Category Archives: Biology

Evolution

Darwin’s Theory

1. Overproduction – Organism reproduce at massive rates, leading to exponential increase in population size.

2. Constancy of Numbers – The capacity of an area is limited, thus the population tends to remain stable and thus, only a fraction of the population would survive, mature and reproduce

3. Struggle for survival – Members of the population struggles to survive, competing for finite resources.

4. Variation – Individuals differ in minute ways in terms of physical, physiological and behavioral characteristics.

5. Survival of the fittest – Certain traits confer upon them an advantage in procuring scarce resources while others will lead to them being disadvantages, resulting in their survival rates differing.

6. Like produce Like – Those that survive and mature would breed and produce off-springs taking after themselves.

7. Formation of a new species – The unequal ability of individuals in the population to survive and reproduce will lead to a shift in the composition of the population over time as with each succeeding generation, the proportion of individuals carrying the advantageous trait increases.

Speciation

Speciation is the process when one or more species arises from a previously existing species as the gene flow in the population is interrupted, as the separated populations cannot interbreed for geographical or behavioral reasons. As biological species are defined in terms of reproductive compatibility, the formation of a new species will require reproductive isolation to eventually form a biological factor preventing interbreeding. These can be in the form of pre or post zygotic barriers.

Prezygotic Barrier:

-        Habitat Isolation : Species occupy different habitats and do not come into contact

-        Temporal Isolation : Species reproduce at different seasons or different times of a day

-        Behavioral Isolation  : Courtship signals differ

-        Mechanical Isolation : Genitalia unsuitable

-        Gamete Isolation : Sperm is unable to fuse with egg

Postzygotic barrier

-        Zygote Mortality : Fertilization occurs but zygote does not survive

-        Hybrid sterility : Hybrid is sterile and is not reproductively viable

-        Fitness : The fitness of the offspring is compromised and survival rates drop

Agents of Evolution

Natural Selection

Differential rates of survival and reproduction alters the allele frequency in a population as favorable allele will be more likely to be passed down as it confers a competitive advantage upon the parent, leading to its higher survival and reproduction likelihood, which in turn results in the allele being passed down to the next generation at a higher frequency. Natural selection first acts on expressed phenotypes before altering genotypes through its effect on allele frequency.

-        Directional selection occurs when the population encounters a different environmental condition favoring a certain extreme phenotype.

-        Disruptive Selection occurs when the gene flow is interrupted and the population may undergo speciation to give separate species.

-        Stabilizing Selection occurs when the environment remains stable over time, promoting phenotypic stability as the species is already optimally suited for the environment.

Evidence of Evolution

-           Anatomical Homology refers to common morphological structures derived from a common ancestor. A basic organism, through descent with modification, would give rise to different populations via natural selection. It can also be reflected in vestigial structures. When the selection pressures that would keep the structure in functional condition are removed, the structure loses its purpose and over generations, would degenerate due to accumulations of mutations that limit its size and shape.

-           Embryological homology refers to development of structures by the embryo. For example, human embryos go through a stage with gill ridges.

-           Molecular Homology can be identifies when cells of the organisms are analyzed at a molecular level, showing similarities in the form of similar genes and amino acid sequences derived from a common ancestor. Closely related species would have less difference in their nucleotide sequence.

-           Fossil records show the succession of organisms and how homologous structures have been modifies through time.

Continental Drift theory shows that identical fossil plants and animals have been discovered on opposite sides of the Atlantic Ocean. This drifting apart of land masses splits organisms by the development of oceanic barriers. Isolating descendent populations and thereby resulting in allopatric speciation

Cell Division

Meiosis

Many steps of meiosis closely resemble corresponding steps in mitosis. Meiosis, like mitosis, is preceded by the replication of chromosomes. However, this single replication is followed by two consecutive cell divisions, called Meiosis 1 and Meiosis 2. These divisions result in 4 daughter cells, each with half as many chromosomes as the parent cell.

Stages of Meiosis

 

Interphase -        Chromosomes replicate during S phase but remain uncondensed

-        The centrosome replicates, forming two centromere

Prophase 1 -        Chromosome begins to condense

-        Homologous chromosomes pair and cross over

Metaphase 1 -        Tetrads line up

-        Pairs of homologous chromosomes arranges on metaphase plate

Anaphase 1 -        Chromosomes move towards the pole, guided by spindle apparatus

-        Homologous chromosomes split, but sister chromatids remain attached at the centromere

Telophase 1 -        In Telophase, cell plates or cleavage furrow forms
Cytokinesis -        In cytokinesis, cell splits
Prophase 2 -        Same as Prophase 1
Metaphase 2 -        Spindle fibres align chromosomes along the equator
Anaphase 2 -        Same as Anaphase 1 but centromeres split
Telophase 2 -        Same as Telophase 1

 

Stage of Mitosis

 

Interphase -        Chromosomes replicate during S phase but remain uncondensed

-        The centrosome replicates, forming two centromere

Prophase -        Chromosome begins to condense

-        Centrioles migrate to the poles of the cell and nuclear membrane disintegrates

Metaphase -        Chromosomes line up

-        Contraction of spindle fibres pull chromatids slightly apart

Anaphase -        Centromere splits and spindle fibre shortens

-        Chromosomes move towards the pole, guided by spindle apparatus

Telophase  -        In Telophase, cell plates or cleavage furrow forms and nuclear membrane forms around the chromatids
Cytokinesis -        In cytokinesis, cell splits

 

 

 

 

 

 

Mitosis vs. Meiosis

 

Mitosis

Meiosis

DNA Replication Occurs during interphase before mitosis begins Occurs during interphase before Meiosis 1
Number of Division One Two
Synapsis of homologous chromosomes Does not occur Occurs during prophase 1, forming tetrads, is associated with crossing over between non-sister chromatids
No. Daughter Cell 2 4
Genetic Composition Genetically identical to the parent cell Containing half as any chromosomes as the parent cell, genetically different from parent
Role in the animal body Enables multicellular adult to arise from zygote. Produces cells for growth and repair Produces gametes, reduces number of chromosomes by half. Genetic variability

 

The key differences between Meiosis and Mitosis are that meiosis reduces the number of chromosome sets from two to one, whereas mitosis conserves the number of chromosome sets. Therefore, mitosis produces daughter cells that are identical to the parent cell and each other whereas meiosis produces cells that differ genetically from their parent cell and from each other.

