Cell Division and Reproduction

1. Cell Division and Reproduction • A basic tenant of biology is that cells arise from preexisting cells. This was first proposed by Rudolf Virchow in 1858 and is known as ‘Virchow’s Principle’. • In eukaryotes, such as plants and animals, cell division allows: • Growth of an individual from a fertilized egg. • The production of sperm and egg cells. • Asexual reproduction by budding, grafting, or air layering.

2. Bacteria and archaea reproduce by binary fission. These groups have a single circular chromosome. As seen in the figure on the right, the chromosome duplicates itself and the cell elongates. Once there is a chromosome in each side of the elongated cell, the cell (plasma) membrane grows inward and divides the cell into two cells. Each of the new cells is just like the original cells. Binary fission can quickly give rise to a large population of identical prokaryotic cells Reproduction in prokaryotes

3. The number of chromosomes in eukaryotic cells is species dependent. Human cells (except sperm and egg) have 46 chromosomes. Chromosomes are a combination of DNA (which carries genetic information) and proteins (which form the framework on which the DNA is held) The DNA of the nucleus is usually diffuse and spread throughout the nucleus. This diffuse DNA is called chromatin. . Eukaryote chromosomes

4. Before cell division can occur each chromosome in the nucleus must be replicated. When this process is complete the two new chromosomes are called chromatids and are connected at one point called the centromere. During the production of two new cells, each cell will get one of the chromatids. Chromosome duplication

5. The Cell Cycle • Every cell goes through a series of events from its production until it, in tern, divides into two new cells. These events are called the cell cycle. • Most of the life of the cell is spent in interphase. At this time no activity can be seen in the nucleus. However, this period is divided into G1, S, and G2 phases. • The G1 phase is when the cell is carrying out its normal activities. This may last for hours, days, or even years. • If the cell is divide, at some point regular activity ceases and the chromosomes in the nucleus duplicate to form the chromatids seen in the previous slide. This is the S pase of the cell cycle. • The cell then passes into the G2 stage. This may last for several hours during which time the cell continues to grow and to complete preparation for nuclear division. • At the end of the G2 stage the cell begins mitosis. At the end of mitosis cytokinesis occurs, and divides the cell into two new daughter cells. • The series of slides which follow examine the cell cycle and the process of mitosis.

6. The Cell Cycle

7. Mitosis - Prophase • The first sign of mitosis is the condensation of DNA into the structure of chromosome • Note the strands that can be seen in the nucleus • Later in prophase the nuclear membrane will disappear, and • Centrioles migrate to opposite sides of the nucleus and spindle fibers appear.

8. Mitosis - Prometaphase • At this point in time the chromosomes can easily be seen. • The microtubles of the mitotic spindle can be seen radiating out from the centrioles. • The nuclear membrane is gone. • Spindle fibers from both centrioles have attached to the centomere region of each chromosome.

9. Mitosis - Metaphase • Metaphase is the point in time when all the chromosomes are lined up in the same plane of the cell .

10. Mitosis - Anaphase • Anaphase begins as the two chromatids separate and are drawn to opposite side of the cell by the receding microtubles which are attached to them. • Eventually the two new sets of chromosomes are positioned at opposite sides of the cell.

11. Mitosis - Telophase • When the nuclear membranes reform, but while the chromosomes are still visible the cell is in telophase. • At this time cytokinesis occurs and the cell is split into two new cells. • Quickly the DNA unravels, the chromosomes are no longer seen, and the cell is back in interphase.

12. animal cell division • In the illustration to the right, a cleave furrow is forming and the cell is dividing into two cells as the plasma membrane pinches inward.

13. Plant Cell Division • Plant cell division involves the production of a cell wall to divide a cell intro two new cells. • Vesicles of new cell wall material accumulate in the mid-plane of the cell. • These fuse forming a partition between the two sides of the cell, then the plasma membrane connects around all sides of the new cells.

14. Control of cell growth • Cell do not replicate themselves randomly, but divide in response to chemical signals that they receive. • These signals are called growth factors, and operate at specific points in the cell cycle. • If no signal is received at the G1 checkpoint, the cell switches to a non-dividing state called G0

15. Control of cell growth • In the illustration below you can see a model of how a growth factor, that attaches to a membrane protein, can start a cascade of reaction that lead to the start of the cell cycle.

16. Uncontrolled Cell Growth When a cell does not respond to normal growth factors, it can replicate unchecked by any control. These cells form tumors which may be benign (confined to their site of origin) or malignant (capable of spreading and invading other tissues or parts of the body).

17. Cancer • The spread of cancer cells to other parts of the body via the circulatory system is called metastasis. • Techniques to destroy tumor cells include: • Surgical removal of the tumor • Chemotherapy • Radiation • Immune system response Different types of tumors may have very different levels of response to the above methods of treatment.

18. Homologous Chromosomes • It is important to remember that a normal somatic (body) cell in humans contains 46 chromosomes. • These chromosomes consist of 23 matched pairs. • Matched pairs of chromosomes are called homologous chromosomes. • In humanspairs 1-22 are called autosomes, and the last pair which may be XX or XY are the sex chromosomes. • Each chromosome of a pair of autosomes carry the same gene, but not necessarily the same exact information.

19. Chromosomes in other species • Each species has a set number of chromosomes. For example: • Humans have 46 • Cats have 38 • Squirrels have 40 • Porpoises have 44 • Chickens have 78 • Alligators have 32 • Drosophila (fruit flies) have 8 • How many chromosomes does a dog have? I’ll let you find the answer to that question!