Mitosis and cell cycles

Question 1

What is the best way to distinguish non-living matter from living matter

Answer 1

the ability of organisms to reproduce

Question 2

What do multi-cellular organisms depend on cell division for?

Answer 2

-Development from a fertilized cell
– Growth
– Repair
(mitosis)

Question 3

What happens before cells divide

Answer 3

They duplicate their genetic material, ensuring that each daughter cell receives an exact copy of it’s genetic material

Question 4

What is a genome?

Answer 4

A cell’s endowment of DNA, A cell’s genetic information, is packaged as DNA.

Question 5

How are DNA molecules packaged?

Answer 5

CHROMOSOMES

Question 6

Somatic cell (non-reproductive)

Answer 6

have two sets of chromosomes (diploid)

Question 7

Gametes

Answer 7

half as may chromosomes as somatic cells (haploid)

Question 8

What do eukaryotic chromosomes consist of?

Answer 8

• Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein(histone) that condenses during cell division

Question 9

What is the centromere

Answer 9

the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

Question 10

What does meiosis yield?

Answer 10

nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell

Question 11

The cell cycle consists of

Answer 11

• Mitotic (M) phase (mitosis and cytokinesis)
  – Interphase (cell growth and copying of chromosomes in preparation for cell division)

Question 12

Interphase sub-phases

Answer 12

– G1 phase (“first gap”)
– S phase (“synthesis”)
– G2 phase (“second gap”)

Question 13

Mitosis is conventionally divided into five phases:

Answer 13

– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase

Question 14

Cytokinesis

Answer 14

 Cytokinesis, division of the cytoplasm, typically follows mitosis.
 In animal cells, cytokinesis occurs by a process called cleavage.
 The first sign of cleavage is the appearance of a cleavage furrow in the cell surface near the old
metaphase plate.
 On the cytoplasmic side of the cleavage furrow is a contractile ring of actin microfilaments
associated with molecules of the motor protein myosin.
 Contraction of the ring pinches the cell in two.
 Cytokinesis in plants, which have cell walls, involves a completely different mechanism.
 During telophase, vesicles from the Golgi coalesce at the metaphase plate, forming a cell plate.
 The plate enlarges until its membranes fuse with the plasma membrane at the perimeter.
 The contents of the vesicles form new cell wall material between the daughter cells.

Question 15

What is a mitotic spindle?

Answer 15

an apparatus of microtubules that controls chromosome movement during mitosis

Question 16

Where does the assembly of spindle microtubules begin?

Answer 16

in the centrosome, the microtubule organizing centre

Question 17

Explain the spindle assembly process

Answer 17

The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them

Question 18

What is an aster?

Answer 18

(a radial array of short microtubules) extends from each centrosome

Question 19

What does the spindle include

Answer 19

the centrosomes, the spindle microtubules, and the asters

Question 20

What do some spindle microtubules do?

Answer 20

attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate

Question 21

The kinetochore

Answer 21

the kinetochore, a proteinaceous multidomain structure, is assembled on the outer surface of the centromere, promoting attachment of the chromosome to spindle microtubules and movement during anaphase.

Question 22

The function of the spindle during anaphase

Answer 22

In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell

Question 23

How do microtubules shorten

Answer 23

The microtubules shorten by depolymerizing at their kinetochore ends

Question 24

What do non-kinetochore microtubules do?

Answer 24

Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell

*In telophase, genetically identical daughter nuclei form at opposite ends of the cell

Question 25

How are chromosomes replicated during binary fission?

Answer 25

the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart

Question 26

Evidence for Cytoplasmic Signals

Answer 26

The cell cycle appears to be driven by specific chemical signals present in the cytoplasm
* Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei

Question 27

The Cell Cycle Control System

Answer 27

The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock
* The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

Question 28

Significance of G1

Answer 28

• If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide
• If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

Question 29

What two types of regulatory proteins are involved in the cell cycle control?

