Control of Growth & Development

Genetic Control - On and Off Genes

This regulation of growth tutorial looks at the various factors involved that affect growth. When it comes to the crunch, it is the genetic coding of our bodies that determine the way we are and how we work, with the external environment either emphasising or inhibiting the effectiveness of some of these genes.

The Role of Genes

Genes are the blueprint of our bodies, a blueprint that creates the variety of proteins essential to any organisms survival. These proteins, which are used in countless ways by our bodies are produced by genetic sequences, i.e. our genes, as described in the cell biology section, protein synthesis pages.

Utilization of Genetic Information

All cells have originated from the single zygote cell that formed it, and therefore possess all the genetic information that was held in that zygote. This means that an organism could be cloned from the genetic information in the nucleus of one cell, regardless of the volume of cells that make the organism (be it one or billions).

However, this brings about the following question, how can cells become differentiated and specialised to perform a particular function if they are all the same? The answer to this is each cell performing its unique role has some of its genes 'switched on' and some 'switched off'.

In light of this, the cells in our body still contain the same genetic information, though only a partial amount of this information is being used in any one cell.

Switched On and Switched Off

Some genes are permanently switched on, because they contain the blueprint for vital metabolites (enzymes required for respiration etc). However, since cells become specialised in multi-cellular organisms such as ourselves, some genes become switched off because they are no longer required to be functional in that particular cell or tissue.

For instance, insulin is produced in pancreas cells, which must have the gene that codes for insulin switched on, and perhaps other genes that are un-related to the role of the pancreas can be switched off.

Some other genes that will be functional during specialisation determine the physical characteristics of the cell, i.e. long and smooth for a muscle cell or indented like a goblet cell

Skin Colour

Skin colour is an excellent example of genetic control at work. Skin colour depends on the degree of melanin found in skin cells. The amount of melanin is pre-determined by the genetic blueprint of some genes in each cell. To be exact, there are two genes that control the production of melanin, each of which has a dominant and recessive expression. This leads to a possible 16 combinations of genotype when coding for skin colour, as seen below.

Possible combinations of genotype coding for skin colour via melanin

Key
# Genotype
1 M1M1M2M2
2 M1M1M2m2
3 M1M1m2m2
4 M1m1m2m2
5 m1m1m2m2


Phenotype
Black Skin
Dark Brown Skin
Brown Skin
Light Brown Skin
White Skin

Although there are 16 possible combinations in expressing the skin phenotype, there are 5 different possible genotypes that the genes of melanin can express for, as indicated above. Each expression of melanin has an accumulating effect on skin tone, until maximum expression of melanin through 4 dominant alleles leads to a black skin phenotype.

Therefore, when any person is born, they will be one of five colours. After this, external factors such as UV sunlight from the sun will change the skin colour away from the genetic expression of its initial colour.

Eye Colour

Melanin is also present in the iris of the eye, therefore its accumulating effect on colour determines the colour of the eye depending on how many dominant and recessive alleles are expressed. The coding for brown eyes is dominant to the coding of blue eyes.

Albinism

Albinism is an occurrence caused by a deficiency of a particular enzyme in a biochemical pathway. The resultant effect is that no melanin is present in the organism, which show pale eyes and white hair/skin. This is not a lethal occurrence in organisms, provided they are not over exposed to ultraviolet radiation from the sun, which can be carcinogenic.

More information about the way genes control and determine the make up of our body is investigated upon on the next page.


Gene Action - Operon Hypothesis

As mentioned in the genetic control page of this tutorial, some genes are switched on and off depending on the function of the cell. Genes that code for vital metabolites, i.e. essential proteins always remain switched on.

Operon Hypothesis - Enzyme Induction

There is a model hypothesised by two famous scientists, Jacob and Monod that illustrates how certain genes can be active or inactive.

The following diagrams the basics of their model of an operon (where it is possible for their to be more than one structural gene and operator gene). The goal of these genes is to breakdown lactose (a sugar) when it is present in the cell.

