Mitosis and Meiosis

• Flemming first described mitosis in root cells in the 1880s. He saw elongated threads forming in the nucleus; watched them shorten and thicken during mitosis
• The chromosomal theory of inheritance was proposed by Boveri in the late 1800s and Sutton in 1902. Chromosomes are linear structures with genes located at specific sites along them.
• The stages of mitosis are prophase, metaphase, anaphase, telophase and cytokinesis.
• Early prophase - condensing replicated chromosome. Mitotic spindle begins to form as microtubules rapidly grow out of the centrosomes, which begin to move away from each other.
• Late prophase/prometaphase - Microtubules emerging from the centrosomes at the poles (ends) of the mitotic spindle extend into the nuclear region, reaching the chromosomes. Some of the spindle microtubules attach to the kinetochores. Other spindle microtubules make contact with microtubules coming from the opposite pole.
• Metaphase - mitotic spindle is fully formed. Chromosomes midway between the spindle poles
• Anaphase - Begins when the two centromeres of each chromosome come apart, separating the sister chromatids. Proteins of the kinetochores, powered by ATP walk the newly separated daughter chromosomes along the microtubules towards the opposite poles of the cell. Spindle microtubules attached to the kinetochores shorten. Spindle microtubules not attached lengthen. As a result poles are moved farther apart.
• Telophase - Nuclear envelope around individual chromosomes.
• Cytokinesis - the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches the cell in two to create two daughters, each with one nucleus.
• Until the 1950s, the stages of interphase were unclear. Root tips of plants were incubated with radioactive phosphorous and it was observed that DNA synthesis occured in S phase.
• Resting G0 phase - Cell has left the cycle and stopped dividing.
• Interphase G1 - cells increase in size
• Interphase S - DNA replication occurs
• Interphase G2 - Cell continues to grow
• After duplication, the chromosome now consists of two chromatids, joined copies of the originial chromosome (i.e. double stranded - two double helices)

  Once separated from its sister, each chromatid (single stranded - one double helix) is considered an individual chromosome

  N = maximum number of alleles at any particular locus

  • somatic cell (n=2)
  • sperm (n=1)
• Diploid-dominant life cycle: In animals, sexually-reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilised egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells are generated early in the development of the embryo.
• Mitosis

  • produces genetically identical cells
  • results in diploid cells
  • Takes place throughout an organism's lifetime
  • involved in asexual reproduction
• Meiosis

  • produces genetically unique cells
  • results in haploid cells
  • takes place only at certain times in an organism's life cycle
  • involved in sexual reproduction
• The stages of meiosis are interphase (DNA replication), prophase I, metaphase I, anaphase I and anaphase II.
• Recombination during meiosis

  • homologous chromosomes held together in early prophase - bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex
  • Crossover occurs between non-sister chromatids of homologous chromosomes.
• Meiosis in females - before ovulation, one primary oocyte, undergoes asymmetric cell division to make one polar body and one secondary oocyte.
• Meiosis in males - spermatogonium undergoes 2x mitosis to become primary spermatocyte, then meiosis to become secondary spermatocyte then meiosis again to become spermatids.
• Primordial germ cells: diploid cells that arises through mitosis. Arise in the primitive streak and migrate via the gut of an embryo to the developing gonads. Once in the gonads are called oogonium and spermatogonium.
• Germ cell: any cell that gives rise to the gametes
• Prophase

  During DNA replication, genetic material is loosely packed as chromatin but during mitosis DNA needs to be more tightly packed to allow for easier separation in anaphase therefore at the start of prophase, chromatin begins condensing into chromosomes.

