CAPE BIOLOGY UNIT 1MODULE III

TEACHINGPROJECTGROUP 5

GROUP MEMBERS:Renee AndersonJunior RobertsDaniek Wallace

GAMETOGENEIS

This is the formation of gametes, or reproductive cells such as ova or sperm.

The production of sperm is called spermatogenesis.

The production of ova (eggs) is called oogenesis.

Both processes involve meiosis.

MEIOSIS

Meiosis is the fundamental process underlying sexual reproduction. It involves two essential outcomes:

1. Reduction Division: the process in which each gamete receives a haploid set (n23) of chromosomes and genes(haploid set is one set of half the chromosomes from each parent.

The diploid number (2n23) is restored on fusion of two gametes(diploid number two set of half the chromosomes from each parent).

2. Rearrangement of genes on the maternal and paternal chromosomes.

This ensures that the offspring are genetically different from one another.

Meiosis involves four main events, and two cell divisions. (In the diagram shown on next slide one pair of homologous chromosomes is shown, to represent 23 pairs in humans).

1

Replication

2

Pairing andCrossing over

4

Separation of chromatids

 

3

Separation of chromosomes

MeiosisI MeiosisII

Each chromosome has two chromatids

(2n; 4c)

Homologouschromosomes(2n; 2c)

DNA replication precedes meiosis, and occurs in the S phase, as in all cell divisions. Recall that the chromosomes in the parent cell contains a diploid set of chromosomes

Pairing of homologous chromosomes and crossing over of chromosome segments occur during prophase of meiosis. They are crucial event in meiosis.

Separation of chromatids occurs as a result of the second meiotic division, and give rise to four daughter cells, each containing a haploid set of chromosomes (1n; 1c) amount ofMeiosis occurs in the germ cells. As a result of meiosis, four daughter cells or gametes, are produced, each containing one of a pair chromosomes and all containing different chromosomes.

Separation of chromosomes occurs as a result of the first meiotic division. The two resulting daughter cells each have one of a pair of replicated chromosomes, or a haploid set (n) with a total of 2c DNA.

Genetic Imprinting- In all diploid cells of an individual the chromosomes occur in homologous pairs. One chromosome of each pair is derived form the mother and father. The maternal and paternal chromosomes are morphologically indistinguishable but have important functional implications because the expression of some genes is dependent on whether they are on the mother or father chromosomes. This is termed genetic imprinting.

Homologous chromosomes

Paternal & maternal

SPERMATOGENSIS The production of sperm takes about 70 days. The

seminiferous tubule has wall with an outer layer of germinal epithelial cells (cells in the outer layer of the organs that produces gametes).

Spermatogenesis is the process by which male

primary cells undergo division, and produce a number of cells termed spermatogonia, from which the primary spermatocytes (diploid) are derived.

Each primary spermatocyte divides into two secondary

spermatocytes(haploid), and each secondary spermatocyte into two spermatids or young spermatoza.

These develop into mature sperm cells.

Between the rows of developing cells are large Sertoli cells stretching from the outer layer of the tubule to the lumen. Spermatocytes become embedded in the many infoldings of the cell surface membrane of the Stertoli cell and develop into spermatids then maturing sperms

SPERMS

The head of the sperm contains the nucleus, which contains the haploid number of chromosomes. It also contains the acrosome, a large lysosome which contains hydrolytic enzymes which penetrates the layer that surrounds the egg(fertilization).

HORMONAL CONTROL OFSPERMATOGENESIS

Spermatogenesis is controlled by the hypothalamus and the anterior pituitary gland.

The hypothalamus secretes gonadotrophin-releasing hormone (GnRH).This hormone travels in a small vein from

the hypothalamus to the pituitary gland. GnRH then stimulates the pituitary gland to secrete two

hormones known as gonadotrophins (this is a hormone that stimulates a gonad, in this case the testis).

The two gonadotrophins are follicle stimulating hormone (FSH) and luteinising hormone (LH).

FSH stimulates spermatogenesis by stimulating Sertoli cells to complete the development of sperms from spermatids.

LH stimulates the synthesis of the hormone testosterone by the Leydig cells (interstitial cell) of the testis. This is known as interstital cell stimulating hormone (ICSH) in the male.

