All living organisms are capable of growing and producing offspring. All eukaryotic organisms including aquatic plants and algae grow through the process of mitosis. Mitosis is a process where one cell divides into two cells. Chromosomes in the original cell are duplicated to ensure that the two new cells have full copies of the necessary genetic information.
The process of mitosis generates new cells that are genetically identical to each other. Mitosis helps organisms grow in size and repair damaged tissue. Some species of algae are capable of growing very quickly. The giant kelp Macrocystis pyrifera can grow as much as 30 centimeters (cm) in length in a single day.
Some organisms can use mitosis to reproduce asexually. The offspring of asexual reproduction are genetically identical to each other and to their parent. Most single-celled, microorganisms reproduce asexually by duplicating their genetic material and dividing in half. For example, phytoplankton reproduce primarily through asexual reproduction. Some single-celled eukaryotes, including some plants and animals, reproduce asexually in a processes called fragmentation or budding.
Sexual reproduction is the production of offspring through the combination of sex cells or gametes. Meiosis is the process of producing gametes, each of which has half of the genetic material needed to create a new organism.
- Chromosomes are duplicated. Meiosis begins in a fashion similar to mitosis with chromosome replication.
- Matched sets of chromosomes pair together.
- Genes are swapped between matched chromosomes. The process of crossing over, or recombination, exchanges genetic information between chromosomes in a cell. The resulting chromosomes are brand new, unique combinations of genetic information.
- First division separates one of each chromosome pair. The parent cell divides in half as in mitosis, producing two cells with a complete amount of DNA (although they are not identical because of crossing over).
- Second division separates each chromosome, leaving one copy of each chromosome per cell. The two new cells divide a second time to produce four new gametes. These gametes contain one-half of the genetic information needed to form a new individual.
- Each parent provides one gamete to the process of fertilization, which results in a cell called a zygote with a full compliment of chromosomes.
- Offspring produced through sexual reproduction are genetically distinct from both parents, since each of their gametes has a unique combination of chromosomes.
In summary, mitosis produces two identical cells, each with the full amount of DNA. Meiosis produces four genetically unique cells, each with half the amount of DNA. See Table 2.10 for a comparison of mitosis and meiosis.
Asexual reproduction occurs when an organism makes more of itself without exchanging genetic information with another organism through sex.
In sexually reproducing organisms, the genomes of two parents are combined to create offspring with unique genetic profiles. This is beneficial to the population because genetically diverse populations have a higher chance of withstanding survival challenges such as disease and environmental changes.
Asexually reproducing organisms can suffer a dangerous lack of diversity – but they can also reproduce faster than sexually reproducing organisms, and a single individual can found a new population without the need for a mate.
Some organisms that practice asexual reproduction can exchange genetic information to promote diversity using forms of horizontal gene transfer such as bacteria who use plasmids to pass around small bits of DNA. However, this method results in fewer unique genotypes than sexual reproduction.
Some species of plants, animals, and fungi are capable of both sexual and asexual reproduction, depending on the demands of the environment.
Asexual reproduction is practiced by most single-celled organisms including bacteria, archaebacteria, and protists. It is also practiced by some plants, animals, and fungi.
THIS VIDEO EXPLAINS ASEXUAL REPRODUCTION
Advantages of Asexual Reproduction
Important advantages of asexual reproduction include:
- Rapid population growth. This is especially useful for species whose survival strategy is to reproduce very fast.
Many species of bacteria, for example, can completely rebuild a population from just a single mutant individual in a matter of days if most members are wiped out by a virus.
- No mate is needed to found a new population.
This is useful for species whose members may find themselves isolated, such as fungi that grow from wind-blown spores, plants that rely on pollinators for sexual reproduction, and animals inhabiting environments with low population density.
- Lower resource investment. Asexual reproduction, which can often be accomplished just by having part of the parent organism split off and take on a life of its own, takes fewer resources than nurturing a new baby organism.
Many plants and sea creatures, for example, can simply cut a part of themselves off from the parent organism and have that part survive on its own.
Only offspring that are genetically identical to the parent can be produced in this way: nurturing the creation of a new organism whose tissue is different from the parents’ tissue takes more time, energy, and resources.
This ability to simply split in two is one reason why asexual reproduction is faster than sexual reproduction.
Disadvantages of Asexual Reproduction
The biggest disadvantage of asexual reproduction is lack of diversity. Because members of an asexually reproducing population are genetically identical except for rare mutants, they are all susceptible to the same diseases, nutrition deficits, and other types of environmental hardships.
