Meiosis & Double Fertilisation

Question 1

What is Mitosis?

Answer 1

• Mitosisproduces genetically identical cells – clones of each other. Diploid ( 2n)
• Mitosisoccursin Meristems

Question 2

What is Meiosis?

Answer 2

• Meiosisoccursinanthers and ovaries and produces sex cells -male and female gametes with half the amount of DNA than the parent cells (one copy of DNA – haploid = 1n)
Sperm (male) egg ( female)
• Meiosisisasourceof genetic variation

Question 3

What is cross pollination?

Answer 3

• Cross pollination or outcrossing is what happens when pollen from one plant lands on the stigma of another plant of the same species.

Question 4

What does the Anther do?

Answer 4

1. Four haploid cells produced by meiosis (Microspores)
2. Mitosis of each microspore creates two nuclei within developing pollen grain
   Tube Nucleus (vegetative) will form pollen tube Generative nucleus (reproductive) – will divide again by
   mitosis to create two sperm either in the pollen grain or Later in the pollen tube just before fertilisation.
   So the tube nucleus produces a pollen tube that grows down
   the style of the ovary and delivers the two sperm to the egg.

Question 5

What is the Ovum, Egg and Egg Sac?

Answer 5

1. Four haploid cells produced by meiosis (megaspores)
2. Three of the four degenerate, leaving one haploid megaspore.
3. The megaspore divides by mitosis three times to produce eight (8) haploid nuclei. Five of these are involved in Double Fertilisation.

Question 6

What happens when double fertilisation occurs?

Answer 6

Double fertilisation is unique to flowering plants and, for each seed that results, involves the delivery of two haploid sperm (male) by pollen tube to the ovary. One fuses with the egg or ovule (female) to produce the 2n (diploid) zygote which grows into the embryo & the other fuses with the two haploid polar nuclei (female) to produce the 3n (triploid) nutritive endosperm tissue.

Question 7

Overview Of Double Fertilisation?

Answer 7

• Double fertilisation is unique to flowering plants
• In the second fertilisation event a sperm fertilises the two polar nuclei to create a triploid (3n) nutritive tissue for the germinating embryo.
( seeds) this endosperm is present in monocot seeds but is usually absorbed into the cotyledons in dicots by the time the seed matures.

Question 8

Double Fertilisation Explained

Answer 8

Pollen tube delivers two haploid (n) sperm to the egg sac, one fuses with the egg to produce a diploid (2n) zygote which grows into the embryo, the other fuses with the two polar nuclei to produce the triploid (3n) endosperm, nutritive tissue for the germinating seed.