Cell Reproduction

1. Cell Reproduction 7th Grade

2. Cell Division and Mitosis Section 1

3. Why is cell division important? Many organisms start as just one cell. That cell divides and becomes two, two become four, four become eight, an so on. Many-celled organisms, including you, grow because cell division increases the total number of cells in an organism.

4. Why is cell division important? (cont) Even after growth stops, cell division is still important. Every day, billions of red blood cells in your body wear out and are replaced. During a few seconds, your bone marrow produced about six million red blood cells. Cell division is important to one-celled organisms, too�it�s how they reproduce themselves.

5. Living Things Grow and Develop All living things, whether they are made of one cell or many cells, grow during periods of their lives. Living organisms have a life cycle. A life cycle begins with the organism�s formation, is followed by growth and development, and finally ends in death.

6. The Cell Cycle Individual cells also complete cycles. The life cycle of a cell is called the cell cycle. An important step of the cell cycle occurs when the cell makes a copy of its DNA (deoxyribonucleic acid ). DNA is the hereditary material of the cell. DNA is organized into structures called chromosomes.

7. Cell Cycle The life cycle of a cell is called the cell cycle. An important step of the cell cycle occurs when the cell makes a copy of its DNA (deoxyribonucleic acid ). DNA is the hereditary material of the cell. DNA is organized into structures called chromosomes.

8. Length of Cycle The cell cycle is a series of events that takes place from one cell division to the next. The time is takes to complete a cell cycle is not the same in all cells.

9. Cell Cycle Short Version During the first stage, called interphase, the cell grows and copies its organelles and chromosomes. After each chromosome is duplicated, the two copies are called chromatids. In the second stage, called mitosis, the chromatids separate in a process. Mitosis ensures that each new cell receives a copy of each chromosome. Mitosis has four phases�prophase, metaphase, anaphase, and telophase. In the third stage, called cytokinesis, the cell splits into two identical cells.

10. Interphase Most of the life of any eukaryotic cell�a cell with a nucleus�is spent in a period of growth and development called interphase. Cells in your body that no longer divide, such as nerve and muscle cells, are always in interphase. Each cell needs a complete set of hereditary material to carry out life functions. Before a cell divides, a copy of the hereditary material must be made so that each of the two new cells will get a complete copy. An actively dividing cell, such as a skin cell, copies its hereditary material and prepares for cell division during interphase.

11. Interphase (cont.) During interphase, each chromosome duplicates. When the nucleus is ready to divide, each duplicated chromosome coils tightly into two thickened, identical strands called chromatids.

12. Mitosis Mitosis is the process in which the nucleus divides to form two identical nuclei. Each new nucleus also is identical to the original nucleus. A chromosome is a structure in the nucleus that contains hereditary material.

13. Steps of Mitosis Prophase Metaphase Anaphase Telophase

14. Mitosis- Prophase During prophase, the pairs of chromatids are fully visible when viewed under a microscope. The nucleolus and the nuclear membrane disintegrate. Two small structures called centrioles move to opposite ends of the cell.

15. Mitosis- Prophase (cont.) Threadlike spindle fibers begin to stretch across the cell between the centrioles. Plant cells also form spindle fibers during mitosis but do not have centrioles.

16. Mitosis- Metaphase In metaphase, the pairs of chromatids line up across the center of the cell. The centromere of each pair usually becomes attached to two spindle fibers�one from each side of the cell.

17. Mitosis- Anaphase In anaphase, each centromere divides and the spindle fibers shorten. Each pair of chromatids separates, and chromatids begin to move to opposite ends of the cell. The separated chromatids are now called chromosomes.

18. Mitosis- Telophase In the final step, telophase, spindle fibers start to disappear, the chromosomes start to uncoil, and a new nucleus forms.

19. Division of the Cytoplasm Cytokinesis is the division of the cytoplasm. In cells that do not have a cell wall, cytokinesis begins at the cell membrane. Cytokinesis begins when the cell membrane begins to pinch inward to form a groove. This groove will eventually pinch all the way through the cell, and two daughter cells form. In animal cells, the cell membrane pinches in the middle, like a balloon with a string tightened around it, and the cytoplasm divides. In plant cells, the appearance of a cell plate tells you that the cytoplasm is being divided. Following division of the cytoplasm, most new cells begin the period of growth, or interphase, again.

