Reproduction in living organisms
Reproduction is the process by which new organisms (offsprings) are
generated. A living organism does not need reproduction to survive, but as a species,
they need that for continuity and to ensure that they are not extinct.
There are two main types of reproduction: these include sexual reproduction
and asexual reproduction.
This involves two individuals of the same species, usually a male and female.
Here the male and female sex cells come together for fertilization to take place. After
this the newly fertilized cell goes on to become a new organism, the offspring. Note
that not all sexual reproduction involve mating.
This form of reproduction occurs without the involvement of another. Asexual
reproduction is very common in single cell organisms and in many plants. There are
many forms of asexual reproduction. Mitosis, fission, budding, fragmentation,
sporulation and vegetative reproduction are all examples of asexual reproduction. In
unicellular organisms, the parent cell just divides to produce two daughter cells. The
term for kind of cell division is Mitosis
Below is an illustration of the process of
a few years and some live for a few days. The term for the length of time an organism
lives is called their ‘Lifespan’. For instance, an adult mayfly lives for only one day, a
mouse lives for 1-2 years and tortoise can live for about 152 years
But can you imagine what will happen to a species if it had no new ones (offspring)
to replace them? They will be extinct. This means reproduction is essential for the
survival of all species. It also ensures that the characteristics of the parents are passed
on to future generations, ensuring continuity.
The Cell Cycle In Living Organisms
The cell cycle is the recurring sequence of events that includes the duplication
of a cell's contents and its subsequent division. This SparkNote will focus on
following the major events of the cell cycle as well as the processes that regulate its
action. In this and the following SparkNotes on cell reproduction, we will see how
the cell cycle is an essential process for all living organisms. In single-cell organisms,
each round of the cell cycle leads to the production of an entirely new organism.
Other organisms require multiple rounds of cell division to create a new individual.
In humans and other higher-order animals, cell death and growth are constant
processes and the cell cycle is necessary for maintaining appropriate cellular
Figure %: The Cell Cycle
reproduction is to create new cells. The cell cycle is the means by which this goal is
accomplished. While its duration and certain specific components vary from species
to species, the cell cycle has a number of universal trends.
DNA packaged into chromosomes must be replicated.
The copied contents of the cell must migrate to opposite ends of the cell.
The cell must physically split into two separate cells.
We will discuss the general organization of the cell cycle by reviewing its two
major phases: M Phase (for mitosis) and interphase. Interphase is generally split into
three distinct phases including one for DNA replication. We will finish with a
discussion of the elements that control a cell's passage through these various stages.
The cell cycle is very highly regulated to prevent constant cell division and only
allows cell that have met certain requirements to engage in cell division.
How long do the different stages of the cell cycle take?
Replication is one of the hallmark features of living matter. The set of
processes known as the cell cycle which are undertaken as one cell becomes two has
been a dominant research theme in the molecular era with applications that extend far
and wide including to the study of diseases such as cancer which is sometimes
characterized as a disease of the cell cycle gone awry. Cell cycles are interesting both
for the ways they are similar from one cell type to the next and for the ways they are
different. To bring the subject in relief, we consider the cell cycles in a variety of
different organisms including a model prokaryote, for mammalian cells in tissue
culture and during embryonic development in the fruit fly. Specifically, we ask what
are the individual steps that are undertaken for one cell to divide into two and how
long do these steps take?
The 150 min cell cycle of Caulobacter is shown, highlighting some of the key
morphological and metabolic events that take place during cell division. M phase is
not indicated because in Caulobacter there is no true mitotic apparatus that gets
assembled as in eukaryotes. Much of chromosome segregation in Caulobacter (and
other bacteria) occurs concomitantly with DNA replication. The final steps of
chromosome segregation and especially decatenation of the two circular
chromosomes occurs during G2 phase.
