1. Chapter 11 How Cells Reproduce

2. 11.1 Henrietta’s Immortal Cells
Henrietta Lacks died of cancer in 1951, at age 31, but her cells (HeLa cells) are still growing in laboratories
HeLa cells are widely used to investigate cancer, viral growth, and other processes important in medicine and research
Understanding the structures and mechanisms that cells use to divide help us understand why cancer cells are immortal and we are not

3. HeLa Cells
Figure 11.1 HeLa cells, an immortal legacy of cancer victim Henrietta Lacks (right). The fluorescence micrograph (left) shows some of the internal components of two HeLa cells in the process of dividing. Blue and green indicate the location of two proteins that help microtubules (red) attach to chromosomes (white). Defects in these and other proteins that orchestrate cell division result in descendant cells with too many or too few chromosomes, an outcome that is a hallmark of cancer.

4. 11.2 Multiplication by Division
Division of a eukaryotic cell typically occurs in two steps: nuclear division followed by cytoplasmic division
The sequence of stages through which a cell passes during its lifetime is called the cell cycle
A sequence of three stages (interphase, mitosis, and cytoplasmic division) through which a cell passes between one cell division and the next

5. The Eukaryotic Cell Cycle
cytoplasmic division
mitosis G2 S i n t e r p h a s G1
A cell spends most of its life in interphase, which includes three stages: G1, S, and G2.
telophase
anaphase
metaphase
prophase
G1 is the phase of growth before DNA replication. The cell’s chromosomes are unduplicated.
S is the phase of synthesis, during which the cell makes copies of its chromosome(s) by DNA replication.
G2 is the phase after DNA replication and before mitosis. The cell prepares to divide during this stage.
The nucleus divides during mitosis, the four stages of which are detailed in the next section.
After mitosis, the cytoplasm may divide. Each descendant cell begins the cycle anew, in interphase.
Built-in checkpoints stop the cycle from proceeding until certain conditions are met.
Figure 11.2 The eukaryotic cell cycle. The length of the intervals differs among cells. G1, S, and G2 are part of interphase.
1 A cell spends most of its life in interphase, which includes three stages: G1, S, and G2.
2 G1 is the phase of growth before DNA replication. The cell’s chromosomes are unduplicated.
3 S is the phase of synthesis, during which the cell makes copies of its chromosome(s) by DNA replication.
4 G2 is the phase after DNA replication and before mitosis. The cell prepares to divide during this stage.
5 The nucleus divides during mitosis, the four stages of which are detailed in the next section.
6 After mitosis, the cytoplasm may divide. Each descendant cell begins the cycle anew, in interphase.
Built-in checkpoints stop the cycle from proceeding until certain conditions are met.

6. Interphase
Interphase consists of three stages, during which a cell increases in size, doubles the number of cytoplasmic components, and replicates its DNA
G1: Interval of cell growth and activity
S: Interval of DNA replication (synthesis)
G2: Interval when the cell prepares for division

7. Interphase and the Life of a Cell
Most cell activities take place during G1
Control mechanisms work at certain points in the cell cycle
Most cell cycle checkpoints are in the stages of interphase
Some can keep cells in G1
Loss of control may cause cell death or cancer

8. Mitosis and Asexual Reproduction
Mitosis is a nuclear division mechanism that maintains the chromosome number
In multicelled species, mitosis and cytoplasmic division increase cell number during development, and replace damaged or dead cells later in life
In asexual reproduction, a single individual can reproduce by mitosis and cytoplasmic division

9. Homologous Chromosomes
Pairs of chromosomes having the same length, shape, and genes
Typically, one member of a pair is inherited from the mother, the other from the father
Human cells have 46 chromosomes (23 pairs)
Except for a pairing of sex chromosomes (XY) in males, the chromosomes of each pair are homologous

10. A An unduplicated pair of chromosomes in a cell in G1.
B By G2, each chromosome has been duplicated.
Figure 11.4 How mitosis maintains the chromosome number.
C Mitosis and cyto-plasmic division package
one copy of each chromosome into each of two new cells.
Stepped Art

11. How Mitosis Maintains the Chromosome Number
6 Mitosis and cytoplasmic division package one copy of each chromosome into each of two new cells.
2 Pair of homologous chromosomes in a cell during G1. Both are unduplicated.
4 By G2, each chromosome has been duplicated.
Figure 11.4 How mitosis maintains the chromosome number.

