Gametogenesis Flashcards
Outline sequence of names in Oogenesis
Primordial germ cell 2n
Oogonia 2n (3 of these degenerate)
Primary oocyte 2n (Growth from oogonia)
Secondary oocyte + Polar Body n (Produced from meiosis I of primary oocyte)
Ovum + 3 polar bodies n (Produced from meiosis 2 of previous stage)
Outline sequence of names in Spermatogenesis
Primordial Germ cell 2n
Spermatogonia 2n (Produced from 2 mitotic divisions of primordial germ cell)
Primary spermatocyte 2n (growth from spermatogonia)
Secondary spermatocyte n (meiosis I produces these)
Spermatids n (Meiosis II produces these)
Spermatozoa n ( Differentiation and growth)
Key features of Sperm cell
Acrosome - Sac of digestive enzymes that digests layers around ovum and allows sperm to penetrate egg
Nucleus - Highly condensed haploid chromosomes. Condensed state reduces energy required for transport
Mitochondria - Tightly packed in middle section of sperm. Provides ATP for lashing of tail
Microtubules - These produce whip like movements of tail that keep mature sperm in suspension
Tail - Flagellum which propels sperm in liquid environment
Key features of oocyte
Cytoplasm with many food reserves
Cell surface membrane of oocyte
Haploid polar body, which dies
Zona pellucida - Clear ‘jelly like’ layer
Why are many sperm released during ejaculation?
For sufficient acrosome enzyme activity to break down the zona pellucida and some of the follicle cells. This is so cell surface membrane can be accessed. One sperm doesn’t have sufficient enzymes to fully break down this outer layer.
How is polyspermy prevented?
Initially ion channels cause the inside of the ovum to change it’s charge from negative to positive. This prevents further sperm entering temporarily.
Fertilisation membrane then forms. This is due to cortical granules from oocyte being released, which combines with the Zona pellucida to form the membrane
How are the chromosomes actually mixed?
Head of the sperm enters the oocyte. The tail region is left outside. The head absorbs water and swells. Chromosomes are released which fuse with ovum and form diploid zygote.
Outline gametogenesis in plants
Two phases
The sporophyte generation is diploid and produces spores by meiosis
The gametophyte generation which happens after produces haploid gametes through mitosis
In flowering plants, both of these phases have been combined into 1 plant
Outline formation of pollen
Each anther contains 4 pollen sacs where pollen grains develop
In each pollen sac there are large numbers of diploid microspore mother cells.
These microspore mother cells divide by meiosis to form haploid microspores(Gametophyte generation)
Gametes are then formed from haploid microspores by meiosis
The haploid microspores consist of a tube nucleus and a generative nucleus. The pollen microspore also has a thick wall
A further division occurs where the generative nucleus divides by mitosis after pollination.
Therefore final pollen grain on stigma will have 3 nuclei in total. 1 tube and 2 generative
Basically from microspore mother cell, Meiosis I and II followed by mitosis
Outline formation of egg cell in plants
megaspore mother cell 2n
Meiosis produces 4 haploids cells. 3 of these degenerate and 1 grows.
Megaspore left then divides by mitosis 2 times
This consequently produces a mature embryo sac
This embryo sac consists of 3 antipodal cells at the top. 2 Polar nuclei in the middle. 2 synergids and 1 female gamete at the bottom
Basically meiosis I and II followed by 2 mitotic divisions
Outline fertilisation in plants
Pollen grain lands on surface of stigma. Pollen grain interacts with stigma
Tube nucleus produces pollen tube which travels through style into ovary and then ovule.
Two generative nucleus due to mitosis. This leads to double fertilisation.
One nuclei fuses with 2 polar nuclei to form 3n endosperm
Other nuclei fuses with female gamete to produce 2n diploid zygote
What is the role of the endosperm
Involved in supplying the embryo plant with food when it begins to germinate
What happens to the zygote
Zygote is said to be totipotent. Can differentiate into any type of cell
Cleavage is first stage of process. Special kind of mitosis where cells divide repeatedly without normal interphase for growth between divisions
Cleavage occurs as the embryo travels down through the oviduct
This produces a mass of small identical, undifferentiated cells in shape of a sphere. This is a blastocyst.
These embryonic stem cells can differentiate into many different types of cell.
Outer layer of blastocyst is totipotent. Inner layer is said to be pluripotent.
What is the role of the synergids
Guidance of pollen nuclei to targets. 2 polar nuclei and the female gamete.
What is the role of the antipodal cells
Main role is nutrition. Nourishes the embryo sac and allows for growth of the endosperm. Large amount of starch, lipids and proteins. These are used up by the developing endosperm and embryo