Arguably the best-characterized prokaryotic cell cycle is that of the model
organism Caulobacter crescentus. One of the appealing features of this bacterium is
that it has an asymmetric cell division that enables researchers to bind one of the two
progeny to a microscope cover slip while the other daughter drifts away enabling
further study without obstructions. This has given rise to careful depictions of the
≈150 minute cell cycle (BNID 104921) as shown in Figure 1. The main components
of the cell cycle are G1 (first Growth phase,
≈30 min, BNID 104922), where at least
some minimal amount of cell size increase needs to take place, S phase (Synthesis,
≈80 min, BNID 104923) where the DNA gets replicated and G2 (second Growth
≈25 min, BNID 104924) where chromosome segregation unfolds leading to
≈15 min). Caulobacter crescentus provides an
interesting example of the way in which certain organisms get promoted to “model
organism’’ status because they have some particular feature that renders them
particularly opportune for the question of interest. In this case, the cell-cycle
progression goes hand in hand with the differentiation process giving readily
visualized identifiable stages making them preferable to cell-cycle biologists over,
say, the model bacterium E. coli.
The behavior of mammalian cells in tissue culture has served as the basis for
much of what we know about the cell cycle in higher eukaryotes. The eukaryotic cell
cycle can be broadly separated into two stages, interphase, that part of the cell cycle
when the materials of the cell are being duplicated and mitosis, the set of physical
processes that attend chromosome segregation and subsequent cell division. The rates
of processes in the cell cycle, are mostly built up from many of the molecular events
such as polymerization of DNA and cytoskeletal filaments whose rates we have
already considered. For the characteristic cell cycle time of 20 hours in a HeLa cell,
almost half is devoted to G1 (BNID 108483) and close to another half is S phase
(BNID 108485) whereas G2 and M are much faster at about 2-3 hours and 1 hour,
respectively (BNID 109225, 109226). The stage most variable in duration is G1. In
less favorable growth conditions when the cell cycle duration increases this is the
stage that is mostly affected, probably due to the time it takes until some regulatory
size checkpoint is reached. Though different types of evidence point to the existence
of such a checkpoint, it is currently very poorly understood. Historically, stages in the
cell cycle have usually been inferred using fixed cells but recently, genetically-
encoded biosensors that change localization at different stages of the cell cycle have
made it possible to get live-cell temporal information on cell cycle progression and
the cell cycle devoted to each of the primary stages of the cell cycle. The area of each
chart is proportional to the overall cell cycle duration. Cell cycle durations reflect
minimal doubling times under ideal conditions. (Adapted from “The Cell Cycle –
Principles of Control” by David Morgan.)
How does the length of the cell cycle compare to the time it takes a cell to
synthesize its new genome? A decoupling between the genome length and the
doubling time exists in eukaryotes due to the usage of multiple DNA replication start
sites. For mammalian cells it has been observed that for many tissues with widely
varying overall cell cycle times, the duration of the S phase where DNA replication
occurs is remarkably constant. For mouse tissues such as those found in the colon or
tongue, the S phase varied in a small range from 6.9 to 7.5 hours (BNID 111491).
Even when comparing several epithelial tissues across human, rat, mouse and
hamster, S phase was between 6 and 8 hours (BNID 107375). These measurements
were carried out in the 1960s by performing a kind of pulse-chase experiment with
the radioactively labeled nucleotide thymidine. During the short pulse, the radioactive
compound was incorporated only into the genome of cells in S phase. By measuring
the duration of appearance and then disappearance of labeled cells in M phase one
can infer how long S phase lasted The fact that the duration of S phase is relatively
constant in such cells is used to this day to estimate the duration of the cell cycle from
a knowledge of only the fraction of cells at a given snapshot in time that are in S
phase. For example, if a third of the cells are seen in S phase which lasts about 7
hours, the cell cycle time is inferred to be about 7 hours/(1/3)
≈20 hours. Today these
kinds of measurements are mostly performed using BrdU as the marker for S phase.
We are not aware of a satisfactory explanation for the origin of this relatively
constant replication time and how it is related to the rate of DNA polymerase and the
density of replication initiation sites along the genome.
