The Continuity of Life: How Cells Reproduce

1. The Continuity of Life: How Cells Reproduce

2. mitotic cell division mitotic cell division (a) (b) (c)

3. A layer of dead cells forms on the surface of the skin. Cells move up. Mitotic cell division occurs here. (d)

4. mitotic cell division and growth juvenile mitotic cell division and growth adults fertilized egg mitotic cell division (in testes) mitotic cell division (in ovaries) sperm fusion of gametes egg

5. Chromosomes • W. Flemming, 1880s found clues to physical nature of gene • Had fine scope lenses • Used chemical dyes that cling to certain cellular structures • Published drawings of dividing cells from salamander embryos

6. centromeres sister chromatids

7. pair of homologous,duplicatedchromosomes sisterchromatids ofone duplicated homologue nuclear envelope

9. Homologous Chromosomes • Chromosomes come in matched pairs • Their number is characteristic of species • Humans – 46 • Chimpanzee – 48 • Fruit flies – 8

10. Homologous Pairs of Chromosomes • Human cells have 23 homologous pairs • After mitosis • 2 daughter cells • Each have 46 chromosomes with 23 homologous pairs

11. Homologous Pairs of Chromosomes • In males of some species • 1 pair of homologous chromosomes is not exactly homologous • Y vs. X – sex chromosomes

12. II. Cell division and the cell cycle • Mitosis is used for asexual reproduction, growth, maintenance, and repair • Meiosis is used for sexual reproduction

13. II. Cell division and the cell cycle • Cell division is a regulated process • 1. The frequency and timing of cell division is controlled • 2. Cells receive signals to divide or not to divide

15. II. Cell division and the cell cycle Different types of cells have different rates of cell division • 1. Some cells in developed tissues never divide (nerve cells, skeletal muscle cells, red blood cells) • 2. Some cells are in a resting stage (G0) and do not enter into the cell cycle until signaled to do so • 3. Other cells are continually in the cell cycle

16. Chromosomes • Chromosomes are apportioned to daughter cells during mitosis • Flemming named mitosis sequence of events during cell division • Occurs in eukaryotic cells & is used for growth, repair, cell replacement • Involves vast reorganization of cell interior

17. III. Mitosis Each new nucleus is genetically identical to the parent nucleus

18. III. Mitosis • Four phases • 1. Prophase—chromosomes condense, spindle apparatus forms, nuclear envelope breaks down • 2. Metaphase—chromosomes line up at equator of cell • 3. Anaphase—sister chromatids separate • 4. Telophase—new nuclear envelopes form, chromosomes unwind

19. (a) Interphase in a seed cell: The chromosomes (blue) are in the thin, extended state and appear as a mass in the center of the cell. The spindle microtubules (red) extend outward from the nucleus to all parts of the cell. (b) Late prophase: The chromosomes (blue) have condensed and attached to the spindle microtubules (red). (c) Metaphase: The chromosomes have moved to the equator of the cell. (f) Resumption of interphase: The chromosomes are relaxing again into their extended state. The spindle microtubules are disappearing, and the microtubules of the two daughter cells are rearranging into the interphase pattern. (e) Telophase: The chromosomes have gathered into two clusters, one at the site of each future nucleus. (d) Anaphase: Sister chromatids have separated, and one set has moved toward each pole.

20. Each new nucleus is genetically identical to the parent nucleus Daughter Cells Each cell has the same genetic makeup as the parent cell Parent Cell Chromosomes have been replicated Mitosis

21. Chromosomal Theory of Inheritance • Sexual reproduction where • Male and female gametes fuse to form zygote (fertilization) • Number of chromosomes donated by male and female • Are equal, but chromosome number within species stays constant

22. Meiosis produces gametes for sexual reproduction • Multiplies number of cells but also reduces chromosome number in each daughter cell to exactly half the number present before meiosis • Daughter cells get 1 member of each homologous pair, i.e. 1 allele for each gene • Mitosis produces 2 daughter cells • Meiosis produces 4 daughter cells

23. Meiosis produces gametes for sexual reproduction • Cells with both members of each homologous pair are diploid (2n) • All body cells in humans are diploid, except gametes • Cells with 1 member of each homologous pair are haploid

24. IV. Meiosis Characteristics of meiosis • 1. Occurs in sex cells (germ cells) and produces gametes • 2. A reductional division resulting in haploid cells • 3. Involves two sequential divisions resulting in four cells • 4. Produces cells that are genetically different

25. Daughter Cells (1n) each chromosome has 2 chromatids Gamete Cells (1n) Parent Cell (2n) 2nd division 1st division

26. IV. Meiosis Meiosis is a source of genetic variability • 1. Genetic recombination—result of crossing over • a. Mixes genes between homologous chromosomes, creating new combinations of genes not previously contained on a single chromosome

27. chiasmata pair ofhomologous,duplicatedchromosomes sisterchromatids ofone duplicatedhomologue nuclear envelope

28. Exchange & Movement of Genetic Material • Crossing-over • Parts of one member of pair cross over to homologue

29. IV. Meiosis Meiosis is a source of genetic variability 2. Independent assortment—numerous combinations of maternal/paternal chromosomes in gametes resulting from random alignment of homologous pairs of chromosomes in metaphase I of meiosis

31. 1 Cells from the udder of a Finn Dorset ewe are grown in culture with low nutrient levels. The starved cells stop dividing and enter the non- dividing phase of the cell cycle. Finn Dorset ewe Blackface ewe 2 Meanwhile, the nucleus is sucked out of an unfertilized egg cell taken from a Scottish Blackface ewe. This egg will provide cytoplasm and organelles but no chromosomes. eggcell udder cell nucleus is removed DNA eggcell donor cell from udder cell divides, formingan early embryo fused cells 4 The embryo is maintained in the culture for 6 days, during which time it develops into a hollow ball of cells. Then it is implanted into the uterus of a second Blackface ewe. electric pulse 3 The egg cell without a nucleus and the quiescent udder cell are placed side by side in a culture dish. An electric pulse stimulates the cells to fuse and initiates mitiotic cell division. Dolly, a Finn Dorsetewe, is born 5 The Blackface ewe gives birth to a female Finn Dorset lamb. A genetic twin of the Finn Dorset ewe. implant in surrogate mother Blackface ewe

32. The Cell Cycle • Mitosis and meiosis are single steps in cell cycle • Cells not in process of dividing are in interphase • Chromosomes are duplicated in preparation for the next round of division during interphase

33. The Cell Cycle

34. Control of the Cell Cycle • The cell cycle is highly regulated • If a cell breaks free of its parent organ • It starts to grow uncontrollably – metastasize

35. Regulating Agents of the Cell Cycle • Regulating agents are • Proteins whose concentrations rise & fall in a controlled manner • If agent concentration is high, cycle progresses; if it is low, cycle is suspended • Cell cycle control – focused 2 places

36. Control of the Cell Cycle • Cell cycle control is focused at 2 places: • Before S phase (DNA synthesis) • At transition between G2 and M phase

37. Control of Regulating Agents • Regulating agents can be controlled by proximity of other cells • Regulating agents can be controlled by factors inside the cell itself • Internal & external stimuli activate or inhibit regulating agents • These, in turn activate enzymes to help cells divide

38. Control of the Cell Cycle • Internal checkpoints & guardians monitor cell health • Errors in this process can lead to uncontrollable growth and cancer