Chapter 8

1. Chapter 8 Sections 1-3 Chromosomes ,Cell Division & Meiosis

2. Section 1 Chromosomes: Objectives • Describe the structure of a chromosome • Identify the differences in structure between prokaryotic chromosomes and eukaryotic chromosomes • Compare the numbers of chromosomes in different species • Explain the differences between sex chromosomes and autosomes • Distinguish between diploid and haploid cells

3. Chromosome Structure DNA Coiling Animation • Chromosomes are rod-shaped structures made of DNA and protein. • In eukaryotes, DNA wraps around proteins called histones to help maintain the compact structure of chromosomes so it fits within the cell • Each human cell has ~1.8m of DNA; wound on histones it’s about 0.09mm

4. Chromosome Structure • Chromatids are duplicate halves of a chromosome • The point at which both chromatids are attached is called centromere

5. Chromosome Structure • Looking more closely at prokaryotes and eukaryotes… • Prokaryotic chromosomes are simpler than eukaryotic chromosomes

6. Chromosome Numbers • Every species has a characteristic number of chromosomes in each cell

7. Chromosome Numbers • Sex chromosomes are chromosomes that determine the sex of an organism • In humans, sex chromosomes are either X or Y • Females: XX; Males: XY • All other chromosomes in an organism are autosomes • Humans have 46 chromosomes total: • 2 sex chromosomes • 44 autosomes • A karyotype is a pictomicrograph of chromosomes in a normal dividing cell found in a human

8. Chromosome Numbers • Diploid cells: have two homologues of each chromosome, are designated by the symbol ‘2n’, and have chromosomes found in pairs • Haploid cells: are reproductive cells (sperm and egg) in humans, have ½ the number of chromosomes present in a diploid, and are designated by the symbol ‘1n’ • When a sperm cell (1n) and an egg cell (1n) combine, they create a new diploid cell (2n). If they were diploid, the new cell would have too many chromosomes to function

9. Section 2 Cell Division: Objectives • Describe the events of cell division in prokaryotes • Name the two parts of the cell that are equally divided during cell division in eukaryotes • Summarize the events of interphase • Describe the stages of mitosis • Compare cytokinesis in animal cells with cytokinesis in plant cells • Explain how cell division is controlled

10. Cell Division in Prokaryotes • Cell division is the process by which cells reproduce themselves • Binary fission is the process of cell division in prokaryotes • Bacteria = prokaryotes → divide by binary fission

11. Cell Division in Eukaryotes • Cell Cycle • The cell cycle is the repeating set of 5 events in the life of a cell • It consists of interphase and cell division • Cell division in eukaryotes includes: • Nuclear division (mitosis) • Division of the cytoplasm (cytokinesis)

12. Cell Division in Eukaryotes • The cell cycle starts at G1 and continues through cytokinesis • In order: G1-S-G2-M-C • Interphaseconsists of: • G1: growth • S: DNA replication (“synthesis”) • G2: preparation for cell division • Interphase occupies most of a cell’s life How the Cell Cycle Works Animation

13. Stages of Mitosis • Mitosis is divided into 4 stages: • Prophase • Metaphase • Anaphase • Telophase • Mitosis results in two offspring cells identical to the original cell

14. Stages of Mitosis • Phase 1: Prophase • Uncoiled DNA coils into chromosomes inside the nuclear membrane • Nuclear membrane breaks down and disappears • Centrosomes move to the poles of the cell and start forming spindle fibers (specialized microtubules) • Phase 2: Metaphase • Fibers from centrosomes move chromosomes to the center of the splitting cell

15. Stages of Mitosis • Phase 3: Anaphase • Chromatids of each chromosome split from their centromere, becoming individual chromosomes • Phase 4: Telophase • Individual chromosomes reach either end of the original cell • Spindle fibers disassemble • Chromosomes are packed together in a nuclear envelope

16. Cytokinesis • Cytokinesis • NOT a phase of mitosis • Splits a cell into two cells that are genetically identical to original cell How the Cell Cycle Works Animation

17. Stages of Mitosis Prophase Interphase Metaphase Cytokinesis Anaphase Telophase

18. Stages of Mitosis • Put the four pictures in order. • At which stage are the centromeres dividing? • Which picture is the starting point for mitosis?