The Irish Potato Famine was one example of the downside of asexual reproduction: Ireland’s potatoes, which had mainly reproduced through asexual reproduction, were all vulnerable when a potato-killing plague swept the island. As a result, almost all crops failed, and many people starved.
The near-extinction of the Gros-Michel banana is another example – one of two major cultivars of bananas, it became impossible to grow commercially in the 20th century after the emergence of a disease to which it was genetically vulnerable.
On the other hand, many species of bacteria actually take advantage of their high mutation rate to create some genetic diversity while using asexual reproduction to grow their colonies very rapidly. Bacteria have a higher rate of errors in copying genetic sequences, which sometimes leads to the creation of useful new traits even in the absence of sexual reproduction.
Types of Asexual Reproduction
There are many different ways to reproduce asexually. These include:
- Binary fission. This method, in which a cell simply copies its DNA and then splits in two, giving a copy of its DNA to each “daughter cell,” is used by bacteria and archaebacteria.
- Budding. Some organisms split off a small part of themselves to grow into a new organism. This is practiced by many plants and sea creatures, and some single-celled eukaryotes such as yeast.
- Vegetative propagation. Much like budding, this process involves a plant growing a new shoot which is capable of becoming a whole new organism. Strawberries are an example of plants that reproduce using “runners,” which grow outward from a parent plant and later become separate, independent plants.
- Sporogenesis. Sporogenesis is the production of reproductive cells, called spores, which can grow into a new organism.
Spores often use similar strategies to those of seeds. But unlike seeds, spores can be created without fertilization by a sexual partner. Spores are also more likely to spread autonomously, such as via wind, than to rely on other organisms such as animal carriers to spread.
- Fragmentation. In fragmentation, a “parent” organism is split into multiple parts, each of which grows to become a complete, independent “offspring” organism. This process resembles budding and vegetative propagation, but with some differences.
For one, fragmentation may not be voluntary on the part of the “parent” organism. Earthworms and many plants and sea creatures are capable of regenerating whole organisms from fragments following injuries that split them into multiple pieces.
When fragmentation does occur voluntarily, the same parent organism may split into many roughly equal parts in order to form many offspring. This is different from the processes of budding and vegetative propagation, where an organism grows new parts which are small compared to the parent and which are intended to become offspring organisms.
- Agamenogenesis. Agamenogenesis is the reproduction of normally sexual organisms without the need for fertilization. There are several ways in which this can happen.
In parthenogenesis, an unfertilized egg begins to develop into a new organism, which by necessity possesses only genes from its mother.
This occurs in a few species of all-female animals, and in females of some animal species when there are no males present to fertilize eggs.
In apomoxis, a normally sexually reproducing plant reproduces asexually, producing offspring that are identical to the parent plant, due to lack of availability of a male plant to fertilize female gametes.
In nucellar embryony, an embryo is formed from a parents’ own tissue without meiosis or the use of reproductive cells. This is primarily known to occur in citrus fruit, which may produce seeds in this way in the absence of male fertilization.
Examples of Asexual Reproduction
All bacteria reproduce through asexual reproduction, by splitting into two “daughter” cells that are genetically identical to their parents.
Some bacteria can undergo horizontal gene transfer – in which genetic material is passed “horizontally” from one organism to another, instead of “vertically” from parent to child. Because they have only one cell, bacteria are able to change their genetic material as mature organisms.
The process of genetic exchange between bacterial cells is sometimes referred to as “sex,” although it is performed to change the genotype of a mature bacterium, not as a means of reproduction.
Bacteria can afford to use this survival strategy because their extremely rapid reproduction makes harmful genetic mutations – such as copying errors or horizontal gene transfer gone wrong – inconsequential to the whole population. As long as a few individuals survive mutation and calamity, those individuals will be able to rebuild the bacterial population quickly.
This strategy of “reproduce fast, mutate often” is a major reason why bacteria are so quick to develop antibiotic resistance. They have also been seen to “invent” whole new biochemistries in the lab, such as one species of bacteria that spontaneously acquired the ability to perform anaerobic respiration.
This strategy would not work well for an organism that invests highly in the survival of individuals, such as multicellular organisms.
Slime molds are a fascinating organism that sometimes behave like a multicellular organism, and sometimes behave like a colony of single-celled organisms.
Unlike animals, plants, and fungi, the cells in a slime mold are not bound together in a fixed shape and dependent on each other for survival. The cells that make up a slime mold are capable of living individually and may spread or separate when food is abundant, much like individuals in a colony of bacteria.
