L13 Cell Cycle

Front 1

What is the purpose of the cell cycle?

Back 1

• To copy the genome and partition the copiesequally between the daughter cells(unicellular and multicellular organisms)

• To enable a multicellular organism to grow toadult size.

• To maintain the total cell number of anadult organism

• To replace lost or damaged cells.

Front 2

What happens when prokaryotes divide by binary fission?

Back 2

DNA attached to cytoplasmicmembrane

cell enlarges & DNAduplicates

septum forms

cell divides in two, DNApartitioned into each cellcells separate

Front 3

What 2 pathways must be regulated?

Back 3

①Replication of DNA (and the partition ofthe two copies)

②Cytokinesis (cell separation)

Front 4

What happens in the DNA replication stage?

Back 4

Circular chromosome ofprokaryotes has one origin(ori) of replication

Two replication forks (RF)form at the origin;replication is bidirectional

Two identical copies of thecircular chromosome

Front 5

What happens in cytokinesis?

Back 5

FtsZ is distributedrandomly throughout thecytoplasm of the cell

early step in bacterial cytokinesis isformation of a ring of a protein, FtsZ, onthe inner surface of the cytoplasmicmembrane at the future division site

FtsZ ring contracts

Front 6

What is the problem in coordinating the two pathways?

Back 6

However:Cell cycle of rapidly growing bacteria is shorterthan the time needed to copy DNA

Cell division takes 20 minutes

DNA replication takes 40 minutes

Implies that some cells will not contain DNA, becauseDNA replication can’t keep up with cell division.

Front 7

How is this problem overcome?

Back 7

The paradox implied by this mis-match in timing isresolved by DNA replication being initiated beforecompletion of the previous round (multifork replication)

Multifork replication ensures that at least one round ofreplication is finished before cytokinesis.

Front 8

What are the The additional complications of the eukaryotic cell cycle?

Back 8

①Genome is composed of multiple linearchromosomes (necessitating co-ordinatedreplication of all of them as well as theirfaithful segregation).

②Multicellularity: Cells in the context of organsand tissues.

③Numerous organelles (must partition intodaughter cells).

Front 9

What details of the cell cycle are universal?

Back 9

• Details of the cell cycle vary from organism toorganism and at different times in anorganism’s life.

• Certain characteristics are universal

o DNA must be faithfully replicated.

o Replicated chromosomes must beaccurately segregated.

Front 10

What happens in the G1 phase?

Back 10

G1 (Gap 1): growth phase, doubling the mass oforganelles and protein, including synthesis of enzymesthat will drive DNA replication.

Front 11

What happens in the S phase?

Back 11

S: DNA Synthesis phase. Chromosome duplication

Front 12

At the end of S phase, What does each replicated chromosome consist of?

Back 12

At the end of S phase, each replicated chromosome consists ofa pair of identical sister chromatids.

The sister chromatids must not be allowed to separate from eachother, otherwise bipolar attachment to the mitotic spindle would bedifficult to achieve

Front 13

What prevents the sister chromatids from drifting apart?

Back 13

Cohesin ensures that sister chromatids do not drift apart.

Front 14

What happens in the G2 phase?

Back 14

G2: preparation for mitosis. The beginning of mitosisis marked by two events

Front 15

What happens in 1. Chromosome condensation?

Back 15

1. Interphase:Chromosomes not1 visible2.

M phase begins:duplicatedchromosomescondense &become visible

Front 16

What does condensin do?

Back 16

Condensin encircles loops of DNA and compresses the sisterchromatids….to give a compact structure

Front 17

What happens in the Formation of mitotic spindle?

Back 17

The spindle pole bodies lie outside the nucleus

The nuclear membrane must breakdown early inmitosis so that the spindle has access to thechromosomes.

Front 18

What is the Kinetochore?

Back 18

Kinetochore – complex ofproteins attached to thecentromere

Front 19

What does tension in the spindle do?

Back 19

The tension in the spindle is pulling thechromosomes away from eachother…but the sister chromatids arestill held together by cohesin.

……concentrated along the chromosome axis(red fluorescent antibody against cohesin).

Front 20

When are the chromatids segregated?

