INTRODUCTION TO SEXUAL REPRODUCTION
Sexual reproduction is a mode of reproduction in which an offspring is formed by two parents (different individual of opposite sex).
Sexually reproducing organisms are needed to reach certain stage of growth and maturity before reproducing.
Despite several variations in morphology and internal structure organisms show same pattern of life or we can say that there are 3 stages of life in every organisms .
1. Juvenile phase
2. Reproductive phase
3. Senescent phase
This is the stage of growth and development that is the period between end of infancy and onset of adulthood. In plants this period of growth is known as vegetative phase. The duration of this varies in different organisms. In this phase organisms cannot reproduce offspring
Juvenile phase is followed by Reproductive phase. In reproductive phase, organism are capable of reproducing offspring. In different organism, onset of reproductive phase is marked by certain characters.
In plants it is marked by:
1. In flowering plants, appearance of flowers (flowers are reproductive organ of plants) .
2. In non – flowering plants, formation of cones which produce seeds.
In animals, it is marked by:
1. Development and maturity of sex organs.
2. Development of secondary sexual characters.
In animals, reproductive phase is also marked by some cyclic changes. They are:
1. Oestrus cycle: Cyclic changes during reproductive phase in non – primates in known as oestrus cycle.
2. Menstrual cycle: Cyclic changes during reproductive phase in primates is known Menstrual cycle
On the basis of the duration of reproductive phase plants are divided into 2 categories.
1. Monocarpic plants: Plants that flowers once in its life time. Example: Bamboos, wheat etc.
2. Polycarpic plants: Plants that flowers at regular intervals.
On the basis of the duration of reproductive phase animals are divided into 2 categories.
1. Seasonal breeders: Reproduce of particular time of year; E.g. birds, lizards, frogs
2. Continuous breeders: Reproduce throughout their sexually maturity stage.
SENESCENT PHASE: It is the stage between end of maturity and death of organism. This phase is characterised by gradual ceasation of functional activities and cellular breakdown leading to the death of organism.
CHAHARECTERISTICS OF SEXUAL REPRODUCTION
1. It involves both parents (biparental).
2. Fusion of gametes (fertilization) takes place.
3. Cell division involves both meiosis as well as mitosis.
4. Offspring are not identical to parents.
5. It is relatively slower mode of reproduction.