But slime mold cells are eukaryotic, and can display a high degree of cooperation to the point of creating a temporary extracellular matrix and a “body” which may become large and complex. Slime molds whose cells are working cooperatively can be mistaken for fungi, and can perform locomotion.
Slime molds can produce spores much like a fungus, and they can also reproduce through fragmentation. Environmental causes or injury may cause a slime mold to disperse into many parts, and units as small as a single cell may grow into a whole new slime mold colony/organism.
New Mexico Whiptail Lizards
This species of lizard was created by the hybridization of two neighboring species. Genetic incompatibility between the hybrid parents made it impossible for healthy males to be born: however, the female hybrids were capable of parthenogenesis, making them a reproductively independent population.
All New Mexico whiptail lizards are female. New members of the species can be created through hybridization of the parent species, or through parthenogenesis by female New Mexico whiptails.
Possibly as a remnant of their sexually reproducing past, New Mexico whiptail lizards do have a “mating” behavior which they must go through to reproduce. Members of this species are “mated with” by other members, and the lizard playing the female role will go onto lay eggs.
It is thought that the mating behavior stimulates ovulation, which can then result in a parthenogenic pregnancy. The lizard playing the “male” role in the courtship does not lay eggs.
Sexual reproduction in animals
During sexual reproduction in animals, a haploid sperm and unites with a haploid egg cell to form a diploid zygote. The zygote divides mitotically and differentiates into an embryo. The embryo grows and matures. After birth or hatching, the animal develops into a mature adult capable of reproduction. Some invertebrates reproduce by self-fertilization, in which an animal’s sperm fertilizes its own eggs.
Self-fertilization is common in tapeworms and other internal parasites, which lack the opportunity to find a mate. Most animals, however, use cross fertilization, in which different individuals donate the egg and the sperm. Even hermaphrodites animals (such as the earthworms) that produce both types of gametes use cross-fertilization.
Animals exhibit two patterns for bringing sperm and eggs together. One is external fertilization, whereby animals shed eggs and sperm into the surrounding water.
The flagellated sperm need an aquatic environment to swim to the eggs, the eggs require water to prevent drying out. Most aquatic invertebrates, most fish, and some amphibians use external fertilization. These animals release large numbers of sperm and eggs, thereby overcoming large losses of gametes in the water. In addition, courting behavior in some species brings about the simultaneous release of the gametes, which helps insure that sperm and egg meet.
The other pattern of sexual reproduction is internal fertilization, whereby the male introduces sperm inside the females reproductive tract where the eggs are fertilized. Internal fertilization is an adaption for life on land, for it reduces the loss of gametes that occurs during external fertilization. Sperms are provided with a fluid (semen) that provides an aquatic medium for the sperm to swim when inside the male’s body. Mating behavior and reproductive readiness are coordinated and controlled by hormones so that sperm and egg are brought together at the appropriate time.
After internal fertilization, most reptiles and all birds lay eggs that are surrounded by a tough membrane or a shell. Their eggs have four membranes, the amnion, the allantois, the yolk sac and the chorion. The amnion contains the fluid surrounding the embryo; the allantois stores the embryo’s urinary wastes and contains blood vessels that bring the embryo oxygen and take away carbon dioxide. The yolk sac holds stored food, and the chorion surrounds the embryo and the other membranes. After the mother lays her eggs, the young hatch.
Mammals employ internal fertilization, but except for the Australian montremes such as the duckbill platypus and the echidna, mammals do not lay eggs. The fertilized eggs of mammals implant in the uterus which develops into the placenta, where the growth and differentiation of the embryo occur. Embryonic nutrition and respiration occur by diffusion from the maternal bloodstream through the placenta. When development is complete, the birth process takes place.
THIS VIDEO EXPLAINS SEXUAL REPRODUCTION IN ANIMALS
Sexual reproduction in lower organisms
In single-celled organisms (e.g., bacteria, protozoans, many algae, and some fungi), organismic and cell reproduction are synonymous, for the cell is the whole organism. Details of the process differ greatly from one form to the next and, if the higher ciliate protozoans are included, can be extraordinarily complex. It is possible for reproduction to be asexual, by simple division, or sexual.
In sexual unicellular organisms the gametes can be produced by division (often multiple fission, as in numerous algae) or, as in yeasts, by the organism turning itself into a gamete and fusing its nucleus with that of a neighbour of the opposite sex, a process that is called conjugation.
In ciliate protozoans (e.g., Paramecium), the conjugation process involves the exchange of haploid nuclei; each partner acquires a new nuclear apparatus, half of which is genetically derived from its mate. The parent cells separate and subsequently reproduce by binary fission. Sexuality is present even in primitive bacteria, in which parts of the chromosome of one cell can be transferred to another during mating.
