Biology Today — May 2017

Stages in Oogenesis

(Multiplication

phase)

(Maturation

phase)

(Growth phase)

Fetal life

mitosis

Oogonia

Primary oocyte

Secondary oocyte

2 nd meiotic division

(completed after

fertilisation)

1 st meiotic division

(begins in utero,

completed prior to

ovulation)

Mitosis

differentiation

Ovum

Childhood

Adult

reproductive

life

First polar

body

Second

polar

body

Puberty

Birth

Chromosomes

number per

cell

Events

46 Cells of germinal epithelium divide to

produce oogonia which multiply by mitotic

divisions forming the primary oocytes.

46 Meiosis begins in the primary oocytes soon

after their formation. However, the oocytes

are arrested in the meiotic prophase I. This

is the first resting stage. They undergo a

round of DNA synthesis, and chromosome

pairing takes place. There is accumulation

of food materials and other resources for

nourishment of the oocyte.

23 Primary oocyte continues meiosis I and

produces first polar body and secondary

oocyte.

The secondary oocyte proceeds with meiosis

II but gets arrested in metaphase II stage.

This is the second resting stage.

Only those primary oocytes destined for ovulation complete the first meiotic division which occurs just before the mature

follicle is ovulated. The second meiotic division occurs in Fallopian tube after ovulation, but only if the secondary oocyte is

fertilised, i.e., penetrated by a sperm.

Hormonal Control of Oogenesis

•    Induces rupture of mature

Graafian follicle and release of

secondary oocyte (ovulation).

•    Stimulates development of

corpus luteum which secretes

progesterone.

•    Stimulates growth of Graafian

follicle and development of oocyte

within follicle to complete meiosis I

to form secondary oocyte.

•    Stimulates formation of estrogen.

Hypothalamus

Ovary

Anterior lobe of pituitary

GnRH

FSHLH

Significance of Oogenesis

Formation of one ovum and three

polar bodies.

Crossing over in meiosis I brings

about variations.

Maintains half the number of chromosomes in

ovum (haploid condition) by forming polar bodies.

MPP-1 CLASS XI ANSWER KEY

1. (a) 2. (b) 3. (c) 4. (d) 5. (c)


2. (b) 7. (d) 8. (d) 9. (d) 10. (c)


3. (d) 12. (b) 13. (d) 14. (b) 15. (a)


4. (b) 17. (a) 18. (b) 19. (b) 20. (c)


5. (d) 22. (a) 23. (b) 24. (c) 25. (d)


6. (a) 27. (c) 28. (b) 29. (d) 30. (c)


7. (c) 32. (d) 33. (d) 34. (d) 35. (d)


8. (c) 37. (a) 38. (c) 39. (b) 40. (c)

MARCH 2017

1-c- PLASMOTOMY 2-i- NYCTINASTY

3-a- PELOTONS 4-f-ELECTROFUSION

5-h- CHAPERONES 6-e-FIBRILLATION

7-j- HISTAMINE 8-d- HOLOGYNIC

9-g- TAVISM A 10-b-VITELLOGENESIS

Winners : Joydeep Saha (West Bengal) , Rida Sood (Delhi), Tanmay Kumar (Ranchi),

Gaurang Sodani (Gurugram), Aadvik Ahuja (Nainital)