cell cycle, mitosis & meiosis

purpose of cell divison

growth, development & repair in multicellular organisms

what does cell division result in

genetically identical daughter cells — requires exact duplication/equal division of DNA

cellular organization of genetic material

• prokaryotes: genome = single, circular strand of DNA & same circular pieces, plasmids (carry a few genes)

• eukaryotes: genome consists of several linear DNA molecules packaged as chromosomes

humans & chromosomes

46 in total — 2 sets of 23 (1 from each parent)

• 22 diploid (cells that have two sets of chromosomes): the somatic cells

• 1 non-diploid — gametes, sex cells

gametes

sperm (males) & eggs (females) —- haploid cells (only 1 pair)

• bc during fertilization the zygote will be diploid & have correct number of chromosomes

how does dna exist in a cell that is not activity dividing vs non-dividing

not actively dividing: chromosome exists as very long DNA molecules with attached proteins (for structure & control of genes)

non-dividing: exists as chromatin

• genes accessible for enzymes: transcription

• entire dna molecule can be copied

• not seen as chromosomes

• not easily organized/moved

what are the stages & checkpoints of the cell cycle

• g1 & g1 checkpoint

• s

• g2 & g2 checkpoint

• m & m checkpoint

g1 & g1 checkpoint

intense period of growth (makes organelles, membrane & enzymes/proteins)

• checkpoint: controlled by protein kinase enzymes: cyclin-dependent kinases, if enough goes to s stage, if not goes to g0

protein kinase enzymes

enzymes that activate or deactivate other proteins through phosphorlyation

g0

stage where cell is alive & active but not in dividing process

s stage

duplication & copy of entire DNA —- DNA synthesis

• if copied without damage or errors moves to g2

g2

second growth period: making enzymes & proteins needed for cell division

• checkpoint: cyclin-cdk complex: mpf, m-phase promoting factor (phosphorylates proteins that start mitotic events & activates protein which breaks down its own cyclin (anaphase)

m phase

includes cytokinesis & mitosis —- needs to reach checkpoint before: kinetochores: release signal molecule when all spindle fibers are attached

mechanisms that regulate cell divsion

• density-dependent inhibition: stops crowded cells from dividing

• anchorage-dependence": must be attached to a substratum (connective tissue)

• cell division will also stop when running out of essential nutrients & growth factors

loss of cell cycle controls

can lead to unrestricted growth — cancer (cells will no longer have the characteristics of the cell from which they came)

• stays in same spot = benign tumor

• moves around due to them metastasizing = malignant tumor

interphase

where a cell spends most of its time

• includes g1, s, g2, g0

what does g2 of interphase look like during mitosis

the chromatin has been duplicated, centrosomes are present & nuclear envelope is intact

what does prophase looking during mitosis

chromatin is coiling & condensing to form chromosomes

• centriole pairs move toward the ends of the cel & begin to make spindle fibers

what does metaphase look like during mitosis

each chromosome is attached by spindle fibers & the duplicated chromosomes are individually lined up at the metaphase plate

what does anaphase look like during mitosis

the sister chromatids are separated due to the contraction of the spindle fibers

what does telophase & cytokinesis look like during mitosis

cleavage furrow forms (animal cells only, plants have to build new wall), the nuclear envelope begins to reform & chromosomes return to the chromatin structure (uncondenses)

homologous pair

same size, carry same genes on the same place (locus) (could be a different version)

• don’t have to be made of sister chromatids — bc crossing over

• only on somatic cells & allows for 50:50 chance to get certain genes

heredity (inheritance)

passing on of traits from one generation to the next — allows for variation

genes

segments of DNA — can be thousands of nucleotide bases in length — there’s a specific sequence of the bases

• code of specific proteins/enzymes

asexual reproduction

one is the sole parent & the offspring is a genetic clone — no variation

sexual life cycle

begins with conception (zygote), then growth/development until sexual maturity & ends with organism producing its own gametes

sex chromosomes

• male: xy —- y codes for male characteristics

• female: xx

meiosis

process begins with 1 diploid cell & two cell divisions resulting in 4 genetically distinct daughter cells called gametes

what does prophase I look like in meiosis

homologous chromosomes pair up (synapsis) to form tetrads & crossing over (exchange of DNA between non-sister chromatids)

what does metaphase I look like in meiosis

tetrads line up along metaphase plate & are attached to spindle fibers

what does anaphase I look like in meiosis

homologous pairs are separated & eventually end up in separate cells (during telephase/cytokinesis)

how does the second half of meiosis compare to mitosis

same exact process

mechanisms that lead to genetic variation in sexually reproducing organisms

• independent assortment during metaphase 1: can line up on different sides

• crossing over during prophase 1

• random nature of fertilization = variation in zygote, each gamete is 1 out of 8 million — zygote is a 1 in 64 trillion possibility

• mutations also contribute to variation

evolutionary significance of genetic variation in population

those best suited to their environment will reproduce the most — pass on favorable genes, those without less favorable genes won’t

• ability to adapt depends on genetic variation