Http:// Cellular Basis of Reproduction and Inheritance Lecture 8, Mitosis

http://www.uq.edu.au

Cellular Basis of Reproduction and InheritanceLecture 8, Mitosis

2

Much of the text material in the lecture notes is from our textbook, “Essential Biology with Physiology” by Neil A. Campbell, Jane B.

Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship. Other sources were sometimes used, and are noted.

3

Where We Are

Cells

Genetics

Evolution and Diversity

Ecology

Animal Structure and Function

http://www.robslade.demon.co.uk

4

Outline

• Overview• Asexual and sexual reproduction• Chromosomes• Mitosis and the cell cycle• Cytokinesis• Cancer cells• Words and terms to know• Possible test items

5

Reproduction

• Reproduction is often associated with the formation of new offspring.• Reproduction, however, is not only about forming new organisms—it

also occurs in many types of cells.• Our skin has an outer layer of dead epithelial cells—underneath are

layers of epithelial cells dividing and undergoing chemical reactions.

6

Human Skin

http://publications.nigms.nih.gov

http://www.web-books.com

7

Cell Division

• New epithelial cells move toward the skin surface to replace cast-off dead cells.

• New cells are formed in our tissues to help heal wounds when we are injured.

• This form of cellular reproduction, or cell division, is a lifelong process for growth to tissue repair.

8

Genetic Transmission

• When a cell divides, the two daughter cells are identical to each other and the parent cell.

• In this context, daughter—the term used by biologists—does not imply gender.

• The parent cell duplicates its set of chromosomes before it splits into two daughter cells.

• During cell division, the two sets of chromosomes are distributed to the daughter cells.

• Each daughter cell receives identical sets of chromosomes containing identical genes.

9

Asexual Reproduction

• Some single-cell organisms, such as amoeba, reproduce through simple cell division.

• The offspring are genetic replicas of the one parent.• The process is known as asexual reproduction since it does not involve

fertilization of an egg by a sperm.• Growing clippings from houseplants often involves asexual reproduction.• In asexual reproduction, the parent and its offspring have identical genetic

material.• The process is known as mitosis.

10

Amoeba

http://biophysik.uni-bremen.de

Microscopic view

http://www.gutenburg.org

Cell division

Amoeba reproduce asexually.

11

Sea Stars and Crab

http://jacobtanenbaum.com

Two sea stars and a crab—sea stars and crabs can replace their appendages through mitosis.

12

Sexual Reproduction

• Sexual reproduction requires fertilization of an egg by a single sperm.• The production of egg and sperm cells involves a form of cell division

known as meiosis.• The two types of cell division, mitosis and meiosis, are involved in the

lives of all sexually-reproducing organisms.• Next week, we will cover meiosis and how the chromosomal material is

transmitted in sexual reproduction.

Computer-generated imagehttp://neurophilosophy.files.wordpress.com

Egg and sperm

13

Genes• A genome is a complete set of an organism’s genes—about 35,000 in

humans.• The vast majority of the genome is located in the chromosomes in the

cell nucleus of each eukaryotic cell.• The genes are formed from the four nucleotide pairs in the cell’s DNA.• Genes are also found on small DNA molecules in the mitochondria and

chloroplasts.

http

://w

ww

.scf

bio-

iitd.

res.

in

14

DNA

http://www.csulb.edu

Two different molecular representations of DNA—modified ball-and stick and space-filling models.

15

DNA Nucleotide Structure

Nitrogenous bases—

thymine (T) and cytosine (C) have single-ring structures

Adenine (A) and guanine (G)have double-ring structures

Sugar(deoxyribose)

Phosphategroup

CH2

Deoxyribose + phosphate group = sugar-phosphate backbone

DNA base pairing rules: A with T, and G with C

16

Eukaryotic Chromosomes

• A eukaryotic chromosome contains a single long DNA molecule bearing most of the organism’s genes.

• The number of chromosomes varies by species—human cells usually have 46, dog cells have 78, and mouse cells have 40.

• Chromosomes consist of chromatin, made-up of DNA and a ‘wrapping’ of protein molecules.

• These proteins help organize the chromosomes and control gene activity.

Chromatin in packed form, computer-generated imagehttp://www.cgl.ucsf.edu

17

Chromosomes Prior to Cell Division

• For much of a cell’s lifecycle, the chromosomes are a mass of long fibers much longer than the diameter of the cell nucleus.

• As a cell prepares to divide the chromatin fibers coil up and form compact chromosomes.