Back 20

Chromatids segregated when the kleisin subunit ofcohesin is cleaved by a protease:

Front 21

What does cytokinesis signal?

Back 21

Cytokinesis - the end of the cell cycle

Front 22

What happens Once the sister chromatids have reached opposite polesof the cell?

Back 22

• Nuclear membrane begins to re-form.

• Cytoplasm is divided in two by a contractile ringof filaments composed of actin and myosin II.

Pinches the cell intwo, giving twodaughters, eachwith one nucleus.

Front 23

What happens in cytokinesis in plants?

Back 23

In animal cells the contractile ring divides the cytoplasm from theoutside in.

In plants, a contractile ring does not form. Instead a new cell wall is constructed between the daughter nucleii, so cytoplasm is partitioned from the inside out.

New wall synthesisguided by thephragmoplast –containsmicrotubules derivedfrom the mitoticspindle.

Golgiderived vesicles aretransported alongthese microtubules.

Front 24

Does the cell cycle vary from organism to organism?

Back 24

Details of the cell cycle vary from organism to organismand at varying times, but the basic processes commonto all cycles are faithful replication and segregation ofchromosomes

Front 25

What happens in early embryonic cycles?

Back 25

Early embryonic cycles – division without growth

Somatic cells maintain a constant size, because cells growafter each division.

The early embryo divides without growing, producing smallercells with each successive cycle.

Early embryonic cycles are 20x faster thanthose of somatic cells

Front 26

What is closed mitosis?

Back 26

Unicellular organisms operate a closed mitosis – nuclearenvelope remains intact throughout.

In baker’s yeast (Saccharomyces cerevisiae), the spindle polebody (SPB) is the sole site of microtubule organization. TheSPB is embedded in the nuclear envelope.

Front 27

What is open mitosis?

Back 27

Multicellular organisms operate an open mitosis – nuclearenvelope breaks down and then reforms.

…because SPB is outside the nucleus. So the membrane mustbreak down so that spindle has access to chromosomes

Front 28

Do cells ever divide assymetrically?

Back 28

There are examples where cells divide asymetrically (thedaughter cells can differ in size or cytoplasmic content or both)

Usually the daughter cells will develop along differentpathways.

Front 29

How does assymetric mitosis occur?

Back 29

The mother cells must segregate cell fate determinants to oneside and then position the plane of division so that one daughterinherits the determinants.

e.g. P granule segregation in development of the nematode wormCaenorhabditis elegans

Front 30

What is an example of this polarity?

Back 30

e.g. P granule segregation in development of the nematode wormCaenorhabditis elegans (a useful model organism; transparent,only 959 cells; including germ cells in the gonad at one end)

Front 31

What is another example of polarity in cell division?

Back 31

e.g. stem cell division

Stem cells are attached to niche cells, blocks their differentiation,but cell division is allowed.One daughter is released and is free to differentiate. The otherdaughter remains attached to the niche cells and remains a stemcell.

Front 32

What is anchorage dependence in cell cycle control?

Back 32

displayed by manyanimal cells, inwhich cells must beattached to asubstratum in orderto divide.

In cell tissue culture:

Surface neededfor division.

Cells divide toform a singlelayer, and thenstop.

Front 33

What is density dependent inhibition?

Back 33

in which cellsstop dividing once theycontact each other(contact inhibition)

If some cellsa removed,divisionoccurs to fillthe gap (andthen stops).

Front 34

What are the features of The cell cycle control system?

Back 34

1. Cell cycle engine

The protein complex that drives the cycle

2. Co-ordination

e.g. replicated DNA must go through mitosisbefore replication occurs again

3. Checkpoints

The cycle will stop if the cell is deprived ofnutrients or DNA is damaged, or if chromosomesfail to attach to the spindle.

Front 35

What are phases of the cell cycle driven by?

Back 35

Phases of the cell cycle are driven byaction of a protein kinase, the cyclindependent protein kinase (CDK)

Front 36

Is it the levels of activity of the kinase that activates phases of the cycle?

Back 36

Levels of the kinase itself remain constantthroughout the cell cycle.

It is the activity (not the levels) of thekinase that activates the phases of thecycle.

Front 37

When is the PK active?

Back 37

The kinase is only active when complexedwith another protein, cyclin

Front 38

Why are cyclins key to regulating the cell cycle?

