dna replication and mitosis

dna replication

* how dna copies exactly to prepare for cell division
* semi-conservative - one new strand, one original to make two new helices

enzymes involved

* helicase
* dna polymerase
* ligase

helicase

* breaks the hydrogen bonds in the double helix, separating the two strands

replication fork

* the place where the helicase is separating the two strands

rna primer

* enzyme called primase attaches to each strand
* primase puts an rna primer as a foundation upon which the polymerase can build
* later, the primer is removed and filled in with dna

dna polymerase

* wraps around the strand at the primer
* attaches new nucleotides to the exposed ones according to the rule of base pairings
* only attaches single nucleotides w/ hydrogen bonds - no backbone yet

base pairing

* a with t, g with c
* two hydrogen bonds between a and t
* three between g and c

ligase

* forms backbone and joins new nucleotides together
* uses atp to link oh (hydroxyl group) on the 3’ to the phosphate group on the 5’ end of another

okazaki fragment

* pieces of new dna that haven’t been connected by ligase yet
* found on the lagging strand

polymerase direction

* only moves from 5’ to 3’
* this creates one leading and one lagging strand bc of antiparallel structure

cell cycle

* the process of a cell growing, replicating dna, going through mitosis, and finally cell division
* stages: g1, synthesis, g2, mitosis, cytokinesis

interphase

* made up of of g1, s, and g2
* cell grows and replicated dna

g1 phase

* growth 1 or gap 1
* cell grows larger
* everything is duplicated except dna
* g1 → s checkpoint: cell size, nutrients, dna damage - becomes irrevokably committed to cell division

s phase

* synthesis
* dna is copied
* two strings of chromatin called sister chromatids - attached duplicates you can’t see because it’s still spaghetti-like
* centrosomes are duplicated

g2 phase

* grows more, makes proteins and organelles, reorganizes contents in preparation for mitosis
* g2 → m checkpoint: no errors in dna replication, dna has been totally replicated
* this is the last chance the cell has to pause for repairs - it may repair damaged dna or finish replicating dna fully. if the damage is too severe, the cell may undergo apoptosis.

mitosis

* process of nucleic division in preparation for cell division
* four stages: prophase, metaphase, anaphase, telophase

prophase

* dna is condensed into chromosomes

mitotic spindle

* formed during early prophase
* made in the centrosome by centrioles
* a structure made out of microtubules (strong fibers)
* grows between the centrosomes as they move apart

prometaphase

* nuclear envelope breaks down, releasing chromosomes
* spindles begin attaching to chromosomes
* they bind to chromosomes at kinetochore (patch of proteins on centromere)

metaphase

* chromosomes are lined up in the middle of the cell, all attached to mitotic spindle microtubules
* both kinetochores on each chromosome are attached to microtubules from each side (one tubules from the top, one tubule from the bottom so they can be pulled apart)
* m checkpoint/spindle checkpoint: making sure all chromosomes are properly attached to the spindle

anaphase

* microtubules pull sister chromatids apart to make each its own chromosome
* they are pulled to opposite poles of the cell

telophase

* cell starts to return to normal
* chromosomes decondense, nuclear membrane returns, spindle disappears
* there are now two nuclei on either side of the cell

cytokinesis

* starts in the final phases of mitosis - may overlap with anaphase or telophase
* cell is pinched in two and finally separate properly