• There are two processes by which the?nucleus?of a?eukaryotic cell?can?divide. These are:
  • Mitosis
  • Meiosis
• Mitosis?gives rise to?genetically identical cells?and is the type of cell division used for?growth,?repair?of damaged tissues,?replacement?of cells and?asexual reproduction
• Meiosis?gives rise to cells that are?genetically different?from each other and is the type of cell division used to produce?gametes?(sex cells)
• During meiosis, the nucleus of the original 'parent' cell undergoes?two rounds of division. These are:
  • Meiosis I
  • Meiosis II

Meiosis I

• The nucleus of the original 'parent' cell is?diploid?(2n) i.e. it contains?two sets of chromosomes
• Before meiosis I, these chromosomes?replicate
• During meiosis I, the?homologous pairs?of chromosomes are?split up, to produce?two?haploid?(n) nuclei
  • At this point, each chromosome still consists of?two chromatids
• Note that the?chromosome number halves?(from 2n to n) in the?first division?of meiosis (meiosis I), not the second division (meiosis II)

Meiosis II

• During meiosis II, the?chromatids?that make up each chromosome?separate?to produce?four?haploid?(n) nuclei
  • At this point, each chromosome now consists of a?single chromatid

One diploid nucleus divides by meiosis to produce four haploid nuclei

NOS: Making careful observations; meiosis was discovered by microscope examinations of dividing germ-line cells

• In the 19th century, microscopes were developed that could be used to view certain?internal cell structures
• Around this time, it was also discovered that certain?dyes?could be used to?stain?(and?observe) the?cell nucleus
• The use of these dyes eventually led to the discovery of?thread-like structures?inside?dividing nuclei
  • These were name?chromosomes
• In the 1880s, a group of German biologists used these new developments to make detailed observations of dividing nuclei
  • Their careful observations led to?the discovery of the process of meiosis?and a basic understanding of how it occurs
• One key observation was made by viewing the chromosomes in specific cells in an organism known as the horse threadworm (Parascaris equorum)
  • It was observed that the?nuclei?of their?egg?and?sperm?cells contained?two chromosomes, whereas the nuclei of a?fertilised egg?contained?four chromosomes
  • This suggested that the chromosome number had been doubled by the process of fertilisation
  • This led to the?hypothesis?that, at some point in every generation, a?special type of nuclear division?must occur that?halves the chromosome number
• The?specific sequence of events in meiosis?was finally discovered by carefully observing cells from the?ovaries?of?European rabbits?(Oryctolagus cuniculus) between 0 and 28 days old
  • This was possible because in females of this species, certain cells in the ovaries start undergoing meiosis?from birth?and the process continues?slowly?over a period of?many days
• The initial discovery of meiosis (as well as the following series of discoveries that revealed to scientists how it occurs) was made possible through?careful scientific observations
• Such careful observations are needed in order to?validate?the?claims?and?discoveries?that scientists make, as?scientific evidence?is required. Careful observations can enable scientists to collect?evidence that allows theories to be developed
• The use of apparatus, dyes, sensors, amongst many other scientific tools, allows scientists to make these?precise and accurate observations
• The methods used to make such observations must be able to be?repeated?so that the results can be confirmed by other scientists if necessary

• The life cycles of organisms can be?sexual?or?asexual?(some organisms are capable of both)
  • In an?asexual?life cycle, the?offspring are genetically identical to the parent?(they have exactly the same chromosomes)
  • In a?sexual?life cycle, the?offspring are genetically distinct from each other and from each of the parents?(their chromosomes are different, causing them to be genetically distinct)
• The?halving of the chromosome number?during meiosis is very important for a?sexual life cycle?as it allows for the?fusion of gametes
• Sexual reproduction is a process involving the?fusion?of the nuclei of two gametes to form a?zygote?(fertilised egg cell) and the production of offspring that are genetically distinct from each other
• This?fusion of gamete nuclei?is known as?fertilisation
  • Fertilisation?doubles?the number of chromosomes each time it occurs
  • This is why it is essential that the chromosome number is also?halved?at some stage in organisms with a?sexual life cycle, otherwise the chromosome number would keep doubling every generation
  • This halving of the chromosome number occurs during?meiosis
  • In animals, this halving occurs during the?creation of gametes

Sexual life cycle