Basic Jacob-Monod Operon Model

Lactose Absent

In the absence of lactose, the repressor molecule is freely available to disable the gene

When lactose is absent, the repressor molecule attaches itself to the operator gene, effectively switching it off.

Lactose Present

In the presence of lactose, the repressor molecule is inhibited and the gene remains switched on, and codes for beta galactosidase

Inborn Error of Metabolism

Sometimes for hereditary reasons, or mutations, the biochemical pathway cannot be fully executed, due to a dysfunctional or missing gene.

Hereditary defects are known as inborn errors of metabolism, and are present since birth. Mutations can occur at the initial meiotic stages in the formation of gametes, and are possible throughout an organisms lifetime preventing the induction of various enzymes.

Gene Control of Hormones

The next page investigates how genes control the production of hormones within the endocrine system of animals.


Hormone Production

Hormones are chemical messengers produced by glands in the endocrine system. Endocrine tissues are specialised to produce such hormones, which have genes switched on according the the hormones they were designed to produce. The protein synthesis pages in the cell biology tutorial go into depth about protein production, which is relevant here as hormones consist of proteins.

The pituitary gland situated at the back of the brain is responsible for the creation of many hormones that are related to growth in animals. The amount of each hormone produce is regulated by the hypothalamus, a part of the brain situated next to the pituitary gland.

Hypothalamus and the Pituitary Gland

As stated, the hypothalamus is responsible for regulating the release of hormones from the pituitary gland. It is responsible for secreting releaser factors which instruct the pituitary gland to secret certain hormones. This is illustrated below.

Hormone target tissue example

Hormone Use

Essentially, the Jacob-Monod hypothesis explained in the genetic control page of this tutorial is the model that is involved in the production of hormones

Essentially hormones are used on demand by the body to instruct cells in particular tissues to devote their resources to the production of a particular protein

The next page investigates the use of these hormones in animals with successive pages studying plant hormones.


Animal Growth Hormones

As mentioned in previous pages of the tutorial, hormones are produced in the endocrine glands of animals. The pituitary gland and hypothalamus are the most important in regards to control and development.

Somatotrophin - Growth Hormone

The pituitary gland is responsible for the production of a hormone called somatotrophin. Somatotrophin is essential in the fact that it promotes mass production of proteins on a body-wide scale, by accelerating the rate of transport of amino acids; the constituents of a protein.

Thyroid Stimulating Hormone

The same part of the pituitary gland is responsible for thyroid stimulating hormone, or TSH for short. This targets the thyroid gland, also a member of the endocrine system, which in turn promotes the production of thyroxine

Thyroxine

Thyroxine is responsible for controlling the body's metabolic rate, and therefore responsible for the amount of energy consumed and the volume of proteins produced.

Growth Extremes

The over production of somatotrophin can cause gigantism while under production can result in dwarfism. If the substance is over produced during adulthood, the person grows overly big jaws, hands and feet, a condition known as acromegaly.

The next page investigates hormones in plants...


Plant Auxins - Phototropism & Geotropism

As with animals, plants also use a variety of hormones to control their growth and development. A family of hormones called auxins are commonly found in plants, and promote (and sometimes inhibit) growth.

Auxins

Auxins are produced in the meristems of plants (meristems are explained on successive pages).

Auxins are responsible in promoting cell elongation, a process that is required before differentiation of a cell. It is able to this by promoting the intake of water, increasing the elasticity of the cell to cope with the increase of water taken in by the cell.

One of the most common auxins is indole acetic acid.

Indole Acetic Acid (IAA)

Indole Acetic Acid affects the root and shoot tips of the plant, as described below.

Shoot Tip - No matter what the concentration, IAA promotes growth in the shoot area of a plant (though higher concentrations promote growth more) .

Root Tip - High concentrations of auxin inhibit growth while small amounts are enough to promote growth in the root with indole acetic acid.

