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.

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    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 is a type of reproduction where the nuclei of male gametes (male sex cells) and the nuclei of female gametes (female sex cells) fuse to form a zygote (fertilised egg).

    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.

    Genetic diversity means having a range of different characteristics and traits within a species, causing different physical features and behaviours within a species.

    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.

    More information on these processes can be found in our article on Human Reproductive System.

    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.

    Reproduction, diagram showing the general structure of a flower including both male and female reproductive organs, StudySmarterFig. 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.

    More information on these processes can be found in our article on Plants Reproduction.

    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 is a type of reproduction that does not involve the fusion of gametes.

    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.

    Reproduction, a diagram showing sexual reproduction and asexual reproduction in fungi, StudySmarterFig. 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.

    Budding is another method of asexual reproduction. Here a bulge forms at the edge of a cell before undergoing cytokinesis and detaching from the mother cell.

    Asexual reproduction of plants

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

    Reproduction, diagram showing vegetative reproduction, StudySmarterFig. 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.

    It is, however, important to note that binary fission is not strictly restricted to prokaryotic organisms. Some single-celled eukaryotic organisms may also reproduce in this way, such as amoeba.

    Sexual vs asexual reproduction

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

    Sexual Reproduction Advantages

    Asexual Reproduction Advantages

    • Increased genetic diversity and genetic variation are found in the offspring.

    • The species can adapt to new and changing environments due to their genetic diversity.

    • The species is largely protected from a disease outbreak. Due to the genetic variation of individuals in the species, the disease is unlikely to affect all individuals.

    • When environmental conditions are suitable, it can trigger a rapid increase in the population of the organisms.

    • There is no need for another mate, only one parent is required.

    • It is faster than sexual reproduction.

    Sexual Reproduction Disadvantages

    Asexual Reproduction Disadvantages

    • Searching for a mate requires time and energy.

    • If an individual is isolated from the rest of the population, that individual is unable to reproduce.

    • There is no increase in genetic diversity or variation in a population.

    • The species may not be able to adapt to changing environments.

    • A disease outbreak is likely to affect all individuals.

    Reproduction - Key takeaways

    • Sexual reproduction can only occur through cells created through the process of meiosis.

    • Asexual reproduction occurs through the process of mitosis.

    • Humans and other mammals reproduce sexually, and cannot reproduce asexually.

    • Plants and fungi can reproduce both sexually and asexually.

    • There are advantages and disadvantages to both types of reproduction.

    Reproduction Reproduction
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    Frequently Asked Questions about Reproduction

    What is sexual reproduction?

    Sexual reproduction is the process where the nuclei of male gametes (male sex cells) and the nuclei of female gametes (female sex cells) fuse together to form a zygote (fertilised egg). 

    What does reproduction mean?

    Reproduction is the process of producing offspring through either sexual reproduction or asexual reproduction

    Which type of reproduction occurs by mitosis?

    Asexual reproduction occurs by mitosis

    What is the difference between sexual and asexual reproduction?

    Sexual reproduction involves haploid sex cells (produced via meiosis) and fertilisation to produce a dyploid zygote, and so requires two parents, whereas asexual reproduction only requires one parent, and does not need gametes or the process of meiosis to occur. The offspring in sexual reproduction are not genetically identical, but in asexual reproduction they are genetically identical.

    What are the advantages of asexual reproduction?

    Advantages of asexual reproduction:

    • When environmental conditions are suitable, it can trigger a rapid increase in the population of the organisms

    • There is no need for another mate, only one parent is required

    • It is more quicker than sexual reproduction

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