Mitosis and the Cell Cycle

• The cell cycle:

  • all complex organisms originated from a single fertilised egg. The single cells forms many cells through processes of the cell cycle. Cells then become specialised for their various roles
  • when cells are going through the cell cycle, the number of cells doubles each time
  • because there will be a large difference in the number of cells after each division, we can convert them to log to plot them on a graph. Using the numbers in log form leads to a more manageable scale on the graph. It also leads to a straight line instead of an exponential curve
• The cell cycle:

  • within multicellular organisms, not all cells retain the ability to divide. Eukaryotic cells that retain the ability to divide show a cell cycle
  • cells do not divide continually but undergo a regular cycle
• Interphase:

  • G1 - cells prepare for DNA replication by growing and synthesising new organelles
  • S - DNA replication occurs
  • G2 - a relatively short gap before mitosis, where proteins needed for division are made, along with the ATP required

  mitosis occurs after this
• Duration of the cell cycle:

  • following mitosis, the daughter cells that are produced will then enter the G1 phase and the cycle usually continues. Not all cells continually divide. Some will stay at the beginning of interphase for years, and never seem to go through the full cycle (G0) e.g. neurones
  • the duration of interphase gives us information about the rate of cell division. Cells with a shorter interphase divide more often (cancer cells or skin cells), whereas other cells (liver) can remain in interphase for months or years before dividing
• Chromosomes:

  • DNA replicates during synthesis phase and separated during nuclear revision
  • chromosomes are made up of long, linear structures of DNA tightly coiled around histone proteins. They contain genes throughout the strand
  • the nucleus of all eukaryotic cells contains a set number of chromosomes, depending on the species. In humans the chromosomes exist as 23 homologous pairs (each consist of a maternal and a paternal). The diploid number of chromosomes is the total number in a normal body cell, consisting of both the maternal and paternal copy. In humans this is 46
• Homologous pairs:

  • homologous chromosomes are a pair of chromosomes containing the same genes in the same position (loci). The pair will include one maternal and one paternal chromosome. There may be different alleles on each chromosome
• Chromosome replication:

  • homologous chromosomes can look different depending on where in the cell cycle they are viewed. Before replication, each chromosome will be single stranded. Afterwards, they will be double stranded
  • during synthesis phase DNA in the chromosomes replicates all the way along its length to produce another identical strand of DNA. The original strand of DNA (original chromosome) and the new identical strand are now called chromatids and are held together at the centromeres. However, they are not visible as individual chromosomes at this stage
• Chromosomes:

  • as cell division begins, the chromosomes shorten and thicken (condense) because the DNA coils up more tightly around the histone proteins. The chromosomes therefore become clearly visible as separate structures
  • during mitosis you refer to sister chromatids. During interphase you refer to chromosomes
• Mitosis:

  in mitosis, a parent cell divides to produce two daughter cells. Each daughter cell contains an exact copy of the DNA of the parent cell; they're genetically identical. This is only possible due to the DNA replicating prior to the start of mitosis
• Functions of mitosis:

  • mitosis increases the number of cells during growth
  • it replaces tissue during repair (mitosis replaces the entire lining of the gut in 5 days). It doesn't repair cells, damaged cells die (apoptosis) and are replaced via mitosis
  • it allows asexual reproduction which produces genetically identical offspring. This is ideal when organisms are rapidly establishing a population and variation would be a disadvantage

  cell division maintains the same chromosome number from one generation to the next. Variation arises by mutations
• The stages of mitosis:

  when a eukaryotic cell divides the genetic material and nucleus divide first (so two new nuclei are formed), then the cytoplasm divides to form two new daughter cells. Mitosis itself occurs in a series of stages. These are;

  • prophase
  • metaphase
  • anaphase
  • telophase

  by the end of telophase there will be two nuclei. Immediately following mitosis, division of the cytoplasm usually occurs - this is known as cytokinesis. However this process often begins before mitosis has completed (during telophase)
• Prophase:

  • DNA supercoils around the histones
  • chromosomes condense and become visible
  • nuclear membrane breaks down and centrioles migrate to poles
• Metaphase:

  • centrioles make the protein strands known as spindles
  • chromosomes line up on the equator of the cell
  • the centromere attaches to the spindle fibres
• Anaphase:

  • centromeres divide, separating each pair of sister chromatids
  • spindles contract pulling chromatids to the opposite poles by their centromeres
• Telophase:

  • chromatids uncoil and become long and thin again
  • they are now indistinct, and we refer to them as chromosomes again
  • a nuclear envelope forms around each group of chromosomes - now 2 nuclei
  • spindles disintegrate and cytokinesis begins (splitting of the cytoplasm)
• Cytokinesis:

  • division of the cytoplasm (cytokinesis) follows mitosis quickly
  • two new daughter cells that are genetically identical are formed
• Mitotic index:

  • this is a ratio showing the number of cells undergoing mitosis in proportion to the total number of cells
  • number of cells undergoing mitosis ÷ total number of cells
  • it can be expressed as a percentage (multiply by 100)
  • the number of cells at each stage of mitosis is proportional to the time spent in that stage. Therefore, if the duration of the cell cycle is known the duration of a particular stage can be estimated
• Cancer:

  • mitosis is a controlled process. Cancer results from mutations in the genes that control cell division, causing rapid, uncontrolled growth and division of cells
  • this results in the formation of a mass of abnormal cells (called a tumour)
  • cancerous tumours do not respond to signals from nerves and hormones as normal cells would and do not undergo programmed cell death (apoptosis), which would normally occur when irreplaceable damage is detected
• Tumours:

  Normally, there is a gene which controls how much cells divide, if this is mutated it can lead to a tumour. Tumours can be;

  • benign - benign tumours and not cancerous - they grow slowly and don't invade other tissues
  • malignant - malignant tumours are cancerous - they grow faster and can spread around the body (to form secondary tumours)
• RP2 - Mitotic index:

  how to set up a microscope and prepare a temporary mounted slide;

  • pipette a small drop of water onto the slide
  • use tweezers to place a thin layer of specimen to the top of the water drop
  • add a drop of stain (iodine, eosin, methylene blue)
  • add a cover slip using a mounting needle. Take care not to get any air bubbles in as they will obstruct the view of the specimen
• RP2 - Mitotic index:

  how to prepare a root tip cell;

  • heat specimen in acid then rinse well in water and dry on a paper towel
  • place 2mm tip from a root tip on a microscope slide and use a mounting needle to break the tip open and spread the cells thinly
  • add drops of stain then place on cover slip and a piece of folded filter paper on top
  • press down gently to squash the tissue, without smearing sideways
• RP2 - Mitotic index:

  the reason for the steps;

  • acid is used to break the cell wall to allow stain to penetrate the cell and stain the chromosomes
  • stain is used to make the chromosomes visible
  • use the first 2-5mm of the root tip - where the dividing cells are found
  • the cell layer should be one cell thick - allows light to pass through so structures are visible
  • smearing the specimen could damage the chromosomes
• Summary of required practical 2:

  • if chromosomes are visible, mitosis is occurring
  • mitotic index = number of cells in mitosis ÷ total number of cells
  • the proportion of cells in a stage of mitosis is equivalent to the proportion of overall time spent in that stage (10% of total cells means it will take up 10% of the total time)