Warm Up Quote: “ Team Competition

The Cell Cycle

Why the cell must divide: Reason 1: The cell only contains one copy of DNA to serve as the instructions for everything that occurs in the cell. If the cell were too large it would not be capable of providing instruction for all necessary functions. Reason 2: The volume of an object will increase faster than the surface area as the object increases in size. The cell’s needs are based on how large the cell is (volume) but everything the cell needs, such as food, oxygen, water, and getting rid of waste, must enter or leave through the cell membrane (surface area). Therefore the cell’s needs will increase faster than the cell’s ability to provide for the needs.

The diagram shows what would happen to the volume to surface area ratio if a cube were to double in size A cube that is 1 mm by 1 mm by 1 mm has a volume of 1 mm3 and a surface are of 6 mm2. The ratio is _______ A cube that is 2 mm by 2 mm by 2 mm has a volume of 8 mm3 and a surface are of 24 mm2. The ratio is _______ This demonstrates that a cell’s needs increase faster than the cell can provide for its needs. Surface area = 6 mm2 Volume = 1 mm3 Surface area = 24 mm2 Volume = 8 mm3 1 mm 2 mm 4 mm

Steps of the Cell Cycle The cell cycle is the events that occur in the cell from creation to division into new cells. This process has several steps. Interphase: the period in between cell divisions. This will take up the majority of the cell’s life. This phase is divided into three parts: G1 phase (growth phase one): the cell will increase in size and carry out its specific function. Some cells will remain in this stage for their entire life. S phase (synthesis phase): creates a copy of the cell’s DNA through a processes called replication. This allows the cells that are going to be created in division to receive identical copies of the DNA creating identical cells. G2 phase: the creation of other materials needed for division occurs in this phase. M phase (mitosis): series of steps used to organize and divide the DNA in the nucleus to ensure each cell gets an identical and complete copy. Mitosis has four steps:

Steps of the Cell Cycle Prophase: Normally the longest step of mitosis. DNA will coil into individual chromosomes in order to be sorted and divided easily. The chromosomes are the two copies of the same section of DNA held together by a protein which gives them and X shape. Before this the DNA was uncoiled in a mass of strings. The nucleus disappears. In animal cells centrioles will move to the poles. Spindle fibers are created from the poles that will be used to move and divide the DNA.

Steps of the Cell Cycle Metaphase: Normally the shortest phase of mitosis. The chromosomes line up single file in the center of the cell. Anaphase: The spindle fibers separate the centromeres that hold the two copies of each piece of DNA together and pull one half of each chromosome (one copy of the DNA) to each side of the cell. Telophase: The DNA uncoils. The spindle fibers break down and the cetrioles move back to their original location. Two nuclei form.

Steps of the Cell Cycle C Phase (cytokinesis): This phase will divide the cell into two identical cells. Animal cells: the cell membrane pinches in until it completely divides the cell. Plant cells: the cell wall is not flexible so the cell will construct a new cell wall down the center by using a cell plate that has appeared as a starting point.

Interphase S: DNA synthesis and replication G2: Centrioles replicate; cell prepares for division G1: Rapid growth and metabolic activity M: Divides the DNA C: Divides the cell

Purpose of Mitosis This process is used to create two new identical cells. This can be done to replace dead or damaged cells or increase the size of an organism (growth). Mitosis can also be used as a form of reproduction. Since the new cells only had one parent this is called asexual reproduction. Benefits: asexual reproduction can occur very quickly and is relatively simple to accomplish. Negatives: all of the organisms are genetically identical so this does not allow for diversity or evolution.

Alternative to Asexual Reproduction Sexual reproduction uses cells that contain only half the DNA founds in normal body cells from two different individuals to create the next generation. This way the next generation does not have double the DNA which would be too much for the cell to go through. Benefits: This produces genetic variations among organisms of the same species. Genetic variation allows a species to have options so that they can select the best version of a trait for the environment to pass on to the next generation. Negatives: This process takes much longer than mitosis and requires two parents.

The Cells of Sexual Reproduction The cells used in sexual reproduction only contain half of the DNA from the parent. However, this is still one copy of every trait that the species creates. Each parent has two chromosomes to code for the production of each trait. The two copies may be the same or they may be different. The two chromosomes that code for the same traits are called homologous chromosomes. Typically only one copy of the DNA is expressed. This explains why a parent with brown eyes can have a baby with blue eyes. The parent has both DNA sequences, but only expresses the brown eye sequence. When the cells from both parents fuse during fertilization the offspring will have two DNA sequences for every trait.

The Cells of Sexual Reproduction Human traits are carried on 23 chromosomes. The typical human body cell contains 46 chromosomes (23 that were inherited from each parent). These cells are called diploid because they have two of each chromosome. The cells used in sexual reproduction contain 23 of the 46 chromosomes which means they are haploid. These cells are called gametes and can be either eggs or sperm. Since the cells used in sexual reproduction contain ½ the amount of DNA as the parent they cannot be created using mitosis. The process that makes these cells is called meiosis.

Steps of Meiosis Interphase: this occurs the same as the interphase before mitosis which means the cell will grow, make a copy of each piece of DNA so that each chromosome will have two copies of the same DNA, and prepare to divide. Prophase I: DNA coils into chromosomes where the two copies of each piece of DNA are held together by a protein giving them the X shape. Nucleus disappears. Spindle fibers form and centrioles move to the poles. The two chromosomes that code for the same trait (homologous chromosomes) will pair together. Pieces of the chromosomes can become tangled, break and switch places in a process called crossing over. This increases the number of different cells the parent can create so that all offspring are different.

Steps of Meiosis Metaphase I: Anaphase I: Telophase I: Cytokinises I: Chromosomes are moved to the center of the cell and lined up. Chromosomes are lined up in homologous pairs. Anaphase I: One whole chromosome from each pair is pulled to each pole. Telophase I: Chromosomes are uncoiled, nuclei form, spindles break down, and centrioles move back to their original location. Cytokinises I: The cell divides into two cells that have two copies of DNA for each trait (diploid).

Steps of Meiosis Interphase: DOES NOT OCCUR AGAIN SO THIS HALF OF MEIOSIS STARTS WITH 46 PIECES OF DNA NOT WITH 2 COPIES OF 46 PIECES OF DNA. Prophase II: DNA coils, the nucleus disappears, centrioles move to poles and spindle fibers form. Metaphase II: Chromosomes line up at the center of the cell single file.

Steps of Meiosis Anaphase II: Half of each chromosome is pulled to each pole. Telophase II: DNA uncoils, nuclei form, the centrioles move back to their original location, and spindle fibers break down. Cytokinises II: Both cells divide creating four genetically different cells that each contain half the amount of DNA (haploid).