How do cells reproduce?

1. How do cells reproduce? • Cell division is at the heart of reproduction • Multicellular organisms originate from a rapidly dividing fertilized egg (cell); eggs and sperm are themselves created from a special type of cell division • Cell division replaces worn-out or damaged cells, keeping the total number of cells relatively constant • There are two types of cellular division: mitosis and meiosis

2. Cell division and reproduction • Asexual reproduction involves the creation of genetically-identical offspring from a single parent; no eggs or sperm are involved • Involves replication of chromosomes, the structures containing the organism’s DNA • Bacteria, yeast, protists, and certain plants and animals

3. Asexual Reproduction • Asexual reproduction is a very efficient means of reproduction • Faster than sexual reproduction • Increases numbers of organisms quickly • Ability to reproduce in absence of mate (male doesn’t need female and vice versa) • Genetic diversity, however, is sacrificed

4. Sexual Reproduction • The ability for an organism to form gametes, or sex cells (eggs and sperm), results in the formation of similar, but not identical, offspring • In sexual reproduction, the resulting offspring are genetically similar, but not identical to either parent; offspring inherits a combination of genes from each parent

5. Cells arise from pre-existing cells • Cell division allows an embryo to develop into an adult, and is the basis of egg and sperm formation • It also ensures the continuity of life from one generation to the next • In the case of unicellular organisms, cell division can reproduce an entire organism

6. Binary fission • Prokaryotes (Bacteria and Archaea) reproduce by a type of cell division called binary fission; “dividing in half” • These cells possess a single chromosome, which is replicated prior to the cell dividing into 2

7. What about eukaryotic cells? • A bacteria contains ~3,000 genes; human cells contain ~25,000 which are grouped into multiple chromosomes located in the nucleus • Each chromosome consists of 1 long DNA strand, with hundreds or thousands of genes • Integrated into this chromosome are proteins!, which help maintain its structure and control the activity of its genes

8. Chromosomes • Human cells have 46 chromosomes • Before a eukaryotic cell can divide, it must replicate its chromosomes • The DNA molecule of each chromosome is copied and new proteins attach as needed A duplicated chromosome

9. Chromosome duplication Sister chromatids Centromere Chromosome distribution to daughter cells

10. The cell cycle • The process of cell division is a key component of the cell cycle, an ordered sequence of events beginning with the ‘birth’ of the cell from a dividing parent and ending with its own division into 2 cells • The cell cycle consists of a growing stage called interphase, and the actual cell division, called the mitotic phase

11. The Cell Cycle • Most of the cell cycle in spent in interphase • During this time, the cell performs its various functions within the organism • Additionally, the cell acquires a rich supply of proteins, creates more organelles such as mitochondria and ribosomes, and grows during this time • Chromosomes are replicated during interphase

12. The Cell Cycle • Interphase is divided into 3 stooges, er, stages… • The G1 phase: cell grows • The S phase: cell grows, chromosomes replicated • The G2 phase: cell grows G stands for “gap” (first and second gap) S stands for “synthesis” (DNA)

13. INTERPHASE S (DNA synthesis) G1 G2 Cytokinesis Mitosis MITOTIC PHASE (M)

14. The Cell Cycle • During interphase, the cell grows (G1), continues to grow while DNA is replicated (S), and then grows more as it completes preparations for cell division (G2) • Cell division occurs in the mitotic phase (also called the M phase) • Accounts for only 10 of the total time required for the cell cycle

15. The Cell Cycle • Like interphase, the mitotic phase is divided into (2) stages • Mitosis: the nucleus (and all its contents, including the duplicated chromomes) divide and are evenly distributed to the ‘daughter’ cells • Cytokinesis: the cytoplasm is divided into 2 Mitosis and cytokinesis produces 2 genetically identical cells, each with a single nucleus, surrounding cytoplasm and plasma membrane

16. Mitosis • Mitosis (the division of nuclear material) is subdivided into 5 main stages: • Prophase • Prometaphase • Metaphase • Anaphase • Telophase

17. Mitosis • During mitosis, chromosome movement is dependent on the mitotic spindle, a football (go Giants! Go Jets!) shaped structure of microtubules that guides the separation of the 2 sets of separating chromosomes • During interphase, chromosomes are not distinguishable because they exist as loose fibers of chromatin; chromatin becomes more tightly packed and visible as mitosis ensues, allowing easy tracking of each step of mitosis