Answer 29

cyclins and cyclin-dependent kinases (Cdks)

Question 30

Stop and Go Signs: Internal and External Signals at the Checkpoints

Answer 30

An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase
* Some external signals are growth factors,
proteins released by certain cells that stimulate other cells to divide
* For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture

Question 31

Loss of Cell Cycle Controls in Cancer Cells

Answer 31

Cancer cells do not respond normally to the body’s control mechanisms
* Cancer cells form tumours, masses of abnormal cells within otherwise normal tissue
* If abnormal cells remain at the original site, the lump is called a benign tumour
* Malignant tumours invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumours

Question 32

What does each duplicated chromosomes have

Answer 32

two sister chromatids that separate during cell division

Question 33

Animal Cells

Answer 33

cytokinesis occurs by a process known as cleavage, forming a cleavage furrow

Question 34

Plant cells

Answer 34

a cell plate forms during cytokinesis

Question 35

Bacterial Cell Division

Answer 35

Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.
Replication continues. One copy of the origin is now at each end of the cell.
Replication finishes.
The plasma membrane grows inward, and a new cell wall is deposited.
Two daughter cells result.

Question 36

Evolution of Mitosis

Answer 36

Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission
* Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis

Question 37

The mitotic spindle distributes chromosomes to daughter cells: a closer look.

Answer 37

Each sister chromatid has a kinetochore of proteins and chromosomal DNA at the centromere.
 The kinetochores of the joined sister chromatids face in opposite directions.
 During prometaphase, some spindle microtubules (called kinetochore microtubules) attach to the
kinetochores.
 When a chromosome’s kinetochore is “captured” by microtubules, the chromosome moves
toward the pole from which those microtubules come.
 When microtubules attach to the other pole, this movement stops and a tug-of-war ensues.
 Eventually, the chromosome settles midway between the two poles of the cell, on the metaphase
plate.
 Nonkinetochore microtubules from opposite poles overlap and interact with each other.
 By metaphase, the microtubules of the asters have grown and are in contact with the plasma
membrane.
 The spindle is now complete.
 Anaphase commences when the proteins holding the sister chromatids together are inactivated.
 Once the chromosomes are separate, full-fledged chromosomes, they move toward opposite poles
of the cell

Question 38

Centrosome

Answer 38

Assembly of the spindle microtubules starts in the centrosome.
 The centrosome (microtubule-organizing center) is a nonmembranous organelle that organizes
the cell’s microtubules.
 In animal cells, the centrosome has a pair of centrioles at the center, but the centrioles are not
essential for cell division.
 During interphase, the single centrosome replicates to form two centrosomes.
 As mitosis starts, the two centrosomes are located near the nucleus.
 As the spindle microtubules grow from them, the centrioles are pushed apart.
 By the end of prometaphase, they are at opposite ends of the cell.

Question 39

What is the function of the nonkinetochore microtubules?

Answer 39

Nonkinetochore microtubules are responsible for lengthening the cell along the axis defined by the poles.
 These microtubules interdigitate and overlap across the metaphase plate.
 During anaphase, the area of overlap is reduced as motor proteins attached to the microtubules
walk them away from one another, using energy from ATP.
As microtubules push apart, the microtubules lengthen by the addition of new tubulin monomers
to their overlapping ends, allowing continued overlap

Question 40

Chemical Signals

Answer 40

 Rhythmic fluctuations in the abundance and activity of cell cycle control molecules pace the
events of the cell cycle.
 These regulatory molecules include protein kinases that activate or deactivate other proteins by
phosphorylating them.
 These kinases are present in constant amounts but require attachment of a second protein, a
cyclin, to become activated.
 Levels of cyclin proteins fluctuate cyclically.
 Because of the requirement for binding of a cyclin, the kinases are called cyclin-dependent
kinases, or Cdks.
 Cyclin levels rise sharply throughout interphase, and then fall abruptly during mitosis.
 Peaks in the activity of one cyclin-Cdk complex, MPF, correspond to peaks in cyclin
concentration.
 MPF (“maturation-promoting factor” or “M-phase-promoting-factor”) triggers the cell’s passage
past the G2 checkpoint to the M phase.
 MPF promotes mitosis by phosphorylating a variety of other protein kinases.
 MPF stimulates fragmentation of the nuclear envelope by phosphorylation of various proteins of
the nuclear lamina.
 It also triggers the breakdown of cyclin, dropping cyclin and MPF levels during mitosis and
inactivating MPF.
 The noncyclin part of MPF, the Cdk, persists in the cell in inactive form until it associates
with new cyclin molecules synthesized during the S and G2 phases of the next round of the
cycle.
 At least three Cdk proteins and several cyclins regulate the key G1 checkpoint.
 Similar mechanisms are also involved in driving the cell cycle past the M phase checkpoint.