25. Sexual reproduction. Gametogenesis. Fertilisation. Not typical forms of sexual reproduction. Sexual reproduction:

Question 1

Sexual reproduction:

Answer 1

Sexual reproduction:

Involves:

1. Meiosis - production of gametes
2. Fertilisation - involves fusion of gametes and increase of initial amount of chromosomes

Question 2

Gametogenesis:

Answer 2

Gametogenesis:

• Formation of ova or sperm (oogenesis/ spermatogenesis)
• spermatogenesis occurs in seminiferous tubules of the testes - during puberty
• spermatogonia produce primary spermatocytes, each of which undergoes meiosis I to produce 2 secondary spermatocytes
• each secondary spermatocyte undergoes meiosis II to form 2 spermatids which mature without further division
• the process starts in the wall and along with maturation they move towards the lumen of the seminiferous tubules
• Sertoli cells synchronise spermatogenesis

Question 3

Oogenesis - most occurs during fetal life

• oogonia produce primary oocytes, which initiate meiosis I - around 3rd month
• primary oocytes are arrested in prophase I in stage diplotene, until the female reaches sexual maturity
• with ovulation, meiosis I is completed with the production of a secondary oocyte + first polar body
• Meiosis II isn’t completed until fertilisation, when it becomes the ovum and second polar body is released
• in this way, female meiosis produces only 1 gamete and 3 polar bodies, in comparison to 4 of spermatogenesis

Answer 3

Oogenesis - most occurs during fetal life

• oogonia produce primary oocytes, which initiate meiosis I - around 3rd month
• primary oocytes are arrested in prophase I in stage diplotene, until the female reaches sexual maturity
• with ovulation, meiosis I is completed with the production of a secondary oocyte + first polar body
• Meiosis II isn’t completed until fertilisation, when it becomes the ovum and second polar body is released
• in this way, female meiosis produces only 1 gamete and 3 polar bodies, in comparison to 4 of spermatogenesis

Question 4

Fertilisation:

• when sperm encounters ovum - undergoes capacitation and acrosomal reaction (hyluronidase and acrosin)
• to penetrate the layers of granulosa cells and zona pellucida
• first sperm to reach is not the one to fertilise the egg, since hundreds may be needed to clear the path
• finally, sperm binds to the zona pellucida + releases its enzymes to reach the eggs entering the head only

Answer 4

Fertilisation:

• when sperm encounters ovum - undergoes capacitation and acrosomal reaction (hyluronidase and acrosin)
• to penetrate the layers of granulosa cells and zona pellucida
• first sperm to reach is not the one to fertilise the egg, since hundreds may be needed to clear the path
• finally, sperm binds to the zona pellucida + releases its enzymes to reach the eggs entering the head only

Question 5

Not typical forms of sexual reproduction: Autogamy - self fertilisation (hermaphrodites)
Internal fertilisation - sperm fertilise egg in internal environment

External fertilisation - release egg and sperm into external environment, sperm fertilise egg outside organism
Allogamy - fertilisation of egg from one individual, from a sperm of another

surrogacy IVF

Answer 5

Not typical forms of sexual reproduction: Autogamy - self fertilisation (hermaphrodites)
Internal fertilisation - sperm fertilise egg in internal environment

External fertilisation - release egg and sperm into external environment, sperm fertilise egg outside organism
Allogamy - fertilisation of egg from one individual, from a sperm of another

surrogacy IVF