The diversity of cell cycles is shown in Figure 2 and depicts several model
organisms and the durations and positioning of the different stages of their cell
cycles. An extreme example occurs in the mesmerizing process of embryonic
development of the fruit fly Drosophila melanogaster. In this case, the situation is
different from conventional cell divisions since rather than synthesizing new
cytoplasmic materials, mass is essentially conserved except for the replication of the
genetic material. This happens in a very synchronous manner for about 10
generations and a replication cycle of the thousands of cells in the embryo, say
between cycle 10 and 11, happens in about 8 minutes as shown in Figure 2 (BNID
103004,103005, 110370). This is faster than the replication times for any bacteria
even though the genome is
≈120 million bp long (BNID 100199). A striking example
of the ability of cells to adapt their temporal dynamics.
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 make up, differentiation is a
function of which particular genes are either expressed or repressed. 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,
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.
Heredity And Variability
Heredity refers to the genetic transmission of traits from parents to offspring.
Heredity helps explain why children tend to resemble their parents, as well as how a
genetic disease runs in a family. Some genetic conditions are caused by mutations in
a single gene. These conditions are usually inherited in one of several straightforward
patterns, including autosomal dominant, autosomal recessive, X-linked dominant, X-
linked recessive, codominant, and mitochondrial inheritance patterns. Complex
disorders and multifactorial disorders are caused by a combination of genetic and
environmental factors. These disorders may cluster in families, but do not have a
clear-cut pattern of inheritance.
Evolution : a process of development in which an organ or organism becomes
more and more complex by the differentiation of its parts; a continuous and
progressive change according to certain laws and by means of resident forces
bathmic or orthogenic evolution : evolution due to something in the organism
itself independent of environment
convergent evolution : the appearance of similar forms and/or functions in two
or more lines not sufficiently related phylogenetically to account for the similarity.
The concept that chance reigns supreme may ring less true when it comes to complex
behaviors. A study of the similarities between the webs of different Tetragnatha
spider species on different Hawaiian Islands provides fresh evidence that behavioral
tendencies can actually evolve rather predictably, even in widely separated places.
The spiders' webs vary significantly, with tissue-like 'sheet webs', disorganized
cobwebs and spiral-shaped 'orb webs' as three of the most common types. Each
species had its own characteristic type of web. But the scientists found that in several
cases, separate species of Tetragnatha spiders on different islands constructed
extremely similar orb webs, right down to the number of spokes, and the lengths and
densities of the sticky spiral that captures bugs. Was this an example of similar
environments producing the same complex behavior, or did the spiders with
corresponding webs share a common ancestor? The tree that linked spiders through
their web-constructing behavior proved highly improbable as it was very
complicated, and contradicted the relationships suggested by their DNA. It is likely
that similar forest types support similar mixes of prey, which could elicit similar web
structures. Previous research has found that physical traits, for example legs or wings,
can arise independently in similar environmental conditions. And various groups
have looked at the evolution of simple behaviors, such as where species locate
themselves within a habitat, like a branch or lake. But the evolution of complex
behaviors is less well understood : predictable evolutionary convergence of behavior
applies far beyond spiders, and happens more often then some believe
- emergent evolution : the assumption that each step in evolution produces
something new and something that could not be predicted from its antecedents.
- organic evolution : the origin and development of species; the theory that
existing organisms are the result of descent with modification from those of past
- parallel evolution : the independent evolution of similar structures in two or
- salutatory evolution : evolution showing sudden changes; mutation or
halmatogenesis / salutatory variation : a sudden alteration of type from
- darwinism / darwinian theory : the theory of evolution by Charles Robert
Darwin according to which higher organisms have developed from lower ones
through the influence of natural selection
adaptive plasticity in response to environmental pressures : snake
that specify a large head size, or head growth may be increased in individual snakes
to meet local demands (adaptive developmental plasticity).
