4.1 Gamete Formation

4.0 Reproduction & Growth

4.1 Gamete formation

The necessity for organism to reproduce  The

continuation of the species.  The passing down of an individual’s genes to future generations.

Types of reproduction Two types: b) Asexual reproduction c) Sexual reproduction 

a) Asexual reproduction The reproductive process in which new individuals are produced without the formation of gametes.  Does not involve the fusion of haploid gametes 

b) Sexual reproduction  The

creation of offspring through the fusion of haploid gametes (male gamete and female gamete) to form a diploid zygote (fertilised egg).

Reproduction

Sexual

Binary fission

Asexual

Spore formation

Vegetative reproduction

Budding

Regeneration

Differences between sexual reproduction and asexual reproduction Differences Sexual reproduction

Asexual reproduction

The production of new individuals involving SEX CELLS @ GAMETES

The production of new individuals without involving SEX CELLS @ GAMETES

Gametes are formed by MEIOTIC DIVISION

The process relies entirely on MITOTIC DIVISION

Involves 2 INDIVIDUALS (parents)

Involves only 1 INDIVIDUAL (single parent)

Sexual reproduction

Differences

Asexual reproduction

New individuals inherit genes from BOTH parents through the fusion of the egg & sperm.

New individuals get their genes from ONE parent without the fusion of egg & sperm.

Present in humans, other mammals, birds, reptiles, amphibians, fish & flowering plants

Present in simple organisms like Amoeba sp., Paramecium sp. & Hydra sp., yeast & plants such as mosses & potatoes.

Produces OFFSPRING that are GENETICALLY DIFFERENT from their parents to ensures the survival of the species if the environmental condition undergo changes constantly

Produces OFFSPRING that are GENETICALLY IDENTICAL to the parent to ensures the adaptations of the parent for survival are passed down unchanged to the offspring. An advantage if the environment is stable

The formation of gametes  In

humans, the male gametes called sperms @ spermatozoa are produced in the male reproductive organs called testes.  The female gametes called ova are produced in the female reproductive organs called ovaries.  These 2 types of gametes are produced through the process of MEIOSIS.

The formation of gametes  The

gametes are genetically different from the parent cells.  During fertilisation, a sperm fuses with an ovum to form zygote.  As the sperm & ovum have a haploid number of chromosomes (n = 23) each, the zygote formed will have a diploid number of chromosomes (2n = 46)  During mitosis, the zygotes then divides mitotically to produce millions of cells, each cell having 46 chromosomes.

The necessity for the formation of gametes  The –

–

–

formation of gametes ensures : The diploid chromosomal number is maintained from one generation to the next generation. Genetic variations in the offspring to give them better survival chances in an ever changing environment. The continuity of a species.

Test Yourself 1. 2. 3. 4. 5.

Why it is important for organisms to reproduce? How many types of reproduction and what are they? Give two differences between the two types of reproduction. How many types of gametes do we have in human and what are they? Why it is necessary for the formation of gametes?

The male reproductive system  Male

reproductive system consists of :

 gonads

that produce male gametes (sperms) & sex hormones.  Accessory glands that secrete fluids essential to sperm movement.  ducts that carry the sperms & secretions.

The male reproductive system  The

male reproductive structures includes the testes, scrotum, epididymis, vas deferens (sperm ducts) & penis.  The male reproductive system also includes the seminal vesicles & prostate gland.  The male gonads @ the testes (singular testis) produce both the male gametes (sperms) & sex hormones.

The male reproductive system

The male reproductive system  Testes

are enclosed in the external sac called scrotum.  The scrotum houses the testes outside the main body cavity.  Outside the body, the temperature of the testes is about 2 - 4oC lower than the normal body temperature of 37oC. Sperms cells cannot develop at normal body temperature, this lower temperature is ideal for the development of healthy sperms.

The male reproductive system  Each

testis has about one thousand seminiferous tubules (fine, long coiled tubes arranged compactly).  The tubules merge to form a tightly coiled tube called epididymis, lies just outside the testes.  Sperms are formed in the seminiferous tubules & mature within the epididymis.

The male reproductive system  The

epididymis connected to the vas deferens @ sperms duct which also stores the sperms.  The vas deferens leaves the scrotum & enters the abdominal cavity.  The 2 sperm ducts join the urethra which connects the bladder to the penis.  The sperm pass from the seminiferous tubules through the epididymis & vas deferens into urethra.

The male reproductive system  The

urethra is the exit route for both sperm & urine.  During the journey of the sperm from the testis to the urethra, accessory glands add secretions for transport & survival of the sperm.  The seminiferous vesicles secrete a thick, clear fluid containing nutrients for the sperm.