20. Results of Mitosis Mitosis is the division of the nucleus, and it produces two new nuclei that are identical to each other and the original nucleus. Each new nucleus has the same number and type of chromosomes. Every cell in your body, except sex cells, has a nucleus with 46 chromosomes�23 pairs. Each of the trillions of cells in your body, except sex cells, has a copy of the same hereditary material. All of your cells use different parts of the same hereditary material to become different types of cells.

21. Results of Mitosis Cell division allows growth and replaces worn out or damaged cells. If you cut yourself, the wound heals because cell division replaces damaged cells. Another way some organisms use cell division is to produce new organisms.

22. Reproduction and Meiosis Section 2

23. Asexual Reproduction Reproduction is the process by which an organism produces others of its same kind. Among living organisms, there are two types of reproduction�sexual and asexual. In asexual reproduction, a new organism is produced from ONE organism (rarely more than one). The new organism will have hereditary material identical to the hereditary material of the parent organism.

24. Cellular Asexual Reproduction Organisms with eukaryotic cells asexually reproduce by cell division (mitosis). However, bacteria do not have a nucleus so they can�t use mitosis. Instead, bacteria reproduce asexually by binary fission. During fission, an organism whose cells do not contain a nucleus copies its genetic material and then divides into two identical organisms.

25. Budding Budding is a type of asexual reproduction made possible because of cell division. When the bud on the adult becomes large enough, it breaks away to live on its own.

26. Regeneration Some organisms can regrow damaged or lost body parts. Regeneration is the process that uses cell division to regrow body parts. Sponges, planaria, sea stars, and some other organisms can use regeneration for asexual reproduction.

27. Sexual Reproduction In sexual reproduction, two parent cells join to form offspring that are different from both parents. The parent cells are called sex cells. Sex cells are formed from cells in reproductive organs. A female sex cell joins with a male sex cell to form a fertilized egg (zygote) which has genetic information from each parent. Sperm are formed in the male reproductive organs. Eggs are formed in the female reproductive organs.

28. Sexual Reproduction The joining of an egg and a sperm is called fertilization, and the cell that forms is called a zygote Following fertilization, cell division begins. Zygote cells multiple to form a new organism. A new organism with a unique identity develops.

29. Diploid Cells Human body cells have 23 pairs of chromosomes or 46 chromosomes. Each chromosome has a mate that is similar to it in size and shape and has similar DNA. When cells have pairs of similar chromosomes, they are said to be diploid

30. Haploid Cells Because sex cells do not have pairs of chromosomes, they are said to be haploid They have only half the number of chromosomes as body cells. Haploid means �single form.� Human sex cells have only 23 chromosomes�one from each of the 23 pairs of similar chromosomes. For example, a human egg cell has 23 chromosomes, and a sperm cell has 23 chromosomes. The new cell that forms when an egg cell and a sperm cell join has 46 chromosomes.

31. Meiosis and Sex Cells A process called meiosis is the copying process that produces cells with half the usual number of chromosomes (haploid cell)

32. Meiosis and Sex Cells Meiosis ensures that the offspring will have the same diploid number as its parent. After two haploid sex cells combine, a diploid zygote is produced that develops into a new diploid organism. During meiosis, two divisions of the nucleus occur. These divisions are called meiosis I and meiosis II.

33. Meiosis I- Prophase I The events of prophase I are similar to those of prophase in mitosis. In meiosis, each duplicated chromosome comes near its similar duplicated mate. In mitosis they do not come near each other.

34. Meiosis I- Metaphase I In metaphase I, the pairs of duplicated chromosomes line up in the center of the cell The centromere of each chromatid pair becomes attached to ONE spindle fiber, so the chromatids do not separate in anaphase I.

35. Meiosis I- Anaphase I In anaphase I, the two pairs of chromatids of each similar pair move away from each other to opposite ends of the cell. Each duplicated chromosome still has two chromatids.

36. Meiosis I- Telophase I In telophase I, the cytoplasm divides, and two new cells form. Each new cell has one duplicated chromosome from each similar pair.

37. Meiosis II The two cells formed during meiosis I now begin meiosis II. The chromatids of each duplicated chromosome will be separated during this division.

38. Meiosis II - Prophase II In prophase II, the duplicated chromosomes and spindle fibers reappear in each new cell.

39. Meiosis II - Metaphase II In metaphase II, the duplicated chromosomes move to the center of the cell. Unlike what occurs in metaphase I, each centromere now attaches to two spindle fibers instead of one.

