Cell Reproduction

1. Cell Reproduction Chapter 8

2. Chapter overview • 3 SECTIONS: • SECTION 1 CHROMOSOMES • SECTION 2 CELL DIVISION • SECTION 3 MEIOSIS

3. CHROMOSOMES • SECTION 1 • P. 151-153

4. CHROMOSOME STRUCTURE • CHROMOSOMES…. • ARE MADE OF DNA AND PROTEIN • ARE LINEAR • HAVE A SYSTEM OF COILING TO COMPACT DNA TO FIT IN EUKARYOTIC NUCLEUS • HISTONES = proteins around which the chromosome wraps to compact its length • ARE TECHNICALLY MOLECULES OF DNA • Each chromosome = 1 molecule of DNA

5. Chromosome structure (con’t) • Are controlled by NONHISTONE proteins • Activate/deactivate areas on DNA molecules • Consist of 2 parts in a dividing cell • CHROMATIDS = chromosome + its exact copy • CENTROMERE = protein disk which connects chromatids in center • Exists as CHROMATIN in a nondividing cell • Long and thin DNA • Not as tightly coiled • Enables DNA’s code to be read and used as needed

6. Chromosome structure (con’t) • PROKARYOTIC CELLS AND CHROMOSOMES • Usually have 1 chromosome in each cell • Are circular DNA molecules • Are compacted • Found in nucleoid region • Attached to inside of cell membrane

7. Chromosome numbers • Number of chromosomes found in each species is a distinguishing characteristic • Number of chromosomes in each species is not unique • Number of chromosomes in each species is not reflective of intelligence, social skills, etc….

8. Chromosome numbers • SEX CHROMOSOMES AND AUTOSOMES • Sex chromosomes determine if an organism is male or female • Each organism has 2 sex chromosomes (XX or XY) • Sex chromosomes can also carry GENES = section of DNA which code for a certain trait • All chromosomes other than the sex chromosomes are called AUTOSOMES • 1 set of autosomes is received from each parent • Therefore each organism has 2 sets of chromosomes • The same number pair of chromosome from each parent is called a HOMOLOGOUS PAIR • Same size, shape, + carry same genes • The chromosomes are referred to as HOMOLOGUES

9. Chromosome numbers • Pictures of chromosomes can be made by taking the DNA out of cells (blood) • Cells are stimulated to divide • At METAPHASE (foreshadow SECTION 2!) the cells are photographed • Homologous chromosomes are paired and the picture of the chromosomes are analyzed for genetic defects + sex of organism can be determined • Picture is called a KARYOTYPE

10. Karyotype

11. Chromosome number • DIPLOID AND HAPLOID CELLS • Cells with 2 sets of chromosomes=DIPLOID CELLS • Includes SOMATIC cells (regular body cells) • Referred to as 2n • Human diploid = 46 • Cells with 1 set of chromosomes = HAPLOID CELLS • Includes SEX cells (egg and sperm) • Referred to a 1n or n • Human haploid = 23 WHY ARE SEX CELLS HAPLOID? • So when egg + sperm combine, their combined chromosome number = the correct diploid number • Cells without exact diploid # may be dysfunctional • The pairing of sex cells created homologous chromosome pairs!

12. Cell division Section 2 p. 154-159

13. Cell division in prokaryotes • Each prokaryotic cell…. • Has onesinglecircularDNA molecule which is attached to cell membrane • Is not coiled around histones • Reproduces through BINARY FISSION • Splitting into 2 cells • Occurs in a series of steps…

14. CELL DIVISION IN PROKARYOTES • The steps of BINARYFISSION… • Step 1: DNA is copied • Step 2: Cell membrane grows between the 2 copies of DNA • Step 3: Cell grows • Step 4: New cell wall forms • Step 5: 2 new IDENTICAL CELLS!

15. Binary fission

16. Binary fission

17. CELL DIVISION IN EUKARYOTES • 2 STAGES TO CELL DIVISION IN EUKARYOTES • MITOSIS • Division of the nucleus in a series of steps • CYTOKINESIS • Division of the cytoplasm in series of steps • Varies in animal and plant cells

18. Cell reproduction • Mitosis occurs in organisms undergoing growth, development, repair, or asexual reproduction • Asexual reproduction • Production of offspring from one parent • Sexual reproduction • Production of offspring from 2 parents • Creates geneticvariety • Requires GAMETES or sex cells which are created through the process of MEIOSIS

19. THE CELL CYCLE • Defined: • Repeating set of events in cell’s life • Divided into: • Interphase : time between divisions • Includes G1, S, and G2 • Mitosis: division of the nucleus • Includes prophase, metaphase, anaphase, telophase • Creates haploid cells from haploid cells • Creates diploid cells from diploid cells • Occurs in SOMATIC (body) cells • Cytokinesis: division of the cytoplasm

