Cell Growth and Division(Mitosis and Meiosis)

Prentice Hall pg. 240-255, 275-278Chapters 10, 11.4

Biology 12004-2005

10–1 Cell GrowthA. Limits to Cell Growth

1. DNA “Overload”2. Exchanging Materials3. Ratio of Surface Area to Volume 4. Cell Division

10–2 Cell DivisionA. ChromosomesB. The Cell CycleC. Events of the Cell CycleD. Mitosis

1. Prophase2. Metaphase3. Anaphase4. Telophase

E. Cytokinesis10–3 Regulating the Cell Cycle

A. Controls on Cell DivisionB. Asexual Reproduction C. Cell Cycle Regulators

1. Internal Regulators2. External Regulators

D. Uncontrolled Cell Growth

11–4 MeiosisA. Chromosome NumberB. Phases of Meiosis

1. Meiosis I2. Meiosis II

C. Gamete FormationD. Comparing Mitosis and Meiosis

10-1 Cell GrowthWhy do cells divide rather than continuing to grow indefinitely?

• On average, the cells of an adult animal are no larger than those of a young animal—there are just more of them.

• The larger a cell becomes, the more demands the cell places on its DNA and the more trouble the cell has moving enough nutrients and wastes across the cell membrane.

Limits to Cell GrowthDNA “Overload”• information that controls a cell’s function is in DNA

(eukaryotic cells, DNA is found in the nucleus of the cell)• when a cell is small, the information stored in that DNA

meets the cell’s needs• cell must have enough DNA to support the protein needs

of the cell• in many large cells, more than one nucleus is present• large amounts of DNA in many nuclei ensure that cell

activities are carried out quickly and efficiently

Exchanging Materials • food, oxygen, and water enter a cell through its

cell membrane. Waste products leave in the same way.

• the rate at which this exchange takes place depends on the surface area of the cell, which is the total area of its cell membrane

• the rate at which food and oxygen are used up and waste products are produced depends on the cell’s volume (the space inside the cell)

• understanding the relationship between a cell’s volume and its surface area is the key to understanding why cells must divide as they grow

Ratio of Surface Area to Volume in Cells

• as the cell gets bigger, the volume increases much more rapidly than the surface area, causing the ratio of surface area to volume to decrease

• this decrease creates serious problems for the cell

Cell Size

Surface Area (length x width x 6)

Volume (length x width x height)Ratio of Surface Area to Volume

Surface area-to-volume ratio

• If cell size doubled, the cell would require eight times more nutrients and would have eight times more waste to excrete.

• The surface area, however, would increase by a factor of only four.

• The cell would either starve to death or be poisoned from the buildup of waste products.

Surface area = 6 mm2 Volume = 8 mm3

Surface area = 24 mm2 Volume = 8 mm3

1 mm 1 mm

1 mm

2 mm 2 mm

2 mm

4 mm 4 mm

4 mm

10-2 Cell Division• Cell division is the process by which

two daughter cells are produced from one cell.

• Cell division results in two cells that are identical to the original, parent cell.

includes

is divided into (Mitosis) is divided into

Cell Cycle

Cell DivisionM phaseInterphase

G1 phase S phase ProphaseG2 phase Metaphase Telophase

Anaphase Cytokinesis

G2 phase

S phase

G1 phase

M P

hase

The discovery of chromosomes• Chromosomes are the carriers of the genetic material that is copied and passed from

generation to generation of cells• Accurate transmission of chromosomes during cell division is critical• The chromosomes condense into compact, visible structures at the beginning of cell

division• The cells of every organism have a specific number of chromosomes

fruit flies-8 ; human cells-46; carrot cells-18

• A Human Chromosome. This is a human chromosome shown as it appears through an electron microscope. Each chromosome has two sister chromatids attached at the centromere.

Sister chromatids

Centromere

Chromosome Supercoil within

chromosome

Continued coiling within

supercoilHistone H1

Nucleosome

DNA

The Cell Cycle• During the cell cycle, a cell

grows, replicates its DNA, prepares for division, and divides to form two daughter cells, each of which then begins the cycle again.  

• The majority of a cell’s life (75%) is spent in the growth period known as interphase.

• Following interphase, a cell enters its period of nuclear division called mitosis.

• Following mitosis, the cytoplasm divides, separating the two daughter cells.

Interphase

Mitosis

Interphase• Interphase, the busiest

phase of the cell cycle, is divided into three parts (G1, S, G2) .– G1: cells increase in size

and synthesize new proteins, organelles and do most of their growing.

– S: chromosomes are replicated and the synthesis of DNA molecules takes place. Key proteins associated with the chromosomes are made.

– G2: After the chromosomes have been duplicated, the cell enters another shorter growth period in which mitochondria and other organelles are manufactured and cell parts needed for cell division are assembled.

DNA synthesis and replication

Centrioles replicate; cell prepares for

division

Rapid growth and metabolic activity

Interphase

G1

G2

S

Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase.

