Chapter 8

1. Chapter 8 How Cells Reproduce Part 1

2. Henrietta Lacks’ Immortal Cells • Beginning in the mid-1800’s, researchers tried and tried to culture a lineage of immortal cells in the laboratory. • What are immortal cells? • Cells that divide forever. • Usually cells grown in the laboratory divide a few times then die within a few weeks. • However, after more than thirty years, in 1951, George and Margaret Gey, and their assistant Mary Kubicek cultured a sample of human cancer cells, which they named HeLa, after the person from whom they came, Henrietta Lacks.

3. Henrietta Lacks’ Immortal Cells • Henrietta was a thirty-one year old African-American wife and mother of five from Baltimore, Maryland. • She had been diagnosed with cervical cancer. • Within a few months, her cancer cells had invaded tissues throughout her body and she died. • However, her cells lived on and still do today, over fifty years after her death.

4. Henrietta Lacks’ Immortal Cells • Why did scientists want to culture an immortal lineage of human cells? • Why do scientists all over the world still use HeLa cells today? • This immortal cell lineage allows researchers to study human diseases and potential cures for them without experimenting on people. • HeLa cells were used by the Geys to determine which strain of the poliovirus causes polio in humans, which led to the development of a polio vaccine. • Today, Henrietta’s cells are still being used to investigate cancer, viral growth, protein synthesis, and the effects of radiation. • They have helped researchers win Nobel Prizes in medicine and chemistry and even travelled into space for experiments on the Discoverer XVII sateillite.

5. Henrietta Lacks’ Immortal Cells • Even though Henrietta is gone, her legacy lives on through the HeLa lineage of cells. • Henrietta is able to continue to help people through her cells that are dividing again and again, more than fifty years after her death. • In this chapter, we will learn about mechanisms that cells use to divide and why cancer cells are immortal and we are not.

6. Multiplication by Division • A cell must divide in two in order to reproduce. • Each new daughter cell must inherit a complete set of chromosomes from the parent cell, as well as some of the parent’s cytoplasm, so that it will have all of the enzymes, organelles, and other metabolic machinery necessary to keep it running until it can make its own. • Usually, a eukaryotic cell can’t simply just split in two because only one of its daughter cells would get the nucleus. • Therefore, a eukaryotic’s cytoplasm divides only after its nucleus divides.

7. Multiplication by Division • There are two ways by which a cell’s nucleus can divide: • Mitosis is division of the nucleus in which the chromosome number of the cell is maintained. Mitosis and cytoplasmic division (called cytokinesis) are used by the body to increase body size during development and to replace damaged and/or dead cells. Many eukaryotes also use mitosis and cytokinesis to reproduce asexually (make identical copies of themselves). Prokaryotes use a process called binary fission to reproduce asexually. • Meiosis is division of the nucleus in which the chromosome number of the cell is halved and is the basis of sexual reproduction. In humans and all other mammals meiosis results in the production of sex cells or gametes (eggs and sperm). Spores, which are the reproductive cells in many fungi, plants, and protists, are also produced by meiosis.

8. Multiplication by Division Table 8.1

9. The Life of a Cell • The time during a cell’s life cycle from when it forms until it divides is called the cell cycle. • The cell cycle consists of : • Interphase • G1 phase • S phase • G2 phase • M phase (Mitosis) • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis

10. The Life of a Cell

11. The Life of a Cell • During Interphase, the cell increases its mass, doubles the number of its cytoplasmic components, and replicates its DNA. • Specifically, during the G1 phase (or first interval or gap phase) the cell grows. • During the S phase, the cell replicates its DNA. • During the G2 phase ( or second interval or gap phase), the cell synthesizes the proteins that it will use during mitosis.

12. The Life of a Cell • Different types of cells go through the cell cycle at different rates. • For example, stem cells in the red bone marrow complete the cell cycle about every 12 hours. • This is because their descendants have the task of replacing 2 to 3 million worn out red blood cells every second. • Other examples of cells that complete the cell cycle include cell of the root tips of plants as well as animal embryonic cells. • On the other hand, human liver cells may take up to a year to complete the cell cycle, since they divide only occasionally to replace cells that have been lost due to injury or cell death.

13. The Life of a Cell • Gene expression controls affect the rate of cell division. • There are specific checkpoints that can halt the cell cycle. • The first checkpoint (called the restriction point) is at the end of the G1 phase, right before the cell enters the S phase. This checkpoint helps the cell to decide if it should continue into the phase, delay cell division, or enter a resting state. Many cells such as liver cells enter a resting state, called G0, where they remain for an extended period of time. Liver cells spend much of their time in this resting state and only go into the next phases about twice a year. Other cells, such as nerve cells, never exit the G0 phase, but remain permanently in this resting phase and are incapable dividing.

14. The Life of a Cell • The second checkpoint occurs at the end of the G2 phase. In order to pass this checkpoint, the cell cheks a number of protein factors to ensure that the cell is ready to proceed with mitosis. • This is the checkpoint at which the cell also determines of there is damage to the DNA (mutation). If DNA damage damage is detected, the cell cycle is arrested at this checkpoint to prevent the mutation from being passed on to daughter cells. • Once this checkpoint is passed, this triggers the beginning of the M phase (mitosis).

15. The Life of a Cell • A third checkpoint (called the spindle checkpoint)exists during metaphase at the point at which all of the chromosomes should be aligned at the equator (center) of the cell. • This checkpoint ensures that all sister chromatids are aligned and ready to separate during anaphase.

16. Chromosomes during the Cell Cycle • The 46 human chromosomes can also exist as 23 homologous pairs. • Homologous refers to the fact that each chromosome in a pair has the same length, shape, and collection of genes as its counterpart in the pair. For each pair, one of the chromosomes was inherited from each parent. • After mitosis and cytokinesis, a diploid (2n) parent cell will have passed a copy of each of its chromosomes to the two diploid daughter cells that were produced. This is how mitosis maintains the chromosome numebr of the parent cell.

17. Chromosomes during the Cell Cycle • In the G1 phase, each chromosome consists of one double-stranded DNA molecule. • During the S phase, the DNA is replicated so that each chromosome consists of two identical DNA molecules connected to each other at an area called the centromere. This is what gives chromosomes the characteristic shape that we are all familiar with, the “X” shape. • Until they are separated, the two identical copies of double-stranded DNA making up each chromosome are called sister chromatids.

18. Chromosomes during the Cell Cycle After S phase and during prophase and metaphase of mitosis During G1 phase