AS-LEVEL AQA BIOLOGY NOTES 
TOPIC 4: all cells arise from other cells

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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

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

The Main Stages of Mitosis

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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

Binary fission

• Most common method of cell division in prokaryotes
  
• Asexual
  
• Genetic material is duplicated
  
• Cell splits into 2
  

Viruses

• Non-living therefore do not replicate
  
• RNA injected into host cell
  
• Host cell replicates viral genetic material