1. BIO 2, Lecture 9 REPRODUCTION I: ASEXUAL REPRODUCTION: BINARY FISSION, MITOSIS, AND THE CELL CYCLE

2. All living organisms replicate their DNA (imperfectly) and then pass it on to “daughter cells” through cell division, a process called reproduction Because replication is imperfect, daughter cells will contain new mutations These mutations are then subject to increasing or decreasing in frequency in the population due to natural selection and other forces that drive evolution

3. There are two types of reproduction: asexual and sexual All living organisms contain at least some cells that reproduce asexually Some living organisms also contain specialized cells that reproduce sexually Asexual reproduction is nature’s way of cloning a cell The two daughter cells produced by asexual reproduction are genetically identical to the parent cell (except for rare mutations)

4. 100 µm (a)Asexual Reproduction : Produces 2 daughter cells genetically identical to the original parent cell

5. Sexual reproduction is nature’s way of producing genetically diverse daughter cells Four daughter cells (eggs or sperm) are produced by sexual reproduction that each contain exactly half the genetic material of the parent cell and are genetically different from the parent cell and from each other This lecture will focus on asexual reproduction The next lecture will focus on sexual reproduction

6. Asexual reproduction comes in two forms: binary fission and mitosis Binary fission is used by prokaryotes to distribute the duplicated copies of their single circular chromosome to two daughter cells Because prokaryotes are single-celled organisms, binary fission asexually reproduces not only the cell but also the entire organism

7. Origin of replication Two copies of origin E. coli cell Bacterial chromosome Plasma membrane Cell wall Origin

8. Mitosis is used by eukaryotic organisms, which have multiple linear chromosomes, and is a much more complex process than binary fission Some single-celled eukaryotes use mitosis as the primary method of reproducing the whole organism (e.g. yeast) Most, however, use it for growth and replacement of dead cells and reproduce the whole organism by sexual reproduction

9. Yeast cells reproducing the whole organism (cell) asexually by mitosis Human white blood cell reproducing asexually by mitosis

10. To understand mitosis, it is necessary to first look at the eukaryotic cell cycle In preparation for cell division, DNA is replicated and the chromosomes condense Each duplicated chromosome has two sister chromatids, which separate during cell division The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

11. 0.5 µmChromosomes Chromosome duplication (including DNA synthesis) Chromo- some arm Centromere Sister chromatids DNA molecules Separation of sister chromatids Centromere Sister chromatids

12. In 1882, the German anatomist Walther Flemming developed dyes to observe chromosomes during mitosis and cytokinesis (cell division following mitosis) Using these dyes, it was possible to divide the cell cycle into two phases: –Mitotic (M) phase (mitosis and cytokinesis), at which time chromosomes are visible –Interphase (cell growth and copying of chromosomes in preparation for cell division), at which time chromosomes are not visible

13. Interphase (about 90% of the cell cycle) can be divided into sub-phases: –G 1 phase (“first gap”) –S phase (“synthesis”) –G 2 phase (“second gap”) The cell grows and performs its ceullar functions during all three phases, but chromosomes are duplicated only during the S phase

14. S (DNA synthesis) MITOTIC (M) PHASE Mitosis Cytokinesis G1G1 G2G2

15. Mitosis is conventionally divided into five phases: –Prophase –Prometaphase –Metaphase –Anaphase –Telophase Cytokinesis is well underway by late telophase

16. PrometaphaseProphase G 2 of Interphase Nonkinetochore microtubules Fragments of nuclear envelope Aster Centromere Early mitotic spindle Chromatin (duplicated) Centrosomes (with centriole pairs) Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule

17. MetaphaseAnaphase Telophase and Cytokinesis Cleavage furrow Nucleolus forming Metaphase plate Centrosome at one spindle pole Spindle Daughter chromosomes Nuclear envelope forming

18. The mitotic spindle is an apparatus of microtubules (long cytoplasmic motor proteins) that controls chromosome movement during mitosis During prophase, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them