20. Cell cycle

21. interphase • Majority of cell’s life spent in this… • Includes: • G1(~90%) • “first gap” - cell grows following a division • S • “synthesis” - DNA is copied- replication • Each chromosome connected to its copy by a protein disk called a centromere and are now called sister chromatids • G2 • “second gap” - cell prepares for division • G0 • can follow G1 for certain cells which do NOT divide (EXAMPLES– NERVE + IMMUNECELLS)

22. Stages of mitosis • Divided into 4 stages • Flows like a movie: 1 stage leads into next • Each stage can be divided into early, mid, and late • Cytokinesis usually occurs simultaneously with the last phase: telophase

23. Stages of mitosis

24. prophase • Shortening & coiling of DNA into chromosomes • Nuclear membrane & nucleolus disappear • 2 centrosomes containing a pair of centrioles each appear & move to opposite poles of cell • Plant cells lack centrioles • Mitotic spindle forms (microtubules) • POLAR fibers connect centrosome to centrosome • KINETOCHORE fibers connect from centrosome to the kinetochore located on the centromere of sister chromatids

25. prophase

26. metaphase • Sister chromatids are moved to center of cell by spindle fibers • Side note: karyotypes typically use cells from metaphase

27. metaphase

28. anaphase • Centromeres are broken as sister chromatids are pulled to opposite poles • Sister chromatids are now individual CHROMOSOMES again

29. anaphase

30. telophase • Chromosomes reach opposite sides of cell • Spindle fibers break apart • Chromosomes loosen and become chromatin • Nuclear envelope reforms • Nucleolus forms

31. telophase

32. cytokinesis • Division of the cytoplasm • In animal cells, the cell membrane pinches in and forms a cleavage furrow through the use of microfilaments • In plant cells, vesicles from the Golgi body fuse to form a cell plate, which becomes the cell wall • Newly formed cells are ½ the size of the original (why we need the “first gap” of interphase!) • New cells are IDENTICAL to original cell

33. cytokinesis

34. A look @ mitosis…

35. Control of cell division • There are 3 places where a cell “decides” if it is going to divide • Cell growth checkpoint • Proteins check size of cell • DNA synthesis checkpoint • DNA copying process is checked for accuracy by enzymes • Cell cycle continues if passed • Mitosis checkpoint • If passed, cell finishes mitosis

36. Cell division control

37. When cell control is lost • In the genes of DNA, there are codes for the proteins which control cell division • Mutations (change in DNA’s code) can lead to abnormal or missing proteins • If cell growth is uncontrollable, cancercan result

38. meiosis Section 3 p.161-164

39. meiosis • Purpose: • To create haploid cells from a diploid cell • Result: • 4 cells with half the number of chromosomes as in the original cell • Reason: • When sexual reproduction occurs and gametes (sex cells) join, they have the correct number of chromosomes

40. meiosis

41. Formation of haploid cells • Cell cycle begins and includes all the phases of interphase • DNA IS COPIED DURING S PHASE! • Cell division occurs TWICE • MEIOSIS I: prophase I, metaphase I, anaphase I and telophase I • MEIOSIS II: prophase II, metaphase II, anaphase II and telophase II • The chromosome number is reduced to half during the first division

42. Formation of haploid cells

43. MEIOSIS I – “Separate the Homologues” • PROPHASE I • DNA coils into chromosomes • Spindle fibers appear • Homologous chromosomes pair in SYNAPSIS forming TETRADS • Crossing over may occur • Increases genetic variety • Each organism receives one set of chromosomes from each parent. During this stage, the chromosome from the mother pairs with the SAME chromosome from the father • SAME= has same GENES in same order, but not necessarily the same TRAITS (characteristics)

44. Meiosis I: prophase I

45. Meiosis I • Metaphase I • Tetrads line up in middle of cell • Spindle fibers attach to centromere of homologous chromosome • Spindle fibers from opposite pole attach to other centromere of homologous pair • FYI: • Due to random orientation of chromosomes, you will never look exactly like your parents….. • There are 8, 388, 608 possible combinations for our chromosomes!

46. Meiosis I: metaphase I

47. Meiosis I • Anaphase I • Homologous pairs are separated randomly • INDEPENDENT ASSORTMENT

48. MEIOSIS I • Telophase I and Cytokinesis I • Chromosomes reach opposite poles • Sister chromatids are still attached at centromere • 2 new cells result • Each cell has half the original number of chromosomes, but still have copies attached • In other words, are still SISTERCHROMATIDS

49. MEIOSIS II – “Separate the Chromatids” • ******Not preceded by copying of DNA****** • Can occur immediately after meiosis I • Can be delayed for considerable time

50. Meiosis II (similar to mitosis) • Prophase II • Spindle fibers reform • Metaphase II • Sister chromatids moved to center of cell • Anaphase II • Sister chromatids separated at centromere and move to opposite poles of cell • Telophase II • Nuclear membrane reforms • Cytokinesis II • 4 new HAPLOID cells created