Phototropism

Auxins also play a part in phototropism, an occurrence that involves plants bending or moving away from light. The shoot tip is responsible for directional movement by the plant in response to sunlight, as this is the area where auxins can be found.

Sunlight eradicates auxin, meaning that the part of the shoot tip of the plant which is receiving direct sunlight will have the least amount of auxin.

The extra auxin present on the shaded side promotes more cell division and elongation, causing the plant to bend towards the sunlight after this lop-sided growth.

Geotropism

Geotropism is a similar occurrence to phototropism where the plant exhibits directional growth in response to gravity. The shoot tip illustrates negative geotropism (grows against force of gravity) while the root tip exhibits positive geotropism (grows in the same direction as gravity).

Apical Dominance

The presence of auxins in the lateral areas of the plant (in between the root and shoot tip) prevent lateral growth. If you cut off the shoot tip of a plant, the lack of 'diffusable' auxins means that they cannot inhibit growth in these lateral areas. This is known as apical dominance.

Leaf Abscission

The presence of auxins in the lateral areas also prevents leaf abscission. In the colder months, auxin concentrations and the rate of photosynthesis drops.

This lack of auxin in the lateral areas results in the forming of an abscission layer at the stalk of the leaf, which weakens its connection with the plant and soon falls off it.

The next page looks at another family of growth hormones, the gibberellin family, with continuing pages looking at the meristems, the sites of plant growth.


Gibberellins and Gibberellic Acid

The family of gibberellins have a similar effect to that of auxins; they promote cell division and elongation. The major difference is that gibberellins in no way inhibit growth.

Gibberellic Acid

Gibberellic Acid is an example of one of the gibberellin family. Regardless of genotype (tall or small plants), more gibberellin equals more lateral growth.

Gibberellins and Germination

Gibberellins are responsible for promoting growth in the embryo of a seed. It does this the following way

Gibberellins initiate this process in Summer, when the external environment exhibits favourable conditions for plant growth.

The previous pages have investigated the various hormones involved in plant growth. The next page investigates the sites of this growth, the meristems.


Plant Meristems and Growth

A common mistake that many people assume is that an increase in size means an increase in growth. This is not the case. Growth is the irreversible increase of cell number, and essentially its dry mass. This is because "fresh mass" is an inaccurate indicator because water levels in organisms fluctuate at different points in its life cycle.

Meristems

Plant growth occurs in areas called meristems, that are the site of repeated cell division of unspecialised cells. These cells differentiate, and become specialised in relation to the function they will perform.

There are two types of meristems; lateral and apical.

Apical Meristems

Apical meristems are the site of primary growth in a plant, and can be found at the root and shoot tips. Here you can find unspecialised cells, which undergo the following sequence to become a functional part of the plant

Lateral Meristems

Lateral meristems coincidentally can be found growing laterally to the plant, they grow out the side of it. Lateral meristems are responsible for secondary thickening, which is required by perennial plants that grow year after year, and need the structural support to continue doing so.

This thickening occurs at the stem and root sections of the plant, and the secondary growth responsible for this thickening occurs in the cambium and cork cambium of the perennial plant.

The cambium completes rings for each successive growth, meaning the plant grows wider in girth. The larger the plant, the wider the girth will be required to support the plant upright. This cambium tissue continues to grow outwards forming layer upon layer of new living mass. On the outer layer of the plant, cork cambium forms to provide a protection against pathogens.

New layers formed also form vascular bundles consisting of phloem and xylem, which will aid transporting resources around the plant such as water and minerals. Unspecialised cells called parenchyma form the medullary rays which reach out laterally across a plant and are present for the transport of water to the outer regions.

As the continually growing outer layer expands, small gaps in the cambium called lenticels are found to assist gaseous exchange in the plant. Essentially, minerals and water come from the inner areas for the cambium and required gas (CO2) comes from the immediate external environment.