• The chromosomes are visible under a light microscope as shown below.• When a cell is not dividing, the chromosomes are too thin to be visible in a

light micrograph.

http

:/z.

abou

t.co

mChrysanthemum (plant) cell

18

DNA Packing

• The DNA in each chromosome is packed into a multi-level system of coils and folds.

– At the first level, the DNA has the appearance of beads on a string. – Each bead is called a nucleosome, consisting of DNA wound around

eight histone molecules.– At the second level, the beaded chain is wrapped into a tight helix-like

fiber.– At the third level, the helical fibers are arranged into a much thicker coil.

• The coils and folds enable large amounts of DNA to be packed into a small cell nucleus.

19

Nucleosome

First level of packing—the DNA is the red cord-like material.

http://www.bio.davidson.edu

20

DNA Packing

http://www.mun.ca

Second and third levels of packing.

‘Recording’ the Golden Gate Bridgehttp://www.sounddesign.unimelb.edu.au

21

Chromatids

• A cell duplicates all of its chromosomes through the process of replication before cell division begins.

• Each chromosome now has two copies called sister chromatids (the term does not imply gender).

• The sister chromatids are joined at their waists at a junction known as the centromere.

22

Sister Chromatids

http://www.cbs.dtu.dk

Sister chromatids

23

Chromatid Separation

• The sister chromatids separate from each other during cell division to form an identical chromosome in each daughter cell.

• Each daughter cell receives a complete, identical set of chromosomes.• A dividing human skin cell has 46 duplicated chromosomes.• Each of its two daughter cells will have 46 single chromosomes to form

23 pairs.• Millions of cells must divide every second to maintain over 60 trillion cells

in the human body.

24

Cell Cycle

• The rate at which cells divide depends on their role within the organism.• Some cells divide as often once a day, others less often, and others (such

as muscle cells and neurons) usually not at all.• The cell cycle is a sequence of events from the time a new cell is formed

until it divides and forms two new cells.• The cell division phases are the process of mitosis, which occurs after an

interphase.

25

Cell Cycle—Interphase

http://bhs.smuhsd.org

26

Interphase—G1 Stage

• About 90 percent of the cell cycle is spent in interphase when the cell performs its normal metabolic functions.

• In the first growth stage (G1) the cell doubles the number of organelles in preparation for cell division.

• The cell grows in size to accommodate this ongoing doubling process.

27

Interphase—S and G2 Stages

• In the synthesis stage (S), the DNA is doubled and the chromosomes are duplicated.

• Each chromosome consists of two sister chromatids in preparation for cell division.

• The sister chromatids have identical genetic content for the formation of daughter cells.

• In the second growth stage (G2), further cell growth occurs.

28

Cell Cycle—Mitotic Phase


29

Mitotic or M Phase

• The part of the cell cycle when the cell divides is called the mitotic or M- phase.

• The M-phase has two overlapping components: mitosis and cytokinesis.• In mitosis, the chromosomes are evenly distributed into the two daughter

cell nuclei.• In cytokinesis, the cytoplasm of the parent cell is divided in two individual

compartments (daughter cells).

30

End of M-Phase

• At the end of the M-phase, each daughter cell contains its own nucleus, cytoplasm, organelles, and plasma membrane.

• Mitosis and cytokinesis produce two genetically-identical daughter cells.

31

Mitosis

• Mitosis is an accurate mechanism for allocating genetic material to two daughter cells.

• In yeast cells, errors in chromosomal division occur about once in every 100,000 cell divisions.

• Mitosis is unique to eukaryotic cells.• Prokaryotic cells have one chromosome, and use a simpler mechanism

for allocating DNA to daughter cells.

32

Stages of Mitosis

• Although mitosis is a continuum of cell activity, four stages are commonly described: – Prophase– Metaphase– Anaphase– Telophase

• During mitosis, the movement of chromosomes is dependent on the the mitotic spindle, a football-shaped structure of microtubules consisting of proteins.

• The spindle grows from centrosomes, clouds of material in the cytoplasm that contain centrioles.

33

Mitotic Spindle

http://www.ornl.gov

http://mcb.berkeley.edu

Centrosome and centrioles

The spindle has two poles and an equator

34

Onion Field

Elba, New Yorkhttp://content.answers.com

Onions are often grown in rich soil that was once lake or river bottom.

35

Onion Root Cell

• Onion root cells are commonly used to demonstrate mitosis—it is one of the biological materials we will use in a lab session.

