REPRODUCTION IN ORGANISMS

 REPRODUCTION IN ORGANISMS

§  Life Span :Period from birth till natural death. Every organism live only for a certain period of time. 

Reproduction –Producing young-ones of their kind, generation after generation. Two types:

·  Asexual reproduction :single parent capable of producing offspring. also known as somatogenic reproduction

·   Sexual reproduction :two parents are involved in producing offspring.

Asexual Reproduction	Sexual Reproduction
Single parent involved	Two parents involved
Fusion & formation of gametes is absent	Fusion & formation of gametes is present
Only mitotic divisions	Involves both (mitosis and meiosis)
Progeny genetically identical (clones)	Progeny with variations (Offsprings.)

Modes of Asexual Reproduction:

§ Binary fission: parent body divides into two halves, genetically identical to parent. Eg.-Amoeba, Paramecium.

§ Multiple fission: parent body divides into many daughter organisms. Eg. Plasmodium.

§ Budding: daughter organisms grow from small buds arising in parent body. eg. Hydra, Yeast.§ Gemmule in sponge.

§ Conidia: non-motile, exogenous spores in chains eg. Fungi (Penicillium).

§ Zoospores: microscopic, haploid, motile structures eg. Algae (Chlamydomonas).

§ Sporulation : Under unfavourable conditions, When the products of  multiple  fission  become individually surrounded by resistant coats, the cyst walls before their release from the parent,  the  process is known as sporulation and the encysted products are termed spores. Eg- Amoeba

§ Fragmentation : Filamentous organism divided into two or more individuals eg. Spirogyra, Spirulina.

§ Regeneration: it is the process of renewal, restoration and growth. Eg. Hydra, Planarian flatworm, lizard tail & human liver.

Vegetative Reproduction in plants

§  Vegetative reproduction frequently used instead of asexual reproduction, units of vegetative propagation called vegetative propagules. Eg.- runner (Grass, Oxalis), rhizome (ginger), sucker (Mentha), tuber (Potato), offset (Eichhornia), bulb (Onion), Bulbil (Agave).

Sexual Reproduction : PHASES OF LIFE SPAN.

§  Juvenile phase: The phase of growth before reproductive maturity. In plants- Vegetative Phase.

§  Reproductive phase: Reproductive maturity.

§  Senescent phase: Phase between reproductive maturity and death. 

Special Flowering : Bamboo- once in life, generally after 50-100 years.

      Strobilanthus kunthiana (Neelakuranji) – flowers once in 12 years, (In Sep-Oct 2006,blue stretched mass flowering attract tourists in Kerala, Karnataka and Tamil Nadu).

The main events of sexual cycle are: Pre-fertilization, Fertilization & Post Fertilization.

A.  Pre-fertilisation events:

a.     Gametogenesis: The process of formation of male and female gametes by meiosis (cell-division)

.·       Homogamete (Isogamete): - gametes similar eg. Algae

·       Heterogamete (anisogamete): - morphologically dissimilar gamete, male gamete (antherozoid or sperm), female gamete (egg or ovum) eg. Human.

·       Sexuality in organisms: In plants Bisexual term is used for Homothallic and Monoecious plants

·       Monoecious: Reproductive organs at different positions in same plant eg. Chara, Maize.·       Hermophrodite : Reproductive organs at different positions in same animal eg. Earthworm.

·       Unisexual term used for Heterothallic and Dioecious plants.

·       Male and female reproductive-structure on different plants.

·       Flowering plants: male flower (staminate flower) & female flower (pistillate flower) eg. papaya and date, palm.

·       Animals – Bisexual term is used for Hermaphrodite animals-eg. Earth-worm, Tape-worm, Leech,

·       Unisexual animals have male & female sexes in separate individuals-e.g. insects, frogs, human beings

Cell division during gamete formation:

Haploid-parent (n) produces haploid gametes (n) by mitotic division, eg. Monera, fungi, algae and bryophytes.

Diploid parent (2n) produces haploid gametes (n) by meiosis division (possess only one set of chromosomes) and such specialized parent cell is called meiocyte or gamete mother cell (2n).

·       Example-

Name of organism	Meiocyte (2n)	gamete          (n)
Human	46	23
Housefly	12	6
Ophioglossum (fern)	1260	630
Potato	48	24

b)  Gamete transfer:- to facilitate fusion.

·    Male gametes - motile & female- non-motile, exception few fungi and in algae.

·    Gamete transfer through water : algae, Bryophytes, Pteridophytes & amphibians.

·    Higher plants, pollen-grains are transferred by pollination. 

B.  Fertilization : fusion of male and female gamete.

·    Syngamy : Process of fusion of male and female gamete.

Two type of Fertilization: external and internal . 

