Cell Division

What is cell division?

how cells replicate to increase their number

Chromosomes

dense packaging of chromatin, existing during mitosis and meiosis

chromatid

one half of a duplicated chromosome

centromere

the region where the two chromatids are connected to make a duplicate chromosome

Haploid cell

cell containing 1 set of chromosomes (n)

Diploid cells

a cell containing 2 sets of chromosomes (2n)

Homologous chromosomes

found in diploid cells

• two sets of every chromosome (one from each parent), forming a pair called this

• similar in length, gene position, and centromere position

• not genetically identical

Humans have how many chromosomes?

46 chromosomes (23 homologous pairs)

MTOC

microtubule organizing center

• composed of 2 centrioles

MTOCs in animals

centrosomes

spindle fibers

mictrotubules that emerge from the centrosome

spindle fibers allow the chromosomes and chromatids to…

be separated during specific phases of cell devision

Kinetochores

the site where spindle fibers attach

kinetochores are proteins located on

the centromere region of the chromosomes

centromere

the region where sister chromatids are connected

Mitosis occurs…

in all somatic and germ cells

Meiosis occurs…

only in germ (gamete producing) cells

Prophase

• chromatin condenses into chromosomes

• nucleolus disappears

• mitotic spindle begins to form

• centrosomes begin to move towards opposite ends of cell

Prometaphase

• nucleus disassembles

• chromosomes condense even further

• each chromatid is attached to a kinetochore

• mitotic spindle further develops

• spindle fibers begin to attach to kinetochores of chromosomes

Metaphase

• chromosomes are lined up across the center of the cell - metaphase plate

• centrosomes have reached opposite ends of the cell

• mitotic spindle is fully developed

• all chromosomes are attached to spindle fibers via kinetochores

• karyotyping performed here

Anaphase

• microtubules shorten

• chromatids are pulled apart

• each sister chromatid is now considered to be an individual chromosome

• chromosomes are pulled to opposite ends of the cell

telophase

• nucleoli reappear

• two nuclear envelopes develop

• chromosomes decondense back into chromatic

• spindle fibers disappear

cytokinesis

physical division to form two cells

cytokinesis - animal cells

cleavage furrow - contractile ring formed by actin and myosin

cytokinesis - plant cells

• cell plate

• no cleavage furrow

End of mitosis

go from one parent cell (diploid) to two daughter cells (diploid)

• DNA is genetically identical to parent cell

Karyotyping

a complete set of chromosomes found during the metaphase stage

• can help identify genetic disorders

When do you perform karyotyping

during metaphase

Gametes

an organisms reproductive cells

• haploid, and only carry half of genetic info that all other cells carry

How many stages of meiosis?

2

• meiosis I

• meiosis II

Meiosis I

separation of homologous chromosomes

• genetic recombination only occurs here

Meiosis II

separation of sister chromatids

Meiosis I

1. Prophase I

2. Metaphase I

3. Anaphase I

4. Telophase & Cytokinesis

Prophase I

• nucelolus and nucleus disappear

• chromatin condenses into chromosome

• meiotic spindle begins to form

• centrosomes begin to move towards opposite ends of the cell

• homologous chromosomes pair up

• crossing over occurs

• microtubules attach to kinetochores of each homologous chromosomes

crossing over

process of genetic recombination producing chromosomes that are unique at the genetic level

chiasmata

region when crossing over occurs, creating genetic diversity in offspring

synapsis

when homologous pair up into tetrads

tetrads

term used to describe paired up homologous chromosomes

metaphase I

• homologous pairs are lined up across metaphase plate

• microtubules are attached to kinetochores

anaphase I

• meiotic spindle shortens

• homologous pairs uncouple and are pulled to opposite poles (disjunction)

telaphase I

• nuclear envelope re-develops

• chromosomes begin to decondense

• each new daughter cell has a new nucleus that has half the number of chromosomes

Diploid parent cells → (after meiosis I)

haploid daughter cells

Meiosis II

sister chromatids will be separated to opposite ends of the cell

Prophase II

• nucleus and nucleolus disappear

• chromosomes condense

• meiotic spindle develops and begins to attach to chromosomes

• centrosomes move towards opposite ends of cell

• no crossing over occurs

Metaphase II

• chromosomes line up on plate

• meiotic spindle fully formed and attached to every chromosome via knetochores

• half the number of chromosomes as here were in metaphase I

• sister chromatids are no longer identical due to crossing over during prophase I