• Mitosis is the division of the cell nucleus

Centrioles

Chromatin

Interphase

Nuclear envelope

Cytokinesis

Nuclear envelope reforming

Telophase

Anaphase

Individual chromosomes

Metaphase

Centriole

Spindle

CentrioleChromosomes

(paired chromatids)

Prophase

Centromere

Spindle forming

Prophase• centrioles, two tiny structures located in the cytoplasm near the nuclear envelope, separate and take up positions on opposite sides of the nucleus

• condensed chromosomes become attached to spindle fibers at a point near the centromere

• chromosomes coil more tightly (the chromosomes become visible)

• the nucleolus disappears• the nuclear envelope breaks down

Spindle fibers

Disappearing nuclear envelope

Doubled chromosome

Sister chromatids

Centromere

Metaphase • the chromosomes line up across the center of the cell,

called the equator (MIDDLE)• microtubules connect the centromere of each

chromosome to the poles of the spindle

Centromere

Sister chromatids

Anaphase• the centromeres that join the sister chromatids separate,

allowing the sister chromatids to separate and become individual chromosomes

• chromosomes continue to move until they have separated into two groups near the poles of the spindle

• Anaphase ends when the chromosomes stop moving.

Telophase • the chromosomes, which were distinct and condensed, begin

to disperse into a tangle of dense material (unravel)• nuclear envelope re-forms around each cluster of

chromosomes• the spindle begins to break apart, and a nucleolus becomes

visible in each daughter nucleus. • Mitosis is complete. However, the process of cell division is

not complete.

Nuclear envelope reforming

Cytokinesis• As a result of mitosis, two nuclei—each with a duplicate set of

chromosomes—are formed, usually within the cytoplasm of a single cell.

• Cytokinesis usually occurs at the same time as telophase.• cytokinesis is the division of the cytoplasm itself. • Cytokinesis can take place in a number of ways.

Nuclear envelope reappears

Two daughter cells are formed

Cytokinesis in plant and animal cellsAnimal cells• the cell membrane

is drawn inward until the cytoplasm is pinched into two nearly equal parts. Each part contains its own nucleus and cytoplasmic organelles.

Plants Cells• a structure known as the cell plate forms midway between the divided

nuclei, as shown below. • The cell plate gradually develops into a separating membrane. A cell wall

then begins to appear in the cell plate.

10–3 Regulating the Cell Cycle • cell growth and cell division are highly controlled• different cells move through the cell cycle at various rate

– muscle cells and nerve cells do not divide at all once they have developed

– skin cells,cells of the digestive tract, and cells in the bone marrow that make blood cells, grow and divide rapidly throughout life. Such cells may pass through a complete cycle every few hours.

• This process provides new cells to replace those that wear out or break down.

• Scientists spend a great deal of time studying how cell division is regulated

Control of Cell Division1. place some cells in a petri dish containing nutrient broth (broth provides food) -most cells will grow until they form a thin layer covering the bottom of the dish2. when cells come into contact with other cells, they respond by not growing3. if cells are removed from the center of the dish, 4. the cells bordering the open space will begin dividing until they have filled the

empty space (5) -These experiments show that the controls on cell growth and cell division can be

turned on and off.

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34

5

Control of Cell Division in the body• Something similar to the previous experiment happens

within the body. • When an injury such as a cut in the skin or a break in a

bone occurs, cells at the edges of the injury are stimulated to divide rapidly.

• This action produces new cells, starting the process of healing.

• When the healing process nears completion, the rate of cell division slows down, controls on growth are restored, and everything returns to normal.

Normal Growth and Repair• Control of Mitosis - normal growth and repair occur as

cells produce new cells by mitosis and as new cells grow larger

1) differentiation - changes that take place in cells as they develop; determined in part by the surrounding cells- cells may differentiate into muscle, blood, bone, skin, etc.

2) growth factors - chemicals that stimulate the division and differentiation of new cells during growth; highly specific & cause mitotic growth of only one type of cell

3) regeneration - process of growing back lost body parts; cells at wound site become less specialized and divide; new cell differentiate into needed tissues

- lizard - may regenerate lost tail which includes many types of tissues

- birds & mammals - may generate some tissues but not entire limbs

Asexual reproduction - one parent produces offspring by cell division

• DNA identical to parent

• unicellular organisms - can reproduce through mitotic division

• ex: bacteria, fungi, plants, some animals

Multicellular organisms - 3 forms of mitotic division

1) budding - organisms grown from a piece of another organism- bud forms on some part

of parent; identical to parent

- separates from parent to continue growth on

its own- yeasts, hydra

2) fragmentation - separated pieces of parent

organism develop into new individuals- flatworm - cut in half & each half forms new

organism

- sea star - any piece containing part of central disk can grow into a new organism

3) vegetative reproduction - new plants grow from the

stems, roots, or leaves of an existing plant- runners - spider plants

- bulbs of tulips

- potato tubers, etc.

