Sexual reproduction is a process that creates a new organism by combining the genetic material of two organisms. It occurs both in eukaryotes and prokaryotes: in multicellular eukaryote organisms, an individual is created anew; in prokaryotes, the initial cell has additional or transformed genetic material. In a process called genetic recombination, genetic material (DNA) originating from two different individuals (parents) join up so that homologous sequences are aligned with each other, and this is followed by exchange of genetic information. After the new recombinant chromosome is formed, it is passed on to progeny.
Sexual reproduction is the primary method of reproduction for the vast majority of macroscopic organisms, including almost all animals and plants. The evolution of sexual reproduction is a major puzzle. The first fossilized evidence of sexual reproduction in organisms such as eukaryotes is in the Stenian period, about 1 to 1.2 billion years ago.There are two main processes during sexual reproduction in eukaryotes: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over to achieve homologous recombination. Evolutionary thought proposes several explanations for why sexual reproduction developed and why it is maintained. These reasons include fighting the accumulation of deleterious mutations, increasing rate of adaptation to changing environments(see the red queen hypothesis), dealing with competition (see the tangled bank hypothesis) or as an adaptation for repairing DNA damage. The maintenance of sexual reproduction has been explained by theories that work at several different levels of selection, though some of these models remain controversial. New models presented in recent years, however, suggest a basic advantage for sexual reproduction in slowly reproducing, complex organisms, exhibiting characteristics that depend on the specific environment that the given species inhabit, and the particular survival strategies that they employ.
1. What is pollination?
- The transfer of pollen grains from anther to stigma of the gynoecium is called pollination.
2. What is fertilization?
The fusion of male and female gametes is called fertilization.
3. What is thalamus ?
- The upper swollen part of the pedicel is called thalamus.
4. Describe flower and its parts ?
- There are Four parts in flower. They are
1. Sepals (calyx)
2. Petals(corolla)
3. Androecium
4. Gynoecium
Sepals (calyx):
Sepals are green in colour. It is the first whorl of the flower. sepals given protection to the flower.
Petals (corolla):
It is the second whorl of the flower these are usually bright in colour (red,yellow) which helps to attract insects for cross pollination.
Androecium:
It is third whorl of the flower. It is consist of stamens. each stamens . is consist of two parts called anther and filament. Another consist of pollen grains which are formed by pollen mother cells.
Gynoecium:
It is fourth whorl of the flower. it is also known as pistil. It consist of three parts called stigma, style and ovary. Inside the ovary, future seeds known as ovules are present
5. Describe the structure of ovule or explain how female gametes are formed?
1. Ovule is a female gametes that develops from a cushion like part of the ovary called placenta.
2. Ovule is connected to the placenta through stalk like structure called funicle.
3. The tissue enclosed inside the ovule is called nucellus.
4. Ovule is covered with layer called integument, they are outer integuments, and inner integuments,.
5. Two integuments, leave a small pore known as micropile.
6. Basal part of ovule where two integuments, arise is known as "chalaza"
7. From the cells of the Nucellus one cell differentiates as megaspore mother cell which is diploid.
8. The megaspore mother cell divides by meiosis to from four megaspores out of which only are develops as embryo sac. This is haploid and also called female 'gametophyte'.
9. Inside the ovule the megaspore cell divides mitotically three times and from '8' nucleus which will be total '7' cells arranged in three groups in mature embryo sac.
11. They are 'one' Egg(female gamete)" two " synergids one central cell called secondary nucleus and 'three' antipodals. While all the cells are in haploid conditions ('n') only the secondary nucleus is diploid ('2n').
Fill in the blanks
1. Floral structure arise on a swollen part of pedicel known as ________
2. The '3n' nucleus is formed from fusion of ___________ nucleus with male nucleus inside embryo sac.
3. Root part is represented in mature embryo by ___________
4. The first diploid condition in embryo sac after fertilization is seen in _________
5. Carpels are present in __________
6. The diploid cell inside the nucleus that undergo meiosis to given rise to embryo sac is known as ________
7. The flower with stalk is called _______
8. The flower without stalk is called _______
9. The study of pollen grains are called _______
10. Gynoecium is also known as _______
11. The cushion like pad present in the ovule is called _______
12. Stamen is consist of _____and _____
13. Synergids is also known as ______helper cells
14. Pollen tube have ______nuclei
15. Endosperm is ________ in state
ANSWER
1. Thalamus
2. Secondary
3. Radicle
4. Zygote
5. Pistil /gynoecium
6. Mega spore mother cell
7. Sessile flower
8. Pedicellate flower
9. Palynology
10. Pistil
11. Placenta
12. Anther and filament
13. Helper 14. Two male
15. "3n" triploid
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