NEGATIVE FEEDBACK

Testosterone is a steroid hormone and is made from cholesterol. Testosterone stimulates growth and development of spermatozoid to form sperm, and also works with FSH to stimulate the Sertoli cells. A negative feedback mechanism operates whereby an increase in the level of testosterone results in a decrease in secretion off GnRH from the hypothalamus. This results in declining levels of LH and FSH.

INHIBIN

The Sertoli cells secrete inhibin which is involved in the negative feedback control of sperm production. If

spermatogenesis proceeds too rapidly, inhibin is released. It inhibit the pituitary gland which reduces the secretion of FSH. If spermatogenesis is low, inhibin is not secreted and

FSH stimulates spermatogenesis.

Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the anterior pituitary to secrete two hormones.

Follicle-Stimulating Hormone (FSH) acts on Sertoli cells, which nourish developing sperm.

Sertoli cells also secrete Inhibin, which reduces FSH secretion by negative feedback.

Luteinizing Hormone (LH) acts on Leydig cells, which produce androgens, chiefly testosterone.

Testosterone regulates production of the brain hormones by negative feedback.

Oogenesis

This the production of eggs. This process in females begins before birth .During he development of the fetus many oogonia are produced. Oogonia is produce by the first division of the germinal epithelial cells.

Oogonia undergo mitosis and form primary oocytes which remain at prophase of meiosis I throughout childhood.

Primary oocytes are enclosed by a single layer of cells, the granulosa cells (or follicle cells) and form structures known as primordial follicles.

Approximately two million of these follicles exist in the female just before birth, but only about 450 ever develop secondary oocytes which are realesed from the ovary during the menstrual cycle.

During puberty one primordial follicle per month develops into a mature follicle called Graafian follicle. This process is caused by a release of FSH (follicle stimulating hormone).

Within each developing follicle, a primary oocyte starts to develop into a egg. The primordial follicle first becomes a primary follicle as the granulosa cells multiply and form many layers of cells around the primary ooctye.

Cells from the stroma of the ovary form futher layers outside these cells known collectively as the theca. The outer part of the theca contains blood vessels and the inner part secretes female sex hormones, the granulosa cells does the same.

As the primary follicle develops, a fluid is secreted by the granulosa cells which contains oestrogen. A fluid filled space antrum, develops in the follicle this is called the secondary follicle. Oestrogen stimulates the growth of the follicle which matures and is know as Graafian follicle

It consist of a secondary oocyte and the first polar body formed when the primary oocyte divides by meiosis I. The secondary oocyte is haploid. The second meiotic division proceeds as far as metaphase but does not continue until a sperm fuses with the oocyte.

At fertilisation the secondary ooctye undergoes the second meiotic division producing a large cell, the ovum and a secondary polar body. All polar bodies are small cells. The have no role in oogenesis and they eventually degenerate.

HORMONAL CONTROL OF OOGENESIS AND THE MENSTRUAL CYCLE

As in male, the hypothalamus and the pituitary gland secretes GnRH which stimulates FSH and LH. These also stimulates the production progesterone and oestrogen.

When GnRH is release it stimulates FSH which travels to the ovaries. On entering the ovary the FSH molecules fits into a receptor sites in the primordial follicles. They the development of several follicles.

The granulose cells of the developing cell starts to develop oestrogen. Oestrogen is a steroid hormone which is produce more rapidly in the first half of the cycle. It repairs the uterus lining, the endometrium. This is the preparation for the possibility of pregnancy. In the pituitary gland the oestrogen inhibits the secretion of FSH (negative feedback) it prevents the possibility of further follicles being stimulated.

In the pituitary gland the oestrogen inhibits the secretion of FSH (negative feedback) it prevents the posibility of futher follicles being stimulated.

At the midpoint the oestrogen builds up to a high level which triggers the secretion of LH.

LH causes ovulation to take place. Ovulation is the process by which a mature ovarian follicle (Graafian follicle) ruptures and discharges an ovum also known as an oocyte.

The ovulated ooctye is surrounded by a layer known as the zona pellucida and the layer of granulosa cells known as the corona radiata which protects the oocyte up to fertilisation.

The remaining part of the Grafian follicle is stimulated by LH to develop into the corpus luteum.