12. Chromosomes During Division
A cell cannot function without a full complement of DNA
Each cell needs to have one copy of each chromosome
The cell replicates its DNA in preparation for mitosis

13. Stages of Mitosis
Four stages of mitosis parcel sister chromatids into separate nuclei
Prophase
Metaphase
Anaphase
Telophase
Cytoplasmic division results in two diploid cells, each with the same number and kind of chromosomes as the parent

14. Controls Over the Cell Cycle
Most differentiated human cells are in G1 – some types never progress past that stage
When a cell divides is determined by gene expression controls
Some cause the cell cycle to advance
Others stop the cycle from proceeding
These built-in checkpoints allow problems to be corrected before the cycle proceeds

15. 11.3 A Closer Look at Mitosis
When a nucleus divides by mitosis, each new nucleus has the same chromosome number as the parent cell
What are the four stages of mitosis?

16. Preparing for Mitosis
During interphase, a cell’s chromosomes are loosened to allow transcription and DNA replication
In preparation for nuclear division, chromosomes condense into their most compact “X” forms
Tight condensation keeps the chromosomes from getting tangled and breaking during nuclear division

17. Interphase and Early Prophase
Plant cell nucleus
Animal cell nucleus
4 Metaphase
All of the chromosomes are aligned midway between the spindle poles.
5 Anaphase
Spindle microtubules separate the sister chromatids and move them toward opposite spindle poles. Each sister chromatid has now become an individual, unduplicated chromosome.
Figure 11.5 Mitosis. Micrographs show nuclei of plant cells (onion root, left), and animal cells (fertilized eggs of a roundworm, right). A diploid (2n) animal cell with two chromosome pairs is illustrated.

18. Prophase What happens during prophase? Centrosome Chromosomes condense
Microtubules form a bipolar spindle
Nuclear envelope breaks up
Microtubules attach to the chromosomes
Centrosome
A region near the nucleus that organizes spindle microtubules; usually includes two centrioles

19. Mitosis – Prophase 6 Telophase
Plant cell nucleus
Animal cell nucleus
6 Telophase
The chromosomes reach opposite sides of the cell and loosen up.
Mitosis ends when a new nuclear envelope forms around each cluster of chromosomes.
Figure 11.5 Mitosis. Micrographs show nuclei of plant cells (onion root, left), and animal cells (fertilized eggs of a roundworm, right). A diploid (2n) animal cell with two chromosome pairs is illustrated.

20. The Spindle
A dynamic network of microtubules that forms during nuclear division
Grows into the cytoplasm from opposite poles of the cell and attaches to duplicated chromosomes
Microtubules from opposite poles attach to different sister chromatids and separate them

21. The Spindle – Illustrated
spindle pole
© Conly L Rieder
Figure 11.6 The spindle. In this dividing lung cell of a salamander, microtubules (green) have extended from two centrosomes to form the spindle, which has attached to and aligned the chromosomes (blue) midway between its two poles. Red shows actin microfilaments.

22. Metaphase and Anaphase
All duplicated chromosomes line up midway between the spindle poles
Anaphase
Microtubules separate the sister chromatids of each chromosome and pull them to opposite spindle poles

23. Mitosis – Metaphase and Anaphase
Plant cell nucleus
Animal cell nucleus
4 Metaphase
All of the chromosomes are aligned midway between the spindle poles.
5 Anaphase
Spindle microtubules separate the sister chromatids and move them toward opposite spindle poles. Each sister chromatid has now become an individual, unduplicated chromosome.
Figure 11.5 Mitosis. Micrographs show nuclei of plant cells (onion root, left), and animal cells (fertilized eggs of a roundworm, right). A diploid (2n) animal cell with two chromosome pairs is illustrated.
left, Michael Clayton/University of Wisconsin, Department of Botany; right, ISM/Phototake; far right, © Cengage Learning.

