Revision:Sexual Reproduction

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Sexual Reproduction

Genetic Variation

Due to

• Random alignment of bivalent at metaphase at the equator
• Formation of chiasmata
• Meiosis .. shuffles the genes further more

Spermatogensis

• Occurs in seminiferous tubules
• Stored in epididymis

Process

1. Diploid Spermatogonia divide by mitosis from germinal epithelium
2. Some of them grow to produce diploid primary spermatocytes
3. Diploid spermatocytes undergo first meiotic division to form 2 haploid secondary spermatocytes
4. Haploid 2ndary spermatocytes undergo 2nd meiotic division to form 2 haploid spermatids
5. These grow in shape and become spermatozoon
6. Sertoli cells provide nutrition and protection against body’s immune system

Oogenesis

At Birth

1. Diploid cells in ovary divide by mitosis to form oogonia
2. Oogonia undergoes 1st meiotic division to from primary oocyte but stops at prophase 1
3. These remain in the follicles

At Puberty

1. Primary oocyte complete the 1st meiotic division to form polar bodies and secondary oocyte
2. 2ndary oocyte undergoes 2nd meiotic division and stops at metaphase 2
3. 2ndary oocyte is shed from ovary, if fertilized completes its 2nd meiotic division to form ovum and polar bodies

Menstrual Cycle

1. Day 1 and 2 Anterior Pituatory gland releases FSH and LH
2. These hormones travel by blood and reach the ovary
3. They stimulate the granulose cells to secrete oestrogen
4. Oestrogen thickens endometrium and inhibits FSH and LH
5. Day 12 LH level rises
6. It stimulates granulose cells to stop producing oestrogen and start releasing progesterone
7. Causes ovulation
8. At day 14, 2ndary oocyte bursts out of ovary
9. Granulosa cells filled with yellow substance to form corpus Leutem
10. Corpus Leutem secretes progesterone
11. Endometrium thickens
12. Inhibits FSH and LH
13. Drop in FSH and LH stops progesterone and oestrogen level going up because Granulosa cells are no longer stimulated
14. Endometrium no longer thickened and lack of prog and oest stimulate the anterior pituatroy gland to release FSH and LH
15. Cycle begins again

Remember the 2 graphs very well (it came in even unit 4 Jan 2006)

Y axis --- hormones X axis Time

Upper level line is – LH

Lower level line is – FSH

At day 14 LH peaks out

For the graphs of Oestrogen and Progesterone

Oestrogen is first then progesterone (level)

Adaptations

Xerophytes

1. thick Cuticle
2. Rolling of leaves
3. Stomata in sunken pits and grooves
4. Presence of epidermal hair
5. Reduction in no. of leaves
6. Succulence

Hydrophytes

There are two types

Totally sunken plants –

1. No stomata
2. No cuticle
3. Small leaves
4. Lacunae
5. Reduced root system
6. Reduced vascular support system

Partially submerged –

1. Stomata on upper surface
2. Thick mesophyll layer
3. Elongated lignified cells to prevent rolling

Adaptations of invertebrates

1. Large surface area to body ratio
2. Flattened bodies
3. External gills
4. Siphons
5. Mantle cavity
6. Waving tails
7. Respiratory pigments
8. Can form air bubbles

Comments

• Suitable for: Edexcel Biology A Level modules 2, but also other exam boards.
• Written by: =Keith=
• From this thread