The Cell Cycle

The repeating sequence of

growth & division of a

cell.

G1 S

G2

Cell Cycle: Outline

I. Interphase A. G1B. SC. G2

II. Cell DivisionA. Mitosis

1. P2. M3. A4. T

B. Cytokinesis

Why do cells divide?

1. They need to “grow” the organism or repair parts of the organism that are dead or damaged.

Why do cells divide?

2. As cells grow larger:• they take in more food through the cell

membrane• they produce more waste which must exit

through the cell membrane SO: they can’t just get bigger and

bigger……they need more cell membrane!

Why do cells divide?

If cells GROW too large, there is not enough cell membrane to be efficient. The ratio of surface area to volume is too LOW:• difficulty obtaining nutrients• difficulty eliminating wastes• diffusion across the cell membrane is

impaired

Why do cells divide?

Cells divide so they can increase surface area to volume ratio stay small and efficient.

In other words…THEY divide to get MAXIMUM SURFACE AREA to VOLUME ratio!

Two types of cell division:

Produces EXACT copies of first or “parent” cells[“biological xerox”]

Occurs only in sex cells[producing egg & sperm]

Mitosis Meiosis• Occurs in ALL regular

body cells[like skin cells]

• Produces genetically DIFFERENT cells from the “parent” cells

[variation in DNA]

Get your note cards!

Put them in chronological order based on what you see…

Interphase

Nucleus clearly seen Growth and repair G1, S, G2 phases Chromosomes are not

visible; DNA is in the form of

chromatin DNA is copied or

REPLICATED Centrioles duplicate

centrioles

nucleus chromatin

Mitosis: Prophase

Chromatin coils to form visible chromosomes

Nuclear membrane and nucleolus disappear

Spindle fibers form between centrioles

Spindle fibers

Sister chromatid

Centromere

Mitosis: Metaphase

Chromosomes move to the equator

spindle fibers attach to chromatids by centromeres

Equator

Mitosis: Anaphase

Centromeres divide Sister chromatids

pulled apart toward opposite poles

Sister chromatid

Mitosis: Telophase/ Cytokinesis

Chromosomes arrive at the opposite poles

NEXT: Cytoplasm divides Nucleolus and nuclear

membrane reappear Chromosomes begin

to uncoil

Nuclear membrane

Cytokinesis

Animals

•Cytoplasm pinches near equator

•called cytokinesis

•Two new cells form

Plants

•Plasma membrane does not pinch

•Cell plate forms across equator

•Two new cells form

Division is Complete

Results

Guaranteed genetic continuity

Two new cells with identical chromosomes to the parent cell

What happens when the cell cycle goes wrong?

CANCER! Cancer is uncontrolled cell division Caused by one of (or a combination

of):• Genetic predisposition• Environmental exposure to carcinogens

Meiosis

The “OTHER” cell division

FAQ for MEIOSIS

1. Why meiosis? For making gametes (egg or sperm), the

specials cells for sexual reproduction. Meiosis reduces the genetic number

for gametes. Without meiosis, sexual reproduction

would make offspring with too much DNA!!

2. What happens? •cell division two times, with no

interphase in between

(Meiosis I then Meiosis II)•Each division goes through P-M-A-T

FAQ for MEIOSIS (cont’d)

3. What’s the result? •4 new daughter cells •each have half the number of

chromosomes as original cells •DNA that is NOT identical to original

cell or to each other

FAQ for MEIOSIS (cont’d)

Meiosis IInterphase I

Prophase I Metaphase I Anaphase ICells undergo a round of DNA replication, forming duplicate Chromosomes.

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Spindle fibers attach to the chromosomes.

The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Interphase I

Prophase I*

Metaphase I Anaphase I

Cells undergo a round of DNA replication, forming duplicate Chromosomes.

Each chromosome pairs with its corre-sponding homo-logous chromo-some to form a tetrad. Crossing over occurs here…

Spindle fibers attach to the chromosomes.

The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Meiosis I

Crossing-Over

Crossing-Over

Crossing-Over

Meiosis I

Interphase I Prophase I

Metaphase I

Anaphase I

Cells undergo a round of DNA replication, forming duplicate Chromosomes.

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Spindle fibers attach to the chromo-somes.

The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Meiosis I

Interphase I Prophase I Metaphase I

Anaphase I

Cells undergo a round of DNA replication, forming duplicate Chromosomes.

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Spindle fibers attach to the chromosomes.

The fibers pull the homologous chromosomes toward the opposite ends of the cell.

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 results in four haploid (N) daughter cells.

Meiosis II

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

Prophase IIMetaphase 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 results in four haploid (N) daughter cells.

Meiosis II

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

Prophase IIMetaphase 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 results in four haploid (N) daughter cells.

Meiosis II

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

Prophase II Metaphase IIAnaphase 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 results in four haploid (N) daughter cells.

Meiosis II

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 II

The 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 results in four haploid (N) daughter cells.

Meiosis II

Gamete formation in humans

In males, every cell that undergoes meiosis produces 4 gametes (sperm).

…called spermatogenesis In females, meiosis occurs once

each menstrual cycle and only one of the four cells made in meiosis actually matures into an egg.

… called oogenesis

Comparing Mitosis & Meiosis

Mitosis1. Occurs in all REGULAR

body cells.2. Only 1 division.3. Produces 2 daughter

cells that are identical to parent cell and are diploid (2N).

4. No genetic variability (differences)

5. For growth and repair!!!!!

Meiosis1. Occurs only in sex cells

(sperm and egg)2. 2 divisions.3. Produces 4 daughter cells that

are NOT identical to the parent cell and are haploid (1N).

4. Genetic variability from one generation to the next.

5. For sexual reproduction!