40. Meiosis II- Anaphase II In anaphase II, the centromere divides, and the chromatids separate and move to opposite ends of the cell. Each chromatid now is an individual chromosome.

41. Meiosis II- Telophase II As telophase II begins, the spindle fibers disappear, and a nuclear membrane forms around the chromosomes at each end of the cell. When meiosis II is finished, the cytoplasm divides.

42. Meiosis Reminder Remember meiosis produces haploid sex cells

43. Meiosis Mistakes Meiosis occurs many times in reproductive organs. Mistakes can produce sex cells with too many or too few chromosomes. Sometimes, zygotes produced from these sex cells die. If the zygote lives, every cell in the organism that grows from that zygote usually will have the wrong number of chromosomes. Organisms with the wrong number of chromosomes may not grow normally.

44. DNA Section 3

45. What is DNA? A cell uses a code in its hereditary material. The code is a chemical called DNA or deoxyribonucleic (dee AHK sih ri boh noo klay ihk) acid. It contains information for an organism�s growth and function. DNA is stored in cells that have a nucleus. When a cell divides, the DNA code is copied and passed to the new cells. In this way, new cells receive the same coded information that was in the original cell.

46. Discovering DNA Since the mid-1800s, scientists have known that the nuclei of cells contain large molecules called nucleic acids. By 1950, chemists had learned what nucleic acid DNA was made of, but they didn�t understand how the parts of DNA were arranged.

47. Structure of DNA In 1952, scientist Rosalind Franklin discovered that DNA is two chains of molecules in a spiral form. By using an X-ray technique, Dr. Franklin showed that the large spiral was probably made up of two spirals.

48. DNA Model In 1953, scientists James Watson and Francis Crick made a model of a DNA molecule. According to the Watson and Crick DNA model, each side of the ladder is made up of sugar-phosphate molecules. Each molecule consists of the sugar called deoxyribose and a phosphate group.

49. DNA Model (Cont) The rungs of the ladder are made up of other molecules called nitrogen bases. Four kinds of nitrogen bases are found in DNA Adenine Guanine Cytosine Thymine

50. DNA Model (cont.) Adenine always pairs with thymine, and guanine always pairs with cytosine. The amount of adenine always equals the amount of thymine. The amount of cytosine in cells always equals the amount of guanine.

51. Copying DNA When chromosomes are duplicated before mitosis or meiosis, the amount of DNA in the nucleus is doubled. The two sides of DNA unwind and separate. Each side then becomes a pattern on which a new side forms. The new DNA has bases that are identical to those of the original DNA and are in the same order.

52. Genes Most of your characteristics, such as the color of your hair, your height, and even how things taste to you, depend on the kinds of proteins your cells make. Proteins build cells and tissues or work as enzymes. DNA in your cells stores the instructions for making these proteins. The instructions for making a specific protein are found in a gene which is a section of DNA on a chromosome. Walter Sutton determined genes are located on chromosomes

53. Genes (cont.) Each chromosome contains hundreds of genes Proteins are made of chains of hundreds or thousands of amino acids. The gene determines the order of amino acids in a protein. Changing the order of the amino acids makes a different protein.

54. Making Proteins Genes are found in the nucleus, but proteins are made on ribosomes in cytoplasm. The codes for making proteins are carried from the nucleus to the ribosomes by another type of nucleic acid called RNA, or ribonucleic acid.

55. Ribonucleic Acid RNA is made in the nucleus on a DNA pattern. RNA is different from DNA. RNA is like a ladder that has all its rungs sawed in half. RNA has the bases A, G, and C like DNA but has the uracil (U) instead of thymine (T). The sugar-phosphate molecules in RNA contain the sugar ribose, not deoxyribose.

56. Ribonucleic Acid (cont.) The three main kinds of RNA made from DNA in a cell�s nucleus are : Messenger RNA (mRNA) Ribosomal (rRNA) Transfer RNA (tRNA). Protein production begins when mRNA moves into the cytoplasm. There, ribosomes attach to it.

57. Ribonucleic Acid (cont.) Ribosomes are made of rRNA. Transfer RNA molecules in the cytoplasm bring amino acids to these ribosomes. Inside the ribosomes, three nitrogen bases on the mRNA temporarily match with three nitrogen bases on the tRNA.

