Fertilisation Part One Flashcards
Describe sexual reproduction including the different types of sexual reproduction that exist.
Sexual reproduction requires 2 parents, generally a male and a female.
And egg and sperm fuse (a zygote) to form an entirely new organism.
Sexual reproduction results in offspring that are genetically different from the parent organisms.
Sexual reproduction produces a greater chance of variation within a species than asexual reproduction would.
Variation improves the chances that a species will adapt to their environment and survive.
Sexual reproduction methods include pollination, external fertilisation and internal fertilisation. Pollination occurs when pollen grains containing sperm nuclei are transferred from the anther to the stigma, down the pollen tube and to the egg. External fertilisation usually requires a medium such as water which the sperm use to swim towards the egg. External fertilisation usually occurs in fish and amphibians. Internal fertilisation occurs within the females and happens in mammals, insects, birds and reptiles.
What are some advantages of asexual reproduction?
1) . It does not require special cells or a lot of energy.
2) . It can produce offspring quickly.
3) . In a stable environment it creates a large thriving population.
What are some disadvantages of asexual reproduction?
1). Limited ability to adapt and so face massive die-off of the environment changes.
What are some of the advantages of sexual reproduction?
1) . Creates lots of variation within a species.
2) . Able to live in a variety of environmental settings.
3) . Able to adapt to change in the environment.
What are some disadvantages of sexual reproduction?
1) . Needs time and energy.
2) . Produces small populations.
Describe how mitosis and meiosis may be used in different modes of reproduction.
Mitosis is used in a sexual reproduction whereas meiosis occurs in sexual reproduction.
Each human cell contains 23 pairs of chromosomes (46 individual chromosomes).
Prior to division the cell is in interphase during which it grows and duplicates the centrioles and DNA content in each of its chromosomes. As a result each chromosome consists of 2 identical chromosomes joined at the centromere.
In prophase the chromatids become visible under light microscopy. At the end of prophase the nuclear membrane breaks down.
In metaphase microtubules form the meiotic or mitotic spindle between the two centrioles and the chromosomes lie on the equator.
At anaphase the chromosome get pulled apart and split.
In telophase the nuclear membrane and he nucleoli reform.
In mitosis the cell divides into 2n diploid cells. Meiosis on the other hand reduces the chromosome number by copying the chromosomes once but dividing twice. The first division (meiosis I) separates the homologous chromosomes, then meiosis II separates the sister chromatids (second division).
The timing of meiosis and fertilisation varies among species.
Describe testicular development in the foetus.
During testicular development the y-chromosomal determination of gonadal sex is visible when primordial germ cell colonisation is complete during the 6th week of foetal development.
8-16 weeks:
1) . The primitive sex cords proliferate into the medulla and then establish contact with the rete testis cords.
2) . They then separate from the coelomic epithelium by the tunica albuginea, the fibrous connective tissue. Note the horseshoe shape of the seminiferous cords and their continuity with the rete testis.
3) . It is during this period that the SRY gene is expressed in males only in the serotoli cells and the vas deferens derived from the mesonephric tubules connects the seminiferous tubules with the wolffian duct.
Describe ovarian development.
8-20 weeks:
1) . Any primitive medullary sex cords degenerate and are to be replaced by the well vascularised ovarian stroma.
2) . The cortical cells proliferate and mesenchymal condensations later develop around the arriving primordial germ cells.
3) . Small cell clusters form around the primordial germ cells which will be oogonium, yielding to primary follicles.
4) . Without the medullary cords the oocytes move to the surface of the ovary.
5) . Initially development is SRY dependent but further development in the ovary is dependent on the population of normal germ cells that are available.
What happens to the oogonia between the 2nd and 7th month of development?
In the human embryo the thousand or so oogonia divide rapidly between the 2nd and 7th month of gestation to form approximately 7 million primordial germ cells.
After the 7th month of development the number of germ cells drops precipitously. Most oogonia die during this period while the remaining oogonia enter the first meiotic division. These later cells called the primary oocytes progress thought the first meiotic prophase into the diplotine stage at which point they are maintained until puberty.
At the onset of adolescence groups of oocytes periodically resume meiosis.
List some methods of asexual reproduction.
Binary fission (amoeba, paramecium),
Budding (yeast, cactus),
Fragmentation (starfish, several plants),
Parthenogenesis (several fish and amphibians and reptiles).
