The Cell Cycle

• Cell theory: cells come from pre-existing cells.
• Prokaryotes (bacteria) divide through a process called binary fission to form 2 identical organisms
• eukaryotes divide through mitosis and meiosis
• mitosis results in 2 identical “daughter“ cells
• meiosis results in 2 unique cells
• most cellular division results in genetically identical cells (except meiosis) meaning DNA is copied directly from parents cells to daughter cells
• somatic cells are body cells such as skin cells, liver cells, and nerve cells
• each somatic cell has 46 total chromosomes paired into 23 pairs
• every cell in a human is a somatic cell except for sex cells
• each body cell inherits 23 chromosomes from each parent
• gametes are sex cells
• the two types of sex cells are sperm (male) and ovum (female)
• sex cells divide through meiosis
• all sex cells are genetically unique, and each sex cell has 23 total chromosomes
• a species whose organisms are all genetically identical will die out quickly due to lack of diversity; no evolution will occur
• a chromosome is made of a linear piece of DNA wrapped around a histone
• the amount of chromosomes varies between all organisms, but the number does not affect complexity
• about 90% of a cell’s life cycle is spent in interphase
• interphase precedes mitosis, and it is the stage where the cell is preparing for cell division
• G1 phase is the growth phase in which the cell replicates its organelles. the cell must pass through a checkpoint to test the new organelles. if the checkpoint fails, the cell will undergo apoptosis. if the checkpoint succeeds, the cell will enter the next phase.
• S phase is the synthesis phase in which the cell replicates its entire genome
• G2 phase is the DNA checkpoint that tests the replicated DNA and makes sure it was copied correctly. it is the last phase the cell enters before mitosis.
• cells must always double up before splitting
• a DNA strand is antiparallel to the other strand, meaning that the bases are on the opposite sides of the double helix
• the 2 groups of DNA nitrogenous bases are purines and pyrimidines
• a purine base always attaches to a pyrimidine base
• DNA replication is semi-conservative, meaning that each new strand is made from a copy of the original strand and a new strand
• bacterial chromosomes are circular
• all DNA replication begins on an origin of replication (ori)
• bacterial chromosomes have 1 ori due to the small size of a bacteria
• eukaryotes have multiple oris
• adenine and guanine are purines while tyrosine and cytosine are pyrimidines
• helicase is an enzyme that unzips DNA double helix by breaking hydrogen bonds to expose bases
• SSBPs, aka single strand binding proteins, bind to “unzipped” DNA strands preventing the strands from floating away or reannealing
• for a DNA to anneal means for DNA to combine into a double helix
• topoisomerase is an enzyme that relieves stress on stands being held by SSBPs by briefly breaking bonds in the backbone
• primase synthesizes RNA primers for the lagging strand
• DNA polymerase synthesizes a new daughter strand of DNA in a 5’ -> 3’ direction only
• DNA polymerase I removes RNA primers and replaces primers with DNA to form the lagging strand
• DNA polymerase III adds nucleotides in a 5’ -> 3’ direction to form the leading strand