Packet 6: Cell Reproduction Notes: Asexual Reproduction pg. 1-3

Notes: Asexual Reproduction Every person began their life as a single cell called a zygote. In humans, this fertilized egg should contain 46 total chromosomes. The first cell then divided and became two identical cells, each of which should also have 46 chromosomes.

This division of cells happens over & over (more than a trillion times) to make all of the cells found in your body. Yet each time new cells are made, each one will always contain 46 chromosomes (if all works properly).

Asexual Reproduction -is the process by which a single parent cell divides to become two cells by itself.

Why do we need to make more cells? A. Growth- to become larger, our bodies make more cells (the cells don’t just become bigger). B. Repair- Cells in our bodies are constantly damaged or destroyed and need to be replaced.

Why do cells divide instead of just getting bigger? A. Demand on DNA – as the cell grows too large, the DNA can no longer serve the increasing needs of the growing cell. B. Rate of Diffusion – diffusion is only fast over short distances. Works with small cells (over short distances). Over long distances it is inefficient, so cell size is limited.  C. Surface area to volume – as a cell’s size increases, the volume (the inside) increases much faster than its surface area (the outside). To make sure the cell membrane (the outer surface of the cell) can let in enough food, oxygen, and water to maintain homeostasis, cells decrease their size or modify their shape.

Ex. Look at the shape of the red blood cell:

Variations in asexual reproduction: Bacteria reproduce through a process called Binary Fission. It is a simpler process since they are prokaryotic (meaning their DNA is not enclosed in a nucleus). Eukaryotic cells (which are more complex) must go through a series of steps called the Cell Cycle in order to create new cells (cell division)

Common features in the cell cycle process: Interphase When the cell prepares to divide Mitosis When the nucleus & genetic material divide. Cytokinesis   When the cell splits into two cells. Interphase Mitosis Cytokinesis

Part I: INTERPHASE The first part of the cell cycle in which the cell grows and more organelles are made The DNA is uncoiled (in this form it is called chromatin). Notice that there aren’t any spindle visible. A critical step that must occur during interphase is the replication of the DNA. When finished there should be two identical copies of all the genetic information in the cell.

Part II: MITOSIS ProphaseMetaphaseAnaphaseTelophase The process in which the nucleus divides into two nuclei, each containing the same number and kinds of chromosomes as the parent cell. Mitosis is divided into 4 phases, each with distinct events occuring: ProphaseMetaphaseAnaphaseTelophase

Part II: Mitosis Prophase- 1st stage (and longest phase of mitosis) -DNA condenses into visible X-shaped chromosomes -Each chromosome is made of 2 identical copies called sister chromatids

2. Metaphase- 2nd phase -Chromosomes line up along the middle or equator of the cell

3. Anaphase- 3rd phase X- shaped chromosomes break in half- this is called the separation of sister chromatids -The chromosomes or sister chromatids move towards opposite poles

4. Telophase- 4th phase (Final phase of mitosis) -Chromosomes reach opposite sides of the cell and begin to uncoil -Each nucleus now has exactly the same genetic information (thanks to the replication of the DNA during interphase)

Part III: CYTOKINESIS The process by which the cytoplasm divides, forming two distinct cells. Plant and animal cells do not divide in the same way since plant cells have a cell wall and are less flexible.

Cleavage Furrow Formed when the cell membrane pinches inward causing the cell to divide into two separate cells (only cells that do not have a cell wall).

Cell Plate Rigid cells that have cell walls (such as plant cells) build up a barrier between the two nuclei to divide the cell into two separate compartments.

Most of the cells in your body are made from stem cells Most of the cells in your body are made from stem cells. These cells are unique because they are able to divide and form almost any type of cell in your body Differentiation means that a cell changes so that it can become specialized for a specific job. Once a cell differentiates, it can no longer change to become another type of cell (i.e. skin cells cannot make or become muscle cells).

Uncontrolled Cell Growth: Almost any type of cell in the body can become cancerous. A mutation (or a mistake in the DNA) may cause a cell to grow and divide uncontrollably). Cancer cells are cells of your own body that have just stopped listening when your body has told it to stop making more cells.

For the rest of class: Complete pg. 5 and have me check it when you are finished 