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Chapter 12: The Cell Cycle

1. When is cell division important?- Reproduction – unicellular organisms – binary fission in bacteria- Growth & development from fertilized egg- Repair (& replacement) of damaged cells

Chapter 12: The Cell Cycle

20 µm100 µm 200 µm

(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).

(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).

(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

1. When is cell division important?2. What is an organism’s genome?

- Total hereditary endowment (all genes) in the cell of a species- Nuclear & extra-nuclear (mito & chloro)

3. How many chromosomes do we have?- 46 – somatic cell (cells of the body)

- 2n - Diploid – 2 sets of chromosomes

- 23 – gamete (sex cells – sperm & egg) - n - Haploid – 1 set of chromosomes

4. What are chromosomes made of?- Chromatin- DNA & proteins

Chapter 12: The Cell Cycle

0.5 µm

Chromosomeduplication(including DNA synthesis)

Centromere

Separation of sister

chromatids

Sisterchromatids

Centromeres Sister chromatids

A eukaryotic cell has multiplechromosomes, one of which is

represented here. Before duplication, each chromosome

has a single DNA molecule.

Once duplicated, a chromosomeconsists of two sister chromatids

connected at the centromere. Eachchromatid contains a copy of the

DNA molecule.

Mechanical processes separate the sister chromatids into two chromosomes and distribute

them to two daughter cells.

Fig 12.4 Chromosome duplication & distribution during cell division

INTERPHASE

G1

S(DNA synthesis)

G2Cyto

kines

is

Mitosis

MITOTIC(M) PHASE

1. When is cell division important?2. What is an organism’s genome?3. How many chromosomes do we have?4. What are chromosomes made of?5. What are the 2 major phases of the cell cycle?

- Interphase – 90%- Mitotic phase – 10%

Chapter 12: The Cell Cycle

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1. When is cell division important?2. What is an organism’s genome?3. How many chromosomes do we have?4. What are chromosomes made of?5. What are the 2 major phases of the cell cycle?

- Interphase- Mitotic phase

6. What are the steps of the cell cycle?- IPMAT- Interphase- Prophase- Metaphase- Anaphase- Telophase

Chapter 12: The Cell Cycle

G2 OF INTERPHASE PROPHASE PROMETAPHASE

Centrosomes(with centriole pairs) Chromatin

(duplicated)Early mitoticspindle

AsterCentromere

Fragmentsof nuclearenvelope

Kinetochore

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore microtubule

Nonkinetochoremicrotubules

Combine pro- & prometaphase

Chromosomes appear as Pairs

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate Nucleolus

forming

Cleavagefurrow

Nuclear envelopeformingCentrosome at

one spindle poleDaughter chromosomes

Middle Apart Two

1. When is cell division important?2. What is an organism’s genome?3. How many chromosomes do we have?4. What are chromosomes made of?5. What are the 2 major phases of the cell cycle?6. What are the steps of the cell cycle?7. What is the difference between animal and plant cytokinesis?

- Animal – cleavage furrow – cell forms from outside in- Plants – cell plate – cell forms from inside out

Chapter 12: The Cell Cycle

Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm1 µmVesicles

forming cell plate

Wall of patent cell Cell plate

New cell wall

(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)

Figure 12.9 Cytokinesis in animal and plant cells

1. When is cell division important?2. What is an organism’s genome?3. How many chromosomes do we have?4. What are chromosomes made of?5. What are the 2 major phases of the cell cycle?6. What are the steps of the cell cycle?7. What is the difference between animal and plant cytokinesis?8. How is the cell cycle regulated?

- Checkpoints- Make sure cell has enough “ingredients” to move to next stage

Chapter 12: The Cell Cycle

Control system

G2 checkpoint

M checkpoint

G1 checkpoint

G1

S

G2M

Figure 12.14 Mechanical analogy for the cell cycle control system

Figure 12.15 The G1 checkpoint

G1 checkpoint

G1G1

G0

(a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues      on in the cell cycle.

(b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a non-dividing state.

Most functioning cells are in G0

1. When is cell division important?2. What is an organism’s genome?3. How many chromosomes do we have?4. What are chromosomes made of?5. What are the 2 major phases of the cell cycle?6. What are the steps of the cell cycle?7. What is the difference between animal and plant cytokinesis?8. How is the cell cycle regulated?

- Checkpoints- Make sure cell has enough “ingredients” to move to next stage - Cyclins - Cyclin-dependent kinase (CDK)- MPF

- maturation promoting factor - Cyclin + CDK = MPF

Chapter 12: The Cell Cycle

Accumulated cyclin moleculescombine with recycled Cdk mol-ecules, producing enough molecules of MPF to pass the G2 checkpoint and initiate the events of mitosis.

MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase.

3

During G1, conditions in the cell favor degradation of cyclin, and the Cdk component of MPF is recycled.

5

During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G1 phase.

4

2

Synthesis of cyclin begins in late S phase and continues through G2. Because cyclin is protected from degradation during this stage, it accumulates.

1

Cdk

CdkG2

checkpoint

CyclinMPF

Cyclin is degraded

DegradedCyclin

G 1

G 2

S

M

G1G1 S G2 G2SM MMPF activity

Cyclin

Time

(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle

(b) Molecular mechanisms that help regulate the cell cycle

Rel

ativ

e C

once

ntra

tion

Fig. 12.16 Molecular control of the cell cycle at the G2 checkpoint

1. When is cell division important?2. What is an organism’s genome?3. How many chromosomes do we have?4. What are chromosomes made of?5. What are the 2 major phases of the cell cycle?6. What are the steps of the cell cycle?7. What is the difference between animal and plant cytokinesis?8. How is the cell cycle regulated?9. What happens when there is a loss of cell cycle regulation?

- Tumors &/or cancer - Lack of cell division

Chapter 12: The Cell Cycle