Cellular Reproduction

1. Cellular Reproduction • How to preserve genetic information.

2. When and why do cells divide? • Cells divide when there is a chemical signal to do so. • Skin cells may divide in response to crowding. Certain cells send out a chemical signal that tells neighboring cells to divide. • Cells may divide in response to an injury, to mend damaged tissue. • Growth factors can signal cell division in children to lengthen bones and add other tissues.

3. Specialized cell membrane proteins signal cell division when growth factors are present. growth factor Growth factor binds to receptor and stimulates cyclin synthesis. growth factor receptor (plasma membrane) cyclin Cyclin activates Cdk; active Cdk stimulates DNA replication. cyclin- dependent kinase (cytoplasm) Cdk’s are always present in the cell.

4. Cancers begin when something goes wrong with the system controlling cell growth and division. Normal G1 to S control: Mutated growth factor receptor gene: Mutated cyclin gene: growth factors + receptor growth factors + mutated receptor always “on” growth factors + receptor cyclin synthesis + Cdk cyclin synthesis + Cdk cyclin synthesis always “on” + Cdk phosphorylates Rb phosphorylates Rb phosphorylates Rb Rb P Rb P Rb P DNA replication uncontrolled DNA replication uncontrolled DNA replication

5. Binary Fission • Bacteria and other prokaryotes reproduce by simple binary fission. • The single ring-shaped chromosome is duplicated, and the cell divides in half.

6. cell division by binary fission cell growth and DNA replication

7. 1 3 attachment site cell wall plasma membrane circular DNA New plasma membrane is added between the attachment points, pushing them further apart. The circular DNA double helix is attached to the plasma membrane at one point. 2 4 The plasma membrane grows inward at the middle of the cell. The DNA replicates and the two DNA double helices attach to the plasma membrane at nearby points. 5 The parent cell divides into two daughter cells.

8. Mitosis • One-celled eukaryotic organisms, and individual cells in a multi-cellular organisms, reproduce by mitosis followed by cytokinesis.

9. The problem Eukaryotic cells are often diploid: that is, they have two of each kind of chromosome.

10. W O R K T O G E T H E R • Why do eukaryotic cells have TWO of each chromosome? • If a cell with two of each chromosome divides, how many chromosomes should end up in each of the two daughter cells?

11. A cell with 14 chromosomes divides by mitosis. How many chromosomes will each daughter cell have? • 28 • 14 • 7 • Each daughter cell will have a different, unpredictable number.

12. When is DNA replicated? • Just before cell division. • In preparation for transcription and translation. • Any time.

13. Overview of Mitosis • After DNA is replicated, it is condensed into chromosomes and identical copies are sorted in the process of mitosis. • Mitosis assures that the two daughter cells have exactly the same DNA.

14. Warning: Confusing terminology ahead! After cell division, the single strand is a chromosome again. (Again, think of it as a one-chromatid chromosome.) Before cell division, a strand of DNA is a chromosome. (Think of it as a one-chromatid chromosome.) During cell division, two identical copies of a DNA strand link together into a two-chromatid chromosome.

15. telophase and cytokinesis G1: cell growth and differentiation anaphase metaphase prophase mitotic cell division G0: nondividing G2: cell growth Under certain circumstances, cell may return to cell cycle. interphase S: synthesis of DNA; chromosomes are duplicated Animated cell cycle at http://cellsalive.com

16. Prior to Mitosis, DNA is replicated during the S-phase of the cell cycle. Chromosomes appear late in G2 phase, just prior to mitosis. If you wanted to count the onion root tip cells in this picture that are in mitosis, what feature would be in the cell that tells you they are in mitosis?

17. INTERPHASE nuclear envelope chromatin nucleolus centriole pairs Late Interphase Can we tell if a cell in Interphase is in G1, S, or G2 of the cell cycle?

18. DNA (2 nm diameter) histone proteins nucleosome: DNA wrapped around histone proteins (10 nm diameter) coiled nucleosomes (30 nm diameter) chromosome: coils gathered onto protein scaffold (200 nm diameter) protein scaffold DNA coils A strand (double helix) of DNA wraps around histone proteins to form chromosomes. This protects DNA from damage during cell division.

19. genes centromere telomeres The structure of a condensed chromosome (before pairing).

20. W O R K T O G E T H E R • Make a prediction: during chromosome formation, which will pair up? • Homologous chromatids that carry the same genes but different alleles? • Sister chromatids that carry the same genes and the same exact alleles? • (Hint: the end goal is to get identical genetic information into the daughter cells.)

21. gene 1 gene 2 same alleles different alleles Homologous chromosomes are those that carry the same genes but may have slightly different information (such as dominant or recessive versions of a gene). Homologous chromosomes do not pair together. Chromosomes only pair with their identical sister chromatids.

22. sister chromatids centromere Identical (sister) chromatids pair up during Prophase, and join at a pinched-in point called the centromere.

23. duplicated chromosome (2 DNA double helices) sister chromatids The chromosome at the end of Prophase consists of two strands of condensed DNA. Each sister chromatid carries exactly the same information.

