AS-LEVEL OCR BIOLOGY NOTES 
TOPIC 6: cell division, cell diversity and cell organisation

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1. the cell cycle

• G0 , G1
  • Biosynthesis
  • Growth phase
  • Biosynthetic activities resume at a high rate
  • Amino acids used to form millions of proteins
  • Proteins required in DNA replication
  • Chromosomes still exist as chromatin
  • DNA replication has not yet occurred

• S
  
  • Synthesis of new DNA
    
  • DNA replication begins
    
  • When completed, all chromosomes are replicated
    
  • Occurs very quickly
    
• G2
  
  • Cell continues to grow
    
  • Proof-reading enzymes check the DNA
    
  • Check to see if the chromosomes have been replicated properly
    
• M
  
  • Nuclear division
    
  • The process of mitosis is carried out
    
•         
  
• Copying and Separating
  
  • Molecules of DNA wrapped around proteins called histones
    
    • Allows DNA to be highly condenses
      
    • Compact and condensed DNA is able to fit inside the nucleus
      
  • DNA and histone protein together are called chromatin
    
    • Chromatin thread about 30nm thick
      
  • Before cell division, DNA of each chromosome must be replicated
    
    • 2 replicas produced, both exact copies of the original
      
    • Held together at the centromere
      

2. how the cell cycle is regulated

 ​​​To avoid abnormal cell activity that can lead to, for example, tumour growth, the cell cycle has three checkpoints to regulate itself.

• G2 checkpoint
  
  • Checks for cell size and DNA replication
    
• M checkpoint
  
  • Checks that chromosomes appropriately attached to spindle
    
• G1 checkpoint
  
  • Checks cell size, nutrient availability, growth factor availability and confirms intact DNA

3. the main stages of mitosis

​Mitosis only occupies small percentage of cell cycle. The remaining time is used for copying and checking of genetic information. Mitosis is the process of cell division which results in two daughter cells which are identical to each other and to the parent cell.

Interphase

• DNA replication
  
• Organelle doubling
  
• Proteins made
  
• Cell ‘double checks’ for mutations
  

Prophase

• Replicated chromosomes super coil (shorten and thicken)
  
• Chromosomes already replicated in Interphase
  
• Chromosomes shorten and thicken
  
• Pair of sister chromatids can be seen using a light microscope
  
• Nuclear envelope breaks down and disappears
  
• Centrioles divides in 2, forming 2 daughter centrioles
  
  • Each daughter centrioles moves to opposite ends of cell
    
• This forms the spindle
  
• This is ‘threads’ of protein (look like latitude lines on a globe)
  
• Nuclear membrane breaks down
  

Metaphase

• Replicated chromosomes line up down middle of ell
  
• Move to central region of spindle
  
• Each becomes to spindle thread by its centromere
  

Anaphase

• Replicas of each chromosome pulled apart
  
• Towards opposite ends of the cell
  
• Sister chromatids separated as centromere splits
  
• Each sister chromosomes effectively become individual chromosomes
  
• Each one genetically identical to the parent cell from which it was copied
  
• Spindle fibers shorten
  
• This pulls sister chromatids further and further away from each other
  
• Form a v-shape as spindle fibers, joined to the centromere, lead
  

Telophase

• Two nuclei formed
  
• As each sister chromatids reaches the pole of the cells, new nuclear envelope forms around each set
  
• Spindle breaks down and disappears
  
• Chromosomes uncoil
  
• Can no longer be seen under a light microscope
  

Cytokinesis

• Cytoplasm and surface membrane divide
  

Animal Cells	Plant Cells
Capable of mitosis and cytokinesis Cytokinesis starts from outside (‘nipping’ in the cell)	Only meristem cells can divide this way Do not have centrioles Tubulin protein threads made in cytoplasm Cytokinesis starts with the formation of a new cell plate where spindle ‘equator’ was New cell membrane and wall laid down along this plate

4. the significance of mitosis in life cycles

​​

• All organisms need to produce genetically identical daughter cells
  
• Mitosis is incredibly significant in many different settings
  

Asexual Reproduction

• Single-celled organisms divide to produce 2 daughter cells, that are separate organisms
  
