Reproduction

Did you know that the human reproductive system contains the largest and smallest human cells?! Biological males produce the smallest, known as the sperm and it is smaller than the diameter of a human hair. Biological females produce eggs, which are the largest cells in the human body and can be seen without a microscope. These two cells come together and fuse their genetic material to create life! However, there are also organisms that reproduce asexually and don't require both male and female reproductive cells. Let's explore the different types of reproduction and how this reflects the diversity of this planet.

The definition of sexual reproduction

During sexual reproduction, genetic material from both parents comes together to produce an offspring that is genetically different from its parents.

Sexual reproduction occurs through the process of meiosis. This process produces haploid gametes (sex cells with one complete set of chromosomes). In fertilisation, gametes from the mother and father fuse to form a diploid cell (sex cells with two full sets of chromosomes, one from each parent).

By producing genetically different offspring, sexual reproduction creates genetic diversity.

The function of the reproductive system in humans

The function of the reproductive system is to produce gametes - eggs and sperm.

Sexual reproduction in humans (and other animals) involves the fusion of a sperm cell (male gamete) and an egg cell (female gamete) to produce a diploid zygote (fertilised egg).

The production of gametes in humans occurs through the following processes:

• Oogenesis (production of ova, or egg cells)

• Spermatogenesis (production of spermatozoa, or sperm cells)

In fertilisation, the nuclei of these cells fuse to form a zygote or fertilised egg. Fertilisation occurs through the following series of stages and reactions:

1. Sperm cells bind to the outer layer of the egg cell, the zona pellucida, and digest a tunnel through it using digestive enzymes – this is the acrosome reaction.

2. The membrane of the sperm and egg cells fuse before the sperm nucleus enters the egg cytoplasm. The cortical reaction follows, which is essentially the hardening of the zona pellucida. This reaction prevents multiple sperm cells from fertilising the same egg.

3. When the egg and sperm nuclei fuse, a zygote is formed.

4. The fertilised egg then divides several times to form a blastocyst and implants itself in the uterus lining.

Sexual reproduction of plants

Sexual reproduction in plants follows similar principles to sexual reproduction in humans; the nuclei of male and female gametes fuse to form a zygote. However, there are some significant differences.

Male gametes are produced in the anthers of the flower and can be found in pollen grains, while female gametes are produced in the flower’s ovaries and stored in the ovules. Ovules in plants are structures that develop into seeds when fertilised.

Fig. 1 - Structure of a flower

Fertilisation in plants occurs as double fertilisation. In this process:

1. A pollen grain lands on the stigma of the carpel and forms a pollen tube which grows downwards into the ovule (found in the ovary).

2. As the pollen tube enters the embryo sac wall in the ovule, the tip of the pollen tube bursts, resulting in one male gamete (in the pollen tube) fertilising the egg – forming a zygote.

3. Another male gamete (also in the pollen tube) fuses with two polar nuclei (female nuclei found in the centre of the embryo sac), producing an endosperm nucleus.

4. The endosperm nucleus divides and forms an endosperm, providing nutrients and nourishment to the growing embryo.

These two fertilisation events are known as double fertilisation.

Sexual reproduction in fungi

Fungi (and plants) can reproduce both sexually and asexually. In both cases, the fungi produce spores released into the environment. These spores land and grow into fungi when environmental conditions are suitable.

Sexual reproduction in fungi occurs in three stages: plasmogamy, karyogamy and meiosis.

1. Plasmogamy – the nuclei of two haploid cells come together. They are not yet fused, so two different nuclei are present in the same cell.

2. Karyogamy – the two nuclei finally fuse, forming a diploid zygote nucleus.

3. Meiosis – cells in the gametangia (sexual reproduction organ in fungi) return to a haploid state through meiotic division. These cells are incorporated into spores which are disseminated into the environment.

Asexual reproduction

Asexual reproduction occurs solely by mitosis, whereby a cell divides to produce genetically identical daughter cells. Many organisms use mitosis to reproduce instead of sexual reproduction through gametes.

Fig. 2 - A diagram showing sexual and asexual reproduction of fungi

Asexual reproduction in fungi

Fungi can reproduce asexually through either fragmentation, budding or the production of spores.

The production of spores is the most common form of asexual reproduction. The parent organism produces these spores through mitosis, and they are disseminated into the environment.

Another form of asexual reproduction is fragmentation, where the thallus (body of the fungus) breaks into pieces before growing again. The mycelium (made up of a thread-like network of filaments, called hyphae) can also break off and grow into more mycelia. The fruiting bodies of fungi, such as mushrooms, can grow from a mycelium spore.

Asexual reproduction of plants

Plants can undergo two main types of asexual reproduction: vegetative reproduction and apomixis.

Fig. 3 - Vegetative reproduction. In this example, new individuals bud off from the stolon (runner)

• Vegetative reproduction involves the vegetative structure of the parent plant to form new, genetically identical individuals through processes such as budding. This form of reproduction does not require seeds or spores.

• Apomixis is a form of asexual reproduction that produces seeds without fertilisation. The ovule gives rise to a new seed.

  In some plants, pollination is required to initiate embryo growth, but no genetic material is transferred from the pollen grain to the offspring.

Asexual reproduction in prokaryotes

Prokaryotes, such as bacteria, are single-celled organisms that do not contain a distinct nucleus or other membrane-bound organelles. They reproduce by asexual reproduction, which occurs through a process known as binary fission. The process of binary fission is as follows:

1. The DNA in the prokaryotic cell replicates, doubling the number of chromosomes. Remember that, unlike in eukaryotic cells, the genetic material in prokaryotes is not contained in a nucleus. Therefore, the DNA is found loose in the cytoplasm, albeit in a specialised region called the nucleoid.

2. The chromosomes separate to opposite ends of the cell. The cell also elongates, forming a septum in the middle.

3. A new cell wall is formed from the septum, which splits down the middle, releasing two genetically identical prokaryotic cells.

Sexual vs asexual reproduction

Each reproduction method presents several distinct advantages; however, they each have their own set of drawbacks.