3.1 In the Beginng Revision (SNAB Biology)

pages 102-114 of the snab textbook revision, all the things you need to know. 


Cell Organelles

Prokaryotic Cells 

  • always have a cell wall 
  • very small 
  • bacteria and cyanobacteria 
  • no nuclei 
  • no membrane bound organelles 

Eukaryotic Cells

  • membrane bound organelles
  • not all eukaryotes have a cell wall 
  • mitochondrion: respiration
  • nucelolus: makes ribosomes
  • nucleus: contains DNA and chromosomes (controls protein synthesis)  
  • RER & SER: transports materials to other parts of the cell 
  • Golgi app: modifies and packages proteins 
  • Lysosomes, Ribosomes and Centrioles 
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Protein Production and Route Through the Cell

1. mRNA ---> DNA 

2. mRNA leaves nucleus 

3. Proteins enter rough ER, move through ER assuming 3-D shape en route

4. vesicles pinched to contain protein and fuse to from flattened sacs of golgi app

5. Golgi app modifies protein then vesicles contain modified protein and fuse with CSM releasing protein.  


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How Does Fertilisation Occur?

1. Sperm reach the ovum, which releases chemicals that trigger the acrosome reaction. 
2. Acrosome swells fusing with CSM

3. Digestive enzymes are released that digest through the follicle cells and the zona pellucida
4. Sperm fuses with ovum membrane, and sperm nucleus enters ovum
5. Enzymes thicken ZP so other sperm cant enter, nuclei of ovum and sperm fuse.

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How Gametes Form

Meiosis One: homologous chromosomes pair up then separate. 

Meiosis Two: chromatids seperate and form haploid gametes. 


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Independent Assortment

during meiosis 1 chromosomes line up randomly in different assortments. depending on the ordering of the chromosomes in meiosis 1 the daughter cells will contain different arrangements. 


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Crossing Over

In Meiosis 1, homologous chromosomes pair up and break at chiasmatas where non-sister chromatids exchange corresponding sections of DNA. 

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Fertilisation in Flowering Plants

  • takes place in embryo sac in ovule 
  • pollen grain germinates on style and pollen tube grows down through the style towards the ovary 
  • nucleus divides to form two haploid gamete nuclei (move down pollen tube)
  • tube grows and two (male) gamete nuclei enter sac 
  • one fuses with egg cell (dipoloid zygote), one fuses with two nuclei in embryo sac (triploid cell)
  • diploid zygote --> embryo, triploid cell --> endosperm (seeds storage tissue)
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