The Life & Death of Cells Biology Lesson # 3 The Life & Death of Cells

The Cell Cycle Cells reproduce through a continuous sequence of growth and division called cell cycles. The cell cycle differs in length based on the different types of cells. The cell cycle has two main stages:

Growth & Division GROWTH This cell growth is also called interphase, and is divided into three parts Cell growth (G1 phase) Replication or duplication of DNA (S phase) Final preparation for cell division (G2 phase) DIVISION This division is also called mitosis, and is the division of chromosomes and the nucleus of the “parent” cell to produce two identical “daughter” cells.

Mitosis The three main functions of mitosis are: Growth – we need to increase our cellular amount to increase in size. Healing & tissue aware – to replace worn or old cells Reproduction – sperm and egg cells go through a special type of division called meiosis to create new life. Cytokinesis, the division of the cytoplasm and organelles, also occurs in this stage.

Rates of Cell Growth & Division The growth of cells and rate of division varies depending on the type of cell. For example, nerve cells do not undergo mitosis once they mature (which is why nerve damage is almost always permanent). Skin cells, on the other hand, divide regularly, which is why cuts and scrapes heal rather quickly. Growing organisms undergo rapid mitosis of cells in the areas of growth, whereas mitosis is much slower in mature organisms. If cells are more likely to be damaged as they function, they tend to divide faster as well, such as intestinal cells (since exposed to many chemicals during digestion). Blood cells, on the other hand, last up to four months, because they are less likely to be damaged.

Rates (continued) Antibiotics decrease the rates of mitosis in certain cells to stop bacteria from growing and spreading. Other drugs stop the replication of DNA to stop a certain cell from functioning. If your environment changes, mitosis will occur more rapidly in some areas of your body. For example, your blood cells would divide more rapidly in areas of high altitude, because they receive less oxygen.

Cell Death The life of cells is determined by the cell cycle, but some cells die because they suffer injury or damage throughout their period of growth or division. For example – exposure to poison. Cell death from damage is called necrosis. Cells can also die when they can no longer perform efficiently or are no longer useful. This regular death of cells is called apoptosis. For example – cells used to fight a viral infection. When the infection is gone, they are no longer necessary.

A Closer Look at Mitosis Interphase In this stage, cells grow and perform normal functions. DNA is in the form of chromatin (thin strands which are hard to see). Centrioles and DNA are replicated at this time – each makes an exact copy of itself. Mitosis There are four stages of mitosis. Use the acronym PMAT to remember the order.

Late Interphase (right before mitosis)

Stage 1 - Prophase In this stage, centrioles move to opposite sides (also called poles) of the cell Spindle fibres appear around the centrioles The chromatin condenses (thicken) into chromosomes and are now visible. The chromosomes pair up to form a double strand. Where the strands join together is called the centromere. The nuclear membrane and nucleolus break down and disappear

Prophase Late Prophase Early Prophase

Stage 2 - Metaphase At this stage, the chromosomes line up along the middle of the cell (called the equator). Centrioles are now at the poles with spindle fibres running between them Each double-stranded chromosome is also attached to a spindle fibre at the centromere.

Metaphase

Stage 3 - Anaphase At this stage, the chromosome pair is divided in two at the centromere, with one going to one pole, and the other going to the other pole (opposite ends of the cell). The chromosomes are now single stranded, and are now called chromatids.

Anaphase

Stage 4 - Telophase In this stage, the chromatids have reached the poles The spindle fibre breaks down and disappears The nuclear membrane starts to reform around the chromatids and the nucleolus reappears The chromatids uncoil (thin) and become chromatin again.

Cytokinesis This occurs at the end of telophase, and involves the division of the cytoplasm, including organelles. In animal cells, the cell membrane pinches in, forming a “cleavage furrow” where it will eventually split in two. In plant cells, a new cell wall forms between the two halves called a “cell plate”

Telophase & Cytokinesis

Interphase (again!) Once the division is done, the new cells are called “daughter” cells The daughter cells are genetically identical to each other and to their original parent cell Each daughter cell enters a new period of growth before they will also divide in mitosis.

Cell Division in Plants

Cancer Cells A cell that divides uncontrollably and unregulated is called a cancer cell. They develop when a change occurs in the cell that affects how the cell divides. If the DNA of the cell changes, it is called a mutation. Some mutations occur due to viruses, radiation, and hazardous chemicals. These are called carcinogens. Viruses are commonly linked to leukemia and cervical cancer. Ultraviolet (UV) radiation is commonly linked to skin cancer. Cigarette smoke is commonly linked to lung cancer.

Cancer (continued) A tumour is a mass of cancer cells formed by abnormal rapid cell division, and there are two types: Benign - it is harmless, and remains in a confined area and causes little damage. Malignant - it is harmful, and can spread to other areas of the body. Drugs and radiation can be used to slow or stop the effects of dividing cancer cells. The chemical treatment is called chemotherapy, and the radiation treatment is called radiation therapy.

Comparing Normal vs. Cancer Cells

How Cancer Spreads

Videos Mitosis – Crash Course Cancer vs. Regular Cell Division Cancer & Treatments