58. Controlling Genes Cells must be able to control genes by turning some genes off and turning other genes on. If the incorrect proteins are produced, the organism cannot function properly. In many-celled organisms like you, each cell uses only some of the thousands of genes that it has to make proteins Each cell uses only the genes that direct the making of proteins that it needs. For example, muscle proteins are made in muscle cells but not in nerve cells.

59. Mutations Genes control the traits you inherit. Without correctly coded proteins, an organism can�t grow, repair, or maintain itself. A change in a gene or chromosome can change the traits of an organism.

60. Mutations If DNA is not copied exactly, the proteins made from the instructions might not be made correctly. These mistakes, called mutations, are any permanent change in the DNA sequence of a gene or chromosome of a cell. Outside factors such as X rays, sunlight, and some chemicals have been known to cause mutations.

61. Results of a Mutation If the mutation occurs in a body cell, it might or might not be life threatening to the organism. If a mutation occurs in a sex cell, then all the cells formed from that sex cell will have the mutation. Some mutations are harmful to organisms, often causing their death. Some mutations do not appear to have any effect on the organism, and some can even be beneficial.

62. DNA and Genetics

63. Inheriting Traits Individuals inherit a homologous chromosome from each parent. The genes on these chromosomes determine what traits appear. Some traits are dominant, and other are recessive. Dominant traits appear even if only one homologous chromosome carries the gene for that trait. Recessive traits appear only if both homologous chromosomes carry that gene.

64. Meiosis in Inheriting Traits Sex Chromosomes carry genes that determine the sex of an individual. In humans, females have two X chromosomes; males have one X chromosome and one Y chromosome. Meiosis produces an egg cell with an X chromosome, and sperm with either an X or Y chromosome. An egg fertilized by a sperm with an X chromo-some will produce a female. If the sperm contains a Y chromosome, the offspring will be male. (Beheaded wives)

65. Sex-Linked Disorders The Y chromosome does not carry all of the genes of an X chromosome. Females have two X chromosomes, so they have two copies of each gene found on the X chromosome. This makes a backup gene available if one becomes damaged. Males have only one copy of each gene on their one X chromosome. The genes for certain disorders, such as colorblindness and hemophilia These disorders are called sex-linked disorders Men are more likely to have sex-linked disorders.

66. Genetic Counseling Genetic disorders can be traced through a family tree. If people are worried they might pass a disease to their children, they may consult a genetic counselor. These counselors often make use of a diagram known as a pedigree, a tool for tracing a trait through generations of a family.

67. Natural Selection

68. Four Parts of Natural Selection Overproduction Each species produces more offspring than will survive to reproduce Inherited Variation Individuals within a population have slightly different traits Struggle to Survive Individuals within a population compete with each other for limited resources (food, shelter, etc) Successful Reproduction Individuals that are better equipped to live in an environment are more likely to survive and reproduce

69. Changes in Populations The theory of natural selection explains how a population changes in response to its environment. If natural selection is always taking place, a population will tend to be well adapted to its environment. But not all individuals are the same. The individuals that are likely to survive and reproduce are those that are best adapted at the time.

70. Changes in Populations (cont.) Changes in populations are sometimes observed when a new force affects the survival of individuals. People hunt elephants for their tusks. As a result, fewer of the elephants that have tusks survive to reproduce More of the tuskless elephants survive. When the tuskless elephants reproduce, they pass the tuskless trait to their offspring.

71. Changes in Populations (cont.) Insecticide Resistance Insects can quickly develop resistance because they often produce many offspring and have short generation times. Insecticides have been used so much that now more than 500 kinds of insects have developed resistance to certain insecticides. Generation time is the average time between the birth of one generation and the birth of the next.

72. Changes in Populations (cont.) Competition for Mates In the process of evolution, survival is simply not enough. In organisms that reproduce sexually, finding a mate is part of the struggle to reproduce. Many species have so much competition for mates that interesting adaptations result. For example, the females of many bird species prefer to mate with males that have colorful feathers.

73. Forming a New Species Sometimes, drastic changes that can form a new species take place. In the animal kingdom, a species is a group of organisms that can mate with each other to produce fertile offspring. The formation of a new species as a result of evolution is called speciation. Speciation often begins when a part of a population becomes separated from the rest. Populations constantly undergo natural selection. After two groups have separated, natural selection may act on each group in different ways.

74. Forming a New Species (cont.) Over many generations, two separated groups of a population may become very different Eventually, they can no longer mate with one another. At this point, the two groups are no longer the same species.