Multicellular organisms also reproduce asexually and sexually; asexual, or vegetative, reproduction can take a great variety of forms. Many multicellular lower plants give off asexual spores, either aerial or motile and aquatic (zoospores), which may be uninucleate or multinucleate. In some cases the reproductive body is multicellular, as in the soredia of lichens and the gemmae of liverworts. Frequently, whole fragments of the vegetative part of the organism can bud off and begin a new individual, a phenomenon that is found in most plant groups.
In many cases a spreading rhizoid (rootlike filament) or, in higher plants, a rhizome (underground stem) gives off new sprouts. Sometimes other parts of the plant have the capacity to form new individuals; for instance, buds of potentially new plants may form in the leaves; even some shoots that bend over and touch the ground can give rise to new plants at the point of contact.
Among animals, many invertebrates are equally well endowed with means of asexual reproduction.
Numerous species of sponges produce gemmules, masses of cells enclosed in resistant cases,that can become new sponges. There are many examples of budding among coelenterates, the best known of which occurs in freshwater Hydra. In some species of flatworms, the individual worm can duplicate by pinching in two, each half then regenerating the missing half; this is a large task for the posterior portion, which lacks most of the major organs—brain, eyes, and pharynx.
The highest animals that exhibit vegetative reproduction are the colonial tunicates (e.g., sea squirts), which, much like plants, send out runners in the form of stolons, small parts of which form buds that develop into new individuals. Vertebrates have lost the ability to reproduce vegetatively; their only form of organismic reproduction is sexual.
In the sexual reproduction of all organisms except bacteria, there is one common feature: haploid, uninucleate gametes are produced that join in fertilization to form a diploid, uninucleate zygote. At some later stage in the life history of the organism, the chromosome number is again reduced by meiosis to form the next generation of gametes. The gametes may be equal in size (isogamy), or one may be slightly larger than the other (anisogamy); the majority of forms have a large egg and a minute sperm (oogamy).
The sperm are usually motile and the egg passive, except in higher plants, in which the sperm nuclei are carried in pollen grains that attach to the stigma (a female structure) of the flower and send out germ tubes that grow down to the egg nucleus in the ovary. Some organisms, such as most flowering plants, earthworms, and tunicates, are bisexual (hermaphroditic, or monoecious)—i.e., both the male and female gametes are produced by the same individual. All other organisms, including some plants (e.g., holly and the ginkgo tree) and all vertebrates, are unisexual (dioecious): the male and female gametes are produced by separate individuals.
Some sexual organisms partially revert to the asexual mode by a periodic degeneration of the sexual process. For instance, in aphids and in many higher plants the egg nucleus can develop into a new individual without fertilization, a kind of asexual reproduction that is called parthenogenesis.
THIS VIDEO EXPLAINS SEXUAL REPRODUCTION IN LOWER ORGANISMS
Sexual reproduction in plants
Plants can be unisexual and bisexual depending on the parts they contain. Let us understand these parts for plant reproduction in this section.
Parts of a Flower in Plant Reproduction
A bisexual flower typically contains the male and female parts in it. There are other supporting structures as well apart from the reproductive parts for sexual reproduction.
There are four main layers of the parts of a flower:
It is a collection of sepals. The sepals are the green coloured small florets that are considered the first layers of the flower from the base. In some cases, the sepals have colour. They are called petaloid. Their main function is to protect the flower while it is still in the bud stage.
This layer is a collection of petals. It is the second layer of the flower, superior to the calyx layer. The petals are the colourful part of a flower that helps to attract insects and birds to the flower to facilitate pollination.
It is the third layer of the flower superior to the Corolla. This is a term given to the male parts for sexual reproduction of a plant. The androecium is made up of a collection of stamens. Each stamen has the following parts:
- Anther-It is present at the tip of the filament. It is internally lobed. Pollen grains are inside the Anther Lobe.
- Filament-It is a thin stalk-like structure that holds the anther
A Gynoecium is a collection of carpels. It is the fourth layer of a flower. It has three parts:
- Stigma-It is a small and sticky landing structure. The pollen grain from the same or different flower stick to it. This structure acts as a landing for the insects or birds that act as pollinating agents.
- Style-It is a thin stalk-like structure that holds the stigma.
- Ovary-It is the base of the style and contains the ovules which contain the female gametes.
Pollination and Fertilization in Plant Reproduction
The transfer of pollen grains from the anther of one flower to the stigma of the same or another flower is known as pollination. It can be caused by insects, birds, wind, water and animals including man. These are together called as pollinating agents.
