Unit 8 : Cell Division
reasons why the cell can’t be large
dna overload: if a cell get’s too large, it’s amount of dna will not be sufficient to control all the activities of the cell
surface area to volume ratio: as a cell gets larger, the cell volume will ALWAYS increase FASTER than the surface area. the surface area, however, controls how nutrients move into the cell and wastes move out of the cell. as the cell gets larger, the cell membrane will be unable to get enough nutrients into the cell nor enough wastes out of the cell
importance of normal cell division
for unicellular organisms, cell division is asexual reproduction
for multicellular organisms, cell division is used for:
organismal growth and development
tissue regeneration (healing and repair)
stages of interphase
G1 (first gap phase): cell growth occurs and normal cell function occurs
S (synthesis phase): chromosomes are duplicated
G2 (second gap phase): the cell checks to see if anything else needs to be done prior to mitosis. if anything else needs to be copied, it copies it. it also checks to make sure all of the dna has been copied.
M (mitotic phase): consists of two processes: mitosis and cytokinesis
INTERPHASE is the first 3 stages of the cell cycle: G1, S and G2. interphase is the longest part of the cell cycle and it’s length differs for different types of cells. during interphase, individual chromosomes are NOT visible. the dna is in a chromatin state.
what is G0
G0 is opting out of the cell cycle. if a cell has no need to divide ever again, it may go into G0. most cells cannot come back out of G0, but a few can
phases of mitosis
prophase
chromosomes condense. (the dna duplicates in S phase, but does not condense until prophase of mitosis)
centrosomes move to opposite ends of the cell
the nucleolus disappears
the nuclear envelope disappears
the spindle forms
kinetochore proteins bind the centromeres of each chromosome.
kinetochore microtubules bind the kinetochores of each chromosome
metaphase
the microtubules push and pull the chromosomes until they are lined up at the center of the cell.
anaphase
kinetochore microtubules pull sister chromatids apart to opposite sides of the cell
non-kinetochore microtubules push against each other starting the process of cytokinesis
telophase
there are 2 nuclei (until cytokinesis is complete)
sister chromatids are on opposite sides of the cell. they go back to a chromatin state.
the nuclear envelopes reform
when does cytokinesis occur?
cytokinesis is the division of the cell nucleus and often begins as early as the end of anaphase, but almost always overlaps with telophase.
chromosomes
number in a human somatic cell: 46 (two of each type of chromosome-diploid)
number in a human gamete: 23 (one of each type of chromosome-haploid)
regulations on cell division
external regulators: factors that respond to events outside the cell that direct cells to speed up or slow down the cell cycle.
example of external regulator: contact inhibition (when cells come into contact with other cells, they stop dividing)
internal regulators: factors from inside the cell that directs cells to speed up or slow down the cell cycle
example of internal regulator: cyclins (a family of proteins within the cell that controls the timing of the cell cycle)
definitions
cancer: uncontrolled cell division (caused by p-53)
homeostasis: maintenance of a stable, internal environment
chromosome: discrete units of genetic material composed of DNA and proteins. contains the genetic information of the cell. human somatic cells have 46 chromosomes. human gametes have 23 chromosomes
chromatin: granular form of dna found in resting cells (cells that are in interphase)
mitosis: division of the cell nucleus
cytokinesis: division of the cell cytoplasm
interphase: time between cell divisions. comprised of G1, S, and G2. longest part of the cell cycle. dna is in a chromatin state
cleavage furrow: part of cytokinesis in animal cells. the pinching between the two new cells being formed
cell plate: part of cytokinesis in plant cells. the formation of a new cell wall between two new plant cells
spindle: structure formed by centrosomes and microtubules during mitosis which draws the duplicated chromosomes apart as the cell divides
centriole: one of two structures that makes up a chromosome
plant cells do NOT have centrioles
centrosome: structure that facilitates cell division made up of two centrioles. plant cells do not have these
centromere: the part of the duplicated chromosome that links sister chromatids
chromatid: one of two strands of replicated dna in a duplicated chromosome. sister chromatids are identical to each other
histone: protein around which dna is wrapped in a chromosome. helps to organize dna
tetrad: homologous chromosome pair
homologous chromosome pairs: each somatic cell has 2 of each chromosome. a homologous pair is when both of the same chromosome find each other; they have the same genes but may have different alleles for that gene.
autosome: chromosomes that do not control the gender of the individual. humans have 32 pairs of autosomal chromosomes
sex chromosome: chromosomes that control the gender of the organism. humans have 1 pair of sex chromosomes (XX for females, XY for males)
gamete: cell used for sexual reproduction. egg (ova) for females; sperm for males. gametes are haploid and are produced by meiosis
somatic cell: cells that are not used for sexual reproduction. most cells in humans. somatic cells are diploid and are produced by mitosis
diploid: having 2 of each type of chromosomes. diploid number for humans is 46
haploid: having one of each type of chromosome. the haploid number for humans is 23
karyotype: an image of the number and appearance of chromosomes in a eukaryotic cell
contact inhibition: external regulator in cell division; when cells come into contact with other cells, they stop dividing
cyclins: internal regulator in cell division; a family of proteins within the cell that controls the timing of the cell cycle
fertilization: when a sperm and an egg fuse
zygote: diploid cell formed upon cell division
meiosis: cellular reproduction that halves the number of chromosomes and ensures genetic diversity of gametes
crossing over: when homologs pair up in prophase I, sister chromatids wrap around each other wand may “swap” alleles
independent assortment: random lining up of homologs on the metaphase plate in metaphase I. ensures genetic diversity as it shuffles up the alleles.
importance of cell division
mitosis: used for organismal growth, development, and tissue regeneration
meiosis: produces gametes (halves the number of chromosomes and produces genetic diversity in gametes)
phases of mitosis
prophase: nuclear envelope disappears; chromosomes condense; nucleolus disappears; spindle forms
metaphase: chromosomes line up on the metaphase plate
anaphase: sister chromatids are pulled apart
telophase: nuclei reform; chromosomes go back to a chromatin state
ways mitosis differs from meiosis
mitosis goes from 1 (2N) cell to 2 (2N) cells; meiosis goes from 1 (2N) cell to 4 (1N) cells.
the daughter cells in mitosis are genetically identical; the daughter cells in meiosis are genetically different
cells undergoing mitosis go through a single division; cells undergoing meiosis go through 2 divisions
homologous chromosomes pair up in meiosis; they do not pair up in mitosis
somatic cells undergo mitosis; only sex cells undergo meiosis
regulations on cell division
internal: cyclins and p53
external: contact inhibition and growth factors
asexual reproduction versus sexual reproduction
asexual reproduction: 1 aparent with identical offspring. Good: faster, more offspring. Bad: much less genetic variation (relies on mutations)
sexual reproduction: 2 parents with offspring that a mix of genes from each parent. Good: much more genetic variation. Bad: slower, fewer offspring
phases of meiosis
meiosis I: splits homologous chromosomes (tetrads)
prophase I: homologous chromosomes pair up; crossing over occurs
metaphase I: homologous chromosomes line up on the meta phase plate (how they line up is random→independent assortment)
anaphase I: homologous chromosomes are pulled apart
telophase I and cytokinesis: end up with 2 1N cells (homologs were separated)
between meiosis I and meiosis Ii there is no telophase
meiosis II: separates sister chromatids
prophase II: sister chromatids start to move to the center of the cell
metaphase II: sister chromatids line up at the metaphase plate
anaphase II: sister chromatids are pulled apart
telophase II and cytokinesis: end up with 4 1N cells since sister chromatids were separated