• E.g. Paramecium
  
• Some multicellular organisms can produce offspring from parts of the parent plant
  
• E.g. Hydra
  

Growth

• Multi-cellular organisms grow by producing new cells
  
• Each new cell is genetically identical to parent cell
  

Repair

• Damaged cells need to be replaced
  
• New cells need to perform the same function – so need to be identical
  

Replacement

• Red blood cells and skin cells are regularly replaced by new cells

5. the significance of meiosis in life cycles

Meiosis is the process of cell division which produced four daughter cells which are not identical - they are different from each other. They have half the number of chromosomes as the parent cell, making them haploid. The process involves 2 nuclear divisions.


Importance

• Takes place in sex organs
  
• Gametes produced here
  
• Important to have genetically different gametes
  
• This promotes genetic variation and allows for Natural Selection to take place

6. stages of meiosis

Interphase I to telophase I and cytokinesis I are very similar as in mitosis. The main difference is that during prophase I, recombination occurs. This is what creates genetic diversity and involves chromatids swapping genes between themselves. Recombination ensures that the four daughter cells are not genetically identical.

The first division creates two daughter cells; the second division which leads to the production of 4 daughter cells.
                    
Prophase II

• Chromosomes pair up
  
• Pair of sister chromatids can be seen using a light microscope
  
• Centrioles divides in 2, forming 2 daughter centrioles
  
• Nuclear membrane breaks down
  

Metaphase II

• Replicated chromosomes line up down middle of cell
  
• Centrioles move to opposite poles of the cell, forming spindle
  
• Chromosomes move to central region of spindle
  
• Each becomes to spindle thread by its centromere
  

Anaphase II

• Sister chromosomes pulled apart towards opposite ends of the cell
  
• Sister chromatids separated as centromere splits
  
• Each sister chromosomes effectively become individual chromosomes
  
• Each one genetically unique
  
• Spindle fibers shorten
  
• This pulls sister chromatids further and further away from each other
  

Telophase II

• New nuclear envelope forms around each chromatid
  
• Spindle breaks down and disappears
  
• Chromatids uncoil and can no longer be seen under a light microscope
  

Cytokinesis II

• Cytoplasm and surface membrane divide, creating four independent haploid daughter cells.
  

7. cells of multicellular organisms and their specialised functions

Within organisms, cells differ. Some cells perform one role very well: they become specialized at this role. Other cells are specialized in other roles. This Specialisation of cells is referred to as ‘differentiation’. Cells can differentiate in many ways, including:

• Different numbers of a particular organelle
  
• Varying shape of cell
  
• Varying contents of cell
  

Differentiation may include all three forms of change.

CELL TYPE	SPECIALISATION
Erythrocytes	Come from undifferentiated stem cells in bone marrow Cells lose all organelles Bi-concave shape Maximizes oxygen carrying capacity Maximizes space for haemoglobin
Neutrophils	Lobed nucleus Granular cytoplasm contain lots of lysosomes Potent enzymes in lysosomes specialized to kill microorganisms
Epithelial Cells	Cover external and internal surfaces 2 types Squamous cells Ciliated cells
Sperm Cells	Many mitochondria generate energy for movement Sperm head contain specialized lysosomes to break down wall of egg Small, long and thin to ease movement Tail helps to propel the sperm Diploid, in order to fulfill role as gamete
Palisade Cells	Long shape maximizes light absorption Contain large amounts of chlorophyll Specialized for photosynthesis
Root Hair Cells	Hair like projections Greatly increase surface area Root able to absorb more water and minerals from soil
Guard Cells	Spiral thickenings of cellulose When turgid, cell opens When flaccid, stoma closes Controlling passage of gases

8. tissues, organs and organ systems

Tissue = a group of similar, specialised cells in a multicellular organism, which collectively are able to carry out a specific or several specific functions.
Organ = group of tissues combined to form a distinct structure and which collectively might carry out a specific function which has an effect on the entire organism, e.g lungs, liver, heart
Organ system = collection of organs with related and interdependent functions, e.g. cardiovascular and digestive systems.