This repeated lateral growth gives arise to the question why a trees age can be defined by rings formed by the cambium growth

Annual Rings

In Summer, the growth mentioned above can be executed much faster by the plant. There are a number of reasons for this

Due to these favourable conditions, cambium is at its most active state, and therefore this is when the most growth occurs. Cells are visible more developed, more elongated etc.

The opposite occurs in Winter, when conditions are less favourable, and therefore cell growth occurs over a smaller volume of area. These condensed areas of growth appear like rings to the human eye. This is how humans can tell its age due to the apparent age of the tree being deduced from the amount of winters that the cambium has grown.


Effect of Chemicals on Growth & Development in Organisms

Plants

Plants require a large number of elements to function properly, mainly carbon, oxygen and hydrogen, essentially used in many biochemical pathways.

In addition to these essential elements, plants also require a dose of nitrogen, phosphorous, potassium and magnesium. A well known fertiliser used to increase crop yield is nicknamed NPK, the chemical symbols for the first three of these macro elements.

A lack of any of these elements can result in the following phenotypes

Herbicides

Herbicides are artificially created chemicals that are used by humans on plants considered as 'pests'. These may be weeds in someone's garden, a farmers crop, or an undesired influx of a species into a foreign environment.

They are effective by accelerating the plants metabolism rate to such an extent that food supplies are totally exhausted, leaving the plant to die. Herbicides are applied by humans to destroy these pests and minimise competition for the other desirable plants in the area.

Animals

Animals have to actively find sources of food to be used in their bodies, and therefore a variety of substances are required by animals, i.e. proteins, carbohydrates, vitamins, minerals and fats. Their are also many macro-elements which are of use to the body. However, some elements are also dangerous to the health of animals, as indicated below

Lead

The presence of lead in the bloodstream can cause a variety of complications to the animal at hand. Lead can lead to brain damage, and prevent the action of the enzyme catalase, which is responsible for the breakdown of poisonous hydrogen peroxide which accumulates in the body

Calcium

Calcium is required for the proper formation of bones and teeth in animals. They are also an important constituent in exoskeletons, such as the outer shell of tortoises.

Vitamins

Vitamins are required for a variety of reasons in the animal body.

More information relating to vitamins and a balanced diet is available in the developmental biology tutorial.

Iron

Iron is a constituent of some enzymes, and also forms cytochrome, required in respiration, and haemoglobin, which is a respiratory pigment responsible for absorption of oxygen. Anaemia is a condition resulting in iron lacking in an animals diet, which results in a lack of red blood cells, or a depletion of haemoglobin content.

Nicotine 

Nicotine is a mild stimulant found in tobacco plants, and can be smoked or chewed. It contains thousands of carcinogenic substances, most notably carbon monoxide and tar. Carbon monoxide is poisonous and found to be around 200 times more absorbable into the blood stream than oxygen, therefore reducing the amount of oxygen available to the body through the bloodstream.

Tobacco has been found to cause cancer, narrow the arteries and generally leads to a declining health due to its numerous substances taking their toll on the body. Nicotine is smoke to produce a mild relaxing effect and is believed to induce a feeling of psychological dependence.

Alcohol

Alcohol is also a poison to the body, yet produces a stimulating effect on the nervous system, and produces a relaxing effect as a depressant.

Alcohol has a toxic effect on the liver, and also effects the uptake of certain ions in the body, and in this instance can prove lethal to an unborn child.

For men, it is believed that alcohol can damage sperm production, and lower sperm count over the longer term. Alcohol can be physiologically and psychologically addictive, with withdrawal effects including delirium, high blood pressure and a high risk of liver disease.

To conclude the control of growth and development tutorial, the next two pages look at growth patterns and the effect of light on growth.


Lights' Effect on Growth

Plants are the primary producers of energy in any ecosystem, meaning that they bring in new energy to it which supports the life that lives there. For plants the produce their own energy, they require sunlight to conduct photosynthesis.

Critical Period

In today's diversified culture of plants' varying species require different amounts of light in a day. The amount of time that a plant requires is called the critical period. Some plants, for instance, require more than 14 hours of light, and without this amount of light on a regular basis, their stalks become etiolated and the leaves of the plant fail to grow to their potential.