Cell Cycle Regulators• cells in mitosis contain a protein that when injected into a

non-dividing cell, would cause a mitotic spindle to form• cyclins regulate the timing of the cell cycle in eukaryotic cells.

• Internal regulators- regulatory proteins inside the cell• External regulators- growth factors (usually work most during embryonic development and

wound healing)

A sample of cytoplasm is removed from a cell in mitosis.

The sample is injected into a second cell in G2 of interphase.

As a result, the second cell enters mitosis.

Uncontrolled Cell Growth• cancer cells do not respond to the signals that regulate the

growth of most cells• they form masses of cells called tumors that can damage

the surrounding tissues• cancer cells may break loose from tumors and spread

throughout the body, disrupting normal activities and causing serious medical problems or even death

What causes the loss of growth control that results in cancer? • smoking tobacco• radiation exposure• viral infection

Normal cells Cancer Cells

Meiosis and sexual reproduction• Sexual reproduction - when chromosomes of

two parents combine to produce offspring; chromosomes are contained in reproductive cells called gametes (eggs or sperm)

• Meiosis - forms gametes; only occurs in eukaryotic cells- results in the production of daughter cells that have half the number of chromosomes of the parent- daughter cells are not alike because their chromosomes may be different

• Mitosis = 2 daughter cells with each containing 2 sets of chromosomes

• Meiosis = 4 daughter cells with each containing 1 set of chromosomes

Chromosome Number• diploid cells - (2N=diploid number) contain two complete sets

of chromosomes; almost all body cells are diploid- humans have 23 chromosomes in 1 set; body cells have 46 chromosomes (2 sets)

- different organisms have different numbers of chromosomes

- fruit fly (Drosophila)chromosomes. Each of the fruit fly’s body cells has 8 chromosomes.

• Haploid cells - (N=haploid number) contain one

set of chromosomes; found only in gametes; produced through meiosis- human gametes have 23 chromosomes (1 set)

• fertilization - when an egg cell and a sperm cell of the same type of organism join to produce a new individual- zygote - cell that results from fertilization; first cell of new individual; contains two complete sets of chromosomes (one from each gamete)

- homologous pairs -

matching pairs of chromosomes in a diploid cell (every chromosome in the egg has a matching chromosome from the sperm); pairs code for the same traits

- chromosome pairs are known as tetrads

Crossing Over• Crossing over is the exchange of genes between pairs of

homologous chromosomes• produces new combinations of genes• only occurs during prophase I of meiosis

because homologous pairs are still tangled together

• chromosomes break where they meet and exchange genes

• Crossing over increases genetic variability

Meiosis I

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II

Phases of Meiosis - includes meiosis I and meiosis II

Meiosis I - divides homologous chromosome pairs (tetrads); produces 2 cells from 1 parent

cell1) Prophase I - chromosomes become thick and visible; nucleoli and nuclear envelope disappear; spindle fibers form

2) metaphase I - homologous chromosomes are together and line up in the middle of the cell

3) anaphase I - homologous pairs of chromosomes separate from each other; spindle fibers pull whole sister chromatids to opposite ends of the cell

4) telophase I - nuclear envelope reforms (in most organisms) - cytokinesis takes place - resulting cells have sister chromatids (contain one chromosome from each parent); (cytokinesis is NOT part of meiosis)

Meiosis II - divides sister chromatids; similar to mitosis

1) prophase II - nuclear envelope disappears; spindle fibers form

2) metaphase II - sister chromatids line up in center of cell

3) anaphase II - centromere divides; sister chromatids of each chromosome separate when centromere is pulled by spindle fibers

4) telophase II - nuclear envelope reforms; cytokinesis takes place; 4 single chromosome, haploid daughter cells formed

Gamete Formation• In male animals, the

haploid gametes produced by meiosis are called sperm.

• In some plants, pollen grains contain haploid sperm cells.

• This female gamete is called an egg in animals and plants

• only one of the cells produced by meiosis is involved in reproduction

• This egg cell receives most of the cytoplasm, the other three cells produced are known as polar bodies, which are reabsorbed by the body

Meiosis and evolution• Evolution - process of change in populations over

time; meiosis always brings change• variation - differences among members of a

population; may allow for survival of a population- results from meiosis, fertilization, & may result from crossing over

• environmental changes can be disastrous for organisms; variety of traits within the population increases the chance that some individuals will survive the change

Comparison of Mitosis and Meiosis• Mitosis results in the production of two genetically

identical diploid cells, whereas meiosis produces four genetically different haploid cells.

Mitosis Meiosis# of divisions 1 2

# of daughter cells produced

2 4

Type of reproduction associated with

Asexual Sexual

Diploid or haploid cell produced?

Diploid Haploid

Is daughter cell identical to parent cell

Yes No

Gamete or body cell produced?

Body (somatic) Gamete (egg or sperm)