Anaphase II

• microtubules shorten

• chromatids are pulled apart

• each sister chromatid is now considered to be an individual chromosome

• chromosomes separate to opposite ends of cell

Telophase II and cytokinesis

• nucleolus reforms

• nucleus re-develops around each set of chromosomes

• chromosomes decondense back into chromatin

• spindle fibers disappear

End of meiosis

• four daughter cells that are gametes

• each is haploid

• amount of DNA in each daughter cell is half the amount of the parent cell

• crossing over has created genetically unique chromosomes

Genetic variation - genetic recombination occurs during three events

• crossing over - prophase I

• independent assortment - each gamete is different/randomized

• random joining of gametes - depends on which sperm fertilizes which egg

The cell cycle

sequence of events that occur before and during the process of cell division

Mitotic phase = M

mitosis and cytokinesis

• active division of cells

Interphase

• spend majority of time

• contains G1, S, G2

• needs to do this before it can go through mitotic phase

G1

• Gap phase

• cell growth - increases in size

• protein synthesis begins in preparation for division

S

• synthesis phase

• DNA replication

• All DNA replicated during this phase

• where sister chromatids are formed

• centrosomes replicate

G2

• Gap phase

• final preparation for mitosis

• organelles replicate

• cell continues to grow

• cell checks that everything is ready for mitosis

G0

• resting phase

• state of a cell that is not actively dividing

• cells in this phase will re-enter the cell cycle based on different environmental cues

• ex) muscle and nerve cells

functional limitations

• surface to volume ratio

• genome to volume ratio

surface to volume ratio

• when a cell grows, volume gets larger more rapidly in comparison to the surface area

• Large S:V value = cellular exchange becomes easier

• small S:V value = cellular exchange becomes harder

genome to volume ratio

• genome size remains constant throughout life

• an increase in the cell volume causes the G:V value to decrease

when G:V value decreases

cell exceeds ability of its genome to produce enough amounts of regulation for cellular activities

If the surface area to volume ratio is too small…

the rate of chemical exchange becomes too low

If the genome to volume ratio is too small…

the cell cannot regulate cellular activities

cell cannot sustain itself if…

surface area to volume and genome to volume ratios are too small

• this is why large organisms have several cells

skeletal muscle cells are not capable of dividing, but are capable of…

growing much larger though exercise

hypertrophy

cells getting larger

Cell specific regulations

• cell cycle checkpoints

• density - dependent inhibition

• anchorage dependence

Cell cycle checkpoints (3)

• end of G1 checkpoint (restriction point)

• end of G2

• M checkpoint (during metaphase, spindle checkpoint)

restriction point

cell growth is assessed, and favorable conditions are checked

• sufficient nutrients

• necessary cell products

• adequate cell size

• healthy DNA

End of G2 check point

• cell evaluates the accuracy and completion of DNA replication

• if cell DNA is replicated and healthy, the mitotic phase can proceed

spindle checkpoint

• cell evaluates if all sister chromatids are attached to a spindle fiber

• mitosis stops if spindle fibers are not attached to both sides of each chromosome

Density - dependent inhibition

cells stop dividing when the surrounding cell density reaches a maximum

anchorage dependence

cells will only divide when attached to an external surface

• prevents cells from multiplying while floating throughout the body in an area it isn’t supposed to be

cancer

• uncontrolled cell division

• cancer cells defy regulations of cell division

what causes cancer?

• normal cell divides only when new cells of that type are needed

• mutated cell divides uncontrollably

• mutated cells continue to divide increasing in number

• cells forma a mass called a tumor

• malignment tumer

maligment tumor

when a tumors cells break loose and travel to other tissues and organs, process called metastasis

p53 gene

• actively suppresses tumors

• mutation of the gene causing the cell to continue dividing in an uncontrolled manner, leading to tumor growth

cancer drugs can inhibit….

mitosis

• this stops uncontrolled growth

• disruptes ability of microtubules to separate chromosomes during anaphase

frequency of cell division

• labile

• quiescent

• fixed

labile cells

cells that continuously divide

Quiescent (stable) cells

do not usually divide, but can be stimulated to as needed

fixed cells

have little to no capacity for cell division

Chromatin

general packaging of DNA around histone proteins

chromosome

denser organization chromatin exists in during mitosis and meiosis

• can exist in duplicated or unduplicated states

chromatid

one half of a duplicated/diploid chromosome

are sister chromatids genetically identical?

yes, before they are separated, 2 chromatids = 1 chromosome