CHAPTER 4

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CHAPTER 4 REPRODUCTION & GROWTH

description

reproduction & growth

Transcript of CHAPTER 4

CHAPTER 4

LEARNING OUTCOMESExplain the necessity for organism to reproduceState types of reproductionExplain the necessity for formation of gametesDescribe formation of sperm in humansDescribe formation of ovum in humansCompare the formation of sperm & ovumNecessity for organism to reproduce

To increase their numberTo ensure the continuation of lifeTo ensure the passing down of an individuals gene to future generationTypes of reproduction

Necessity for formation of gametes

In humans, the male gametes, called sperm or spermatozoa, are produced by the reproductive organs called the testes. The female gametes or ova are produced by the reproductive organs known as the ovaries. The production of these two types of gametes occurs through the process of meiosis. The process of meiosis produces haploid gametes which are genetically different from the parental cells.

These variations in the gametes, together with random fertilisation, produce variations in the offspring that enable them to better survive in the ever changing environment.

The normal number of chromosomes in each human somatic cell is 46. This number is the diploid number, 2n. However, sperm and ova have a haploid number, n, of 23 chromosomes. During fertilisation, a sperm fuses with an ovum to form a zygote.As a result of the sperm and ovum each having a haploid number of 23 chromosomes, the zygote formed will have 46 chromosomes. The zygote then divides by mitosis to produce millions of cells in the child, all having 46 chromosomes (Figure 4.3). Thus, the diploid condition (2n) of human is maintained from generation to generation. This characteristic of gamete formation ensures the survival of the human species on Earth.

The human male reproductive system

Testis of semineferous tubuleFunctions of the organs in the male reproductive system

Sperm formation in humansSpermatogenesis, the formation of sperm, occurs in the seminiferous tubules. The seminiferous tubules are lined by germinal epithelial cells (Figure 4.5(b)]. These cells divide many times forming cells known as spermatogonia. Spermatogonia undergo growth to form primary spermatocytes [Figure 4.5(b) and (c)]. These divide by First, meiotic division to produce haploid secondary spermatocytes, and then undergo a second meiotic division to form spermatids (Figure 4.6). The spermatids obtain nourishment from nearby nutritive cells called Sertoli cells, develop tails and mature into sperms.

A human spermThe human female reproductive system

Ovum formation in humans1. Oogenesis - formation of ovum that starts in the 2 ovaries of a female foetus before birth.2. The germinal epithelial cells near the surface of the ovary divide repeatedly by mitosis into diploid oogonia (singular, oogonium).

3. The oogonia grow to form primary oocytes. Each oocyte is surrounded by a layer of follicle cells. The cells provide nutrients to the developing oocyte and secrete the sex hormone, oestrogen. The diploid primary oocyte and the follicle cells form a primary follicle (Figure 4.8).

4. At birth, the female baby has about 1 million primary oocytes but remain dormant until puberty. Less than 400 000 follicles mature while the rest degenerate and die. 5. During foetal stage, all primary oocytes undergo meiosis but stop at prophase I of meiosis I.6. Every month from puberty onwards, many primary oocytes become active and grow but only one oocyte matures. This oocyte completes meiosis I to become a secondary oocyte and a tiny polar body. Both of them are haploid.

7. The secondary oocyte begins meiosis II until metaphase II. It is lined by layers of follicle cells and together they are called a secondary follicle.8. The first polar body may complete meiosis II to form 2 haploid polar bodies.

9. The secondary follicle increases in size and matures to form the Graafian follicle.10. At about 28 days intervals, the Graafian follicle and part of the ovary wall break up and release the secondary oocyte.

11. The release of the secondary oocyte from the ovary is known as ovulation. 12. The secondary oocyte (a.k.a mature egg@ ovum) enters the Fallopian tube. If a sperm penetrates the secondary oocyte during fertilization, meiosis II is completed and 2 haploid cells of different sizes are formed.13. The larger cell is the ovum {plural, ova) whereas the smaller cell is the second polar body.14. Thus, a primary oocyte eventually becomes a haploid ovum (n) and 3 haploid polar bodies (n). All polar bodies degenerate and die.15. The nuclei of the ovum and the sperm fuse to form a diploid zygote (2n).

16. Once the Graafian follicle has released its oocyte, it develops into a yellowish mass of cells called corpus luteum.17. If pregnancy does not take place, the corpus luteum begins to degenerate after approximately 10 days.

18. If pregnancy occurs, the corpus luteum does not degenerate but will continue to produce sex hormones (oestrogen and progesterone).19. The whole process of development and formation of ova is regulated very systematically by hormones

Comparison between the formation of sperm & ovum

Similarities Both are developed from the germinal epithelial cells (2n) of the reproductive organThe cells undergo mitosis to produce many diploid cells ( spermatogonia & oogonia)Both process undergo meiosis to produce four haploid daughter cells or gametesBoth process involve growth

Differences:

Spermatogenesis DifferencesOogenesis Testes in male reproductive organPlace of formationOvary in female reproductive organ4 sperms are formed, all the sperms are activeGametes produced1 ovum & 3 polar bodies are formed, the polar bodies degeneratesSperms are smaller in sizeSize of gametesOvum has a bigger sizeSperms have tails to enable them to swimMovement of gametesThe movement of the ovum is facilitated by the cilia of the Fallopian tubeA life long process which begins after puberty with sperms formed continuoslyFormation of gametesProcess begin at the stage of foetus and stops at menopause. Formation of ovum occurs once in every menstrual cycleDivide to produce spermatogonium; Spermatogonium develop into primary spermatocytesGerminal epithelium cellsDivide to poduce oogonium; oogonium develop into primary oocytesAfter meiosis I, 2 secondary spermatocytes are formedproductAfter meiosis I, one secondary oocyte and one polar body is formed