1. Cell Division

2. Cell Division What is cell division?
Why do cells multiply rather than get larger to grow?

3. Why Cell Divide
Two reasons why cells divide rather than increase in size:
Increased demand on DNA (DNA overload)
Trouble getting nutrients and wastes across the membrane

4. What is Cell Division?
Cell division – a cell divides into 2 daughter cells o Each daughter cell is same size of parent o Each daughter cell receives same DNA (genes)
Prokarytotes - cell is separated into 2 parts
called binary fission
Eukaryotes
Mitosis – division of the nucleus
Cytokinesis – division of cytoplasm • By cell membrane or cell plate (plants)

5. Chromosomes
Chromosomes – made up of DNA, which carries the cell coded genetic information
Every species has a specific number of chromosomes, e.g., humans = 46 o Diploid – two sets (2N) as in an adult o Haploid (1N) as in an egg or sperm
Chromosomes are not visible in most cells except during cell division
Well before cell division, chromosomes are replicated (copied)
Each chromosome consists of 2 identical sister chromatids
Each pair of chromatids are attached at the centromere

6. Chromosomes

7. The Cell Cycle Interphase (90% of cycle) Mitotic phase
G1 phase~ growth
S phase~ synthesis of DNA
G2 phase~ preparation for cell division
Mitotic phase
Mitosis~ nuclear division
Cytokinesis~ cytoplasm division

8. Mitosis

9. Prophase
Chromosomes visible (before it was thin threads called chromatin)
Nucleoli disappear
Sister chromatids
Mitotic spindle forms
Centrosomes move

10. Prophase

11. Metaphase Centrosomes at opposite poles Centromeres are aligned
Kinetochores of sister chromatids attached to microtubules (spindle)
Kinetochores - the protein structure on chromosomes where the spindle fibers attach during division to pull the chromosomes apart

12. Metaphase

13. Anaphase Paired centromeres separate; sister chromatids liberated
Chromosomes move to opposite poles
Each pole now has a complete set of chromosomes

14. Anaphase

15. Telophase Daughter nuclei form Nuclear envelopes arise
Chromatin becomes less coiled
Two new nuclei complete mitosis

16. Telophase

17. Cytokinesis Cytoplasmic division Animals: cleavage furrow
Plants: cell plate

18. Cell Cycle Regulation
Cyclin – proteins that regulate the timing of the cell cycle in eukaryotic cells
Cyclin-dependent kinases (CDKs) – bind to cyclins to start various activities that take place in the cell cycle

19. Cell Cycle Regulation
Uncontrolled cell growth – cancer cells do not respond to the signals that regulate growth of most cells
Causes of cancer are various –
DNA mutations that produce proteins including those that regulate the cell cycle o Carcinogens - tobacco, radiation exposure, o viruses
Control over cell cycle broken down o No longer respond to external regulators or o Fail to produce internal regulators to ensure orderly growth
Apoptosis – preprogrammed cell death; examples: hand and feet development, leaf fall
Stem cells – unspecialized cells (embryonic and adult) Embryonic – after fertilization but before specialization
Adult – in organism, cells that can develop into other types of cells (EX: nervous into brain cells)

20. Meiosis What is different about the chromosomes?
Homologous chromosomes – chromosome that has a corresponding chromosome from the opposite-sex parent
Synapsis – when homologous chromosomes form pairs that are bound together along their length to form tetrad of 4 chromosomes ** Crossing-over – exchange of material (genes) between chromosomes

21. Tetrads (Homologous Pairs)

22. Crossing Over

23. Meiosis
Meiosis – process of reduction division in which the number of chromosomes per cell is cut in half (Haploid: 1N) through the separation of homologous chromosomes in a diploid cell (Diploid: 2N)

24. Meiosis I

25. Meiosis II

26. Gamete Formation
Gamete - o Spermatogenesis – produces 4 sperm o Oogenesis – produces one egg and 3 polar bodies

27. Gamete Formation