WEEK 4 - CELL THEORY + PROKARYOTE AND EUKARYOTE + VIRUSES + PRIONS + VIROIDS

CELL THEORY



Robert Hooke

plant cell

• In 1665, he discovered a honeycomb-like structure in a cork slice by using a primitive microscope
• He saw cell walls as a dead tissue then he started introducing the word cell to world. 
• He also published the first ever scientific bestseller; Micrographia 

drawings of crystal take from
frozen urine

illustration of his compound
microscope

illustration is bee's stings which
have barbs at the end

Anton van Leeuwenhoek 

animalcules, bacteria and spermatozoa

• In 1674, he discovered animalcules meaning tiny animals when he described the algae spirogyra under microscope. He also discovered other cell in his illustration:



a cross- sectional view
of a nerve fiber  

drawings of shape and size
red blood cells

Theodore Schwann

• In 1836, he discovered the pepsin, digestive enzymes
• He also concluded that all animals are made up of cells

animal cells

Rudolf Virchow

• In 1855, he stated the only source for a living cell was from another living cell 

3 main parts of cell theory:

1. all living things are made up from cells
2. the cells can perform all the function of life which is carry out life processes 
3. all cells are came from pre-existing cells

Exceptions to cell theory:

1. viruses
2. prions
3. subcellular organelles which carry info and replicate independently
4. multinucleated single cell;muscle cells

Modern cell theory:

1. the cell contains hereditary information which is passed on from cell to cell during cell division
2. all cells are basically the same in chemical composition and metabolic activities
3. all basic chemical and physiological functions are carried out inside the cells
4. cell activities depends on the activities of subcellular structures within the cell

PROKARYOTE VS EUKARYOTE

VIRUSES

Properties:

• no membranes, cytoplasm, ribosomes or other cellular components
• cannot move or grow but can reproduce inside a host cell-Obligate intracellular parasites
• consist of 2 major parts -  a protein coat and hereditary (DNA or RNA)
• nucleic acid surrounded by a protective protein coat called a capsid
• some also surrounded by an outer membranous layer called envelope, made of lipid and protein
• come in a wide range of shapes
• can infect plants, animals, human and even bacteria and they attack only specific to their host

can be classified according to their:

1. Genetic material                                            
2. Virus shape
3. Symmetry of the capsid
4. Presence or absence of the envelope
5. Type of the host

viruses reproduce in two ways

PRIONS

• an infectious agent that is composed primarily of protein
• propagate by transmitting a mis-folded protein state
• the process is dependent on the presence of the polypeptide in the host organism
• implicated in a number of diseases in a variety of mammals
• prion diseases - alzheimer's and parkinson's diseases, mad cow disease

VIROIDS

viroids can cause severely misshaped potatoes

• small circular RNA molecules without a protein coat
• can contain as few as 250 nucleotides
• infect plants
• induced diseases lead to dramatic economic losses in agriculture and horticulture worldwide