  Mitotic spindles also begin to form. Mitotic spindles are structures made from microtubules that aid in the organisation and arrangement of chromosomes. The spindles attach to an organelle known as the centrosome. Each cell in mitosis has two centrosomes. During prophase, centrosomes begin to move in opposite directions.
• Prometaphase 

  Chromosomes finish condensing into their compact state. The nuclear envelope begins to break down, allowing spindle fibres to attach to the chromosomes at a site called the kinetochore (an area of the centromere found on each sister chromatid). The sister chromatids are attached to spindles that originate from the opposite centrosome, linking the two together.
• Metaphase

  Chromosomes align upon a theoretical line known as the metaphase plate. The centrosomes have finished moving and are located at opposite ends of the cell.

  At this stage, the cell will check that all the chromosomes are aligned along the metaphase plate, with their kinetochores correctly attached. This helps to ensure sister chromatids are split evenly between the two daughter cells. An error in chromosomal alignment or spindle attachment will result in the cell halting further progress until the problem is fixed.
• Anaphase

  Sister chromatids are pulled to opposite ends of the cell. The spindle fibres contract, breaking the chromatids at the centromere and moving them to opposite poles of the cell. Spindle fibres not attached to chromatids will elongate the cell to prepare it for division
• Telophase

  The cell has elongated and is nearly finished dividing. Cell-like features begin to reappear such as the reformation of two nuclei (one for each cell). The chromosomes then de-condense and the mitotic spindle fibres are broken down.
• Cytokinesis

  The division of the cytoplasm to form two new cells. This stage actually begins between anaphase and telophase, however, doesn’t finish until after telophase. To separate the two cells, a ring of protein (actin ring) pinches the cytoplasm along a crease known as a cleavage furrow. This splits the cytoplasm equally between the two cells
• Errors in mitosis typically occur during metaphase due to misalignment of chromosomes along the metaphase plate or failure of the mitotic spindles to attach to one of the kinetochores. The daughter cells can have an unequal distribution of chromosomes, leaving one cell with one too many and the other with one too few.

  The cell missing a chromosome usually dies, however, the cell with the extra chromosome can cause problems. If the extra chromosome carries genes that promote cell growth, this may lead to cancer via constitutive activation of signalling pathways within the cell.
• In meiosis I, homologous chromosomes are separated into two cells such that there is one chromosome (consisting of two chromatids) per chromosome pair in each daughter cell, i.e. two chromosomes total
• Prophase I

  Prior to prophase, chromosomes replicate to form sister chromatids. There are initially four chromatids (c) and two chromosomes (n) for each of the 23 chromosome pairs (4c, 2n). The nuclear envelope disintegrates and chromosomes begin to condense. Spindle fibres appear.

  To further increase genetic diversity, homologous chromosomes exchange small parts of themselves, such that one chromosome contains both maternal and paternal DNA. This process is known as crossing over, and the points at which this occurs on a chromosome are referred to as chiasmata.
• Prometaphase I

  Spindle fibres attach to the chromosomes at points along the chromosomes called centromeres. While this is happening, the chromosomes continue to condense.
• Metaphase I

  Maternal and paternal versions of the same chromosome (homologous chromosomes) align along the equator of the cell. A process called independent assortment occurs – this is when maternal and paternal chromosomes line up and randomly align themselves on either side of the equator. This in turn determines which gamete chromosomes are allocated to, which leads to genetic diversity among offspring
• Anaphase I

  Here, each of the homologous chromosomes is pulled towards opposite poles of the cell as the spindle fibres retract. This equally divides the DNA between the two cells which will be formed.
• Telophase I and Cytokinesis I

  During telophase I, the nuclear envelope reforms and spindle fibres disappear. In cytokinesis I, the cytoplasm and cell divide resulting in two cells that are technically haploid – there is one chromosome and two chromatids for each chromosome (2c, n)
• Prophase II and Prometaphase II

  These stages are identical to their counterparts in meiosis I.
• Metaphase II

  In metaphase II, chromosomes line up in single file along the equator of the cell. This is in contrast to metaphase I, where chromosomes line up in homologous pairs.
• Anaphase II

  Next, sister chromatids are pulled to opposite poles of the equator.