Cell Growth and Reproduction

Cell Cycle

Growth in OrganismsFor an organism to grow, its cells divide instead

of getting larger.

Cell Size Limitations• 1) Diffusion:• If organelles are too far from the cell membrane,

they would have to wait a long time to get molecules that are diffusing through (ex. lysosomes, mitochondria).

Cell Size Limitations• DNA: • The nucleus cannot provide DNA fast enough to

act as the blue print for the high amounts of proteins and enzymes that a large cell would need. (ex. cytoskeleton).

Cell Size Limitations• Surface area-to-volume ratio:• The volume of the cell increases a lot faster than

the surface area. Therefore if the cell gets too big, the cell membrane will not have enough surface area to diffuse the amount of nutrients the cell needs or get rid of the waste the cell produces

1mm

1mm1mm

2mm

2mm

2mm

4mm4mm

4mm

Surface area =6mm2 Surface area= 24mm2 Surface area= 96mm2Volume= 1mm3 Volume = 8mm3 Volume= 64mm3

Cell Cycle• Cell cycle – stages that the cell goes through from growth to

repair to division.

Interphase• Interphase- period where the cell is preparing to divide

through growth and maintenance. (90% of a cells life).

– G1 phase (“first gap”) growth

– S phase (“synthesis”) DNA duplication

– G2 phase (“second gap”) Maintenance

Synthesis• Synthesis – portion of interphase where cells

duplicate their DNA.

• DNA is packaged into sets of chromosomes.

• Humans have 46 chromosomes (23 from each parent)

Synthesis cont’d• Somatic cells normal body cells, have two sets

of chromosomes (diploid 2n)

• Gametes reproductive cells (sperm and eggs) only have one set of chromosomes (haploid n)

Synthesis cont’d• In synthesis each chromosome gets copied and is

attached to its copy. • Sister chromatids –

identical copies of

a chromosome• Centromere – holds

sister chromatids

together.- At this point their

are double the number

of chromosomes

Mitosis• Mitosis - Cell division where 1 diploid parent cell makes 2

identical diploid daughter cells. *• Technically mitosis id the division of one nucleus into two.

Parent cell

Sister chromatids

chromosomes

identical daughter cells

Prophase• the chromosomes coil up and become visible while the

nuclear envelope disappears. Sister chromatids are present.• A centriole forms at each pole and spindle fibers made of

microtubules grow out. • Short microtubules stick out from the centriole in a star

shape known as an aster.

Metaphase• Early in metaphase (prometaphase) spindle fibers

attach to each of the chromatids at a sight called the kinetochore.

• The spindle fibers line the chromatids up at the equator of the cell called the metaphase plate.

Kinetochore

Fig. 12-7

Microtubules Chromosomes

Sisterchromatids

Aster

Metaphaseplate

Centrosome

Kinetochores

Kinetochoremicrotubules

Overlappingnonkinetochoremicrotubules

Centrosome 1 µm

0.5 µm

Anaphase• Anaphase- During this phase, the kinetochores “reel

in” or “gobble up” the spindle fibers to pull the sister chromatids apart by splitting their centromere.

• The split chromatids are pulled towards opposite poles of the cell

Telophase• Once the chromatids have reached the opposite sides

the spindle fibers disappear, the chromosomes unravel, and the nuclear envelope reappears on the two new nuclei.

• In cytokinesis, the cytoplasm then forms a cleavage furrow at the equator to split the cytoplasm. (in plants a cell plate forms at the equator.)*

Two identical daughter cells are formed

Mitosis Vid

Mitosis

*

Fig. 12-UN2

Binary Fission• Prokaryotes (bacteria and archaea) reproduce by a

type of cell division called binary fission. (believed that mitosis may have evolved from binary fission)

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

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Fig. 12-11-4

Origin ofreplication

Two copiesof origin

E. coli cellBacterialchromosome

Plasmamembrane

Cell wall

Origin Origin

Cytoplasmic Signals• The cell cycle is controlled by chemical signals

in the cytoplasm

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Fig. 12-13

Experiment 1 Experiment 2

EXPERIMENT

RESULTS

S G1M G1

M MSS

When a cell in theS phase was fused with a cell in G1, the G1 nucleus immediatelyentered the Sphase—DNA was synthesized.

When a cell in theM phase was fused with a cell in G1, the G1 nucleus immediatelybegan mitosis—aspindle formed andchromatin condensed,even though thechromosome had notbeen duplicated.

The Cell Cycle Control System• cell cycle control system- a set of events that

occurs in a sequence to regulate the cell cycle.

• The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

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• the G1 checkpoint is the most important

• Most cells that make it past G1 go on to divide.

• If the cell does not receive the go-ahead signal, it will go into a non-dividing state called the G0 phase. (ex. Kinetochores didn’t attach correctly)

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The Cell Cycle Control System

Cyclin and Cdks• cyclin – a regulatory protein that builds up in the G2

phase.• Cyclin dependent kinases (Cdks) – a regulatory

protein that is always present waiting for the arrival of cyclin.

• MPF (maturation-promoting factor) - cyclin and Cdks combined to trigger a passage through G2 and Mitosis.

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External Signals• growth factors - proteins released by certain cells that

stimulate other cells to divide• platelet-derived growth factor (PDGF) stimulates the

division of human fibroblast cells (connective tissue). This is released when cut from blood platelets so healing can begin.

• density-dependent inhibition - crowded cells stop dividing

• anchorage dependence – cells must be attached to something in order to divide

External Signals cont’d

Cancer Cells• Cancer cells do not respond normally to the cells cycle

check points or external signals.• Cancer cells may not need growth factors to grow and

divide:– They may make their own growth factor– They may convey a growth factor’s signal without

the presence of the growth factor– They may have an abnormal cell cycle control

system

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• Transformation - A normal cell is converted to a cancerous cell

• If the cancer stays at the original site, the lump is called a benign tumor

• Malignant tumors spread to surrounding tissues through metastasis (transport by blood vessels.)

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Transformation

You should now be able to:1. Describe the organization eukaryotic genome.

2. List the phases of the cell cycle; describe the sequence of events during each phase

3. List the phases of mitosis and describe the events characteristic of each phase

4. Draw the phases of mitosis and describe the structures and events in each phase.

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5. Compare and contrast cytokinesis in animals and plants

6. Describe the process of binary fission in bacteria.

7. Describe cytoplasmic signaling and checkpoints as well as external signaling of cells.

8. Explain how the abnormal cell division of cancerous cells escapes normal cell cycle controls

9. Distinguish between benign and malignant tumors.

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