Examples of tissues:
Squamous epithelia

• Found in lining of oral cavities, blood vessels, alveoli of lung, skin epithelium, and other places
  
• Some variants of squamous epithelium are thin and lubricated to enable efficient gas exchange, e.g. in lung
  
• Others are reinforced with keratin to improve resistance to friction and infection, e.g. the skin
  

Ciliated epithelium

• Found in upper airway tract (trachea) and fallopian tubes
  
• Cells constituting this type of epithelium are specialised with large number of cilia
  
• They are able to waft material in a single direction, e.g. pathogens and mucus away from the lungs and towards the oral cavity, or ova away from the ovary and towards the uterus
  

Cartilage

• Cells in cartilage are specialised to resist pressure and provide lubrication for joint movement
  

Muscle

• Cells in muscle (myocytes) can store and convert large amounts of energy
  

Phloem

• Transports glucose and some other compounds up and down the plant stem - is highly specialised to ensure efficient movement of sugar to areas of the plant that need it most
  

Xylem

• transports water up the stem of a plant
  
• Highly specialised to transport water against the direction of the force of gravity ​

9. stem cells

Stem cells are a renewing source of undifferentiated cells, capable of becoming differentiated to a number of different possible cell types. They can also undergo repeated divisions. There are 3 types of stem cell.

Multipotent	​Forms cells of tissue in which they were formed
Omnipotent	Only forms one type of tissue
Pluripotent	Can develop into every cell type in body

10. erythrocytes & neutrophils derived from stem cells in bone marrow

All blood cells are produced  in the bone marrow from undifferentiated cells; Each potentially able to carry out same function. Each begins with same number of chromosomes, but eventually play different roles.
Erythrocytes

• Red blood cells
  
• Cell loose nucleus, mitochondria, Golgi apparatus and rough E.R
  
• Packed full of hemoglobin
  
• Shape changes, forming biconcave discs
  
• This allows them to transport oxygen
  

Neutrophils

• A type of white blood cell
  
• Keep nucleus
  
• Cytoplasm appears granular
  
• Large number of lysosomes produced
  
• This aids their role of ingesting invading microorganisms

11. xylem vessels & phloem sieve tubes

In plants, stem cells are found in the growing tip and the cambium. They are produced from meristem cells in the meristem, and also found in roots.                
Cambium

• Lies in between xylem and phloem
  
• Layer of meristem cells
  
• Divide to produce new xylem and phloem
  

Xylem

• Transports water and minerals within plant
  
• Meristem cells produce small cells which elongate
  
• Walls are reinforced with waterproof lignin
  
• Ends of cells break down
  
• This forms a long continuous tubes with a wide lumen
  
• Provides support for the plant
  

Phloem

• Transports products of photosynthesis within a plant
  
• Structure consists of sieve tubes and companion cells
  
• Meristem tissue produces cells that elongate and line up to form end to end tubes
  
• Ends do not break down completely
  
• Form sieve plates in between cells
  
• Sieve plates allow for the movement of material up or down tube
  
• Next to each sieve cell is companion cell, providing support ​

12. the potential of stem cells in research & medicine

Stem cells represent a huge interest in scientific research due to their potential in medical therapies.

• Bone marrow transplants
  

• Already widely used
  
• Bone marrow is major source of stem cells - especially stem cells capable of becoming blood cells
  
  • E.g. red blood cells and white blood cells
    
• Used therapeutically to treat blood and immune disorders
  

Drug research

• In theory, stem cells can be used to grow artificial tissues
  
• Drugs can then be tested on these tissues before being tested on human subjects
  

Developmental biology

• Stem cells provide insight into embryological development
  
• Important information about how our organs are formed
  
• Can explain why they fail, or have abnormalities
  
• This information can be used to improve medicine
  

Replacement of lost or damaged tissues

• ‘The dream’ of stem cell research
  
• Mice have been treated for Type 1 diabetes mellitus with artificial pancreatic cells which secrete insulin normally
  
• Stem cells could also be used to grow into specific organs or even limbs
  
• They have the potential to treat many conditions