Gametes, Fertilisation and Stem Cells 3.6, 3.7, 3.9-3.11 Flashcards
Define fertilisation
The exact moment the nuclei of the male and female gametes fuse to create a zygote with a diploid number of chromosomes
How are egg cells specialised for their function? x3
- Large cytoplasm which contains huge amounts of nutrients
- Zone pellucida is a protective glycoprotein layer
- Cortical granules form a protective coating
How are sperm cells specialised for their function?
- Lots of mitochondria in the mid piece to provide energy for swimming
- Flagellum allows sperm to swim towards egg
- Acrosome contains digestive enzymes to break down zona pellucida
Where does fertilisation occur in mammals?
Oviduct
What are the 3 components of fertilisation?
1) Acrosome Reaction = sperm makes contact with zona pellucida and digestive enzymes are released from the acrosome. Enzymes digest the zp so sperm can move to cell membrane of egg cell. Sperm head fuses with cell membrane which triggers cortical reaction.
2) Cortical reaction = egg cell releases the contents of the cortical granules into space between cell membrane and zp which causes zp to thicken and makes it impenetrable to other sperm - only one sperm fertilises the egg.
3) Nuclei fusion = nucleus of the sperm fuses with the nucleus of the egg cell
What does meiosis produce?
4 genetically varied haploid daughter cells = gametes
How many chromosomes do gametes have?
Half the number = haploid
Describe briefly the process of meiosis:
1) DNA replicates so that there are 2 identical copies of each chromosome
2) DNA condenses
3) Chromosomes arrange into homologous pairs
4) First division = homologous pairs are separated (halving chromosome number)
5) Second division = pairs of sister chromatids are separated
6) Four genetically different daughter cells produces
What are the two methods of producing genetically varied gametes?
Crossing over
Independent assortment
Describe crossing over
- Before first division homologous pairs line up
- Two of the chromatids twist around each other and a section breaks off to rejoin onto other chromosome, recombining genetic materials
Chromatids now have the same genes but different combination of alleles
Describe independent assortment
When gametes are produced, different combinations of maternal and paternal chromosomes go into each cell
What are stem cells?
Undifferentiated cells that can become new cells which become specialised through differentiation
What is potency?
The ability for stem cells to differentiate into specialised cells
Define totipotency
The ability to produce ALL cell types, including all the specialised cells in an organism and extraembryonic cells (placenta and umbilical cord)
Only present in mammals in the first few cell divisions of an embryo
Define pluripotency
The ability to produce all the specialised cells in an organism BUT NOT extra-embryonic cells
Why are adult stem cells much less flexible than embryonic stem cells?
They can only develop into SOME cell types
Where are plant stem cells found?
In regions of growth eg. Roots or shoots
Give an example of a stem cell therapy
Treatment for leukaemia (cancer of the bone marrow) kills all the stem cells in the bone marrow so that bone marrow replacements can replace them.
What are the potential benefits of stem cells
- Save many lives eg. Many people waiting for organ transplant die before donor organ is available so stem cells can be used to grow organs instead
- Improve quality of life eg. Used to replace damaged cells in blind people
What are the two potential sources of human stem cells? + explain
Adult stem cells = simple operation with little risk where needle is inserted into centre of bone and a small amount of bone marrow is removed
Not as flexible as embryonic due to limited range of cells
Less risk of rejection if patient uses their own stem cells
Embryonic stem cells = embryos created in IVF and stem cells removed when 4-5 days old with rest of cells being destroyed
Very flexible
High risk of rejection
Name two ethical issues of using embryonic stem cells
1) People believed fertilised embryos have a right to life from the moment of fertilisation
2) People believe it is wrong to destroy viable embryos
What has been established to consider the benefits and ethical issues of embryonic stem cell research?
Regulatory authorities
Name 5 things regulatory authorities do:
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1) Monitoring developments in scientific research (regulated appropriately)
2) Advice to governments and professionals (promote science)
3) Licensing and monitoring centres (ensures only fully trained staff carry out research to avoid unregulated research)
4) Guidelines and codes of practice (scientists working in similar manner and controls methods used to extract eg. Maximum age of embryo)
5) Proposals of research
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