EXTERNAL FERTILISATION	INTERNAL FERTILISATION
Fertilization takes place outside the body	Fertilization takes place inside the body
External medium is required- water	No external medium required
Large no. of gametes produced.	Lesser no. of gametes produced.
Offsprings –less protected	Offsprings- well protected.
Eg. Algae, fish, amphibians	Gymnosperm, angiosperm, aves, mammals.

C.  Post -fertilization events- formation of zygote and embryo.

Zygote. Single celled , diploid, vital link between two generations. In fungi & algae, zygote develops a thick walled that is resistant to dessication and damage.

Haplontic life cycle- zygote (2n) divides by meiosis to form haploid (n) spores.

Diplontic life-cycle- zygote (2n) divides mitotically, develops into embryo (2n).

Embryogenesis: development of embryo from zygote by cell division (mitosis) and cell differentiation.

  On based of Zygote development, animals are grouped into two types-

 

Oviparous Animals	Viviparous Animals
They ley fertilized or unfertilized egg.	They give birth to young ones.
Fertilized eggs have calcareous shell	No calcareous shell
Development of embryo outside female body	Development of embryo inside female body
Less chance of survival.	More chance of survival.
Eg. Reptiles and Birds	Eg. Mammals

 

Cell- division increases the number of cells in the developing embryo

Cell differentiation - groups of cells undergo certain modifications for the formation of different kinds of tissues and organs.

In flowering plants- zygote formed inside ovule.

Changes occur in flowering plants:

Sepal, petals, stamens,	Fall off
Zygote	Embryo
Primary endosperm nucleus	Endosperm (3 N)
Synergid, antipodal cells	Disintegrate
Ovary	Fruit
Ovule	Seed
Ovary wall	Pericarp (epicarp + mesocarp + endocarp)
Outer Integument	testa
Inner Integument	tegmen

Parthenogenesis: Female gamete develops into new organism without fertilization . eg- Honey bee, turkey, lizard, rotifers (Protozoans).

Seedless fruits formed by parthenogenesis

Clone: A group of individuals of the same species that are morphologically and genetically similar to each other & their parents.

Chapter 1: REPRODUCTION IN ORGANISMS QUESTIONS

Q) What is meiocyte?

Ans) It is the gamete mother cells which undergoes meiosis.

Q) Why is date palm referred to as dioecious?

Ans) In date palm the male and female flowers are present in different plants.

Q) What is special in flowering in bamboo?

Ans) Bamboo flowers only once in their life time generally after 50-100 years .

Q) Define clone?

Ans) It is a group of individual of the same species that are morphologically and genetically similar.

Q) What is vegetative propagule?

Ans) The units of vegetative propagule that gives rise to a new plant is called vegetative propagule. Examples: Runner and rhizome.

Q) Diagrammatically represent asexual reproduction in yeast?

 Ans) See Figure 1.2 (page-5)

Q) Cucur bit are called monoecious. Justify?

Ans) They have male and female reproductive structures in different flowers.

Q) If chromosomes number in meiocyte in rat, elephant and rice is 42,56,24 what will be the chromosomes number in their gamets?

Ans) Rat 21, elephant 28,rice 12

3 marks

Q) Why higher organisms have resorted to sexual reproduction in spite of its complexity? Ans) 1. It brings about variations.

2.  Variation helps in better adaptation in nature.

3.  Variation is the basis of evolution.

Q) What is parthenogenesis. Give Example?

Ans) Development of a new individual from an unfertilized egg is called parthenogenesis. It is simpler easier and rapid mode of replication. But there is no chance of any variation. So it does not play any role in evolution. Eg.- Honey bee and some lizards

Q Mention the reason for difference in ploidy of zygote and primary endosperm nucleus in angiosperm Ans Zygote is formed by fusion of haploid male gamete and haploid egg so it is diploid.

Whereas the PEN is formed by fusion of haploid male gamete with two haploid polar nuclei so it is triploid.

5 Marks.

Q) What are vegetative propagules. Name any four of them and give example? Eg.- rhizome of ginger, leaf bud of bryophyllum, bulb of onion, offset of pistia.

HOTS.

Q)Amoeba is immortal. Explain?

Ans) Amobea has no natural death. It undergoes binary fission and forms two new daughter cells.

Q) Which is the first diploid single cell which begins life? 

Ans) Zygote.

Q What is the fate of zygote in organism which show 1 Haplontic life cycle and Diplontic life cycle

Ans In haplontic life cycle zygote undergoes meiosis during its generation In diplontic life cycle zygote divides mitotically during embryogenesis

Q) What type gamete are formed in staminate and pistillate flowers? 

Ans) Pollen grains and egg cell respectively.

Q) Name the organisms with respect to their sexuality?

i)  Monoecious animal

ii)  Dioecious animal

iii)  Monoecious plant

iv)  Dioecious plant

Ans :- i) Earthworm ii) Cockroach iii) Chara iv) Marchantia.