Lecture 7 Mitosis & Meiosis

1. Lecture 7Mitosis & Meiosis

2. Cell Division • Essential for body growth and tissue repair • Interphase • G1 phase • Primary cell growth phase • S phase • DNA replication • G2 phase • Microtubule synthesis • Mitosis • Nuclear division • Cytokinesis • Cytoplasmic division

3. Chromosomes: Carriers of Inherited Traits • Chromosomes were first observed by the German embryologist Walther Fleming in 1882 • Chromosomes exist in somatic cells as pairs • Homologous chromosomesorhomologues • The number of chromosomes varies enormously from species to species • The Australian ant Myrmecia spp. has only 1 pair • Some ferns have more than 500 pairs

4. Human Chromosomes • Humans have 46 chromosomes • The 23 pairs of homologous chromosomes can be organized by size • This display is termed a karyotype

5. Diploid cellshave two copies of each chromosome Basic Terminology • Replicated chromosomes consist of two sister chromatids • These are held together at the centromere

6. Chromosomes • Chromosomes are composed of chromatin • Complex of DNA (~ 40%) and proteins (~ 60%) • A typical human chromosome contains about 140 million nucleotides in its DNA • This is equivalent to • About 5 cm in stretched length • 2,000 printed books of 1,000 pages each! • In the cell, however, the DNA is coiled

7. The DNA helix is wrapped around positively-charged proteins, called histones 200 nucleotides of DNA coil around a core of eight histones, forming a nucleosome The nucleosomes coil into solenoids Solenoids are then organized intolooped domains The looped domains appear to form rosettes on scaffolds Chromosomes: Packaged DNA

8. Xm Im Im Xf Im If If Xm If Im If Im Xm Im If Xm Im Im Im If Xf Im If Xm If If If Xf Metaphase Prophase Anaphase Telophase Xf Xf If If Xf Im Im Xm Condense Line Up Separate Divide Mitosis • The phases of mitosis are: • Cytokinesis • Cleavage furrow formed in late anaphase by contractile ring • Cytoplasm is pinched into two parts after mitosis ends

9. Pair of centrioles Early mitotic spindle Centromere Aster Chromosome, consisting of two sister chromatids Early prophase Early Prophase • Asters are seen as chromatin condenses into chromosomes PLAY Prophase Figure 3.32.2

10. Fragments of nuclear envelope Polar microtubules Kinetochore Kinetochore microtubule Spindle pole Late prophase Late Prophase • Nucleoli disappear • Centriole pairs separate and the mitotic spindle is formed PLAY Prometaphase Figure 3.32.2

11. Metaphase plate Spindle Metaphase Metaphase • Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell • This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate PLAY Metaphase Figure 3.32.4

12. Daughter chromosomes Anaphase Anaphase • Centromeres of the chromosomes split • Motor proteins in kinetochores pull chromosomes toward poles PLAY Anaphase Figure 3.32.5

13. Nucleolus forming Contractile ring at cleavage furrow Nuclear envelope forming Telophase and cytokinesis Telophase and Cytokinesis • New sets of chromosomes extend into chromatin • New nuclear membrane is formed from the rough ER • Nucleoli reappear • Generally cytokinesis completes cell division PLAY Telophase

14. Controlling the Cell Cycle • Cell growth is assessed at the G1 checkpoint • DNA replication is assessed at the G2 checkpoint • Mitosis is assessed at the M checkpoint G0 is an extended rest period • The eukaryotic cell cycle is controlled by feedback at three checkpoints PLAY Control of the cell cycle

15. Programmed cell death Cell Death • During fetal development, many cells are programmed to die Fingers and toes form from these paddle-like hands and feet • Human cells appear to be programmed to undergo only so many cell divisions • About 50 in cell cultures • Only cancer cells can divide endlessly

16. What is Cancer? • Cancer is unrestrained cell growth and division • The result is a cluster of cells termed a tumor • Benign tumors • Encapsulated and noninvasive • Malignanttumors • Not encapsulated and invasive • Can undergo metastasis • Leave the tumor and spread throughout the body

17. What is Cancer? • Most cancers result from mutationsin growth-regulating genes • There are two general classes of these genes • 1. Proto-oncogenes • Encode proteins that stimulate cell division • If mutated, they becomeoncogenes • 2. Tumor-suppressor genes • Encode proteins that inhibit cell division • Cancer can be caused by chemicals, radiation or even some viruses