18. Prophase • The mitotic spindle forms during the first stage, prophase • The chromatin fibers containing DNA become more tightly coiled and folded forming discrete chromosomes that can be seen with a light microscope • Remember, there are 2 pairs of chromosomes at this stage as they were replicated during the S phase of interphase

19. Prophase Visible chromosomes; nuclear envelope still present Early mitotic spindle present

20. Prometaphase • During the second stage of mitosis, prometaphase, the nuclear envelope breaks away • Proteins embedded in the chromatin attach to microtubules of the spindle, and move the chromosomes towards the center of the cell

21. Prometaphase Dissolution of nuclear envelope; chromosomes moved towards the center of the cell Mitotic spindle extend ‘pole’ to ‘pole’

22. Metaphase and Anaphase • During metaphase, the mitotic spindle spreads across the entire cell, with the chromosomes aligned perpendicularly at its center (remember each chromosome has been replicated into 2 prior to mitosis) • In Anaphase, the sister chromatids of each chromosome separate and move away from each other (toward opposing poles)

23. During mitosis, each chromosome has been replicated consisting of 2 sister chromatids; these chromosomes align and separate during metaphase and anaphase, respectively METAPHASE ANAPHASE Metaphase plate Daughter chromosomes Spindle

24. Telophase • During the fifth (and final) stage of mitosis called telophase, nuclear envelopes form around the 2 copies of separated chromosomes; the chromatin fiber uncoils and the mitotic spindle disappears • Sort of a reverse prophase! • Cytokinesis follows this final stage of mitosis, pinching the cell into 2

25. Telophase and Cytokinesis

26. Got all that? • The eukaryotic cell cycle consists of: • Interphase (G1, S, G2) – growth & DNA replication • Mitosis • Prophase – mitotic spindle forms, chromatin condenses • Prometaphase – nuclear envelope dissolves, chromosomes attach to spindle • Metaphase – mitotic spindle spreads pole to pole with chromosomes aligned at center • Anaphase – each sister chromatid of replicatec chromosome separates • Telophase – nuclear envelope reforms, chromatin uncoils • Cytokinesis – cell divides into 2

27. Cell Division • The timing of cell division must be regulated in order to grow and develop normally • Skin cells and stomach cells are replaced regularly as they are constantly abraded and sloughed off • Other cells, such as liver cells, do not divide unless damaged; In this way, cell division repairs wounds and heals

28. Cell Division • Proteins regulate cell division by stimulating cells to divide in their presence • For example, injury to the skin causes blood platelets to release a protein which promotes rapid growth of connective tissue cells that help seal the wound • Proteins control each cycle of mitosis and each stage does not occur until triggered to do so by these proteins

29. Cell Division • Proteins serve as a control system for each stage of the cell cycle • Want a job? Research on controls over the cell cycle is one of the hottest areas in biology today. Why? • Without check points, cells will continue to divide unregulated….. = cancer

30. Cancer • Cancer is a disease of the cell cycle • Cancer cells divide uncontrollably and do not respond normally to the cell cycle control system • Cancer begins when a single cell undergoes transformation from a normal cell to a cancer cell • Cancer cells may proliferate into a tumor, an abnormally growing mass of body cells

31. Cancer • Benign tumors remain at the site and can usually be removed easily with surgery • Malignant tumors spread into neighboring tissues and other parts of the body, interrupting organ function as it goes • Cancer cells may secrete molecules that cause blood vessels to spread toward the tumor, and allow proliferation of the cancer cells via the circulatory system (metastasis)

32. Cancer • Radiation damages DNA in cancer cells moreso than it does in normal cells and can be used as a cancer treatment • Chemotherapy is used to treat metastatic or widespread tumors; involves the use of drugs that disrupt cell division (some drugs prevent the mitotic spindle from forming in the first place); however side effects are seen in normal, rapidly-dividing cells

33. Meiosis • Meiosis is the process of cell division in which the number of chromosome is cut in half • Unlike mitosis, which results in a ‘daughter’ cell containing the exact number of chromosomes as the ‘parent’ cell • Meiosis takes place in reproductive organs and produces gametes, sex cells, such as eggs, sperm, and pollen (plants)

