Test 4. Lecture 34

Question 1

___________ is perhaps the most fundamental characteristic of cells.
All cells reproduce by dividing in two, each parental cell gives rise to two
daughter cells on completion of a cycle of cell division

Cell division must be carefully regulated and coordinated.

Answer 1

Self-reproduction

Question 2

In eukaryotic cells, progression through the cell cycle is controlled by___________that have been conserved from yeasts to mammals.

Answer 2

protein

kinases

Question 3

The division cycle of most cells consists of four coordinated processes:

Answer 3

• Cell growth
• DNA replication
• Distribution of the duplicated chromosomes to daughter cells
• Cell division

Question 4

what are the four phases of the cell cycle? and describe

Answer 4

M phase: Mitosis (nuclear division), usually
ending with cell division (cytokinesis).

• Interphase: period between mitoses, divided
into G1 , S, and G2
.
• G1 phase (gap 1): interval between mitosis
and DNA replication. The cell is
metabolically active and growing.

• S phase (synthesis): DNA replication takes
place.

• G2 phase (gap 2): cell growth continues
and proteins are synthesized in preparation
for mitosis.

Question 5

Duration of phases varies considerably in different kinds of cells.
• Budding yeasts can progress through all four phases in 90 minutes.
• __________ may have cell cycles of 30 minutes, but there is no growth
(G1 or G2) [because it doesnt need to increase cell size]phase.

Answer 5

Early embryos

dont take that much time to divide because it only needs to replicate DNA and it doesnt need to get bigger

Question 6

In contrast, some cells in adult animals cease division altogether (e.g.,
nerve cells).
• Others may divide only occasionally, to replace cells that have been lost.
Figure 17.3 Determination of cellular DNA con

Answer 6

general information

Question 7

Cell cycle analysis requires identification of the ____________

• Phases of interphase must be identified biochemically, usually by DNA
content.
• Animal cells in G1 are diploid (two copies of each chromosome). Their
DNA content is 2n.

Answer 7

phases.

Question 8

DURING S PHASE, replication
increases the DNA _________

• DNA content can be
determined
by incubation of cells with a fluorescent dye that binds to DNA.

• Fluorescence intensity of individual cells is measured in a
FLOW CYTOMETER or _______________

Answer 8

During S phase
content to 4n.

fluorescenceactivated
cell sorter.

Question 9

Cell progression through division cycle: regulated by ______________

Answer 9

extracellular and internal signals

Question 10

Cellular processes, such as growth, DNA replication, and mitosis: regulated by ___________s

Answer 10

a

series of control point

Question 11

A major control point: _________ controls progression from G1
to S, first defined in____________
Once cells pass START, they are committed to entering S phase and undergoing one
division cycle.

Answer 11

START

yeast.

Question 12

Passage through START is highly regulated by external signals, such as nutrient
availability, ________________

If there is a shortage of nutrients, cells can arrest the cycle at START and enter a resting
phase.

Answer 12

mating factors and cell size.

Question 13

In order to maintain constant size, yeast cells must reach a minimum size
to pass ________.

The small daughter cells of budding yeasts spend a longer time in G1 and
grow more than the large mother cell.

Answer 13

START

Question 14

In most animal cells, ___________in late G1
functions like START.

• Passage through restriction point is regulated by extracellular growth factors.

• Once it has passed the restriction point, the cell is committed to proceed through S
phase and the rest of the cell cycle.

• If appropriate growth factors are __________ in G1
, progression stops at the
restriction point and cells enter a resting stage called G0
.
• Skin fibroblasts are arrested in G0 until stimulated by platelet-derived growth factor to
proliferate and repair wound damage.

Answer 14

restriction point

not present `

Question 15

Some cell cycles are controlled principally in_______
.
• The fission yeast Schizosaccharomyces pombe cell cycle is controlled
by transition from G2
to M, the point at which cell size and nutrient
availability are monitored.

Answer 15

G2

Question 16

1. Cell cycle control in G2 also occurs in __________.
   ___________ can remain arrested in G2
   for long periods (decades in
   humans).
2. Progression to M phase is triggered by __________

Events in different stages of the cell cycle must be coordinated so they
occur in the appropriate order.

It is critically important, for example, that the cell not begin mitosis until
replication of the genome has been completed.

Answer 16

animal oocytes
Vertebrate oocytes

hormonal stimulation.

Question 17

• Recent studies have revealed that eukaryote cell
cycles are controlled by a conserved set of protein
kinases, which trigger the major cell cycle
transitions.

• Three experimental approaches contributed to
identification of the molecules responsible for cell
cycle regulation:

1. Studies of frog oocytes, which are arrested in G2
   until hormonal stimulation triggers entry into M
   phase.

• In 1971, researchers found that oocytes could be
induced to enter M phase by microinjection of
cytoplasm from oocytes that had been hormonally
stimulated.
• The cytoplasmic factor responsible was called
_______________

Answer 17

maturation promoting factor (MPF).

Question 18

Coordination of the cell cycle phases is dependent on a series of cell ________

• ____________ ensure
that damaged DNA is not replicated
and passed on to daughter cells.

• The cell cycle is arrested until DNA is
repaired or replicated.

•___________: stops mitosis at metaphase if chromosomes are not properly aligned on the spindle.

Answer 18

cycle checkpoints.

DNA damage checkpoints

Spindle assembly checkpoint

Question 19

Later work showed that MPF is also present in somatic cells, where it induces
entry into M phase.

• MPF thus appeared to act as a general regulator of transition from__________to_________

2. Genetic analyses of yeasts: Investigators found temperature-sensitive mutants that were defective in cell cycle progression (cdc for cell division cycle mutants).

• cdc genes are required for passage through START and entry into mitosis; they
encode protein kinases.

Answer 19

G2

to M.

Question 20

The protein kinase has been shown to be a cell cycle regulator conserved in all
eukaryotes, known as _____

3. Protein synthesis in early sea urchin embryos:

• In 1983, Hunt and colleagues identified two proteins (________) that accumulate
throughout interphase but are rapidly degraded at the end of each mitosis,
suggesting a role in inducing mitosis

Answer 20

Cdk1.

cyclins

Question 21

• The three experimental approaches
converged in 1988, when MPF was
purified and shown to be composed of __________-and ________–

• Cyclin B is a regulatory subunit required for catalytic activity of the Cdk1 protein kinase.

Answer 21

Cdk1 and cyclin B.

Question 22

 Further studies demonstrated the regulation of MPF by phosphorylation and
dephosphorylation of Cdk1.

 During G2
, cyclin B is synthesized and forms complexes with Cdk1.

 Cdk1 is phosphorylated and inhibited, leading to accumulation of inactive
Cdk1/cyclin B complexes during G2
.
 Dephosphorylation activates Cdk1, which phosphorylates several proteins that
initiate the events of M phase.

 Cyclin B is degraded by ubiquitin-mediated proteolysis.

 Destruction of cyclin B inactivates Cdk1, leading the cell to exit mitosis, undergo
cytokinesis, and return to interphase.

Ubiquitylation of cyclin B is mediated by a ubiquitin ligase: the ________________, which is activated as a result of phosphorylation by Cdk1/cyclin B.

Answer 22

anaphasepromoting

complex/cyclosome (APC/C)