing over occurs between non-sister chromatids of bivalents)

  - The point at which the chromatids join is called chiasmata

  - Breaks can occur in genetic material where the chromatids cross over and parts of the chromatids are exchanged between homologous pairs

  - This results in a new combination of alleles

  
• Independent Assortment


  - During Metaphase I, the homologous pairs of chromosomes randomly line up opposite each other on either side of the equator

  - There are 23 different homologous pairs, this means there is there are 2^23 number of ways the pairs could assort themselves

  - As a result, each gamete receives different combinations of maternal and paternal chromosomes

  
• Erythrocytes - Specialisation


  - Biconcave shape which increases surface area for diffusion and to increase the cell flexibility for it to fit through narrow capillaries

  - No nucleus so there is more space to hold haemoglobin to increase transport of oxygen

  - Made from stem cells in the bone marrow
• Neutrophils - Specialisation


  - Has a lobed nucleus

  - The cells are flexible to enable them to surround pathogens and engulf them. They contain lysozymes filled with hydrolytic enzymes

  - Made from stem cells in the bone marrow
• Sperm Cells - Specialisation


  - Flagellum contains lots of mitochondria to release energy to allow the sperm to move to the egg cell

  - The acrosome in the head of the sperm cell contains digestive enzymes so that the sperm can penetrate and fertilise the egg cell
• Palisade Cells - Specialisation


  - Found in mesophyll tissue layer of leaves

  - They are rectangular, tightly packed cells that contain many chloroplasts to absorb and maximise light for photosynthesis

  - Thin cell walls to reduce diffusion distance of carbon dioxide
• Guard cells - Specialisation


  - Have flexible walls (more so on one side) which results in cell bending when turgid to open the stomata and closing when flaccid and this helps control water loss by transpiration
• Squamous Epithelial Cells - Specialisation


  - Single layer of flat cells

  - Provides a short diffusion distance
• Ciliated Epithelial Cells - Specialisation


  - Hair-like projections that sway and move substances, such as mucus, away from the lungs or egg

  - Goblet cells are also located within the epithelium which release mucus to trap molecules
• Cartilage - Organisation


  - Connective tissue that is firm and flexible

  - Provides structural support

  - Made up of elastin and collagen fibres
• Muscle - Organisation

  - Composed of tissues that contract and relax to create movement
• Xylem Tissue - Organisation


  - Responsible for transporting water and mineral ions

  - Tissue is made up of elongated, hollow dead cells with lignin in the walls to strengthen and waterproof the walls

  - Xylem tissue is made from stem cells in the meristem
• Phloem Tissue - Organisation


  - Responsible for transporting organic substances made in photosynthesis

  - Made of sieve tube element cells. They lack most organelles to make transporting sugars easier

  - Phloem sieve tubes are made from stem cells in the meristem
• Stem Cells


  Stem cells are unspecialised cells that can self-renew (continually divide) to become specialised