dna replication and mitosis

dna replication

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