Mitosis and the Cell Cycle DAY C 01/03/07 Objectives: Define the cell cycle. Describe the four phases of the cell cycle and mitosis.

The Cell Cycle In most animal and plant cells the cells increase in size and then they divide into two cells. Then the new cells increase in size and divide again. This occurs repeatedly to create new cells. This is called the Cell Cycle.

Differences in cell division between prokaryotes and eukaryotes Prokaryotes Less complex cells; so division is fairly simple. In division one cell divides into two daughter cells. Must replicate genetic material and separate it into the two daughter cells. Does not go through the complex steps of cell division; can just divide in two after genetic material is replicated. Eukaryotes More complex cells; so division is more complex. Must replicate chromosomes and make sure they separate properly. Goes through similar process of prokaryotes only more complex. Goes through the steps of mitosis.

Cell Cycle Stages The cell cycle is the period of time from the beginning of one cell division to the beginning of the next. What happens during the cell cycle? – The cells doubles its contents so it can divide into two independent cells. What are the stages of the cell cycle? – G, S, and M phases

The Cell Cycle Stages What happens in each stage? – G phases (G 1 and G 2 ) - Growth phases; the cell grows most in G 1 but it will also grow a little in G 2. – S phase – synthesis phase; the cell’s DNA replicates so that it has twice the amount of DNA (and chromosomes) as a normal cell would. – M phase – cell division phase; mitosis occurs in this phase and the cell finally splits into two independent “daughter” cells.

What is the order of the Cell Cycle?

Timing of the Cell Cycle Stages Not all cells move through the cell cycle at the same rate. – Cells in an embryo can go through the cell cycle in as little as 30 minutes. – Average cells in an adult human go through the cell cycle in about 20 hours.

Interphase Interphase is made up of all stages of the cell cycle except mitosis (M Phase). In interphase the DNA and protein that make up chromosomes are unfolded and the most active, using the information they contain to direct cell growth and development.

Mitosis (The M Phase) You observed the four different stages of mitosis, as well as the interphase stage of the cell cycle. The four stage of mitosis are: prophase, metaphase, anaphase, and telophase.

Prophase The first phase of mitosis; it generally takes longer than the rest. First clue that prophase has begun is that the chromosomes become visible. Chromosomes are made of two identical strands called chromatids (sister chromatids). The chromatids are attached to the centromere.

Metaphase The second stage of mitosis. They complete their attachment to the spindle fibers and line up across the middle of the cell.

Anaphase The centromeres that hold the chromatids together split. The chromatids move toward the two poles of the cell; thus separating the duplicated chromosomes.

Telophase The microtubules of the spindle begin to break apart. The chromosomes clustered at either pole of the cell spread out. Nuclear membrane forms around each cluster of chromosomes.

Cytokinesis Process where the cytoplasm divides in half. Occurs along with telophase and results in two separate daughter cells.

Cytokinesis in Plant vs. Animal Cells Plant Cells Cell plate forms midway between the two nuclei of the cells. The cell plate grows outward until a new cell wall has grown between the two cells. Animal Cells The cell membrane is pulled in by a ring of filaments. This continues until the cytoplasm is pinched in two.

Breakdown of Mitotic Steps Prophase: Chromosomes coil up and become visible during prophase. The nuclear envelope dissolves and the spindle forms. Metaphase: Chromosomes move to the center of the cell and line up along the equator. Spindle fibers link the chromatids of each chromosome to opposite poles. Anaphase: Centromeres divide during anaphase. The two centromeres (now called chromosomes) move toward opposite poles as the spindles attached to them shorten. Telophase: A nuclear membrane forms around the chromosomes at each pole. Chromosomes, now at opposite poles, uncoil and the spindle dissolves. The spindle fibers break down and disappear.

Important Vocabulary Chromosomes: – hold the cell’s DNA coiled up with proteins. Chromatids: – parts of chromosome that when paired with its identical sister chromatid constitutes a chromosome. Centromere: – The point where sister chromatids attach in a chromosome.

Important Vocabulary Spindle: – Cluster of microtubules that help separate sister chromatids during mitosis. Centriole: – In animal cells it helps organize the microtubules of the spindle. Cytokinesis: – Division of the cytoplasm during mitosis.