19. Stages of Mitosis • Put the four pictures in order. • 3, 2, 1, 4 • At which stage are the centromeres dividing? • 1 (anaphase) • Which picture is the starting point for mitosis? • 3 (prophase)

20. Cytokinesis – Plants v. Animals • During cytokinesis in animal cells, a cleavage furrow pinches in and eventually separates the diving cell into two cells

21. Cytokinesis – Plants v. Animals • During cytokinesis in animal cells, a cleavage furrow pinches in and eventually separates the diving cell into two cells • In plant cells, a cell plate separates the dividing cells into two cells and eventually transforms into a cell wall

22. Control of Cell Division • Cell division in eukaryotes is controlled by many proteins • Control occurs at three main checkpoints: • G1: if cell is healthy and has suitable size, proteins initiate DNA synthesis (S phase starts). It not, division stops. • G2: DNA repair enzymes check DNA replication. If good, cycle continues; if not, division stops. • Mitosis checkpoint: if the cell passes this checkpoint, proteins signal the cell to exit mitosis and it enters G1 again.

23. When Control is Lost: Cancer • Cancer may result if cells do not respond to control mechanisms • Cancer is uncontrolled cell growth • Some mutations cause cancer by overproducing growth-promoting molecules leading to increased cell division • Other mutations may interfere with the ability of control proteins to slow or stop the cell cycle

24. Section 3 Meiosis: Objectives • Compare the end products of meiosis with those of mitosis • Summarize the events of meiosis I • Explain crossing-over and how it contributes to the production of unique individuals • Summarize the events of meiosis II • Compare spermatogenesis and oogenesis • Define sexual reproduction

25. Formation of Haploid Cells • Meiosis is the process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell. • Cells undergoing meiosis divide twice so diploid (2n) cells that divide meiotically produce four haploid (1n) cells instead of two diploid (2n) cells • Meiosis results in four haploid cells (gametes) and not two diploid cells as in mitosis

26. Meiosis I • Meiosis I includes prophase I, metaphase I, anaphase I, and telophase I • Be careful: these are not exactly the same as the mitosis phases! • Crossing over is when homologous chromosomes exchange corresponding segments of genetic material (DNA) during prophase I resulting in genetic recombination • Crossing over is a source of variation in a species!

27. Meiosis I • Prophase I: DNA coils into chromosomes which line up next to their homolog (this does not happen in mitosis!). This pairing up is called synapsis. Each pair of homologue chromosomes is called a tetrad. During synapsis, the chromatids in the pairs twist around each other and cross over. • Metaphase I: tetrads line up randomly along the midline of the dividing cell. Spindle fibers from the poles connect to the chromosomes.

28. Meiosis I • Anaphase I: each homologous chromosome moves to the opposite pole of the dividing cell. • The random separation of homologous chromosomes is called independent assortment and results in genetic variation • Telophase I & Cytokinesis I: the chromosomes reach opposite ends of the cell and cytokinesis begins. The new cells are haploid.

29. Meiosis I Summary • Meiosis I: the original cell produces 2 new cells, each containing one chromosome from each homologous pair • The new cells contain half the number of chromosomes, but each contains two copies (as chromatids) because the original cell copied its DNA before meiosis I Meiosis and Crossing Over

30. Meiosis II • Meiosis II occurs in each cell formed during meiosis I • It is NOT preceded by the copying of DNA • Meiosis II includes prophase II, metaphase II, anaphase II, and telophase II • Four new haploid cells result

31. Meiosis II • Prophase II: spindle fibers from and begin to move chromosomes toward the midline of the dividing cell. • Metaphase II: chromosomes move to the midline of the dividing cell, with each chromatid facing the opposite end of the dividing cell.

32. Meiosis I • Anaphase II: chromatids separate and move toward opposite poles of the cell. • Telophase II: a nuclear membrane forms around the chromosomes in each of the four new (almost) cells • Cytokinesis II: forms the four new cells. Each cell contains half of the original cell’s number of chromosomes

33. Stages of Meiosis

34. The Chromosome Thing: NOVA Comparison Animation

35. Development of Gametes • Spermatogenesis is the process by which sperm cells are produced. • Oogenesis is that process that produces mature egg cells.

36. Sexual Reproduction • Sexual reproduction is the formation of offspring through meiosis and the union of a sperm and an egg. • Offspring produced by sexual reproduction are genetically different from the parents.