2.2 - cell division & stem cells

• genetic information is found in the nucleus of each body cell
• a chromosome is a linear arrangement of genes
• chromosomes are normally found in pairs in the nucleus
• in humans, there are 46 chromosomes in each body cell (23 pairs)
• a gene is a section of DNA
• genes are located on chromosomes and these are also normally found in pairs in the nucleus
• genes control specific characteristics in the body
• there are 2 types of cell division:

  • mitosis
  • meiosis
• mitosis takes place for:

  • growth
  • replace worn out cells
  • repair damaged tissue
• during the process of mitosis:

  1. all the chromosomes and genes in the parent cell are copied
  2. the cell divides to produce 2 daughter cells
• the daughter cells are genetically identical to each other, and to the mother cell. the number of chromosomes stays the same
• meiosis takes place to produce gametes. In animals, the gametes are sperm & eggs. In flowering plants, the gametes are pollen & eggs
• during the process of meiosis:

  1. all the chromosomes and genes in the mother cell are copied
  2. the cell divides twice to form 4 gametes. Each gamete has half the number of chromosomes of the original mother cell
• all gametes are genetically different from each other. they show variation
• diploid cells are cells which contains 2 complete sets of chromosomes
• haploid cells have 1 set of chromosomes
• a stem cell is an undifferentiated cell that can develop into a specialised cell. specialised cells include nerve, bone and muscle cells
• Stem cells could be used to replace cells that have been damaged or destroyed, such as cases of spinal cord or brain injury that have led to paralysis.
• stem cells in plants are found in regions called meristems. meristem cells can differentiate to produce all types of plant cells at any time during the life of the plant.
• people may object to the use of embryonic stem cells because potential life is destroyed
• adult stem cells can be taken from several areas of the body such as the bone marrow. The benefits of using your own stem cells include:

  • no rejection
  • no need to find a donor
  • no need for tissue typing
• the main disadvantage of adult stem cells are that they will differentiate into a narrower range of cell types
• males have an X and Y chromosome and females have 2 X chromosomes
• meiosis is important for sexual reproduction because it increases genetic variation
• mitosis occurs in all non-reproductive cells while meiosis occurs only in reproductive cells
• mitosis produces 2 daughter cells while meiosis produces 4 daughter cells
• the daughter cells produced in mitosis are genetically identical while the daughter cells produced in meiosis are genetically different
• in mitosis, daughter cells have 46 chromosomes while in meiosis, daughter cells have 23 chromosomes
• mitosis occurs for the growth, repair and replacement of damaged cells. and meiosis occurs for the formation of gametes during sexual reproduction
• a chromosome is a linear DNA molecule tightly coiled around proteins
• chromosome pairs are not identical and may contain different alleles (versions of a gene)
• uncontrolled cell mitosis causes cancer
• cancer occurs when cells divide and grow uncontrollably, forming primary tumours. once the primary tumours break away, they can spread to other parts of the body, creating secondary tumours
• cell differentiation produces specialised cells with specific functions. once a cell differentiates, it cannot divide to make unspecialised cells
• embryonic stem cells are unspecialised and capable of differentiating into a range of different cell types
• adult stem cells can differentiate into a limited range of different cell types. enable replacement of dead or damaged cells
• The main meristems are close to the tip of the shoot and the tip of the root
• meristem cells are capable of differentiating into any cell type throughout the life of a plant
• advantages of stem cells:

  • used to treat damage or disease
  • not rejected by body
  • can be used to grow organs
• disadvantages of stem cells:

  • can become contaminated and transmit infections
  • can cause tumours
  • long term risks unknown
  • potential side effects