Cell division

• Mitosis is part of the cell cycle and involves the division of the nucleus
• The cell cycle consists of two main phases: interphase and the mitotic phase
• During interphase, the cell increases its energy stores, grows in size, (G1 and G2) replicates its organelles, and copies its DNA (S)
• Chromosomes are not visible as distinct structures during interphase, instead, scientists see chromatin which has a loose open structure
• Chromatin allows DNA to be accessible for transcription and replication
• During interphase, chromosomes are replicated, resulting in two identical molecules of DNA called sister chromatids
• Sister chromatids are joined at the centromere
• Mitosis results in the division of the nucleus and the formation of two identical diploid cells
• Mitosis is used for growth, repair, and asexual reproduction in organisms
• The division of the cell during mitosis is called cytokinesis
• Cell division by mitosis has two main stages: interphase and the mitotic or m phase
• During interphase, each chromosome is copied to form two sister chromatids joined at the centromere
• Chromosomes are not visible as distinct structures in the nucleus during interphase
• There are four stages in mitosis: prophase, metaphase, anaphase, and telophase
• Prophase:

  • Chromosomes condense and become visible in the nucleus
  • Each chromosome consists of two sister chromatids joined at the centromere
  • Nucleolus disappears and nuclear membrane starts to break down
  • Centrioles move to either side of the cell
  • Spindle fibers attach to the centromere of each chromosome and move the chromosomes towards the center of the cell
• Metaphase:

  • Spindle apparatus is fully formed
  • Chromosomes are lined up at the center or equator of the cell
• Anaphase:

  • Centromeres divide into two
  • Spindle fibers shorten, pulling sister chromatids towards opposite poles of the cell
• Telophase:

  • Chromatids reach the poles of the cell and are referred to as chromosomes again
  • Spindle apparatus breaks down, nuclear membranes reform
  • Chromosomes uncoil back to chromatin state, nucleolus reappears in each nucleus
• Cytokinesis:

  • Central cell membrane is pulled inwards by the cytoskeleton, creating a groove that deepens
  • Membranes fuse to form two independent cells
• In plant cells:

  • Centrioles do not play a role in forming the spindle apparatus
  • During cytokinesis, vesicles from the golgi apparatus form membrane structures down the center of the cell, which fuse to form a central cell membrane dividing the cytoplasm into two
  • A new cellulose cell wall forms down the center
• Biological side of cancer:

  • Cells divide to repair damaged tissue or replace worn-out cells
  • Factors influencing cell division rate: nutrient and oxygen availability, growth factors, genetics
  • Inherited genes or random mutations can increase the risk of certain cancers
  • Mutations in genes controlling cell division can lead to uncontrolled cell division and cancer
• Cancer is a group of diseases caused by uncontrolled cell division

  • Mutations in genes controlling cell division can cause cells to grow out of control
• Mutations:

  • Changes to the base sequence of DNA in a cell
  • Increased chance of mutation when exposed to mutagens like carcinogenic chemicals or ionizing radiation
• Cancer cells:

  • Abnormal cells in the body
  • Can have multiple nuclei, odd shapes, and large, dark nuclei
  • Can be recognized and destroyed by the immune system
• Tumor formation:

  • Tumors form when cells divide uncontrollably and layer over each other
  • Tumors can damage organs by pressing on tissues or causing blockages
  • Benign tumors are slow-growing and non-invasive, while malignant tumors are fast-growing and invasive
• Tumor characteristics:

  • Benign tumors are encapsulated and do not invade other tissues
  • Malignant tumors are not encapsulated and can invade other tissues, metastasizing to other organs
• Cancer treatments:

  • Target cell cycle to control cell division in cancer cells
  • Treatments cannot distinguish between cancer cells and normal body cells, leading to side effects like hair loss, reduced immunity, and gastrointestinal issues
• Types of cancer treatments:

  • Surgery: removes specific tumors, with risks of spreading if not contained
  • Radiotherapy: damages DNA to stop cell division
  • Chemotherapy: inhibits enzymes needed for DNA replication to prevent cell division
• Combination treatments:

  • Surgery may be followed by radiotherapy or chemotherapy
  • Different drugs and treatments used for different types of cancers
  • Research ongoing for more targeted therapies to reduce side effects and improve effectiveness
• Prokaryotic cells can exchange plasmids containing useful genes through horizontal gene transfer
• A virus binds to a host cell by recognizing antigens on the cell membrane and injects its genetic material
• Viral DNA is transcribed by the host's machinery, leading to the production of viral proteins
• The death of the host cell occurs when many virus particles leave, causing damage to the cell membrane
• Binary fission is the process by which prokaryotic cells, such as bacteria, reproduce
• Binary fission comes from the words binary, meaning two parts, and fission, meaning to split
• In binary fission, the circular loop of DNA replicates, plasmids can be replicated, and the cell elongates
• The DNA loops move to opposite poles of the cell and new cell walls begin to form between the two new cells
• Two identical daughter cells are formed, each with a copy of the DNA loop, but they may have different numbers of plasmids
• Bacterial division is rapid, leading to faster evolution compared to eukaryotes due to higher mutation rates
• We can calculate bacterial growth rates by knowing the starting population and the number of divisions based on doubling