- monogenesis : the theory of evolution according to which the course of
evolution is fixed and predetermined by law, no place being left for chance
- an adaptations programme has dominated evolutionary thought in England
and the United States during the past 40 years. It is based on faith in the power of
natural selection as an optimizing agent. It proceeds by breaking an organism into
unitary 'traits' and proposing an adaptive story for each considered separately. Trade-
offs among competing selective demands exert the only brake upon perfection; non-
optimality is thereby rendered as a result of adaptation as well. Some criticize this
approach and attempt to reassert a competing notion (long popular in continental
Europe) that organisms must be analyzed as integrated wholes, with Bauplane so
constrained by phyletic heritage, pathways of development and general architecture
that the constraints themselves become more interesting and more important in
delimiting pathways of change than the selective force that may mediate change
when it occurs. Some fault the adaptationist programme for its failure to distinguish
current utility from reasons for origin (male tyrannosaurs may have used their
diminutive front legs to titillate female partners, but this will not explain why they got
so small); for its unwillingness to consider alternatives to adaptive stories; for its
reliance upon plausibility alone as a criterion for accepting speculative tales; and for
its failure to consider adequately such competing themes as random fixation of
alleles, production of non-adaptive structures by developmental correlation with
selected features (allometry, pleiotropy, material compensation, mechanically forced
correlation), the separability of adaptation and selection, multiple adaptive peaks, and
current utility as an epiphenomenon of non-adaptive structures. Some support
Darwin's own pluralistic approach to identifying the agents of evolutionary change
- the theory of intelligent design (ID)makes the claim that the existence of
complex systems and phenomena, lacking any justification for their existence that is
known to us, implies that such systems exist as the purposeful result of the activity of
a powerful, conscious being that designed the visible complexity into them. This is
not a scientific explanation, as it posits the existence of something that cannot be
tested or demonstrated by experiment, but must be taken on faith. The contrast
between the theory of intelligent design and the theory of special creation is that the
latter names the designer "God" and declares the story in the biblical book of Exodus
as the whole truth, whereas the former does not name the designer nor does it declare
any particular story of the designer's works and actions to be historical truth.
However, both of these theories are theology, not biology, and while not identical, are
both out of place in a life science journal. Theologians, and even scientists, are
entitled to logically debate questions of faith surrounding the problems of first causes,
complexity, the existence of evil, and so forth, but not in scientific publications.
Albert Einstein is quoted as having said, "Science without religion is lame; religion
without science is blind." Let us be clear, however: science is about knowledge
gained by hypothesis testing, and religion is about faith gained from reason,
inspiration, and introspection. We must keep them properly separated to understand
the difference between that which we can know and that which we must choose, or
choose not, to believe.
- first proposed by W.D. Hamilton in 1964, the theory of kin selection holds
that altruistic cooperative behavior preferentially directed at helping a relative is
favored because it helps that relative do better and reproduce, which indirectly helps
the cooperator to pass on its genes. Generating siderophores is costly to producer
Pseudomonas aeruginosa (cooperators), but others around it can use the siderophores
to their own benefit without paying the price (cheaters). When relatedness is high, the
cooperators spread to fixation and take over; and when relatedness is low, the
cheaters spread to take over, meaning that higher relatedness had a tendency to favor
selection for more altruism or cooperation. Another more subtle effect of kin
selection is the scale of competition—whether competition is local (competition
between close relatives) or global (competition between unrelated bacteria of the
same species). Relatedness increases cooperation, so that over time, a localized group
of highly related organisms emerges. But eventually, these would also become the
closest competitors in the local area, so they were the ones you had to compete with
for spots in the gene pool in the next generation. The experimental effects of
relatedness on the scale of competition explained > 90% of the variation in the
frequency of cooperators versus cheaters at the end of the experiment. The work has
implications for social insects : if individual insects are close relatives but are going
be dispersing to some other area, or maybe foraging in different areas or looking in
different areas for mates, then the scale at which competition might take place is
going to vary quite a bit depending on the ecology of that particular insect.