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Reproduction

Reproduction, the production of offspring, is essential for continuing life. If organisms could not reproduce and create offspring, life would cease to exist. Organisms have various ways of reproducing, reflecting the diversity of life on this planet. Let’s look at the ways organisms can reproduce to pass on their genes.

Sexual Reproduction and Function

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

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).

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

Sexual Reproduction in Humans

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 Sexual Reproduction.

Sexual Reproduction in 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. Take a look at Fig. 2 to familiarise yourself with the structure of a flower.

Reproduction Structure of a mature flower StudySmarterFigure 2: A figure showing the structure of a flower. Mariana Ruiz, wikimedia.com

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 Sexual Reproduction in Plants.

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 (see figure 2): 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. Meiosiscells 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.

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 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 in Plants

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

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.

Advantages and Disadvantages of Sexual vs Asexual Reproduction

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

Sexual reproduction

Here are some advantages and disadvantages of sexual reproduction.

Advantages

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.

Asexual Reproduction

Here are some advantages and disadvantages of asexual reproduction.

Advantages

  • 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.

Disadvantages

  • 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.

Frequently Asked Questions about 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). 

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

Asexual reproduction occurs by mitosis

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.

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

Final Reproduction Quiz

Question

What cells are contained in the ovaries?

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Answer

Oocytes,  which mature to become egg cells or ova. 

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Question

The fimbriae connect what to what? 

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Answer

Ovary to fallopian tubes

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What is the uterus?

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Answer

A muscular, pear-shaped organ, becomes known as the womb during pregnancy. 

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What is the cervix?

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Answer

The lowering end of the uterus, containing the opening to the vaginal canal. 

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What process creates gametes?

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Answer

Meiosis

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Question

What cell is created in the testicles? 

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Answer

Sperm

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The vas deferens carry what to where? 

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Answer

Sperm from the epididymis to the ejaculatory ducts

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Question

What process forms egg cells? 

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Answer

Spermatogenesis

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What process forms sperm cells

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Answer

Spermatogenesis

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Question

What is the outer layer of the egg cell that sperm cells bind to called?

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Answer

Zona Pellucida

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Question

The acrosome reaction does what?

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Answer

Creates a tunnel into the zona pellucida to allow sperm cells to reach the membrane, by using enzymes to digest the outer layers of the egg.  

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What reaction occurs after fertilisation of the egg cell?

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Answer

The cortical reaction

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Question

What occurs during the cortical reaction?

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Answer

Calcium ions released by nuclear fusion stimulates the release of cortical granules into the zona pellucida. These harden the zona pellucida and remove binding proteins, preventing further sperm entry. 

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Question

A fertilised egg is known as?

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Answer

A zygote

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Question

The binding of the blastocyst to the uterus wall is known as?

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Answer

Implantation

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Question

What term best describes the female reproductive system of a flower?

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Answer

Gynoecium

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What is the role of synergids during fertilisation?

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Answer

Promote the growth and extension of the pollen tube by secreting chemicals

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Question

What is the ‘testa’?

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Answer

Seed coating

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What is the ‘pericarp’?

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Answer

Fruit wall

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What is the difference between monoecious and dioecious flowers?

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Answer

Monoecious flowers possess both male and female reproductive organs while dioecious flowers only possess one type; either male or female.

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Question

Another term for ‘flowering plant’ is angiosperm. True or false?

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Answer

True.

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Question

The outermost whorl of a flower contains the androecium. True or false?

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Answer

False, the sepals, collectively known as the calyx, make up the outermost whorl of the flower

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Question

The gynoecium contains the male reproductive system, while the androecium contains the female reproductive system. True or false?

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Answer

False, androecium contains the male reproductive system, while the gynoecium contains the female reproductive system.

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Question

Fill in the blanks. The androecium is the _____ whorl of the flower which contains the male reproductive structures, including _____ and anthers. 

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Answer

Third, stamens.

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Question

Fill in the blanks. The gynoecium makes up the _________ whorl – this contains the female reproductive system, including the stigma, style, and ovary, collectively known as the ______.

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Answer

Innermost, carpel.

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What is the role of the endosperm during embryo development?

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Answer

The endosperm provides a regular supply of nutrients and plant hormones to the embryo.

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Question

Why is water uptake so crucial in the first stages of germination?

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Answer

Water allows the hydrolysis of biomolecules to form reactants used in cellular respiration, as well as activating a number of enzymes used in cellular reactions. The embryonic shoot, root and cotyledons respire and grow.

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Question

Fill in the blanks. During ovule development, the inner and outer integuments form, which surround and protect the ______. A small opening at the tip of the ovule remains, called the _______.

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Answer

Nucellus, micropyle.

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Question

Why may self-pollination be considered a disadvantage for some plants?

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Answer

Since both gametes come from the same parent, it will result in inbreeding. Thus, any disadvantageous recessive characteristics in the parent will be greatly amplified, potentially resulting in defects in the plant.

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What are the 4 stages of mitosis

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Answer

Prophase, Metaphase, Anaphase, Telophase

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What is mitosis used for?

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Answer

Growth, repair of damaged cells, asexual reproduction

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What happens during prophase?

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Answer

The nuclear envelope is dismanteled, chromosomes condense to visible structures, centrioles migrate to oppposite poles of the cell and form mitotic spindles, special protein structures on the centromere (central region of the chromosome) called kinetochores attach the chromosome to the mitotic spindle.

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What happens during metaphase?

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Chromosomes line up in the equator.

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What happens during anaphase?

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The aligned chromosomes are pulled apart by the spindles, pairs of sister chromatids are separated and pulled to the opposite poles of the cell.

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What happens during telophase?

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The nuclear envelope reforms around each set of chromatids at the poles, the chromosomes atart to decondense into chromatin, the mitotic spindles disappear.

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What happens during cytokinesis?

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Answer

A belt of contractile fibres forms at the equator of the cell, this creates an indentation at the centre, known as the cleavage furrow, this contractile belt tightens and eventually divides the cytoplasm into two identical cells.

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Question

What stage of the cell cycles does this How are cyclins and cyclin-dependent kinases (Cdks) involved in the regulation of the cell cycle?image show?

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Answer

Cyclins can only regulate cells if they are tightly bound to Cdks, forming Cdk/cyclin complexes. These complexes are phosphorylated, and they subsequently transfer their phosphate group to a target protein which makes the protein to change shape and become activated - thus advancing the cell cycle.

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Question

Name some examples of tumour suppressor genes.

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Answer

Retinoblastoma protein (Rb), p53 and p2.1

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What is the purpose of tumour suppressor genes?

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Answer

Tumour suppressor genes regulate the cell cycle by preventing cells from dividing uncontrollably due to damage or mutation in the DNA.

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What is asexual reproduction?

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Answer

Asexual reproduction is a process that occurs via mitosis and does not create any genetic variation. It is also known as cloning.

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Question

What happens in the G1 step of the interphase?

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Answer

The G1­ step's function is to increase the size of the cell as well as the number of organelles (such as mitochondria and ribosomes). Biosynthesis of different molecules is increased, including an increase in the production of mRNA and proteins in preparation for DNA replication.

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Question

What occurs in the S phase?

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Answer

The genetic material of the cell is duplicated through DNA replication.

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