Chapter 8

Cell Cycle - Mitosis

WHAT CELL REPRODUCTION ACCOMPLISHESCell Reproduction –

– may result in the birth of new organisms

– more commonly involves the production of new cells

• When a cell undergoes reproduction, or cell division, two “daughter” cells are produced that are genetically identical

– to each other

– to the “parent” cell.

Cell Theory – Cells Come From Cells

• Cell division plays important roles in the lives of organisms.

• Cell division

– replaces damaged or lost cells

– permits growth

– allows for reproduction

WHAT CELL REPRODUCTION ACCOMPLISHES

• Before a cell divides, it duplicates all of its chromosomes through the process of replication, resulting in two copies called sister chromatids containing identical genes.

• Two sister chromatids are joined together tightly at a narrow “waist” called the centromere.

Eukaryotic Chromosomes

• When the cell divides, the sister chromatids of a duplicated chromosome separate from each other.

• Once separated, each chromatid is

– considered a full-fledged chromosome

– identical to the original chromosome.

Eukaryotic Chromosomes

Figure 8.UN01

Duplicationof all

chromosomes

Geneticallyidentical

daughter cells

Distribution viamitosis

• In asexual reproduction,

– single-celled organisms reproduce by simple cell division in which the parent cell and daughter cells have identical DNA

– there is no fertilization of an egg by a sperm.

• Some multicellular organisms, such as sea stars, can grow new individuals from fragmented pieces.

• Growing a new plant from a clipping is another example of asexual reproduction.

• Producing more skin cells is yet another example of asexual reproduction.

WHAT CELL REPRODUCTION ACCOMPLISHES

• Sexual reproduction requires fertilization of an egg by a sperm using a special type of cell division called meiosis.

• Thus, sexually reproducing organisms use

– meiosis for reproduction

– mitosis for growth and maintenance.

WHAT CELL REPRODUCTION ACCOMPLISHES

• Chromosomes are

– made of chromatin, fibers composed of roughly equal amounts of DNA and protein molecules

– not visible in a cell until cell division occurs.

Eukaryotic Chromosomes

Each eukaryotic chromosome contains one very long DNA molecule, typically bearing thousands of genes.

Figure 8.3

Chromosomes

LM

Duplicated chromosomes(sister chromatids)

TE

M

Tight helical fiber

Thick supercoil

TE

M

Centromere

Nucleosome

“Beads on a string”

Histones

DNA double helix• The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding.

• Histones are proteins used to package DNA in eukaryotes.

• Nucleosomes consist of DNA wound around histone molecules.

TE

M

Nucleosome

“Beads on a string”

Histones

DNA double helix

Figure 8.2

Number of chromosomes in body cells

Indian muntjac deer

Species

Opossum

Koala

Human

Mouse

Giraffe

Buffalo

Dog

Red viscacha rat

Duck-billed platypus

102

78

60

54

46

40

30

22

16

6

The Cell Cycle

• A cell cycle is the ordered sequence of events that extends

– from the time a cell is first formed from a dividing parent cell to its own division into two cells.

• The cell cycle consists of two distinct phases:

1. interphase

2. the mitotic phase.

Figure 8.UN03

Interphase Cell growth and

chromosome duplication

G2

Mitotic(M) phase

S phase DNA synthesis; chromosome duplication

G1

Genetically identical“daughter”cells

Cytokinesis (division ofcytoplasm)

Mitosis (division ofnucleus)

• Most of a cell cycle is spent in interphase.

• During interphase a cell

– performs its normal functions

– Replicates all of its DNA – sister chromatids

– doubles everything in its cytoplasm

– grows in size

The Cell Cycle

• Interphase can be broken down into 3 distinct phases

1. G1 phase – cells increase in size, make new proteins and organelles

2. S phase – chromosomes replicate, DNA synthesis

3. G2 phase – completion of organelle production

The Cell Cycle - Interphase

• The mitotic (M) phase includes two overlapping processes:

1. mitosis, in which the nucleus and its contents divide evenly into two daughter nuclei

2. cytokinesis, in which the cytoplasm is divided in two

The Cell Cycle

Mitosis and Cytokinesis

• During mitosis the mitotic spindle, a football-shaped structure of microtubules, guides the separation of two sets of sister chromatids.

