Chapter A3 Animal Growth and Heredity Lesson A3.1 How organisms grow

Growth You began life as a single cell but by the time you were born you were already made up of billions of cells. You know that body systems are made up of organs, organs are made of tissues, tissues are made up of cells. Nearly all body cells produce EXACT copies of themselves. This allows the new cells to perform just like the old cells and lets your organs perform normally.

Cell Division Every square centimeter of our skin has about 150,000 skin cells. Draw a centimeter box on your skin. The skin’s cells are always dying and getting replaced. The nucleus controls everything a cell does, much like a brain controls our bodies. Inside the nucleus are threadlike strands called chromosomes.

DNA Chromosomes are made up of a chemical called DNA, which forms a chemical code. Everyone’s DNA is unique, or different. DNA determines the shape and function of a cell. It also determines when a cell will divide.

Mitosis Whenever a cell divides it must get an exact copy of the “parent” cell’s chromosomes. The process of cell division is called mitosis. Mitosis occurs when the nucleus first makes exact copies of its chromosomes. The cell has DNA for 2 cells. Then the chromosomes pull apart and form two groups. The cell membrane pinches in the middle, separating the two groups. Finally, the two groups break apart and TA-DAH! You h have two cells; one the original one, and one the copy of that original cell. The two cells then repeat and repeat this over and over as cells need to replace, as the DNA code tells them to.

Regeneration As soon as the body is injured it begins the process of repairing itself. This is called regeneration. Regeneration is a form of tissue replacement. In humans, regeneration is limited to mostly to healing wounds. Plants and some animals can regenerate major body parts. Planaria, which is a type of flatworm, can regenerate most of its body.

If you chop up a sponge into tiny pieces, each of those tiny pieces then regenerate into a full sized sponge. You actually end up with many more sponges than you started with.

Asexual Reproduction Many one-celled organisms can reproduce by simple cell division or fission. Fission is a type of reproduction that does not need male and female cells to join. This is also called asexual reproduction. MITOSIS is used to produce offspring.

Budding Another form of reproduction that involves the process of asexual reproduction is called budding. A parent cell or organism begins to form an identical copy of itself, but it grows out of its own form. The new copy, or bud, continues to grow until it reaches full adult size. When it is done, it simply detaches or breaks off and it is ready to live on its own.

Sexual Reproduction Most organisms reproduce sexually. In sexual reproduction, cells from two parents are needed to produce offspring. The one-celled organism they produce is called a zygote. The zygote contains the chromosomes from both the female and male parents. Nearly all human body cells have 46 chromosomes each. If a body cell has less than that, it will not function properly.

If two body cells were to unite and keep their count of 46, the resulting cell would have 92 and that would be too many. Because of this, sexual reproduction requires that the cells divide first then unite. This way only half of the chromosomes will pass on, and they result in only 46 total.

Humans produce reproductive cells, which only have 23 chromosomes. Meiosis is the process that causes the cells to reduce the number of chromosomes in half. The first stage of meiosis the cell copies its chromosomes, like mitosis we have studied. The second stage of meiosis the two cells then divide again, having only half of the original copied chromosomes. They do NOT copy at this point. Each of the four new cells is a reproductive cell is called a gamete. A gamete only has half the number of its original cells.