Sexual Reproduction

Introduction

Reproduction is the formation of new individuals of a species.

It can be achieved sexually or asexually (usually a species will only reproduce by one or other of these methods)

In asexual reproduction there is only one parent. Offspring are produced by mitosis and are clones of the parent, there is, therefore, no genetic variation (except that introduced by genetic mutation) and so no tolerance for changes in environmental conditions.

In sexual reproduction there are two parents each of which produce a gamete. Two gametes (one from each parent) fuse to form a zygote which will undergo repeated mitotic divisions to produce a new mature individual.

Meiosis

Meiosis is reductive - the process starts with a diploid cell and produces haploid cells i.e. the number of chromosomes has been reduced

Meiosis is a two stage process with both stages being divided up (for convenience) into a number of phases:

Stage 1 involves separation of homologous chromosomes

Stage 2 involves separation of chromatids

This description states the key events at each phase of the process

• Prophase 1

bivalents form

chiasmata appears

crossing over occurs

• Metaphase 1

centromeres attach to spindle

• Anaphase 1

homologous pairs separated

so the chromosome number is reduced

• Telophase 1

new nuclear membranes develop

• Prophase 2

does not occur if no interphase as chromosomes are already condensed

• Metaphase 2

chromosomes attach to spindle

Note that the spindle is at right angles to that in metaphase 1

• Anaphase 2

sister chromatids separate

• Telophase 2

new nuclear envelopes develop

Significance of Meiosis

genetic variation - each gamete carries only one copy of each gene

orientation of bivalents in metaphase 1 is random (this is independent assortment of chromosomes)

orientation of chromatids in metaphase 2 is random

Human Reproduction

Structure of the male reproductive system

They are produced in the testes and mature in the epididymis.

When required they pass along the vas deferens (aka ductus deferens or sperm duct) towards the urethra.

As they pass along the vas deferens they mix a with liquid called seminal fluid to form semen which is ejaculated. Seminal fluid is secreted by the seminal vesicles and the prostate gland and provides nutrition and lubrication for the sperm

Structure of the female reproductive system

They are released at a regular rate (of about one a month) into the Fallopian tubes (oviducts) and pass along this tube towards the uterus.

Fertilisation normally takes place in the fallopian tube.

If fertilisation occurs the zygote embeds in the endometrium (wall) of the uterus and the foetus develops.

If fertilisation does not take place the ovum passes out through the vagina during menstruation.

Gametogenesis

In both males and females diploid cells called primordial germ cells multiply, by mitosis, to form cells which then undergo meiosis to produce gametes.

Is the production of spermatozoa.

It takes place in structures called the seminiferous tubules within the testes.

The seminiferous tubules are lined with spermatogonia (germ cells). These multiply by mitosis to form spermatocytes which undergo meiosis

At the end of meiosis a single spermatocyte will have produced 4 spermatids (immature spermatozoa).

Each spermatid then matures to become a sperm

The maturation process is supported by Sertoli cells within the seminiferous tubules (these provide nutrition to the developing sperm).

The mature sperm has three main sections:

a head region - contains the nucleus and the acrosome which is an enzyme-filled structure used to dissolve away the coverings of the ovum allowing the sperm nucleus to enter the ovum.

a midpiece - this is filed with mitochondria which provide the energy needed for movement

a tail region - this provides the propulsion using the energy supplied by the mitochondria

The tail enables the sperm to move through the female reproductive structures towards the ovum. This movement is assisted by contractions of the muscles of the uterus

New spermatogonia are made throughout the male's life. In contrast, by the time a female reaches puberty all of the mitotic activity involved in producing ova, i.e. the production of oocytes, has been completed forever.

The ovaries at puberty contain many primary oocytes surrounded by protective follicular cells forming structures called primordial follicles.

Each month, from the onset of puberty, several primordial follicles begin to mature. Usually only one of these will complete the maturation process and have its ovum released. The remainder will degenerate to form atretic follicles.

The main stages of follicle maturation are:

• primary oocyte (as part of a primordial follicle). This is the stage at which all oocytes exist until puberty
 

Maturation begins

The first meiotic division occurs to form

• secondary oocyte (as part of a secondary follicle). Fluid is secreted into the follicle by the follicle cells resulting in enlargement
 

Further enlargement results in

• mature (or Graafian) follicle

The Graafian follicle ruptures releasing the ovum - this is

• ovulation

The ovum passes along the Fallopian tube. If it fuses with a sperm this triggers the second meiotic division

The follicle left behind undergoes further changes. The cells become larger and form a

• corpus luteum
 

The corpus luteum has endocrinal functions i.e. it releases hormones; specifically oestrogen and progesterone, which are involved in maintaining a pregnancy.

If no fertilisation takes place the corpus luteum degenerates to form a corpus albicans which has no function

The maturation of an ovum and the associated changes in the follicle structures of the ovary are known as the ovarian cycle and take approximately 28 days

A series of changes take place in the uterus over a 28 day period and are referred to as the menstrual cycle.

The menstrual cycle describes changes in the endometrium (lining of the uterus) in readiness for receipt of a fertilised ovum.

In the days before ovulation the endometrium becomes thicker.

If implantation does not occur the endometrium is shed during menstruation.

Oestrogen and progesterone are secreted by the corpus luteum and are called ovarian (or sex) hormones. These control the changes to the endometrium

Luteinising hormone (LH) and follicle-stimulating hormone (FSH) are released by the anterior pituitary gland (at the base of the brain) and are called gonadotrophic hormones. These control the events in the ovary and thus control the sex hormones.

The anterior pituitary secretes FSH into the blood stream. This travels through the blood and has its effect at the ovaries

The rise in FSH results in a rise in oestrogen level

The oestrogen begins the repair of the endometrium which has been lost at menstruation

The rise in oestrogen also stimulates the production of LH

The rise in LH inhibits the production of FSH

The fall in FSH level results in a fall in oestrogen level

The peak in LH level stimulates ovulation

and the corpus luteum then begins to produce progesterone

The rise in progesterone promotes the development of the endometrium

and inhibits the production of both FSH and LH

The fall in LH causes the corpus luteum to degenerate

and hence the progesterone level drops

The fall in progesterone level removes the inhibition of FSH

the fall in progesterone stops the maintenance of the endometrium and it is shed (menstruation)

so the FSH level begins to rise

these two events represent a return to the start of the cycle