19. During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes to the center of the cell Kinetochores are centromeres bound by proteins that attract the microtubules At metaphase, the chromosomes are all lined up at the metaphase plate, the midway point between the spindle’s two poles

20. Microtubules Chromosomes Sister chromatids Metaphase plate Centrosome Kineto- chores Kinetochore microtubules Overlapping nonkinetochore microtubules Centrosome 1 µm 0.5 µm

21. In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell The microtubules shorten by depolymerizing at their kinetochore ends

22. EXPERIMENT Kinetochore RESULTS CONCLUSION Spindle pole Mark Chromosome movement Kinetochore Microtubule Motor protein Chromosome Tubulin subunits

23. In telophase, genetically identical daughter nuclei form at opposite ends of the cell In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow In plant cells, a cell plate forms during cytokinesis

24. Cleavage furrow 100 µm Contractile ring of microfilaments Daughter cells (a) Cleavage of an animal cell (SEM)(b) Cell plate formation in a plant cell (TEM) Vesicles forming cell plate Wall of parent cell Cell plate Daughter cells New cell wall 1 µm

25. Chromatin condensing Metaphase AnaphaseTelophase Prometaphase Nucleus Prophase 1 2 3 5 4 Nucleolus Chromosomes Cell plate 10 µm

26. The frequency of cell division varies with the type of cell These cell cycle differences result from regulation at the molecular level The cell cycle appears to be driven by specific chemical signals present in the cytoplasm Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei

27. Experiment 1 Experiment 2 EXPERIMENT RESULTS SG1G1 M G1G1 M M S S When a cell in the S phase was fused with a cell in G 1, the G 1 nucleus immediately entered the S phase—DNA was synthesized. When a cell in the M phase was fused with a cell in G 1, the G 1 nucleus immediately began mitosis—a spindle formed and chromatin condensed, even though the chromosome had not been duplicated.

28. The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock The cell cycle control system is regulated by both internal and external controls The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

29. Accurate translation requires two steps: –1. An enzyme called aminoacyl-tRNA synthetase adds an amino acid to all the tRNAs that carry the anticodon that is complementary to the codon in the mRNA that codes for that amino acid –2. The tRNA anticodon recognizes and base- pairs to its mRNA codon Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon

30. S G1G1 M checkpoint G2G2 M Control system G 1 checkpoint G 2 checkpoint

31. For many cells, the G 1 checkpoint seems to be the most important one If a cell receives a go-ahead signal at the G 1 checkpoint, it will usually complete the S, G 2, and M phases and divide If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G 0 phase

32. G1G1 G0G0 G 1 checkpoint (a)Cell receives a go-ahead signal G1G1 (b) Cell does not receive a go-ahead signal

33. An example of an internal signal that stops the cell cycle: Kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase Some external signals that promote the cell cycle are growth factors, proteins released by certain cells that stimulate other cells to divide For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture

34. Petri plate Scalpels Cultured fibroblasts Without PDGF cells fail to divide With PDGF cells prolifer- ate 10 µm

35. Another example of external signals is density-dependent inhibition, in which crowded cells stop dividing Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide

36. Anchorage dependence Density-dependent inhibition (a) Normal mammalian cells (b) Cancer cells 25 µm

37. Cancer cells exhibit neither density- dependent inhibition nor anchorage dependence Cancer cells do not respond normally to the body’s control mechanisms Cancer cells may not need growth factors to grow and divide They may make their own growth factor They may convey a growth factor’s signal without the presence of the growth factor They may have an abnormal cell cycle control system

38. A normal cell is converted to a cancerous cell by a process called transformation Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue If abnormal cells remain at the original site, the lump is called a benign tumor Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors

39. Tumor A tumor grows from a single cancer cell. Glandular tissue Lymph vessel Blood vessel Metastatic tumor Cancer cell Cancer cells invade neigh- boring tissue. Cancer cells spread to other parts of the body. Cancer cells may survive and establish a new tumor in another part of the body. 1 2 3 4