Cell & Nuclear Division

what does mitosis produce?

two genetically identical cells

what is the purpose of mitosis

growth and repair

what does meiosis produce

gametes

what is the purpose of meiosis

reproduction

where does meiosis occur?

only in ovaries and testes

what is binary fission?

cell division in prokaryotes

what is budding?

cell division in fungi

what is a meristem?

area of specialised tissues with stem cells

what are the two types of meristem?

• apical meristem = at root tips/branch tips, causes plants to lengthen

• lateral meristem = lateral side of plants and roots, causes growth in width

what happens in interphase of mitosis?

• dna replicates

• dna is not condensed

• diploid nucleus (2n)

what happens in prophase of mitosis?

• dna supercoils and condenses around histone proteins

  • forming nucleosomes

• the nuclear envelope disintegrates

• mitotic spindle forms

  • centrosome makes microtubules

  • kinetochore microtubule ataches to kinetochore (protein at centromere/link between mother and father copy)

• centrosomes start to move

what happens in metaphase of mitosis?

• chromosomes move to metaphase plate (line up on equator)

• centrosomes reach opposite poles

what happens in anaphase of mitosis?

• this is the shortest phase

• overlap microtubules come from centrosome

• motor proteins bend microtubules in opposite directions

• pushes centrosomes apart, pulling kinetochore and separating sister chromatids

• sister chromatids move to opposite poles

what happens in telophase of mitosis?

• nuclear envelope forms around each set of chromosomes

• chromosomes are no longer condensed

• spindle apparatus start to disappear

• cell is elongated (ready for cytokinesis)

what is cytokinesis?

division of the cytoplasm to form two identical daughter cells

what happens during cytokinesis in animal cells?

actin and myosin proteins form a contractile ring that pinches the cell membrane to split the cytoplasm (outside to inside)

what happens during cytokinesis in plant cells?

a cell plate is built up by vesicles that collect between the poles of the cell and lay down cell membrane and cell walls (inside to outside)

what is meiosis?

• a reduction division (diploid to haploid)

  • makes gametes

what happens during interphase of meiosis?

cell grows and replicates dna making 2 identical sets of chromosomes

what happens during prophase I of meiosis?

• duplicated DNA supercoils around histone proteins

  • chromatin to chromosomes

• nuclear envelope disintegrates

• mitotic spindle forms

  • centrosome builds new microtubules that will be used to pull centrosomes into position

• kinetochore attaches to microtubules

• centrosomes move towards opposite poles of the cell due to lengthening microtubules

• homologous chromosomes (pairs of matching chromosomes, one from each parent) pair up

• crossing over occurs, promoting variation as there is exchange of genetic information between non-sister chromatids

what happens during metaphase I of meiosis?

• homologous chromosomes line up on the cell equator

• chromosome alignment is random = independent assortment, also promotes variation

  • spindle fibres attach randomly to kinetochores so chromosome pair on each side is random

what happens during anaphase I of meiosis?

• homologous chromosomes separate to the opposite poles of the cell

• move due to motor proteins pushing microtubules

what happens during telophase I of meiosis?

• there is a set of chromosomes located at each pole

• a new nuclear envelope forms around each set of chromosomes

• chromosomes are no longer condensed

• spindle apparatus disappears

• cell elongated and ready for cytokinesis

what happens during prophase II of meiosis?

• dna supercoils and condenses around histone proteins

• nuclear envelope disintegrates

• mitotic spindle forms

• centrosomes move towards opposite poles of the cell due to lengthening microtubules

what happens during metaphase II of meiosis?

• chromosomes line up on the equator randomly (random assortment)

• chromosomes move due to action of mitotic spindle

• the centrosomes are at opposite poles of the cell

what happens during anaphase II of meiosis?

sister chromatids separate due to motor proteins pushing microtubules

what happens during telophase II of meiosis?

• a set of chromosomes is located at each pole

• a new nuclear envelope forms around each set of chromosomes

• chromosomes elongate

• spindle apparatus disappears

• cell elongated and ready for cytokinesis

why can unequal cytokinesis occur?

occurs in oogenesis to provide the zygote with the resources it needs to survive, as it maintains most of the original cytoplasm

what is crossing over?

• two non-sister chromatids exchange DNA

• allows for mixing of alleles

what is independent assortment?

• chromosomes line up at the metaphase plate in a random fashion

• so they are sorted into separate cells randomly

what is non-disjunction?

• where there is improper separation of chromosomes during mitosis

• this results in a gamete with an abnormal chromosome number

• if this happens on chromosome 21 (trisomy 21), this is Down’s Syndrome

how does binary fission work?

• the original cell has a single chromosome

• this is duplicated

• the cell splits and the copies of DNA are separated

• two identical cells are created, one with original DNA and one with a copy

how does budding work?

• nucleus duplicates and a bud forms in the cytoplasm

• one nucleus moves into the bud

• the bud closes up so there is a smaller daughter yeast cell and the original yeast cell

what are cyclins?

groups of proteins that controll the cell’s progression through the cell cycle

how do G1 cyclins work?

• in growth phase 1, cyclins bind to cyclin-dependent kinases and phosphorylate and activate them

• high concentrations of activated CDKs triggers DNA replication and cell growth

how do G2 cyclins work?

• in growth phase 2, mitotic cyclins (M cyclins) bind to CDKs and phosphorylate and activate them

• high concentrations of activated CDKs = m-phase promoting factor, triggers mitosis

what does G1 cyclin do specifically?

proofreads DNA for mutations, so concentration remains high for entire interphase

what is the mitotic index?

ratio of cells undergoing mitosis : total cells

MI = cells undergoing mitosis / total number of cells

what is apoptosis?

natural, programmed cell death

what is a mutagen?

an external factor that may trigger a mutation eg smoking or radiation

what are tumour suppressor genes?

code for proteins that regulate the cell cycle (preventing too rapid growth)

what are proto-oncogenes?

genes that can turn into an oncogene, causing the cells to divide