24. Telophase During telophase:
Two clusters of chromosomes reach the spindle poles
A new nuclear envelope forms around each cluster
Two new nuclei are formed, each with the same chromosome number as the parent cell

25. Mitosis – Telophase 6 Telophase
Plant cell nucleus
Animal cell nucleus
6 Telophase
The chromosomes reach opposite sides of the cell and loosen up.
Mitosis ends when a new nuclear envelope forms around each cluster of chromosomes.
Figure 11.5 Mitosis. Micrographs show nuclei of plant cells (onion root, left), and animal cells (fertilized eggs of a roundworm, right). A diploid (2n) animal cell with two chromosome pairs is illustrated.
left, Michael Clayton/University of Wisconsin, Department of Botany; right, ISM/Phototake; far right, © Cengage Learning.

26. 11.4 Cytokinesis: Division of Cytoplasm
In most kinds of eukaryotes, the cell cytoplasm divides between late anaphase and the end of telophase, but the mechanism of division differs between plants and animals
Cytokinesis
The process of cytoplasmic division

27. Cytokinesis in Animal and Plant Cells
Animal cells
A cleavage furrow partitions the cytoplasm
A band of actin filaments rings the cell midsection, contracts, and pinches the cytoplasm in two
Plant cells
A cell plate forms midway between the spindle poles
Partitions the cytoplasm when it reaches and connects to the parent cell wall

28. Cytoplasmic Division in Animal Cells
Animal cell cytokinesis
1 In a dividing animal cell, the spindle disassembles as mitosis ends.
2 At the midpoint of the former spindle, a ring of actin and myosin filaments attached to the plasma membrane contracts.
3 This contractile ring pulls the cell surface inward, forming a cleavage furrow as it shrinks.
Figure 11.7 Cytoplasmic division of animal cells and plant cells.
4 The ring contracts until it pinches the cell in two.

29. Cytoplasmic Division in Plant Cells
Plant cell cytokinesis
5 In a dividing plant cell, vesicles cluster at the future plane of division before mitosis ends.
6 The vesicles fuse with each other, forming a cell plate along the plane of division.
7 The cell plate expands out- ward along the plane of division. When it reaches and attaches to the plasma membrane, it parti- tions the cytoplasm.
Figure 11.7 Cytoplasmic division of animal cells and plant cells.
8 The cell plate matures as two new cell walls. These walls join with the parent cell wall, so each descendant cell becomes enclosed by its own wall.

30. 11.5 Marking Time With Telomeres
The ends of eukaryotic DNA strands consist of noncoding sequences called telomeres
Telomeres protect eukaryotic chromosomes from losing genetic information at their ends
Vertebrate telomeres have a short DNA sequence, 5′-TTAGGG-3′, repeated thousands of times
Shortening telomeres are associated with aging

31. Telomeres
Telomeres are important because a polymerase can’t copy the last hundred or so bases of the 3′ end of a chromosome
When telomeres get too short, checkpoint gene products stop the cell cycle, and the cell dies

32. Stem Cells and Telomerase
In an adult, only stem cells retain the ability to divide indefinitely
What are stem cells immortal?
They retain the ability to make telomerase, a molecule that reverses the telomere shortening that normally occurs after DNA replication
Research suggests that the built-in limit on cell divisions may be part of the mechanism that sets an organism’s life span

33. Telomeres – Illustrated
Figure 11.8 Telomeres. The bright dots at the end of each DNA strand in these duplicated chromosomes are telomeres.

34. 11.6 When Mitosis Becomes Pathological
When enough checkpoint mechanisms fail, a cell loses control over its cell cycle forms a neoplasm – a group of cells that lost control over how they grow and divide
A neoplasm that forms a lump (abnormal mass) in the body is called a tumor

35. Oncogenes and Proto-Oncogenes
An oncogene is any gene that helps transform a normal cell into a tumor cell
Genes encoding proteins that promote mitosis are called proto-oncogenes – mutations can turn them into oncogenes
Growth factors are molecules that stimulate a cell to divide and differentiate
A gene that encodes the epidermal growth factor (EGF) receptor is an example of a proto-oncogene

36. Oncogenes and Overactive EGF Receptors
Figure 11.9 Effects of an oncogene. In this section of human breast tissue, a brown-colored tracer shows the active form of the EGF receptor. Normal cells are lighter in color. The dark cells have an overactive EGF receptor that is constantly stimulating mitosis; these cells have formed a neoplasm.

