Meiosis and Gametes

• What are gamete?

  Sex cells (sperm cells and egg cells) that fuse together during sexual reproduction
• Are gametes haploid or diploid?

  Haploid
• What does it mean to be haploid?

  Contain one set of chromosomes
• When the gametes fuse together, a new diploid organism is formed.
• How are gametes produced?

  Meiosis
• Meiosis produces four genetically different daughter cells.

  Each daughter cell receives a different combination of chromosomes.
• Sexual reproduction involves the random fusion between gametes. Any two gametes could fuse together to produce a random combination of chromosomes.
• Meiosis and random fertilisation further increase genetic diversity.
• What is crossing over?

  Crossing over is the exchange of genetic material between homologous chromosomes during meiosis.
• • The chromosomes of a homologous pair are arranged down the middle of the cell before the first cell division in meiosis I.
  • Sections of DNA are swapped between the chromosomes in a pair in a process called crossing over.
  • Crossing over produces new combinations of alleles.
  • This increases genetic variation.
• What is independent assortment?

  The random distribution of alleles for different genes during the formation of gametes.
• • The chromosomes in a homologous pair are separated in meiosis I in a random formation to produce two genetically different daughter cells.
  • The combination of chromosomes in the two daughter cells is random.
  • The process of separating chromosomes into random combinations is called independent assortment.
  • Independent segregation increases genetic variation.
• In fertilisation, a sperm and egg fuse to produce a diploid zygote, which divides to become an embryo.
• How are sperm cells specialised for their function?

  To swim towards the egg they have a flagella, the tail moves with energy provided by the mitochondria in the mid piece of the sperm, the head is covered by acrosome which releases enzymes to break down the zona pellucida (the eggs outer-membrane)
• Egg cells are much bigger than sperm cells, females produce less eggs than males produce sperm.
• The cytoplasm of the egg is high in nutrients which are used when the embryo is growing, the outer membrane of the egg is called the zona pellucida which must be penetrated in order for the egg to be fertilised, once a sperm has entered the egg, the zona pellucida plays a role in preventing polyspermy (where more than one sperm fertilises the egg).
• When a sperm and egg cell fuse together, this creates a zygote.
• A zygote divides by mitosis and keeps doing so until an embryo is formed
• Acrosome reaction:

  • Sperm cells swim towards the egg cell to fertilise it.
  • Sperm find the egg by following chemicals the egg releases. This is called chemotaxis.
  • Once they reach the egg cell, the acrosome reaction is initiated.
  • The inner membrane of the acrosome fuses with the plasma membrane of the head of the sperm.
  • This exposes the acrosome, which releases enzymes to break down the zona pellucida.
  • Sperm can then break through to the cell surface membrane of the egg and bind to it.
• Cortical reaction:

  • Once the head of the sperm has fused to the cell surface membrane, cortical granules (a type of vesicle) move to the cell surface membrane in the egg, and fuse with the membrane.
  • The contents of the cortical granules are exocytosed, and it prevents any other sperm from binding.
• Once the head of the sperm cell has entered the egg cell, the second meiotic division occurs in the egg, so that a haploid egg nucleus can fuse together with the nucleus from the sperm - fusion of the two produces a diploid zygote.
• What is In vitro fertilisation (IVF)?

  Where an egg is fertilised by a sperm in a laboratory and then the embryos are implanted into the womb.
• Usually, a maximum of two embryos are implanted, in case all the embryos that are implanted are successful.
• Why are mitochondria only maternally inherited?

  The mitochondria from the mother are in the egg cell, so remain when the nuclei of the sperm and egg fuse and divide. However, the mitochondria from the father are in the tail of the sperm, which never enters the egg cell.