IGCSE Biology Paper-3: Specimen Questions with Answers 341 - 341 of 358

Passage

Flowers contain the male and female reproductive structures of a plant.

The female reproductive structure is the carpel.

The male reproductive structure is the stamen.

Question 341 (3 of 3 Based on Passage)

Edit

Describe in Detail

Essay▾

This question is about reproduction in humans.

Outline the events that occur during spermatogenesis.

Explanation

Spermatogenesis

It consists of two parts:

  • Formation of spermatids
  • Metamorphosis of spermatids into sperms/spermatozoa (Spermiogenesis/Spermatoleasis)

Formation of Spermatids

It is completed in three phases:

  • Multiplication phase
  • Growth phase
  • Maturation phase

Hormones which regulate spermatogenesis are FSH of anterior pituitary and testosterone of testis. Androgen binding hormone/ factor (ABH) is secreted from Sertoli cell and concentrates testosterone in the seminiferous tubules.

  • During multiplication phase the primordial germ cells divide several times by mitosis to produce many cells, called spermatogonia. These cells are generally differentiated into type-A and type-B spermatogonia. The type-A spermatogonia continue as stem cells while type-B spermatogonia differentiate and enter growth phase.
  • During growth phase the spermatogonia differentiate into primary spermatocytes. The size of primary spermatocyte increases due to accumulation of nourishing material, obtained from the lumen of the seminiferous tubules. During this phase the preparation for meiotic division also begin. Both, the spermatogonia of multiplication phase and primary spermatocyte of growth phase are diploid .
  • During maturation phase there are two divisions in meiosis, first reduction division and second equational division. The germ cells formed after reduction division are haploid (n) and are called secondary spermatocytes. Each secondary spermatocyte then produces two daughter cells called spermatids. Thus, from each primary spermatocytes 4-spermatids are produced. The spermatids are non-motile, haploid cells which get attached to the Sertoli cells for nourishment during metamorphosis.

Spermiogenesis/Spermatoleasis

Following are the important changes during metamorphosis of spermatid into sperm:

  • Nucleus: It becomes compact due to removal of nuclear sap and RNAs etc. From spherical it changes into anteriorly pointed structure.
  • Golgi bodies: All the Golgi bodies which are scattered in the cytoplasm of spermatid assemble at the anterior side of nucleus. The secretory granules of vacuoles fuse to form acrosome whereas the covering becomes acrosomal vesicle. The vesicle protects the nucleus from anterior side and is known as head cap.
  • Centrosome: Both the centrioles of the centrosome move towards posterior end of nucleus and form proximal (close to nucleus) and distal (away from nucleus) centrioles. The microtubules (9-sets) of the distal centriole elongate to form axial fi lament which later becomes the part of middle piece and tail.
  • Mitochondria: All these organelles fuse end-to-end to form a mitochondrial sheath which get spirally twisted around the anterior part of axial filament, which later becomes middle piece.
Primordial Germ Cell

Developed by: