How do Organisms reproduce? Flashcards
Why is reproduction at its most basic level will involve making
copies of the blueprints of body design.?
Organisms look similar because their body designs are similar. If body
designs are to be similar, the blueprints for these designs should be
similar. Thus, reproduction at its most basic level will involve making
copies of the blueprints of body design
Why sis the DNA called the basic event in reproduction?
the chromosomes in the nucleus of a cell contain information for inheritance
of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. The DNA in the cell nucleus is the information source for making proteins. If the information is changed, different proteins will be made. Different proteins will eventually lead to altered body designs.
What is the importance of DNA copying in reproduction?
- the chromosomes in the nucleus of a cell contain information for inheritance
of features from parents to next generation in the form of DNA (DeoxyriboNucleic Acid) molecules. The DNA in the cell nucleus is the information source for making proteins. If the information is changed, different proteins will be made. Different proteins will eventually lead to altered body designs. - Therefore, a basic event in reproduction is the creation of a DNA
copy. Cells use chemical reactions to build copies of their DNA. This
creates two copies of the DNA in a reproducing cell, and they will need to
be separated from each other. However, keeping one copy of DNA in the original cell and simply pushing the other one out would not work, because the copy pushed out would not have any organised cellular
structure for maintaining life processes. Therefore, DNA copying is
accompanied by the creation of an additional cellular apparatus, and
then the DNA copies separate, each with its own cellular apparatus.
Effectively, a cell divides to give rise to two cells. - To determine if the two cells will be identical will depend on how
accurately the copying reactions involved occur. No bio-chemical reaction
is absolutely reliable. Therefore, it is only to be expected that the process
of copying the DNA will have some variations each time. As a result, the
DNA copies generated will be similar, but may not be identical to the
original. Some of these variations might be so drastic that the new DNA
copy cannot work with the cellular apparatus it inherits. Such a newborn
cell will simply die. On the other hand, there could still be many other
variations in the DNA copies that would not lead to such a drastic
outcome. Thus, the surviving cells are similar to, but subtly different
from each other. This inbuilt tendency for variation during reproduction
is the basis for evolution
Why is variation beneficial to the species but not necessarily
for the individual?
- Populations of organisms fill well-defined places, or niches, in the
ecosystem, using their ability to reproduce. The consistency of DNA
copying during reproduction is important for the maintenance of body
design features that allow the organism to use that particular niche. Reproduction is therefore linked to the stability of populations of species. - However, niches can change because of reasons beyond the control
of the organisms. Temperatures on earth can go up or down, water levels
can vary, or there could be meteorite hits, to think of a few examples. If
a population of reproducing organisms were suited to a particular niche
and if the niche were drastically altered, the population could be wiped
out. However, if some variations were to be present in a few individuals
in these populations, there would be some chance for them to survive.
Thus, if there were a population of bacteria living in temperate waters,
and if the water temperature were to be increased by global warming,
most of these bacteria would die, but the few variants resistant to heat
would survive and grow further. Variation is thus useful for the survival
of species over time.
Different types of fission.
- Many bacteria and protozoa simply split into two equal halves during
cell division. In organisms such as Amoeba, the splitting of the two cells
during division can take place in any plane. - some unicellular organisms
show somewhat more organisation of their bodies, such as is seen in Leishmania (which cause kala-azar), which have a whip-like structure at one end of the cell. In such
organisms, binary fission occurs in a definite orientation in relation to these structures. - Other single-celled organisms, such as the malarial parasite, Plasmodium, divide into many daughter cells simultaneously by multiple fission.
- Yeast, on the other hand, can put out small buds that separate and
grow further.
Modes of Reproduction used by single organisms.
- Fission
- Fragmentation
- Regeneration
- Budding
- Vegetative propagation.
- Spore formation
Why cell by cell division is not possible in multicellular organisms?
- In multi-cellular organisms with relatively simple body organisation,
simple reproductive methods can still work. Spirogyra, for example, simply breaks up into smaller pieces upon maturation. These pieces or fragments grow into new individuals. - This is not true for all multi-cellular organisms. They cannot simply divide cell-by-cell. The reason is that many multi-cellular organisms, as we have seen, are not simply a random collection of cells. Specialised cells are organised as tissues, and tissues are organised into organs, which then have to be placed at definite positions in the body. In such a carefully organised situation, cell-by-cell division would be impractical.
Multi-cellular organisms, therefore, need to use more complex ways of reproduction. - A basic strategy used in multi-cellular organisms is that different cell types perform different specialised functions. Following this general pattern, reproduction in such organisms is also the function of a specific cell type.
- reproduction can be achieved from a single cell type, if the organism itself consists of many cell types if there is a single cell type in the organism that is capable of growing, proliferating and making other cell types under the right circumstances.
Define regeneration.
Many fully differentiated organisms have the ability to give rise to new individual organisms from their body parts. That is, if the individual is somehow cut or broken up into many pieces, many of these pieces grow
into separate individuals. For example, simple animals like Hydra and Planaria can be cut into any number of pieces and each piece grows into a complete organism. This is known as regeneration.
What is development in regeneration?
Regeneration is carried out by specialised cells. These cells proliferate
and make large numbers of cells. From this mass of cells, different cells
undergo changes to become various cell types and tissues. These changes take place in an organised sequence referred to as development. However,
regeneration is not the same as reproduction, since most
organisms would not normally depend on being cut up to be able to reproduce.
Define budding.
Organisms such as Hydra use regenerative cells for reproduction in the process of budding. In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site . These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.
Why is vegetative propagation practised for growing some types of
plants?
- There are many plants in which parts like the root, stem and leaves develop into new plants under appropriate conditions. Unlike in most animals, plants can indeed use such a mode for reproduction. This
property of vegetative propagation is used in methods such as layering or grafting to grow many plants like sugarcane, roses, or grapes for agricultural purposes. - Plants raised by vegetative propagation can bear
flowers and fruits earlier than those produced from seeds. Such methods also make possible the propagation of plants such as banana, orange,
rose and jasmine that have lost the capacity to produce seeds. Another advantage of vegetative propagation is that all plants produced are genetically similar enough to the parent plant to have all its characteristics. - Similarly buds produced in the notches along the leaf
margin of Bryophyllum fall on the soil and develop into new plants.
How will an organism be benefitted if it reproduces through spores?
- Even in many simple multi-cellular organisms, specific reproductive
parts can be identified. The thread-like structures that developed on
the bread are the hyphae of the bread mould (Rhizopus). They are not reproductive parts. On the other hand, the tiny blob-on-a-stick structures are involved in reproduction. The blobs are sporangia, which contain cells, or spores, that can eventually develop into new Rhizopus individuals . The spores are covered by thick walls that protect them until they come into
contact with another moist surface and can begin to grow