37. Tumor Suppressors
Checkpoint gene products that inhibit mitosis are called tumor suppressors because tumors form when they are missing
The products of the BRCA1 and BRCA2 genes are examples of tumor suppressors – they regulate the expression of DNA repair enzymes
Viruses such as HPV (human papillomavirus) cause cells to make proteins that interfere with their own tumor suppressors

38. Checkpoint Genes in Action
Figure Checkpoint genes in action. Radiation damaged the DNA inside this nucleus.
A Red dots show the location of the BRCA1 gene product.
B Green dots pinpoint the location of the product of a gene called 53BP1.
Both proteins have clustered around the same chromosome breaks in the same nucleus; both function to recruit DNA repair enzymes. The integrated action of these and other checkpoint gene products blocks mitosis until the DNA breaks are fixed. A B
© Phillip B. Carpenter, Department of Biochemistry and Molecular Biology, University of Texas - Houston Medical School.

39. Cancer
Benign neoplasms (such as ordinary skin moles) grow slowly, stay in one place, and are not cancerous
Malignant neoplasms (cancers) disrupt body tissues, both physically and metabolically
Get progressively worse
Dangerous to health
Cancer causes 15-20% of all human deaths in developed countries

40. Three Characteristics of Cancer Cells
Cancer cells grow and divide abnormally
Capillary blood supply to the cells may increase abnormally
Cytoplasm and plasma membrane are altered
Malignant cells typically have an abnormal chromosome number
Metabolism may shift toward fermentation
Altered or missing proteins impair the function of the plasma membrane

41. Three Characteristics of Cancer Cells (cont’d.)
Cancer cells break loose and invade other parts of the body (metastasis)
Due to altered recognition proteins and weakened adhesion, they do not stay anchored properly in tissues
Can slip easily into and out of circulatory and lymphatic systems vessels
Use these vessels to migrate elsewhere in the body

42. Neoplasms and Malignancy
Benign neoplasms grow slowly and stay in their home tissue.
Cells of a malignant neoplasm can break away from their home tissue.
The malignant cells become attached to the wall of a lymph vessel or blood vessel (as shown here). They release digestive enzymes that create an opening in the wall, then enter the vessel.
The cells creep or tumble along inside vessels, then exit the same way they got in. Migrating cells may start growing in other tissues, a process called metastasis.
Figure Neoplasms and malignancy

43. Reducing the Risk of Cancer
Life style choices can reduce the risk of acquiring mutations that lead to cancer
Not smoking, avoiding exposure of unprotected skin to sunlight, etc.
Some neoplasms can be detected with periodic screening procedures such as gynecology or dermatology exams

44. Skin Cancers
C Melanoma spreads fastest. Cells form dark, encrusted lumps that may itch or bleed easily.
B Squamous cell carcinoma is the second most common form of skin cancer. This pink growth, firm to the touch, grows under the skin’s surface.
A Basal cell carcinoma is the most common type of skin cancer. This slow-growing, raised lump may be uncolored, reddish-brown, or black.
(A) Dr. Allan Harris/Phototake; (B) Biophoto Associates/Science Source; (C) James Stevenson/Science Source.
Figure Skin cancer can be detected with early screening.
A Basal cell carcinoma is the most common type of skin cancer. This slow-growing, raised lump may be uncolored, reddish-brown, or black.
B Squamous cell carcinoma is the second most common form of skin cancer. This pink growth, firm to the touch, grows under the skin’s surface.
C Melanoma spreads fastest. Cells form dark, encrusted lumps that may itch or bleed easily.

45. Points to Ponder
Exactly where and when does mitosis occurs in any organism?
What would happen if cytokinesis did not occur in plants and animals?
Many of the drugs used in chemotherapy cause loss of hair in the individual being treated. Why is this?