UNIT 2 ; section 4 the cell cycle + differentiation

1. the enzyme DNA helicase breaks the h-bonds between the 2 polynucleotide strands = 2 single strands

2. original strands act as template strands. free DNA nucleotides join to the template strand through specific base pairing

3. nucleotides on the new strand joined together by DNA polymerase . h-bonds form between the bases on the original and template strand 

 each new dna molecule has one original strand + one new one 

SEMI - CONSERVATIVE REPLICATION 
As half of the new dna molecules are from the orignal piece of dna

all body cells from multi cellular organisms use to grow + divide 

consists of a period of cell growth + dna replication  = interphase 

a period of cell division = mitosis 

interphase is seperated into 3 seperate growth stages G1, S and G2 

G1 = cells grow + new organelles made 
S= cell replicates dna 
G2= cell keeps growing + prepares for division 

Interphase
cell carries out normal functions and prepares to divide
dna is unravelled + replicated to double its genetic content 
the organelles are also replicated so it has spare ones + ATP content is increased 

2 types of cell divisions - mitosis + meiosis

mitosos occurs during the cell cycle

a parent cell divides to form 2 genetically identical daughter cells 

growth/repairing damaged cells / tissues 

chromosomes are made of 2 strands joined in the middle by a centromere 

seperate strands are called chromatids

2 strands on the same chromosome are called sister chromosomes 

in mitosis , 1 stranded chromatids end up as one stranded chromosomes in the new daughter cells 

1. Interphase - the cell is not dividing , carries out normal functions , prepares to divide

2. Prophase - chromosomes condense geting shorter+ fatter. Centrioles begin to move to opposite ends of cells. nucleur envelope breaks down + chromosomes lie free in the cytoplasm  

3. Metaphase - the chromosomes each with 2 chromatids line up along the middle of the cell + become attached to the spindle by their centromere 

4. Anaphase - the centromeres divide, seperating each pair of sister chromatids. The spindles contract, pulling chromatids to opposite ends of the cells , centromere first.

5. Telophase - the chromatids reach the opposite poles on the spindles. they uncoil and become long and thin again, they're now chromosomes again. A nucleur envelope forms around each group of chromosmes = 2 nuclei. The cytoplasm divides and there are now 2 daughter cells that are genetically identical to the original cell and each other.

Each cell goes back into interphase. 

cell growth + division controlled by genes 

if theres a mutation in the gene that controls cell division , the cells grow out of control 

the cells keep dividing which forms a tumour

cancer is tumour that invades surrounding tissues

some treatments disrupt the cell cycle -
these treatments dont distinguish tumour cells from normal cells , so also kill normal body cells that are dividing 
however tumour cells divide much more frequently so are more likely to be killed

G1- (cell growth + production) 
chemical drugs e.g. chemo , prevents synthesis of enzymes needed for dna rep. 
without these the cell us unable to enter the S- phase , disurpting the cell cycle and forcing the cell to kill itself

S phase - (Dna rep.) 
radiation damages the dna , when the cell enters the s-phase it checks for damaged dna and if any is detected it kills itself, preventing further tumour growth 

steps are taken to reduce the impact on normal body cells eg. 
repeated treatments with periods of non treatment in between 
a chunk of tumour is often first removed 

multicellular organisms are made up from many different cell types , eg nerve, wbc, muscle
all cell types are specialised = designed to carry out functions
structure of a specialised cell is adapted to its job
process of becoming specialised = DIFFERENTIATION

tissues = similar cells grouped together , eg. squamous epithelium
organs = groups of different tissues that work together to perform a function eg the lungs
organ systems = organs work together to form organ systems eg. respiratory system