Ritu's Biology Hub

What is Sexual Reproduction?

In sexual reproduction, genomes from 2 different individuals are mixed to produce offsprings that differ genetically from one another and from both the parents

Benefits of sexual reproduction

• Creates novel assortment of genes through meiosis and genetic recombination
• In all multicellular animals, whole life is spent in diploid state and haploid state exists only for a brief period specialized for sexual fusion. These haploid cells are called gametes – one is large and non-motile – the egg or ovum and the other, small and motile – sperm or spermatozoa
• Genetic recombination results in unpredictably dissimilar offsprings
• Reshuffling of genes helps species to survive in an unpredictably variable environment
• Since offsprings with wide variety of gene combinations are generated, there are chance that at least one will possess features that allow survival under adverse conditions
• Allows many deleterious mutations to be eliminated
• May select for rare advantageous mutations that arise in separate individuals

Meiosis

• Term was coined in 1905. In Greek, it means reduction
• Divided into 3 groups based on the stage in the life cycle at which meiosis occurs:
  • Gametic or terminal meiosis – It forms gametes. In male vertebrates, meiosis occurs just prior to differentiation of spermatozoa.
  • Zygotic or initial meiosis – Occurs in protists and fungi. Meiosis happens after fertilization giving haploid spores which undergo mitosis giving rise to haploid adults. Thus, diploid stage is seen only after fertilization for a short period of time
  • Sporic or intermediate meiosis – Seen in plants and some algae.

Primordial germ cells migrate to developing gonads which eventually develop into ovaries or testes where the germ cells undergo mitosis. This generates abundant oogonia or spermatogonia, respectively. They then undergo meiosis to produce sex cells.

Sex Determination

• In many animals, including some vertebrates, the ovum is asymmetrical ie., different regions of the cytoplasm contains different types of mRNAs and proteins
• When the fertilized ovum divides, cells derived from certain regions of the ovum inheriting some specific molecules form the primordial germ cells
• In mammals, the ovum is symmetrical and cells derived from the fertilized ovum are totipotent ie., can develop into any kind of cell of the body. Thus, in them, the primordial germ cells are formed by signals from the neighbouring cells
• For example, in mice, one week after fertilization, 50 cells in a tissue lying outside the embryo are induced by their neighbours to develop into primordial germ cells
• Then they enter into the embryo and migrate to the genital ridge which later develops into the gonads. While passing, extracellular proteins signal the primordial germ cells to survive, proliferate and migrate
• In genital ridge, they continue to proliferate for 2-3 days and then commit to a developmental pathway that will lead them to become either eggs or sperms
• The Y chromosome in the somatic cells of the genital ridge determine which type of gonad it will become. This, in turn, depends on whether X or Y chromosome containing sperm fertilizes the ovum. XX embryo develops into female while XY embryo develops into male
• Some of the somatic cells in the genital ridge contain the Y chromosome which causes it to develop into testis. Such somatic cells develop into sertoli cells
• They Y chromosome conatins a gene – sry – Sex determining region of Y. sry gene encodes a gene regulatory protein – Sry protein. If this functional gene is present, embryo develops into a male
• The Sry protein activates transcription of genes required for sertoli cell development including the production of a protein called Sox 9
• The sertoli cells form the male reproductive system in the following way –
  • Stimulates newly arriving primordial germ cells to develop along a pathway that produces sperm
  • Produces anti-Mullerian hormone which otherwise lead to the development of the Mullerian duct that forms the female reproductive system
  • Causes neighbouring somatic cells around the developing gonad to form the connective tissue
  • Induce other somatic cells in the developing gonad to become Leydig cells which secrete testosterone that is responsible for the appearance of secondary sexual characteristics in males. The Wolffian duct gives rise to prostate gland and seminal vesicles
• In absence of Sry or Sox 9 protein because of a mutant or non-functional gene, the genital ridge by default develops into ovaries regardless of the chromosome it carries. The surrounding cells become follicular cells and secrete oestrogen

Continued to Part 2