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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.

Sexual 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.

Asexual reproduction:

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 mitosis:



Living organisms do not live forever. Some live for many years, others live for 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 yearsautoshape 10


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 conditions.

Figure %: The Cell Cycle

As we discussed in theIntroduction to Cell Reproduction, the goal of cellular 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?


Figure 1:

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 phase, ≈25 min, BNID 104924) where chromosome segregation unfolds leading to cell division (final phase lasting ≈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 arrest.



Figure 2:

Cell cycle times for different cell types. Each pie chart shows the fraction of 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: growthmorphogenesis (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 times.

- parallel evolution : the independent evolution of similar structures in two or more rather closely related organisms

- salutatory evolution : evolution showing sudden changes; mutation or saltation.

o halmatogenesis / salutatory variation : a sudden alteration of type from one generation to another

- 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

o adaptive plasticity in response to environmental pressures : snake populations that persistently encounter large prey may accumulate gene mutations 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.


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