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Unit 3 Lesson 1 What Are Cells? Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? All Made of Cells • Cells are the basic units of structure and function in a living thing. The bodies of all living things are made up of cells, including nerve and skin cells. • Living things are also called organisms and can be made up of one cell or trillions of cells. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? All Made of Cells • People did not know cells existed because cells were microscopic, or not visible without a microscope. • Scientists developed the cell theory that states: all living things are made of cells, all life processes take place in cells, and new cells come from existing cells. • Robert Hooke was the first person to study cells using a microscope. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? What Parts Do Cells Have? • Plants and animals are made of cells, but they need different cell parts to stay alive. • The nucleus makes more cells, produces energy, takes in materials, and gets rid of wastes. • Cell membranes control what enters and leaves the cell. • Mitochondria release energy that the cells use to do their jobs. • The jellylike cytoplasm gives the cell shape and holds the cell parts together. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? What Parts Do Cells Have? • The cell wall protects plant cells and helps the plant keep its shape. • The chloroplast uses the sun’s energy to make sugar to feed the plant. • The vacuole in a plant cell stores water, nutrients, and waste. In animal cells, the vacuole is much smaller. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? Cells Divide and Multiply • Cell division makes two cells from one. Cells divide for two reasons: to grow and reproduce. • As cell division occurs, the organism grows. When organisms reproduce, they make egg and sperm cells through cell division. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? Cells Divide and Multiply • Chromosomes are structures in the cell’s nucleus made of DNA. DNA is made of sections called genes. • Genes control an organism’s characteristics. • An organism’s characteristics, such as height or eye color, come from the parent’s cells when they reproduce. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? How Cells Divide • Mitosis is a six-step process that allows the organism to grow. • Each chromosome is copied before the cell divides. • Copies of each chromosome are joined. • The duplicated chromosomes move to the middle of the cell. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? How Cells Divide • Then, the copied chromosomes separate and move to opposite ends of the cell. • A nuclear membrane is formed around each set of chromosomes. • Finally, the cytoplasm divides and cell division is complete! Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? How Cells Divide • Meiosis forms eggs and sperm, or sex cells, which have half as many chromosomes as body cells. • Egg and sperm cells join to form a new single cell during sexual reproduction. • The single cell divides by mitosis and grows into a new individual. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? Where Do Traits Come From? • Inherited traits are characteristics passed from parents to their offspring. • Examples of these characteristics might include eye color, hair color, freckles, and face shape in humans, and fur color and texture in animals. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? Where Do Traits Come From? • Meiosis causes genetic differences since an organism receives only half of its genes from each parent. • This results in genetic differences between parents and offspring. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? Dominant and Recessive Traits • In the 1800s, Gregor Mendel observed height differences in pea plants. • Mendel investigated how the height trait was passed on from parent plants. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 3 Lesson 1 What Are Cells? Dominant and Recessive Traits • Mendel hypothesized that traits in organisms are controlled by a pair of factors and each parent passes on one factor. • In the pea plants, tallness was a strong, or dominant trait. Shortness was a weak, or recessive trait. Copyright © Houghton Mifflin Harcourt Publishing Company