IB Biology: Level 8 - Cell Cycle & Cancer 1.6, 3.2, & 7.1

The cell cycle comprises the events that take place between the formation of the cell and when it divides.

• Interphase: G1- phase, S-phase, G2-phase • Mitosis • Cytokinesis

Interphase is the time between the formation of a cell and when the nucleus divides. It is a period of intense metabolic activity.

• Cell activities include: Transcription, translation, cell differentiation, cellular respiration, and new organelles are produced. • Cells that will not divide remain in G1.

• Once a cell is instructed to divide it enters S phase. - DNA replication occurs. • Chromosomes must be replicated to make sure each new cell formed has a full set of chromosomes.

• The cell prepares further for nuclear division. • In animal cells, centrioles are replicated.

• Mitosis is a type of nuclear division in eukaryotic cells. • The nucleus divides to produce two daughter nuclei the genetically identical. • Mitosis is an extremely important process. • It is crucial that new cells produced for growth and repair are genetically identical to existing cells. • Asexual reproduction also relies on mitosis.

Mitosis is a continuous process. However, it is split into 4 stages for convenience. 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase

When chromosomes condense, they coil up to become shorter and thicker. - In eukaryotic cells, chromosomes are made up of DNA and proteins (histones). - The proteins help organise the DNA and enable chromosomes to coil. - Chromosomes are least condensed during G1 of interphase. - Individual chromosomes are not visible with a light microscope in the nucleus of cells during interphase. -During mitosis the chromosomes become highly condensed. -This process is called supercoiling. - Chromosomes then become visible with a light microscope. - Fully condensed chromosomes are approx. 250 times thicker that a DNA double helix.

• Centromere = location where 2 sister chromatids are joined together. • Sister chromatids = genetically identical structures produced by DNA replication. • During mitosis the sister chromatids will separate • they are then termed chromosomes. • It is a common misconception that chromosomes are always X-shaped structures. Chromosomes are only X-shaped following DNA replication and before the sister chromatids separate. Otherwise chromosomes in eukaryotes are linear structures.

• Chromosomes condense by supercoiling (and become visible with a light miroscope). • Centrioles move towards opposite poles (ends) of the cell and spindle microtubules form. • Nuclear membrane starts to break down.

• Centrioles are at opposite poles of the cell. • The nuclear membrane is fully broken down. • Chromosomes are fully condensed. • Spindle microtubules attach to the centromere of each chromosome • one spindle microtubule from each centriole attaches to each side of the centromere. • Chromosomes line up at the equator (middle) of the cell • due to spindle microtubules pulling on the centromeres.

• The centromeres divide and the 2 sister chromatids are separated (each chromatid is now called a chromosome). • The spindle microtubules shorten, pulling genetically identical chromosomes to each pole. • At the end of anaphase, each pole has an identical set of chromosomes.

• The chromosomes at each pole become less condense. • A nuclear membrane forms around the 2 sets of chromosomes. • The spindle microtubules break down.

Indicates the proportion of cells undergoing mitosis in a tissue.More actively growing tissues (such as the root tips of a plant or a tumour) will have a higher mitotic index.

• View a slide of your chosen tissue under a light microscope. • Select a higher magnification so the field of view contains approximately 100 cells. • Count all the cells that you can see. • Count all the cells in which you can see condensed chromosomes. • Use the equation: -Number of cells undergoing mitosis divided by the total number of cells observed.

• Division of cytoplasm. • Occurs at the end of mitosis and results in 2 separate daughter cells being produced.

In animal cells: - Plasma membrane pinches in all the way around the equator of the cell. - When the membrane meets the middle of the cell, the cell splits to form 2 identical cells. In plant cells: - A line of vesicles form at the equator of the cell. These join together to form plasma membranes and cell walls, separating the 2 daughter cells.

• Are proteins that control and coordinate the different phases of the cell cycle. • They make sure one phase does not start before another phase is complete and stop progression of the cell cycle if the cell should not divide. • Cyclins pair together with an enzyme cyclin-dependent kinase (CDK) to regulate the activity of other proteins that perform phase-specific roles. -Cyclins are like managers that oversee a team of workers. Each phase transition has many workers and one manager. • Kinases are enzymes that add phosphate to (or phosphorylate) their substrate. Phosphorylation is a common way to regulate the activity of proteins in cells.

• Cyclins were discovered by accident. • Their fortunate occurrence is known as serendipity. • Researchers were studying protein synthesis in sea urchins. • They noticed the levels of some proteins increased and decreased in regular cycles. • The researchers realised the importance of their observations. • These proteins were named cyclins and found to have a key role in the control and coordination of the cell cycle. • Serendipity has regularly happened throughout the history of science. Perhaps the most well-known is the chance discovery of antibiotics by Alexander Fleming. • As Louis Pasteur said, “In the fields of observation, fortune favors the prepared mind”.

• Phase transitions coincide with the high levels of each cyclin. e.g. a high concentration of cyclin A coordinates the transition from S phase to G2 phase. • The cyclins must reach a threshold level for the cell cycle to progress. • The level of CDK (cyclin-dependent kinase) remains constant. • The mechanism of controlling the cell cycle by cyclins is highly conserved throughout evolution and is the same in yeast, insects, plants and animals.

• An agent that causes changes in the DNA sequence (a mutation). • Mutagens include chemical substances and ionising radiation.

• Some oncogenes code for proteins that control the cell cycle such as cyclins. • Mutations in oncogenes may result in their loss of function. • This causes the cells to keep dividing when they should not. • A well-known oncogene is BRCA 1. Mutations in this gene greatly increase the risk of developing breast and ovarian cancer. The normal function of the BRCA 1 protein is to repair damage to DNA. People with a faulty BRCA 1 gene are less able to repair damage to their DNA; they are more likely to accumulate mutations in the genes that control the cell cycle. Primary tumour Formed when a cell that has lost the ability to control the cell cycle keeps dividing to produce a mass of cells.- Many primary tumours are benign (slow growing and their cells do not spread).- Some primary tumours are malignant (fast growing and their cells spread to other parts of the body).- Malignant tumours cause cancer.Some oncogenes code for proteins that control the cell cycle such as cyclins. • Mutations in oncogenes may result in their loss of function. • This causes the cells to keep dividing when they should not. A well-known oncogene is BRCA 1. Mutations in this gene greatly increase the risk of developing breast and ovarian cancer. The normal function of the BRCA 1 protein is to repair damage to DNA. People with a faulty BRCA 1 gene are less able to repair damage to their DNA; they are more likely to accumulate mutations in the genes that control the cell cycle.

• Formed when a cell that has lost the ability to control the cell cycle keeps dividing to produce a mass of cells. • Many primary tumors are benign (slow growing and their cells do not spread). • Some primary tumors are malignant (fast growing and their cells spread to other parts of the body). • Malignant tumors cause cancer.