Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.
The two methods of reproduction
This unit may be expanded or changed in the future
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.
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
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)
>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
Asexual reproduction in bacteria
Asexual reproduction in animals
||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.
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.
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
|Number of parents
||1 (either male or female)
||2 (male and female)
|Makeup of offspring
||genetically identical (to parent and other offspring)
|Cell division process
||normal cell division
following nuclear division (by mitosis)
||special cell division
following nuclear division (by meiosis) producing sex cells (gametes): after fertilisation subsequent divisions: normal
||quick - good for bulking up of numbers to colonise new areas
||produces variation - the basis of evolution
||disease may affect all
||slower - needs special processes to bring together gametes and protect zygote, embryo etc during development
||useful when conditions ideal for growth
||may be synchronised with (end of ?) growing season