Types of Pollination
- Self-Pollination: Self-Pollination is when the pollen of one flower transfers to the stigma of the same flower. Many flowers that are hermaphrodite see this kind of pollination. However, there are many advantages and disadvantages of this type of pollination. Many flowers have various mechanisms to prevent self-pollination or promote cross-pollination.
- Cross-Pollination: Cross-Pollination is when the pollen of one flower transfers to the stigma of another flower. This type of pollination helps brings about genetic variation in the species and allow the plant to withstand changes in the environment better. Once the pollen has landed on the stigma of a flower, the pollen tube develops to transfer the pollen to the ovules which contain the female gamete.
Microsporogenesis results in the formation of Male Gametes and Megasporogenesis results in the formation of Female Gametes.
- The anthers contain the pollen mother cell (2n-diploid) that undergoes meiosis to form microspores.
- Tetrad is the result of the microspore mother cell diving and the formation of 4 microspores.
- The Anther releases the microspores/pollen grains when it is mature.
Megasporangium are the Ovules. They are in the ovary and contain the female gametes. Megasporogenesis is the formation of megaspores from the megaspore mother cell (diploid). The resultant of the meiosis fo the megaspore mother cell is 4 haploid megaspores. Of the four cells that form, only one is functional while the other degenerate.
Double Fertilization happens in angiosperms. This is because the male gamete that enters the ovule has two nuclei. One of the male gametes fuses with the female gamete to form a diploid zygote whereas the other one forms a triploid endosperm by fusing with the diploid polar nuclei. The zygote divides to form the future plant whereas the endosperm provides nutrition to the developing embryo.
Events after fertilization in plant reproduction
After fertilization, the ovary becomes the fruit and the ovules become the seeds. The other structures like the calyx, corolla and the remaining parts of the androecium and gynoecium degenerate or fall off.
THIS VIDEO EXPLAINS SEXUAL REPRODUCTION IN PLANTS
Growth and Development
“Development” and “growth” are sometimes used interchangeably in conversation, but in a botanical sense they describe separate events in the organization of the mature plant body.
Development is the progression from earlier to later stages in maturation, e.g. a fertilized egg develops into a mature tree. It is the process whereby tissues, organs, and whole plants are produced. It involves: growth, morphogenesis (the acquisition of form and structure), and differentiation. The interactions of the environment and the genetic instructions inherited by the cells determine how the plant develops.
Growth is the irreversible change in size of cells and plant organs due to both cell division and enlargement. Enlargement necessitates a change in the elasticity of the cell walls together with an increase in the size and water content of the vacuole. Growth can be determinate—when an organ or part or whole organism reaches a certain size and then stops growing—or indeterminate—when cells continue to divide indefinitely. Plants in general have indeterminate growth.
Differentiation is the process in which generalized cells specialize into the morphologically and physiologically different cells. Since all of the cells produced by division in the meristems have the same genetic makeup, differentiation is a function of which particular genes are either expressed orrepressed. The kind of cell that ultimately develops also is a result of its location: Root cells don’t form in developing flowers, for example, nor do petals form on roots.
Mature plant cells can be stimulated under certain conditions to divide and differentiate again, i.e. to dedifferentiate.
This happens when tissues are wounded, as when branches break or leaves are damaged by insects. The plant repairs itself bydedifferentiating parenchyma cells in the vicinity of the wound, making cells like those injured or else physiologically similar cells.
Plants differ from animals in their manner of growth. As young animals mature, all parts of their bodies grow until they reach a genetically determined size for each species. Plant growth, on the other hand, continues throughout the life span of the plant and is restricted to certain meristematic tissue regions only. This continuous growth results in:
- Two general groups of tissues, primary and secondary.
- Two body types, primary and secondary.
- Apical and lateral meristems.
Apical meristems, or zones of cell division, occur in the tips of both roots and stems of all plants and are responsible for increases in the length of the primary plant body as the primary tissues differentiate from the meristems. As the vacuoles of the primary tissue cells enlarge, the stems and roots increase in girth until a maximum size (determined by the elasticity of their cell walls) is reached. The plant may continue to grow in length, but no longer does it grow in girth. Herbaceous plants with only primary tissues are thus limited to a relatively small size.
Woody plants, on the other hand, can grow to enormous size because of the strengthening and protective secondary tissues produced by lateral meristems, which develop around the periphery of their roots and stems. These tissues constitute the secondary plant body.
THIS VIDEO EXPLAINS PLANT GROWTH AND DEVELOPMENT