OCR a Level Biology B Learner Resource - Gametogensis Fertilisation Sorting

Learner Resource 2

This activity can be used to reinforce learning of gametogenesis and the function of hormones in both males and females. The boxes can be cut and learners then asked to put them in the correct sequences – a column for males, a column for females which join at fertilisation to show the events leading to implantation.

Females

Oogonia (diploid) are present in the ovaries of a fetus before birth, held at prophase I of meiosis.

At puberty, Gonadotrophin releasing hormone (GnRH) is released into the blood. It stimulates the release of LH and FSH from the anterior pituitary gland.

FSH and LH levels rise in the bloodstream binding to follicle cells in the ovary which are stimulated to produce oestrogen.

One follicle becomes dominant and the primary oocyte (diploid) it completes meiosis I to produce a secondary oocyte (haploid).

The dominant follicle, known as the Graffian follicle secretes increasing amounts of oestrogen, which initially inhibits FSH and LH but then stimulates a surge of LH causing ovulation – the dominant follicle releases the secondary oocyte.

After ovulation, the secondary oocyte proceeds to the end of meiosis II to produce a mature ovum which passes into the oviduct. The follicle remains in the ovary as the Graffian follicle and secretes progesterone.

Version 1 1 © OCR 2016 Fertility and assisted reproduction delivery guide Males

At puberty, Gonadotrophin releasing hormone (GnRH) is released into the blood. It stimulates the release of LH and FSH from the anterior pituitary gland.

LH binds to receptors on the Leidig cells which then secrete testosterone. Testosterone affects sertoli cells in the testes, which stimulates spermatogenesis. Excessive production of sperm is prevented by negative feedback – high levels of testeoterone cause the production of inhibin.

During spermatogenesis, spermatogonia (diploid) in the seminiferous tubules divide by mitosis to produce primary spermatocytes (diploid) and then by meiosis I to produce secondary spermatocytes (haploid).

Secondary spermatocytes undergo meiosis II producing spermatids which mature into spermatozoa and are stored as haploid sperm in the epididymis.

At ejaculation, sperm are released into the vas deferens, fluids are added from the seminal vesical and prostate gland. The resulting semen is released through the urethra.

Semen enters the vagina, passing through the cervix, uterus and into the fallopian tube

Fertilisation occurs when the sperm nucleus enters the ovum, meiosis II is completed in the ovum and the nuclei form the sperm and ovum fuse to produce a zygote (diploid).

The zygote undergoes rapid mitosis forming a blastocyst which implants in the endometrium of the uterus and secretes HCG which can be detected in a pregnancy test.

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