CHAPTER 8 The Cellular Basis of Reproduction and Inheritance

1. CHAPTER 8The Cellular Basis of Reproduction and Inheritance Overview: Reproduction: asexual & sexual prokaryotes eukaryotes Mitosis Meiosis Abnormalities

2. The life cycle of a multicellular organism includes • development • Reproduction: the birth of new organisms • Cell division is at the heart of the reproduction of cells and organisms; cell division plays a role in replacement of lost or damaged cells • Organisms can reproduce sexually or asexually • Just in the past second, millions of your cells have divided in two

3. Some organisms make exact copies of themselves, asexual reproduction • Some organisms can also reproduce asexually • This sea star is regenerating a lost arm • Regeneration results from repeated cell divisions

4. The Reproduction of Organisms • In asexual reproduction, single-celled organisms reproduce by simple cell division

5. Cells arise only from preexisting cells • All cells come from cells • Cellular reproduction is called cell division • Cell division allows an embryo to develop into an adult • It also ensures the continuity of life from one generation to the next

6. Passing On the Genes from Cell to Cell • Before a parent cell divides, it duplicates its chromosomes • The two resulting “daughter” cells are genetically identical

7. It requires fertilization of an egg by a sperm • Production of egg and sperm is called meiosis • Sexual reproduction is different

8. Prokaryotes reproduce by binary fission • Prokaryotic cells divide asexually • These cells possess a single chromosome, containing genes • The chromosome is replicated • The cell then divides into two cells, a process called binary fission

9. THE EUKARYOTIC CELL CYCLE AND MITOSIS The large, complex chromosomes of eukaryotes duplicate with each cell division • An eukaryotic cell has many more genes than a prokaryotic cell • The genes are grouped into multiple chromosomes, found in the nucleus • The chromosomes of this plant cell are stained dark purple

10. Is the complete set of an organism’s genes • Is located mainly on chromosomes in the cell’s nucleus • A genome

11. Eukaryotic Chromosomes • Are made of chromatin, a combination of DNA and protein molecules • Are not visible in a cell until cell division • Chromosomes

12. The Cell Cycle • Eukaryotic cells that divide undergo an orderly sequence of events called the cell cycle

13. The cell cycle consists of two major phases: • Interphase, where chromosomes duplicate and cell parts are made • The mitotic phase, when cell division occurs

14. Interphase • Eukaryotic cell division consists of two stages: • Mitosis • Cytokinesis • Mitosis • Is the division of the chromosomes • Is preceded by interphase

15. In mitosis, the duplicated chromosomes are distributed into two daughter nuclei • After the chromosomes coil up, a mitotic spindle moves them to the middle of the cell • Mitosis consists of four distinct phases:

16. The process of cytokinesis divides the cell into two genetically identical cells • The sister chromatids then separate and move to opposite poles of the cell

17. Typically occurs during telophase • Is the division of the cytoplasm • Cytokinesis

18. Cytokinesis differs for plant and animal cells • In animals, cytokinesis occurs by cleavage • This process pinches the cell apart

19. In plants, a membranous cell plate splits the cell in two

20. Anchorage, cell density, and chemical growth factors affect cell division • Most animal cells divide only when stimulated, and others not at all • In laboratory cultures, most normal cells divide only when attached to a surface • They are anchorage dependent

21. This is called density-dependent inhibition • Cells continue dividing until they touch one another

22. Growth factors are proteins secreted by cells that stimulate other cells to divide

23. Growth factors signal the cell cycle control system • Proteins within the cell control the cell cycle • Signals affecting critical checkpoints determine whether the cell will go through a complete cycle and divide

24. The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division

25. Cancer Cells: Growing Out of Control • Normal plant and animal cells have a cell cycle control system • When the cell cycle control system malfunctions • Cells may reproduce at the wrong time or place • A benign tumor may form What Is Cancer? • What is cancer? • Cancer is caused by a breakdown in control of the cell cycle

26. Cancer Treatment • Radiation therapy disrupts cell division • Chemotherapy involves drugs that disrupt cell division • Cancer treatment

27. Cancer Prevention and Survival • Cancer prevention includes changes in lifestyle • Not smoking • Avoiding exposure to the sun • Eating a high-fiber, low-fat diet • Visiting the doctor regularly • Performing regular self-examinations

28. MEIOSIS AND CROSSING OVER Chromosomes are matched in homologous pairs • Somatic cells of each species contain a specific number of chromosomes • Human cells have 46, making up 23 pairs of homologous chromosomes

29. Homologous Chromosomes • Different organisms of the same species have the same number and types of chromosomes • A somatic cell • Is a typical body cell • Has 46 chromosomes in a human • Humans have • Two different sex chromosomes, X and Y • 22 pairs of matching chromosomes, called autosomes

30. Gametes have a single set of chromosomes • Cells with two sets of chromosomes are said to be diploid • Gametes are haploid, with only one set of chromosomes

31. Repeated mitotic divisions lead to the development of a mature adult • The adult makes haploid gametes by meiosis • All of these processes make up the sexual life cycle of organisms • At fertilization, a sperm fuses with an egg, forming a diploid zygote

32. Their cells contain two sets of chromosomes • Their gametes are haploid, having only one set of chromosomes • Humans are diploid organisms • Fertilization • Is the fusion of sperm and egg • Creates a zygote, or fertilized egg

33. Meiosis reduces the chromosome number from diploid to haploid • Meiosis, like mitosis, is preceded by chromosome duplication • However, in meiosis the cell divides twice to form four daughter cells

34. While they are paired, they cross over and exchange genetic information • The homologous pairs are then separated, and two daughter cells are produced • In the first division, meiosis I, homologous chromosomes are paired

35. The sister chromatids of each chromosome separate • The result is four haploid daughter cells • Meiosis II is essentially the same as mitosis

36. Review: A comparison of mitosis and meiosis • For both processes, chromosomes replicate only once, during interphase

37. Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring • Each chromosome of a homologous pair comes from a different parent • Each chromosome thus differs at many points from the other member of the pair

38. The Origins of Genetic Variation • Offspring of sexual reproduction are genetically different from their parents and from one another • The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes • Random fertilization also increases variation in offspring

39. Random Fertilization • The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote Homologous chromosomes carry different versions of genes • The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci

40. Crossing Over • Homologous chromosomes exchange genetic information • Genetic recombination occurs • In crossing over

41. Crossing over further increases genetic variability • Crossing over is the exchange of corresponding segments between two homologous chromosomes • Genetic recombination results from crossing over during prophase I of meiosis • This increases variation further

42. ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE A karyotype is a photographic inventory of an individual’s chromosomes • To study human chromosomes microscopically, researchers stain and display them as a karyotype • A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes • A karyotype is an orderly arrangement of chromosomes

43. Connection: An extra copy of chromosome 21 causes Down syndrome • This karyotype shows three number 21 chromosomes • An extra copy of chromosome 21 causes Down syndrome

44. Accidents during meiosis can alter chromosome number • Abnormal chromosome count is a result of nondisjunction • Either homologous pairs fail to separate during meiosis I

45. Connection: Abnormal numbers of sex chromosomes do not usually affect survival • Nondisjunction can also produce gametes with extra or missing sex chromosomes • Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes

46. Connection: Alterations of chromosome structure can cause birth defects and cancer • Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer • Four types of rearrangement are deletion, duplication, inversion, and translocation