18. Cancer, cell division, and p53 protein PLAY How tumor suppression genes work

19. New molecular therapies for cancer • Potential cancer therapies are being developed to target seven different stages in the cancer process • Stages 1-6 • Prevent the start of cancer within cells • Focus on the decision-making process to divide • Stage 7 • Act outside cancer cells • Prevents tumors from growing and spreading Receiving the signal to divide Stopping tumor growth Stepping on the gas Passing the signal via a relay switch Checking that everything is ready Amplifying the signal Releasing the “brake”

20. Importance of Generating Diversity • Diversity is what allows gene populations to survive changes in their environment • The greater the diversity, the more likely some individuals will have the traits to survive a major change • Genetic diversity is the raw material that fuels evolution • No genetic process generates diversity more quickly than sexual reproduction

21. Sexual reproduction Maximizes diversity Involves the alternation of meiosis and fertilization Meiosis halves the number of chromosomes from each parent cell Fertilization restores the number of chromosomes Contain one set of chromosomes Two Types of Reproduction • Asexual reproduction • Creates a faithful copy • Does not involve fertilization Contains two sets of chromosomes

22. Spend most of life in diploid form Spend half of life in haploid form and half in diploid form Spend most of life in haploid form The Sexual Life Cycle • The life cycles of all sexually-reproducing organisms follows the same basic pattern • Haploid cells or organisms alternate with diploid cells or organisms • There are three basic types of sexual life cycles

23. The Sexual Life Cycle in Humans

24. Discovery of Meiosis • Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887 • Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells • The fusion of gametes is called fertilization or syngamy • It creates the zygote, which contains two copies of each chromosome

25. Metaphase Prophase Anaphase Telophase Xm Im Im Xf Im If If Xm If Im If Im Xm Im If Xm Im Im Im If Xf Im If Xm If If If Xf Xf Xf If If Xf Im Im Xm Condense Line Up Separate Divide Xf mX Xf Xf Xm Xm Xf mX fX Xm Xm Xf Meiosis I fX Xm Xm Xf Xm fX Xm Xf Xm fX Xm Xf Im If Im If If Im Im If Xf Xf Im Xm Xm If If Im Im If If Im Im Xm Xm If Xf Meiosis II Xf Im If Xm Xm Im Im If If Xf Xf Meiosis Mitosis PLAY Unique features of meiosis

26. Meiotic Cell Division: Meiosis I • Tetrads line up at the spindle equator during metaphase I • In anaphase I, homologous chromosomes still composed of joined sister chromatids are distributed to opposite ends of the cell • At the end of meiosis I each daughter cell has: • Two copies of either a maternal or paternal chromosome • A 2n amount of DNA and haploid number of chromosomes • In telophase I: • The nuclear membranes re-form around the chromosomal masses • The spindle breaks down • The chromatin reappears, forming two daughter cells Figure 27.7 PLAY Prophase I, Metaphase I, Anaphase I, Telophase I

27. Meiotic Cell Division: Meiosis II • Mirrors mitosis except that chromosomes are not replicated before it begins • Meiosis accomplishes two tasks: • It reduces the chromosome number by half (2n to n) • It introduces genetic variability PLAY Meiosis II

28. Purpose Create new cells (faithful copies) Create new individuals (diversity) Where it occurs Body cells Special reproductive organs Number of divisions after one chromatin replication event One Two Number of daughter cells Homologous pairs in Meiosis I, Individually in Meiosis II Individually Homologous chromosomes in daughter cells Two Four Makeup of daughter cells vs. parent cell Two (diploid) One (haploid) Same as parent cell Different from parent cell Differences Between Mitosis & Meiosis Meiosis Mitosis How chromosomes Line up at metaphase PLAY Differences between meiosis &. mitosis

29. Sexual reproduction increases genetic diversity through three key mechanisms Independent assortment Crossing over Random fertilization Evolutionary Consequences of Sex

30. Independent assortment • In humans, a gamete receives one homologue of each of the 23 chromosomes • Humans have 23 pairs of chromosomes • 223 combinations in an egg or sperm • 8,388,608 possible kinds of gametes Three chromosome pairs 23 combinations

31. Crossing over • DNA exchanges between maternal and paternal chromatid pairs • This adds even more recombination to independent assortment that occurs later

32. Random fertilization • The zygote is formed by the union of two independently-produced gametes • Therefore, the possible combinations in an offspring • 8,388,608 X 8,388,608 = • 70,368,744,177,664 • More than 70 trillion! • And this number does not count crossing-over