34. Meiosis Homologous pair of chromosomes • Human cells have 46 chromosomes, made up of 23 pairs of homologous chromosomes • Cells with 2 sets of chromosomes are considered diploid One duplicated chromosome Sister chromatids

35. Meiosis • The two chromosomes composing a pair are called homologous because they both carry genes controlling the same inherited characteristics • One exception are the sex chromosomes, X and Y • Females have a homologous pair (XX), while males have 1 X and 1 Y • The other 22 chromosomes are called autosomes

36. Meiosis Homologous pair of chromosomes • For both sex chromosomes and autosomes, we inherit one chromosome of each pair from our mother and the other from our father From Father From Mother

37. Meiosis • The 46 chromosomes in the human cell consists of 23 pairs of homologous chromosomes • Homologous chromosomes are similar, but not identical; they may carry different versions of the same genetic information • For example, one chromosome may code for blond hair, while the other codes for dark hair; or both may contain the same gene (ex. Blue eyes)

38. Meiosis • Human cells contain 22 pairs of autosomes, and 1 pair of sex chromosomes (X and/or Y) Chromosome 1 is the largest; containing 8000 genes Chromosome 21 is the smallest; containing only 300 genes Sex chromosomes http://www.sciencemuseum.org.uk/exhibitions/genes/153.asp

39. Meiosis • Meiosis is a special type of cell division that will produce cells containing half the number of chromosomes • Cells containing half the number of chromosomes are sex cells, or gametes • Gametes contain a single set of chromosomes and are considered haploid (half) • All other cells containing 2 homologous sets of chromosomes is said to be diploid

40. Meiosis • For humans, the diploid number is 46 • Nearly all of our cells are diploid; the exceptions are the gametes! • Sexual reproduction allows a haploid sperm cell to fuse with a haploid egg cell during the process of fertilization producing a zygote • The resulting zygote is diploid; it has 2 sets of homologous chromosomes: 1 from Mom, and 1 from Dad

41. Meiosis • Meiosis occurs only in reproductive organs • During meiosis, a ‘mother’ cell divides and produces 4 genetically distinct ‘daughter’ cells which contain half the number of chromosomes as the ‘other cell • Why 4? This is because meiosis begins with mitosis! (insert “UGH!!!”s here….)

42. Meiosis reduces the chromosome number from diploid to haploid • Just as a cell entering mitosis has duplicated its chromosomes, so too, does a cell entering meiosis (resulting in 92 chromatids) • During prophase 1 (so called because it is the first cycle occuring during Meiosis 1 (out of 2)) the process of crossing over occurs

43. Crossing over • Crossing over is the process by which aligned chromatids of homologous chromosomes exchange genetic segments resulting in a genetically-new chromatid • The driving force of genetic diversity and evolution! • Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring

44. Crossing over

45. Cell division • In both mitosis and meiosis, the chromosomes duplicate only once, in the preceding interphase • Mitosis replicates cells for growth, tissue repair and asexual reproduction and produces daughter cells genetically identical to the parent cell (diploid) • Meiosis produces haploid cells that are genetically distinct from the parent cell

46. Genetic diversity • Changes in an organism’s DNA create different versions of genes (and resulting characteristics) • Reshuffling of these different versions during sexual reproduction produces genetic variation http://www.duggarfamily.com/

47. Genetic diversity • For a human there are 23 chromosomes and 223 combinations of chromosomes that meiosis can package into gametes • 223 equals 8 million (possible combinations)!!! • Each gamete you produce contains 1 of ~8 million possible combinations inherited from your father and your mother • The random fusion of egg and sperm will produce a zygote with any of 64 trillion (8 mil x 8 mil) combinations of chromosomes!!!

48. Points contact each other • DNA is exchanged • Occurs at 1 or more points along adjacent chromatids http://www.accessexcellence.org/AB/GG/crossing.html

49. Homologous chromosomes carry different versions of genes • A pair of homologous chromosomes can bear 2 different kinds of genetic information for the same characteristic

50. Brown coat (C); black eyes (E) Coat-color genes Eye-color genes C E Brown Black C E C E Meiosis c e e c c e White Pink White coat (c); pink eyes (e) Chromosomes of the four gametes