24. Which of these are genetically identical? • Homologous chromosomes • Sister chromatids • Neither of these

25. Which of these will pair up to form chromosomes in mitosis? • Homologous chromosomes • Sister chromatids • Chromatids pair at random, so it’s impossible to predict.

26. MITOSIS: Early Prophase condensing chromosomes beginning of spindle formation Notice that these cells in prophase have barely visible chromosomes as DNA begins to condense.

27. W O R K T O G E T H E R • In prophase, the identical chromatids are paired into chromosomes. The end goal is to get each identical chromatid to opposite ends of the cell. Where should the cell place the chromosomes now to achieve that goal?

28. MITOSIS: Late Prophase pole kinetochore pole As prophase progresses, the chromosomes become more and more visible as they condense.

29. MITOSIS: Metaphase spindle microtubules Chromosomes, with their paired identical chromatids, move to the center of the cell.

30. W O R K T O G E T H E R • In metaphase, the chromosomes are lined up in the middle of the cell. Predict what has to happen next to get the chromatids to opposite poles of the cell.

31. MITOSIS: Anaphase "free" spindle fibers Identical chromatids separate from one another and migrate to opposite poles of the cell.

32. MITOSIS: Telophase chromosomes extending nuclear envelope re-forming Telophase completes Mitosis. Both poles of the cell now have identical DNA, and the cell can divide in half.

33. MITOSIS: Cytokinesis After Mitosis has finished sorting the chromosomes, cytokinesis takes place, dividing the cell into two new cells.

34. INTERPHASE Before S phase, the cell was diploid (two copies of each chromosome). After cytokinesis, are the cells diploid or haploid?

35. W O R K T O G E T H E R • Predict: How does cytokinesis differ between animal and plant cells?

36. The process of cytokinesis 1 Microfilaments form a ring around the cell's equator. 2 The microfilament ring contracts, pinching in the cell's “waist.” 3 The waist completely pinches off, forming two daughter cells.

37. Cytokinesis in plant cells Golgi apparatus cell wall plasma membrane carbohydrate- filled vesicles 1 Carbohydrate-filled vesicles bud off the Golgi apparatus and move to the equator of the cell. 2 Vesicles fuse to form a new cell wall (red) and plasma membrane (yellow) between daughter cells. 3 Complete separation of daughter cells.

38. In which phase do chromosomes condense? • Prophase • Metaphase • Anaphase • Telophase

39. In which phase do sister chromatids separate from one another? • Prophase • Metaphase • Anaphase • Telophase

40. In which phase do chromosomes line up in the middle of the cell? • Prophase • Metaphase • Anaphase • Telophase

41. Meiosis • Meiosis is cell division that involves the reduction of chromosomes in a cell.

42. The problem: • When diploid organisms reproduce sexually, two cells must fuse and share genetic information. • The end result of sexual reproduction is a new diploid organism that has genetic information from both parents.

43. W O R K T O G E T H E R • How can two cells from diploid parents fuse to make a diploid offspring?

44. 2n n meiotic cell division 2n 2n n fertilization diploid parental cells haploid gametes diploid fertilized egg The cells from the parents must be haploid if their offspring is to be diploid. While diploid cells hold two copies of each chromosome (one from each parent), haploid sex cells hold one copy of each chromosome.

45. homologous chromosomes sister chromatids Meiosis is reduction division. It begins with a diploid cell and produces haploid cells. Why does it produce four haploid cells?

46. Meiosis also involves the cell cycle, and takes place after S phase of the cell cycle. DNA is replicated before meiosis. G1: cell growth and differentiation mitotic cell division G0: nondividing G2: cell growth Under certain circumstances, cell may return to cell cycle. interphase S: synthesis of DNA; chromosomes are duplicated

47. MEIOSIS I Homologous chromosomes pair and cross over. Homologous chromosomes line up in pairs. Homologous chromosomes move to opposite poles. paired homologous chromosomes recombined chromosomes chiasma spindle microtubule (a) Prophase I (b) Metaphase I (c) Anaphase I (d) Telophase I First half of meiosis: separation of homologous chromosomes.

48. Prophase I Homologous chromosomes pair and cross over. paired homologous chromosomes spindle microtubule chiasma Notice that four strands — maternal and paternal chromosomes and their identical sister chromatids — join into a single unit, called a tetrad.

49. protein strands joining duplicated chromosomes direction of “zipper” formation Protein strands “zip” the homologous chromosomes together.

50. While in tetrads, homologous chromosomes often swap ends, further mixing up genetic information. recombination enzymes chiasma chiasma Recombination enzymes bind to the joined chromosomes. Recombination enzymes snip chromatids apart and reattach the free ends. Chiasmata (the sites of crossing over) form when one end of the paternal chromatid (yellow) attaches to the other end of a maternal chromatid (purple). Recombination enzymes and protein zippers leave. chiasmata remain, helping to hold homologous chromosomes together.