• The cells have large chromosomes which take stain well.• Mitosis in onion root cells is readily observed through a light microscope.

http://www.mytinyplot.co.uk

36

Mitosis, Onion Cell

http://www.sep.alquds.edu

1. Interphase (G2)2. Prophase3. Metaphase4. Anaphase5. Telophase

Mitosis consists of phases 2 through 5

37

Interphase

http://www.microscopy-uk.org.uk


38

Interphase

• Interphase is the period of cell growth when the cell makes new molecules and organelles.

• In late interphase (G2), the cytoplasm contains two centrosomes and their centriole pairs.

• The chromosomes are duplicated, but they cannot be visually distinguished since they are still loosely packed in chromatin fibers.

• The nucleolus is visible since it is producing ribosomes needed for protein synthesis during cell division.

39

Prophase



40

Early Prophase

• Prominent changes occur in the nucleus and cytoplasm during prophase.• The chromatin fibers coil and become thick enough to be seen through a

light microscope.• Each chromosome appears as two identical sister chromatids joined at

their waists.• The mitotic spindle forms as microtubules grow from the two centrosomes

that are moving away from each other.

41

Late Prophase

• The nuclear envelope breaks-up, enabling the microtubules of the mitotic spindle to reach the chromosomes.

• Some of the microtubules attach to the chromosomes, and place them in an agitated motion.

• Other microtubules make contact with the microtubules from the opposite pole to move the chromosomes to the center of the cell.

42

Metaphase



43

Metaphase

• The mitotic spindle is fully formed, and the chromosomes are positioned on the cell equator.

• Microtubules attach to the two sister chromatids of each chromosome to pull them toward the opposite poles.

• For now, the tug-of-war keeps the chromosomes positioned between the two poles.

44

Anaphase



45

Anaphase

• The sister chromatids of each chromosome pair separate suddenly during anaphase.

• Each is now a daughter chromosome.• Motor proteins ratchet the daughter chromosomes along the microtubules

to the opposite poles of the parent cell.• The microtubules shorten in length to help bring the chromosomes closer

to each pole.• Other microtubules, not attached to the chromosomes, lengthen and push

the poles apart to elongate the parent cell.

46

Telophase



47

Telophase

• Telophase begins when the two chromosomes reach the opposite poles.• The nuclear envelopes form, chromosomes uncoil, and the mitotic spindle

disappears.• Mitosis is know complete.• Cytokinesis, the division of the parent cell into two separate cells, occurs at

the end of telophase.

48

Cytokinesis in Plant Cells

http://trc.ucdavis.edu

The formation of the cell walls and plasma membranes are visible.

49

Cytokinesis in Plant Cells

• Vesicles containing material for the cell walls collect at the equator of the elongated plant cell.

• The vesicles fuse to form a cell plate.• The cell plate fuses with the plasma membrane to produce two separate

daughter cells.

50

Cytokinesis in Animal Cells

Actin molecules pinch-off the parent cell to form two daughter cells.

http://www.molecularexpressions.com

51

Cytokinesis in Animal Cells

• A ring of microfilaments in the cytoplasm produces a cleavage furrow in the elongated animal cell.

• This furrow encircles the equator of the cell halfway between the two poles.

• The ring, consisting of the protein molecule, actin, contracts like the pulling of a drawstring, deepening the furrow and pinching the parent cell in two.

• Actin is also responsible for muscle contractions—it acts like a ratchet device.

52


Review of the Cell Cycle

53

Review of Mitosis


1. Interphase (G2)2. Prophase3. Metaphase4. Anaphase5. Telophase

Mitosis consists of phases 2 through 5

54

Scope of Mitosis

Epithelial cells from the human inner cheekhttp://washington.uwc.edu

Mitosis occurs in eukaryotic cells—plants, animals, and fungi.

55

Cell Control Cycle System

• Eukaryotic cells must control the timing of mitosis to grow and maintain healthy tissues.

• The events of the cell cycle are directed by the cell cycle control system, consisting of special proteins within the cell.

• The proteins integrate information from the environment, and send start and stop signals via signal transduction pathways at key points in the cell cycle.

56

Off-State

• The cell cycle normally halts at the G1 stage unless it receives a signal to proceed.

• If the signal does not arrive, the cell cycle will switch to a permanent ‘off’ state, such as in mature muscle cells and neurons—these cells are said to remain at G0.

57

When Things Go Wrong

• Cells can reproduce at the wrong time if the cell cycle control system malfunctions.