Back 38

Cyclins are the key to regulating the cell cyclebecause:

• They undergo cycles of synthesis anddegradation – so their levels rise and fall

• There are different CDK’s and cyclins,each pair activating a different phase ofthe cell cycle.

Front 39

What examples of cyclins at the differents cell cycle phases?

Back 39

e.g. G1/S phase cyclin (cyclin E)

e.g. M phase cyclin (cyclin B)

Front 40

What does cyclin B do?

Back 40

Promotes entry into mitosis e.g

• activates condensin

•Induces nuclear membranebreakdown

Front 41

What does mitotic CDK do?

Back 41

Mitotic CDK phosphorylates nuclear lamin

• Causes depolymerization of lamin filaments

• Lamina mesh disintegrates, and no longer supports the nuclearmembrane.

Front 42

How does CDK-cyclin trigger different phases of the cell cycle?

Back 42

CDK-cyclin complexes trigger different phases ofthe cell cycle because the cyclin directs thekinase to specific target proteins.

Cyclins are degraded by proteolysis, terminatingthe cell cycle phase they control.

Front 43

What is Cyclin B degradation required for?

Back 43

Cyclin B degradation isrequired for exit from mitosisand ending the cell cycle

Front 44

Why must the cell cycle be tightly regulated?

Back 44

• The cycle is tightly regulated…phases must occurin the proper order. S phase and M phase mustonly happen once during each cycle.

• e.g. mechanism must exist to prevent re-replicationof G2 DNA

• existence of such mechanisms was first provenusing cell fusion experiments.

Front 45

What happened in these cell fusion experiments?

Back 45

Fusing an S phase with a G1 phase cell:The S phase nucleus continues DNA replication.

The G1 DNA is instructed to enter S phase

Fusing an S phase with a G2 phase cell:The S phase nucleus continues DNA replication.

The G2 DNA is NOT forced into S phase

Front 46

What are checkpoints?

Back 46

Surveillance mechanisms (checkpoints) operate continually toensure next phase is not initiated unless the previous one hasbeen completed

Front 47

What is the first checkpoint?

Back 47

1. In G1: Restriction point (R). Apositive signal (growth factor)from the outside will instruct thecell to divide (no signal, nodivision).

Front 48

What is the second checkpoint?

Back 48

2. At G2/M: Is DNA synthesis complete? Cell cycle is suspendedif not (passing less DNA to progeny is not permitted)

Front 49

What is the 3rd checkpoint?

Back 49

3. Spindle checkpoint: is eachchromosome attached to thespindle? If not, the cell cycle issuspended.

Front 50

What is the 4th checkpoint?

Back 50

4. AND…the DNA damage checkpoint (operates throughout thecycle). Arrests cycle while damage is repaired.

Front 51

What happens if there is a Failure of DNA damage checkpoint?

Back 51

• Cycle keeps turning despite DNA damage. Mutationsaccumulate.

• Leads to cancer

Front 52

What happens if the If spindle checkpoint fails?

Back 52

• Unequal segregation of chromosomes

• Causes human aneuploidies (meiotic spindle) e.g.Down’s Syndrome (extra copy of chromosome 21)

Front 53

What do all cancers feature?

Back 53

All cancers feature a de-regulated cell cycle (as aconsequence of mutation)

Front 54

What signals are ignored?

Back 54

signals that start andstop the cell cycle areignored

Front 55

Do checkpoints operate in cancer?

Back 55

Checkpoints that protectthe genome (DNAdamage, mitotic spindle)no longer operate

Front 56

Do cells communicate with eachother in cancer?

Back 56

Cells do not communicate witheach other (e.g. no contactinhibition).

Front 57

What is a summary of the cell cycle?

Back 57

• In the prokaryotic cell cycle, multifork replicationcompensates for the mismatch in timing betweenDNA replication and cell division.

• The eukaryotic cell cycle is divided into four phases,enabling faithful replication and segregation ofchromosomes.

• The cyclin dependent kinase/cyclin complexdrives the eukaryotic cell cycle.

• The cycle is subject to regulation at multiplelevels, including checkpoints that guard againstaccumulation of mutations and mis-segregation ofchromosomes