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The two methods of reproduction

This unit has recently been expanded - 2 notes at the end
Asexual reproduction means reproducing without the interaction of two sexes or genders, whereas sexual reproduction involves the fusion of two special cells called gametes, one from a male source and one from a female source.

Asexual reproduction

Before a cell divides, its nucleus divides. Each chromosome is copied, and each nucleus receives the same genetic material: genes, made of DNA.

As each cell divides into two, the resulting "daughter" cells are therefore exact copies of one another.

This process is responsible for the increase in number of cells which occurs during normal growth and development, and when tissues are replaced following injury.

Normal cell division is also the basis for asexual reproduction. Only one type of cell is involved, with no input from another individual. Because no new genetic material is introduced, there is no variation in the resulting offspring.

Since the offspring from this process contain the same genetic material as one another (and the same as the original single parent), they can be described as a clone.

Examples of asexual reproduction

Asexual reproduction in plants

'Spider plant' Chlorophytum with plantlets There are many examples of asexual reproduction in plants, e.g. the spider plant Chlorophytum which produces plantlets on stolons branching from buds in the parent plant.
These are genetically identical and will grow to look alike, provided that they are raised in the same environment.
What features of the plant's environment would be need to be standardised (for them to look the same)?
>same illumination (amount of light)
>similar temperature
>watered similarly
>same sort of soil (minerals etc)

Many plants used for food can be propagated, i.e increased in number, by the method of asexual reproduction.

Do not confuse asexual reproduction with (sexual) reproduction in flowering plants, which often combine both male and female parts in the same flower.
Fruits and seeds are produced as a result of sexual reproduction.

Some parts of plants become enlarged as a result of normal cell division and this is called vegetative growth. This is often linked with surviving adverse weather conditions and keeping food reserves for the plant in order to grow again the next season. These plants are called vegetables, and Man often uses these reserves for himself.

Each of the examples of food plants below use asexual and sexual reproduction in different ways
Strawberry plants produce new plantlets by asexual reproduction. These then root in the soil on the end of stolons (runners).
Note that strawberry fruits are products of sexual reproduction
More about onions More about apples

Asexual reproduction in bacteria

Bacteria reproduce asexually by a simpler process known as binary fission.
More about bacteria

Asexual reproduction in animals

Hydra with buds Aphid (greenfly) giving birth Asexual reproduction is much less common in animals, but it is often seen in simpler animals e.g. Hydra.

Identical twins are produced by a form of asexual reproduction when the ball of cells making up the embryo breaks into two, and each implants in the uterus and grows independently (after the normal sexual form of reproduction, obviously!)

In animal lifecycles, asexual reproduction sometimes alternates with sexual reproduction. See weblink below.

Sexual reproduction

Both male and female sex cells (sperms and eggs in animals, pollen and ovules in plants) are produced by a special cell division process which halves the number of chromosomes in each resulting cell. The chromosome separation process ensures that each sex cell has a unique combination of genes in its nucleus.

Fertilisation is also a random process and so when the nuclei fuse the resulting fertilised egg (zygote) has an individual genetic makeup.

In contrast to asexual reproduction, sexual reproduction introduces variation into offspring. This is an essential feature in order for evolution to take place.

Sperms and eggs combining to make different fertilised eggs (zygotes)

Further development after fertilisation

This zygote then divides again and again using the normal process of cell division, producing cells containing genes which are exact copies of the original. So each cell of the embryo, and the adult organism into which it develops, contains cells which are genetically identical. This is fortunate because the body's immune system will target any "foreign" cells (normally invading microbes) which differ from the others.

However as each organ develops, the cells within it (collectively known as tissues) become specialised for their particular tasks, e.g. muscle cells, nerve cells, red and white blood cells, and they "read" and use only part of their genetic information to do this.

As they have differentiated into these different cell types, it appears that they have lost the ability to divide again into other cell types. A few unspecialised cells (e.g. stem cells) retain the ability to do this.

Summary of differences

Asexual reproduction Sexual reproduction
Number of parents 1 (either male or female) Usually 2 (male and female) - see note 1 below
Makeup of offspring genetically identical (to parent and other offspring) genetically different
Cell division process normal cell division following nuclear division (by mitosis) special cell division following nuclear division (by meiosis) producing sex cells (gametes) - see note 2 below:
after fertilisation subsequent divisions: normal
Advantages quick - good for bulking up of numbers to colonise new areas produces variation - the basis of evolution
Disadvantages disease may affect all slower - needs special processes to bring together gametes and protect zygote, embryo etc during development
Life cycle useful when conditions ideal for growth may be synchronised with (end of ?) growing season

Note 1

Some organisms are hermaphrodite (bisexual) - i.e. have both male and female sexual organs. This is found in many forms of invertebrates, e.g. molluscs, earthworms.
Files on this site: snail ... earthworm

It is not so common amongst vertebrates, although some fish change from one sex to another.

Most flowering plants are hermaphrodite. In their stamens they produce pollen (spores acting as male sex cells) as well as ovules (female sex cells) produced in their ovaries.
lily (13K)
Lily flower- showing both male and female parts

Female cucumber flower
However there are examples of plant species with separate male and female flowers on the same plant (e.g. cucumber), and plant species that have separate whole plants of one sex or another (e.g. holly). cucumber_male
Male cucumber flower

Hermaphroditism enables all individuals to be producing offspring, and encounters between pairs of organisms to be productive.
There are often mechanisms to prevent hermaphrodite organisms fertilising themselves, such as different timings of the various processes, and chemical incompatibility mechanisms.

When self-fertilisation occurs the result is not the same as asexual reproduction - the offspring being genetically identical to the parent. When sex cells are produced, only half the parent's genetic material is used and then combined with an equal amount of genetic material from the other sex cell. There are elements of randomness in the production of gametes and in the fertilisation process, so resulting offspring are at least partly genetically different from the parent, and one another. There would be always be variation in a population of sexually reproducing organisms so that some organisms are more suited to their environment than others and survival of the fittest will underpin the process of evolution.

[The term hermaphrodite may also be used in human terms to describe rare instances of uncertainty over a person's sex, perhaps resulting from a developmental disorder, but the term intersex is preferred.]

Note 2

Differences between male and female gametes

Male gametes
Female gametess
  • smaller
  • produced in large numbers
  • able to move - either of their own accord e.g. swimming sperm cells
    or by other means e.g. pollen carried by the wind or pollinating insects
  • consequently more likely to suffer wastage, but this could be another example of survival of the fittest
Some (smaller, simpler) plants produce swimming sperm cells.
These are found in wetter places.

  • larger - with food reserves inside the cell
  • therefore fewer in number
  • not so inclined to move - the male gamete moves to them, not vice versa
  • usually more protected - and this protection continues to be given to the developing embryo - competition again?
However there are cases of simple organisms where two very similar sized cells fuse, then after nuclear fusion meiosis occurs - and there is the suggestion that sex itself can be seen as the result of evolution.

Web links

BBC Nature Wildlife videos: Aphid reproduction methods

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