• The result may be a tumor, an abnormal mass of cells—the mass can be either benign or malignant.

• A benign tumor remains at its original site, although it can cause problems if it grows.

• Benign tumors of the brain can be dangerous because the cranial cavity is enclosed.

58

Malignant Tumors

• A growth or lump resulting from the reproduction of cancer cells is known as a malignant tumor.

• Like benign tumors, malignant tumor displace normal tissue as they grow larger.

• Malignant cancer cells, however, can spread to adjacent tissues and other parts of the body.

• This spread—known as metastasis—occurs through the blood vessels and lymphatic system.

• Malignant cancer cells may continue to divide and spread until the host organism dies.

59

Cancer

• Cancer claims the lives of about one out of every five people in the United States and other industrialized nations.

• Cancer is the result of a severely malfunctioning cell cycle control system.• The cells divide excessively as if there were no stop signal—the cells may

also exhibit other unusual behaviors.

Lung cancer cellshttp://www.oralcancerfoundation.org

60

Cancer Types

• Cancers are named according to where they originate.• For example, liver cancer originates in the liver, and may be confined

there or later spread to other tissues.• Cancers are grouped into four broader categories based on their sites of

origin.• Carcinomas originate in external or internal coverings of the body such as

the skin or intestines.• Sarcomas originate in tissues that support the body including bone and

muscle.• Leukemias and lymphomas originate in blood-forming tissues including

bone marrow and lymph nodes.

61

Surgery and Radiation Therapy

• The three main types of cancer treatment are surgery, radiation therapy, and chemotherapy.

• These treatments can be used individually or in combination.• Surgery is often a first step—less invasive surgical techniques are being

developed and introduced.• Radiation therapy can often destroy malignant cells with their high rate of

mitotic cell division, while leaving healthy cells intact. • The side effects of radiation treatment can include hair loss and nausea.

62

Chemotherapy

• Chemotherapy also disrupts the high rate of mitotic division in malignant cancer cells.

• Some antimitotic drugs disrupt the formation of the mitotic spindle prior to cell division.

• Another antimitotic drug freezes the mitotic spindle so that cell division cannot continue.

• Some of these drugs are produced from plants in found in tropical and temperate rain forests, which are endangered due to over-cutting and clear-cutting.

63

Prevention

• At least 50 percent of all cancers are related to lifestyle factors.• These include lack of exercise, poor diet, excessive exposure to UV sun-

light, and smoking.• Many other cancers are treatable if they are detected early.• Websites and literature are available from health organizations to discuss

the risks and what can be done.• Regular visits to a physician can help identify tumors at their early stages

for immediate treatment.

64

Detection

• Some malignancies can be easily detected in their early stages including those of the:

– Skin, by self-exam and physical exam– Oral cavity, by physical exam– Breast, by self-exam (and mammogram for women at high risk)– Prostate, by rectal exam and blood test– Cervix, by pap smear– Testicles, by self-exam– Colon, by colonoscopy and blood test

65

Breast Cancer

Awareness Ribbons

http://www.vitriforms.comPossible early symptoms

http

://w

ww

.nzb

cf.o

rg.n

z

66

Prostate Enlargement

Benign prostatic hyperplasia (BHP) is shown—malignancies can also occur.

http://www.meb.uni-bonn.de

67

Melanoma

http://healthcare.utah.edu

68

Words and Terms to Know

• Anaphase• Asexual reproduction• Benign tumor• Carcinoma• Cell cycle• Cell cycle control system• Cell division• Centriole• Chemotherapy• Chromatin• Chromosome• Cytokinesis• Genome• Interphase

•Leukemia•Malignant tumor•Metaphase•Metastasis•Mitosis•Mitotic phase•Mitotic spindle•Nucleosome•Prophase•Radiation therapy•Sarcoma•Sexual reproduction•Sister chromatids•Telophase

69

Possible Test Items

1. What is asexual reproduction? How does it differ from sexual reproduc-tion?

2. What is a genome, what are its basic elements, and how is it organized?3. Describe how a parent cell transmits identical genetic information to its

daughter cells.4. Describe interphase, the phases of mitosis, and cytokinesis in cellular

reproduction. 5. Describe the basic mechanisms for the formation of cancer cells. How

do benign and malignant tumors differ?6. Should government be involved in promoting healthy lifestyles since

society pays a price for preventable diseases including some types of cancer?

Documents

Http:// Cellular Basis of Reproduction and Inheritance Lecture 8, Mitosis