• Spindle microtubules grow from structures within the cytoplasm called centrosomes.

• Mitosis consists of four distinct phases:

1. Prophase

2.Metaphase

3.Anaphase

4.Telophase

Mitosis and Cytokinesis

© 2013 Pearson Education, Inc.

Bioflix Animation: Mitosis

1. Prophase

– Changes occur in nucleus and cytoplasm

– Chromatin fibers coil in the nucleus into visible chromosomes

– Each chromosome exists as 2 identical sister chromatids connected at the centromere

– Microtubules start to grow from each centrosome and the centrosomes move away from each other

– Nuclear envelope breaks into pieces

– Spindle microtubules attach to the centromeres of the chromosomes and move them toward the center of the cell.

Mitosis and Cytokinesis

Figure 8.7a

Nuclearenvelope

Plasmamembrane

Chromosome(two sister chromatids)

Spindle microtubules

Fragments of nuclear envelopeCentrosome

Centromere

Early mitotic spindle

Centrosomes (with centriole pairs) Chromatin

PROPHASEINTERPHASE

1.Metaphase

– Mitotic spindle is fully formed

– Chromosomes line up between the two poles of the spindle (between the two centrosomes)

– Microtubules attached to the centromeres play tug of war keeping the chromosomes in the middle of the cell.

Mitosis and Cytokinesis

1.Anaphase

– Sister chromatids separate and move toward opposite poles of the cell as the spindle microtubules shorten

– Microtubules not attached to chromosomes get longer and elongate the cell.

Mitosis and Cytokinesis

Figure 8.7b

ANAPHASEMETAPHASE TELOPHASE AND CYTOKINESIS

Spindle Daughterchromosomes

Cleavagefurrow

Nuclearenvelopeforming

1.Telophase and Cytokinesis

– Chromosomes have reached the opposite ends of the cell

– Nuclear envelope forms

– Chromosomes uncoil

– Spindle dissapears

– Cytoplasm divides

– Cell pinches in two (cleavage furrow in animals)

Mitosis and Cytokinesis

• Cytokinesis usually

– begins during telophase,

– divides the cytoplasm

– is different in plant and animal cells.

Mitosis and Cytokinesis

• In animal cells, cytokinesis

– is known as cleavage

– begins with the appearance of a cleavage furrow, an indentation at the equator of the cell.

Mitosis and Cytokinesis

Figure 8.8aa

Cleavage furrowContracting ring ofmicrofilaments

Daughter cells

• In plant cells, cytokinesis begins when vesicles containing cell wall material collect at the middle of the cell and then fuse, forming a membranous disk called the cell plate.

Mitosis and Cytokinesis

Figure 8.8ba

Wall of parent cellCell plateforming Daughter nucleus

LM

Cancer Cells: Growing Out of Control

• Normal plant and animal cells have a cell cycle control system that consists of specialized proteins called cyclins, which send “stop” and “go-ahead” signals at certain key points during the cell cycle.

What Is Cancer?

• Cancer is a disease of the cell cycle.

• Cancer cells do not respond normally to the cell cycle control system.

• Cancer begins when a normal cell undergoes transformation in which a mutation in the genetic code (DNA) occurs. This mutation interferes with the production of proteins vital in the cell cycle control system.

• Cancer cells can form tumors, abnormally growing masses of body cells.

• If the abnormal cells remain at the original site, the lump is called a benign tumor.

• The spread of cancer cells beyond their original site of origin is metastasis.

• Malignant tumors can

– spread to other parts of the body and

– interrupt normal body functions.

• A person with a malignant tumor is said to have cancer.

What Is Cancer?

Figure 8.9

A tumor growsfrom a singlecancer cell.

Cancer cells invadeneighboring tissue.

Metastasis: Cancercells spread throughlymph and bloodvessels to other partsof the body.

Glandulartissue

Bloodvessel

Tumor

Lymphvessels

Cancer Treatment

• Cancer treatment can involve

– radiation therapy, which damages DNA and disrupts cell division, and

– chemotherapy, the use of drugs to disrupt cell division.

Cancer Prevention and Survival

• Certain behaviors can decrease the risk of cancer:

– not smoking

– exercising adequately

– avoiding exposure to the sun

– eating a high-fiber, low-fat diet

– performing self-exams

– regularly visiting a doctor to identify tumors early

THE END