Membrane Systems and Organelles of Animal Cells

Membrane Systems and Organelles of Animal Cells

1. Nucleus

   • Largest and most obvious organelle in cells
   • 10-25um in diameter
   • DNA in chromosomes: contains DNA in close association with histones
   • Histones: help support and protect DNA
   • DNA diffusely organised in a form, chromatin
   • Heterochromatin: Densely packed (dark under EM)
   • Euchromatin: Less densely packed (lighter under EM)
   • Nucleoli
     • 1-3 um
     • Under EM, appears darker than chromatin and is a more discrete structure
     • Contains DNA that codes for ribosomal RNA (rRNA)
       • Makes ribosomes (organelles essential in the process of protein synthesis
         • Once produced, these are transported out of the nucleolus, through the nucleus and into the cytoplasm
   • Control centre of the cell
     • DNA in chromosomes codes for the synthesis of proteins in the cytoplasm
       • Islolating chromosomes (DNA) from cytoplasm and its reactions, DNA is protected from damage
         • ‘DNA code’ for protein synthesis needs to be moved to cytoplasm where protein synthesis takes place and other molecules need to enter nucleus from cytoplasm (nuclear pores in nuclear envelope)
           • Nuclear envelope is a double membrane
             • Outer membrane encrusted with ribosomes (site of RER)
               • mRNA (carrying genetic code from DNA) passes through nuclear pores into cytoplasm