This is partly because their is not enough light available for photosynthesis and therefore not enough ATP available for cellular processes and growth. 

Auxin also plays a part, as light destroys auxin, plants that are immersed in light have cells that do not become as elongated producing a weak stem. Plants who require more than 12 hours of light are deemed 'long day short night plants' because of their light dependant nature.

On the other side of the coin, some plants require longer periods of darkness, these being 'short day long night' plants. These type of plants only flower when the level of sunlight is below 12 hours, or more precise, there is more darkness than sunlight.

Photoperiodism (the length of light that each plant will react to) is stimulated by a pigment called phytochrome. 

Various forms of phytochrome react to these varying wavelengths, depending on which wavelength of light is absorbed by the plant. There is a constant reaction involving phytochrome in plants, which is the plants' indicator of whether it is daytime or night time.

Phytochrome 660 is in abundance in the plant during the day, and converts itself to phytochrome 730 on absorbing light. This unstable form of phytochrome reverts to its original form during the day and once again the cycle starts all over again. This instance can control the degree of photosynthesis that occurs in the plant, while phytochrome is visibly one of the ways that plants react to sunlight.

Phytochrome therefore has a regulating control on how and when photosynthesis occurs, and phytochrome levels during different times of the day will react variously within different species of plant.

When looking at Darwin's outlook on speciation and adaptive radiation, it is more plausible now to see why plants possess dithering characteristics suited to their external environment and their needs. The latitude of where a plant exists also has great bearing on how they will cope in their environment, because, for instance, in the extreme latitudes, there is not sunlight for 6 months, and the following six months their is constant light. Seasons also play a part, and considering both seasons and latitude into account, it is evident why different climates make homes for different varieties of plants.

Plants have a balancing act between flowering, growing and producing energy for the mass already present on the plant. These chemicals, i.e. indole acetic acid, gibberellin and phytochrome all play their part internally, while reacting to the outside environment. With this in hand, all these reactions originate from one thing, the genetic coding that made the plant operate the way it does.

Previous pages have studies the basics of cells, the genes that control them, and some of the cellular processes that occur in animals. The following pages describe how these processes are regulated, by genetic coding and a suitable environment for the reactions to occur in (i.e. enough sunlight to reach a critical period).


Growth Patterns

As mentioned in the tutorial, when measuring growth it is more desirable to use dry mass as a reliable indication. However, since you cant just 'suck' an organism dry, we tend to measure growth as an increase in height or 'fresh' weight.

The following indicates various growth curves for the corresponding organisms. 

Sigmoid Growth Curve

The sigmoid curve is a 'growth average' representing all organisms, where young organisms experience rapid accelerating growth to cope with their environment, followed by a continuous steady growth. Towards maturity, an organisms growth rate slows down until no growth occurs. 

Annual Plant Growth 

Annual plants only live for one year. At first, the embryo of the seed harnesses the seed food supply, and is illustrated by a decrease in growth to begin with as the food supplies shrink. Once this energy kicks in, and photosynthesis can occur in the plant, growth begins to accelerate. Throughout the year the plant will continue to grow, and when it has reached the Winter months, and seeds have been dispersed, it withers and dies, illustrated by the decrease in mass.

Tree Growth 

Perennials such as trees continue to grow year after year, and in the instance of trees, can grow for an immense space of time. As explained on the next page of the control and regulation tutorial, most growth occurs in Spring, illustrated in the graph by the accelerated increases in height.

Human Growth 

Humans have two phases of growth 'spurts', one in infancy and one in adolescance. In between infancy and adolescance there is a period of steady growth while adulthood is when growth halts.

Insect Growth

Insects possess exoskeletons (an exterior skeleton) which means continuous growth cannot occur without this skeleton allowing so. In light of this, the insects must shed their outer layer for growth to continue. This moulting of the skin allows body mass to increase.


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