The male reproductive system  The

prostate gland secretes a milky white fluid that activates @ increases the mobility of the sperm.  The sperm & the secretions from the accessory glands make up the semen.

Formation of sperm in humans  Spermatogenesis

(spermatozoa formation) takes place in the densly coiled seminiferous tubules.  Line up the seminiferous tubules are 2 types of cells (germinal epithelial cells & sertoli cells).  Germinal epithelial cells of a seminiferous tubule divide by MITOSIS to produce DIPLOID SPERMATOGONIA.

Formation of sperm in humans  The

spermatogonia grow to become PRIMARY SPERMATOCYTES which are large DIPLOID cells.  Each primary spermatocytes undergoes MEIOSIS I to form TWO HAPLOID SECONDARY SPERMATOCYTES.  Each secondary spermatocyte divides again during MEIOSIS II to produce TWO SPERMATIDS. This means a total of FOUR spermatids are formed for every primary spermatocyte.

Formation of sperm in humans  Spermatids

obtain nourishment from nearby nutritive cells (sertoli cells), develop tails & mature into SPERMS (SPERMATOZOA) through cell differentiation.  Spermatogenesis takes about 65-75 days in human male.

Pathway of sperm from production to ejaculation.

Structure of the human sperm A

mature sperm consists of 3 distinct parts : a head, a midpiece & a tail.  The head contains a large nucleus & a specialised lysosome called the acrosome, which contains hydrolytic enzymes that help to digest the protective layer surrounding an ovum, enabling the sperm penetrate & fertilise it.  Midpiece contains a large number of mitochondria, which provide the energy needed for the movement of the tail.

Test Yourself 1. 2. 3.

Describe the male reproductive system. How is the formation of sperm in human? Explain the structure of the human sperm.

The female reproductive system  Consist

of 2 ovaries, two fallopian tubes, the uterus, cervix, vagina and the external genital organs called vulva.  Ovaries produce the female gametes (ova) and female hormones (oestrogen and progestrone).

The female reproductive system

Formation of ovum in humans  Oogenesis

(formation of ovum) starts in the ovaries of the foetus before birth.  The formation starts with the formation of precursor egg cells called oogania (singular – ooganium).  Originate from germinal epithelial cells near the surface of an ovary.  Then its multiply by mitosis to form DIPLOID oogonia.

Formation of ovum in humans  Oogonia

will grow up to form PRIMARY OOCYTES (diploid).  Each primary oocyte is surrounded by a layer of follicle cells that nourish the developing oocyte.  It also secrete female sex hormones  Primary oocyte & these follicle cells make up a primary follicle.

Formation of ovum in humans  At

birth a human female has as many as 2 million primary oocytes but the number will reduced during puberty about 400,000.  After birth, the primary oocytes become dormant until puberty  After puberty until menopause, every month a few primary oocytes become active & meiosis resume BUT only SINGLE primary oocytes in primary follicle matures completely.

Formation of ovum in humans  Primary

oocyte undergoes meiosis I to form HAPLOID cells which are not equal in size.  Secondary oocyte, together with layers of follicle cells around it, is now called SECONDARY FOLLICLE.  Secondary oocyte starts MEIOSIS II

Formation of ovum in humans   

 

The secondary follicle increases in size & mature to form GRAAFIAN FOLLICLE. At intervals of approximately 28 days the Graafian follicle merges with the wall of ovary. The ovarian wall & the Graafian follicle RAPTURE, releasing the secondary oocyte @ egg into the Fallopian tubes. The process whereby the ovary released the secondary oocyte known as OVULATION. Ovulation take place in one of the ovaries once in every 28 days.

Formation of ovum in humans  Single

primary oocyte gives rise to SINGLE HAPLOID OVUM & THREE HAPLOID POLAR CELLS/BODY.  Polar bodies will degenerate.  When Graafian follicle released secondary oocyte, it develop into a yellowish mass of cells known as CORPUS LUTEUM.

Formation of ovum in humans  If

no pregnancy occur corpus luteum degenerate approximately at 10th days.  But if pregnancy takes place corpus luteum not generate & play an important role to secrete oestrogen & progesterone.  Whole formation & development of ova regulated by hormones.

Comparison Between Spermatogenesis & Oogenesis

 What

is spermatogenesis?  What is oogenesis?  Where do spermatogenesis takes place?  Where do oogenesis takes place?

Discussion  Discuss

in your group to compare and contrast the stages that occur during formation of sperm and ova.

Exercise  Do

Checkpoint 4.1 on page 128 in the textbook.