1. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 What You Will Learn, continued Cytokinesis differs in animals and plants. Cancer occurs when cells replicate abnormally. Section 2 The Cell Cycle

2. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 The Life of a Cell Your cells pass through different stages in their life cycle, which is called the cell cycle. The cell cycle results in two new cells that are exact copies of the original cell. The cell cycle begins when the cell is formed and ends when the cell divides to form two new cells. Section 2 The Cell Cycle

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 Before a cell divides, it must make a copy of its DNA. The DNA of a cell is organized into structures called chromosomes. Copying the chromosomes ensures that each new cell receives all the DNA of the parent cell. Section 2 The Cell Cycle The Life of a Cell, continued

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 Prokaryotic cells are less complex than eukaryotic cells. Prokaryotes divide by binary fission. Binary fission means “splitting into two parts.” Each of the two new prokaryotic cells has a single, identical, circular chromosome. Section 2 The Cell Cycle The Life of a Cell, continued

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 Eukaryotic cells have many chromosomes, and have more DNA than prokaryotic cells do. The number of chromosomes varies.Fruit flies have 8, potatoes have 48, an humans have 46. Each chromosome has a similar pair, or homologous chromosome. –Humans have 23 pairs of homologous chromosomes, for a total of 46. Section 2 The Cell Cycle The Life of a Cell, continued

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 Control of the Cell Cycle Cells spend most of their lives in interphase, carrying out their functions. Feedback switches monitor the size and health of the cell. When a healthy cell reaches a certain size, the “GO” signal is given to divide. The feedback switches that control the stop and go signals are proteins. Section 2 The Cell Cycle

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 Control of the Cell Cycle, continued Since the directions for making proteins come from DNA, changes in the DNA can change the proteins. If the DNA is damaged, the stop and go signals may not function properly. Cells may begin dividing out of control. Cells dividing uncontrollably may form a tumor and become invasive. This is called cancer. Section 2 The Cell Cycle

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 During mitosis, the chromatids separate so that each new cell receives a copy of each chromosome. Mitosis is divided into four phases: prophase, metaphase, anaphase, an telophase. Section 2 The Cell Cycle The Life of a Cell, MItosis

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 Interphase Most metabolic activity takes place in Interphase Cells increase in mass Cell organelles such as ribosomes, mitochondria and ER are duplicated Chromosomes are duplicated or copied. Together they are considered to be one chromosome comprising of two parts called sister chromatids. Section 2 The Cell Cycle Interphase

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 Prophase During prophase, the long chromosomes condense into rod like structures. Sister chromatids are now visible Nuclear membrane and Nucleolus disappear Spindle begins to form In animal cells centriole pairs begin move to opposite side of the cell. Section 2 The Cell Cycle

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Metaphase During metaphase, the nuclear membrane dissolves Chromosomes with sister chromatids align at the cell’s equator (middle)

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Anaphase During anaphase, the sister chromatids separate and move to opposite ends of the cell. The chromatid is now considered to be a full chromosome.

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Telophase The spindle starts to disappear a new nuclear membrane forms around each set of chromosomes, the rodlike structures de-condense

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Cytokenesis (animal cells) Cytokinesis is the division of cytoplasm and all of the organelles in the cytoplasm. During cytokinesis, the cell splits into two daughter cells. In eukaryotes without a cell wall, the cell is split by a pinching of the cell membrane Sister cells are both identical to the original cell.

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Chapter 5 Cytokenesis (Plant Cell) In plant cells and other eukaryotes with cell walls, the cell is split by a cell plate that forms in the middle of the cell. The cell plate contains materials needed to make the cell membrane and cell wall between the daughter cells. Section 2 The Cell Cycle

16. Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter menu Section 2 The Cell Cycle Chapter 5