The corpus luteum continues to produce oestrogen and progesterone. The hormones stimulates the uterus to maintain its thickness and the pituitary gland which inhibits the release of LH.

If fertilisation does not take place the corpus luteum starts to degenerate about 28 days into the cycle. The endometrium breaks down causing menstruation.

NEGATIVE FEEDBACK

Menopause is the permanent cesssation of monthly periods and the end of a woman’s fertility.

This is when the number of follicles declines and they become less sensitive to FSH. Oestrogen level declines and causes the levels of FSH to increase which causes menopause.

The most commonest symptoms are night sweats, random hot flashes, vaginal dryness

SEXUAL REPRODUCTION Sexual Intercourse

Sexual intercourse is other wise known as copulation. This is when the penis is inserted into the vagina.

Erection of the penis is a caused by the increase in blood content of spongy erectile tissue in the penis. In this erect state the penis can be inserted into the vagina where the friction, produced by a rhythmic movement. This increase the stimulation of sensory cells at the tip of the penis.

This leads to reflex contractions of the muscles in the epididymis and the vas deferens. The sperm are thus moved by peristalsis along the vas deferens into the urethra. Here they mix with the secretion form the seminal vesicles and Cowper‘s glands, a process called ejaculation.

Sexual excitement also causes the clitoris of the female to become erect. The female orgasm involves the muscular contractions of both the vagina and the uterus.

The secretions of the male accessory glands are alkaline and decrease the normal acidity of the vagina to pH 6-6.5 which is optimum pH for sperms.

Once sperm is deposited within the vagina, they proceed on their journey into and through the uterus and on up into the fallopian tubes. It is here that fertilization may occur if an "egg" is present ( it is still a secondary oocyte until after completion of meiosis II).

Although sperm can swim several millimeters each second, their trip to and through the fallopian tubes may be assisted by muscular contraction of the walls of the uterus and the tubes. They respond to a chemical attractant (oxytocin and prostaglandins) produced by the egg or the tissues surrounding it and chemical in the semen respectively.Sperm may reach the egg within 15 minutes of ejaculation. An average human ejaculate contains over one hundred million sperm, but only a few dozen complete the journey. And of these, only one will succeed in fertilizing the egg.

FERTILISATION

Fertilisation is the fusion of the sperm nucleus with the egg nucleus to form a zygote. This occurs in the fallopian tubes.

STAGES OF FERTILISATION

The enzymes releases by the acrosomes of the many sperm digest a path through the material that holds the granulosa cell together.

After penetrating the granulosa cell it reaches the zona pellucia which has special receptors to which the sperm heads can bind.

The enzyme (arosomes) cause the sperm to move freely through the membrane of the egg, where it fuses with the nucleus of the egg.

After this fusion, the lysosomes known as cortical granules release enzymes which cause the zona pellucida to thicken and harden forming the fertilisatoin membrane (cortical reaction).

The enzymes also destroy the sperm receptor sites, so sperm can no longer bind to the zona pellucida.

The Effect of Fertilisation When fertilisation occurs, the zygote

develops into a ball of cells called the blastocyst which embeds itself into the wall of the uterus. The trophoblastic cell (outer cells of blastocyst) secrete a hormone called human chorionic gonadotrophin (HCG). This function pervent the degeneration of the coprus luteum, which is the first sign of pregnacy.

DIAGRAM SHOWING SPERM CLUSTERED AROUND A SECONDARY OCCYTE

IMPLANTATION Following fertilisation, the zygote divides,

(cleavage) mitotically until a hollow ball of cells, the blastocyst.The outer layer of the cells of the blastocyst is called trophblast, develops into the embryonic membranes, the chorion and the amnion.

The chorion develop villi, which grow into the surrounding uterine tissue fro which they absorb nutrients. These villa from part of tissue from which they absorb nutrients. These villi form part of the placenta which is connected to the fetus by the umbilical cord.

The amnion develops as a membrane around the fetus and encloses the amniotic fluid, a watery fluid which protects the fetus by cushioning if from physical damage

http://www.youtube.com/watch?v=ARERGD0neMI

http://www.dnatube.com/video/2076/NurseReviewOrg-Animation-on-Oogenesis

http://www.youtube.com/watch?v=8_rfZ_qj1z4