The Cell Cycle, Mitosis, & Meiosis… A Brief Review

Are the cells of organisms the same size Are the cells of organisms the same size? Do your cells get bigger as you grow, or do you just produce more cells?

Well, technically the answer to both of these questions is… yes! In most cases, living things grow by producing more cells! Adult animal cells are no larger than young animal cells…adults just have MORE cells!

Limits to Cell Growth The LARGER a cell becomes the MORE DEMAND the cell places on its DNA 2 Main Reason’s Cell’s Divide: DNA “Overload”  larger cell = wait time for certain parts of the cell to get the information it needs to get any job done! Exchanging Materials  larger cell = difficulty in exchanging good materials (ex: food, water, & oxygen) with waste materials, both inside and outside of the cell

Cell Size…Smaller is better! The ability of your cells to exchange nutrients and waste depends on the surface area & volume of your cells Surface area = length x Width x # of sides Volume = length x width x height The change in a cell’s size also depends on the surface area of a cell and the volume of that cell. Think of this as a ratio  Surface Area Volume

In other words…. The surface area-to-volume ratio is the reason why cells divide…The BIGGER the cell, the LONGER it takes for things to get inside/ leave the cell. When a cells volume increases…the Surface area begins to spread (i.e. the cell SWELLS). Your bodies way of initiating/ starting CELL DIVISION is to increase the volume inside of the cell  increasing cytoplasm, organelles, & DNA.

Cell Cycle In order for the cell to divide…it must go through a 3 part Cell Cycle. . The Cell Cycle has 3 Main Sections: Interphase (~22 hrs) Mitosis/ Cell Division (~1.5 hrs) Cytokinesis (~30 minutes)

Interphase The cell’s prep time for cell division  cell growth; DNA & centriole replication; nuclear membrane visibility, etc. 3 Parts of Interphase: G1 Phase  Cell Growth S Phase  DNA Replication G2 Phase  Prep for Mitosis/ Cell Division

Cytokinesis – the final division of the cytoplasm Mitosis – often called cell division; process where 1 cell divides into 2 cells. 4 Phases: Prophase Metaphase Anaphase Telophase Cytokinesis – the final division of the cytoplasm

So how does it work… DNA is tightly coiled into structures called chromosomes. Chromosomes = X (or Y) shaped structures composed of DNA that are only visible during cell division (i.e. Mitosis or Meiosis). Humans have 46 chromosomes (23 pairs). The chromosomes consist of 2 “sister” chromatids. The sister chromatids are attached at an area called the centromere.

Chromosome

Here are your 46 Chromosomes 

Two major types of Cell Division: Mitosis  Divides Somatic Cells (body cells); Chromosome # 1 – 22 ***remember…these are in pairs*** Meiosis  Divides Gametes (sex cells); Chromosome #23 ***remember…these are in pairs***

Mitosis In mitosis, a somatic/ body cell is called a DIPLOID cell. Diploid cells are represented by 2N. The 2 represents the number of parents (2) and the N represents the number chromosomes per parent (23 chromosomes per parent). The 2 new cells are genetically identical to the original cell.

Four Main Stages of Mitosis… PMAT Prophase Metaphase Anaphase Telophase

Meiosis Cell Division of gametes (sex cells), which results in the production of eggs and sperm. These cells are HAPLOID, cells that contain a single set of chromosomes (i.e. only 23 chromosomes); it is represented by N. In this type of cell division 4 new cells are created, each being genetically different. Meiosis is divided into 2 stages  Meiosis I & Meiosis II

So what controls the cell division? In the early 1980s scientists discovered a group of proteins that regulate the timing of the cell cycle. These proteins were called Cyclins. 2 Other Regulators: Internal Regulators  monitor events that occur inside the cell (ex: makes sure all DNA replicated before entering into mitosis). External Regulators  monitor events that occur outside the cell (ex: Growth factors that control the speed of the cell cycle ESPECIALLY during embryonic growth)

Uncontrolled Cell Growth Cancer – disorder where the bodies own cells lose the ability to control growth. Cancer cells DO NOT respond to the signals that regulate the growth of most cells (a.k.a cyclins & regulator proteins). Many cancerous cells have genetic defect in the p53 gene…the gene that stops the cell cycle. Result in large masses of cells = TUMORS Tumors damage the surrounding tissue of the body

Examples of Tumors

Now let’s see if you remember the Cell Cycle… Complete the given flow chart using the following terms…. G1 Phase – Mitosis Anaphase – Metaphase Cytokinesis – S Phase Telophase G2 Phase Interphase Prophase