1. Synapsis and crossing over. During prophase 1, duplicated homologous chromosomes line up and become physically connected along their lengths by a zipper-like protein structure, the synaptonemal complex, this process is called synapsis. Genetic rearrangement between non-sister chromatids, known as crossing over also occurs during Prophase 1. Following disassembly of the synaptonemal complex in late prophase, the four chromatids of a homologous pair are visible in the light microscope as a tetrad. Each tetrad normally contains at least one X-shaped region called a chiasma, the physical manifestation of crossing over. Synapsis and crossing over do not occur during mitosis.

2. Tetrads on the metaphase plate. At metaphase 1 of meiosis, paired homologous chromosomes are positioned on the metaphase place, rather than individual replicated chromosomes, as in mitosis.

3. Separation of homologues. At anaphase 1 of meiosis the duplicate chromosomes of each homologous pair more towards opposite poles, but the sister chromatids of each duplicated chromosome remain attached. In mitosis, sister chromatids separate.

Meiosis 1 is called the reductional division because it halves the number of chromosomes sets per cell – a reduction from 2 sets to one set. The sister chromatids then separate during the second meiotic division, meiosis 2, producing haploid daughter cells. The mechanism for separating sister chromatids is virtually identical in meiosis 2 and mitosis.

 

Cancer

Cancer is the result of uncontrolled cell division as the population of cancer cells multiply without regulation. It arises from the transformation of normal cells to become immortal via mutations. If these cells are not destroyed, they undergo rapid controlled mitosis to form neoplastic growths or tumors which may be malignant (spread) or benign (localized). Spreading of the cancerous growth is a process known as metastasis.

Cancer can be triggered by various things as listed below.

-        Carcinogens – leading to mutations

-        Oncogenes – cause uncontrolled mitosis

-        Retroviral oncogenes

-        Genetic predisposition – inherited oncogenes

Characteristics of cancer

-        Acquisition of self-sufficiency in growth signals, leading to unchecked growth

-        Loss of sensitivity to anti-growth signals, leading to unchecked growth

-        Loss of capacity for apoptosis

-        Loss of capacity of senescence, leading to limitless replicative potential possibility due to the presence of telomerase, allowing for cell growth beyond he Hayflick’s limit

-        Acquisition of sustained angiogenesis, allowing tumor to grow beyond the limitations of passive nutrient diffusion

-        Acquisition of ability to invade neighboring tissues, with no contact inhibition, causing cell to continue growing even when it comes into contact with neighboring cells.

-        Loss of capacity to repair genetic errors, leading to an increased mutation rate (genomic instability), thus accelerating all the other changes

Mutations

Epigenetics

The theory of epigenetics is that non-mutational changes to DNA can lead to alterations in gene expression. Normally oncogenes are silent, for example, because of DNA methylation. Loss of the methylation can induce aberrant expression of oncogenes, leading to cancer pathogenesis. Known mechanism of epigenetic changes include DNA methylation, and methylation or acetylation of histone proteins bound to chromosomal DNA at specific locations.

Oncogenes

Oncogenes promote cell growth through a variety of ways. Mutation in pronto-oncogenes, which are the normally quiescent counterparts of oncogenes, can modify their expression and function, increasing the amount or activity of the product protein. When this happens, the proto-oncogenes become oncogenes and this upsets the normal cell cycle regulations, making uncontrolled growths possible.

Tumor suppressor genes

Many tumor suppressor genes effect signal transduction pathways regulate apoptosis. Tumor suppressor genes code for anti-proliferation signals and proteins that suppress mitosis and cell growth. Generally, tumor suppressors arrest the progression of the cell in order to carry out DNA repair, preventing mutations from being passed on to the daughter cells. The p53 protein, one of the most important and thoroughly studied tumor suppressor genes, is a transcription factor activated by many cellular stressors including hypoxia and UV damage. However, a mutation can damage the tumor suppressor gene itself, or the signal pathway which activates it, thus ‘switching it off’. The invariable consequence of this is that DNA repair is hindered or inhibited: DNA damage accumulates without repair, inevitably leading to cancer.

 

 

 

 

Immune System

Innate Immunity

The innate immunity is present before any prior exposure to pathogens and is effective from the time of birth. These are largely unspecific and are quick to recognize and respond to a broad range of microbes regardless of their precise identity.

Innate Immunity [Rapid response]

Acquired Immunity

[Slower response]

External Defenses

Internal Defenses

-        Skin

-        Mucous membrane

-        Secretions

-        Phagocytic cells

-        Antimicrobial proteins

-        Inflammatory Response

-        Natural Killer Cell

-        Humoral Response

-        Cell-mediated response

External defense

Intact skin is a barrier that is normally impenetrable by virus or bacteria, but even tiny abrasions might allow their passage. Likewise, the mucous membranes lining the digestive, respiratory tract bars entry of potentially harmful microbes. Certain cells of these mucous membranes also produce mucus, a viscous fluid that traps microbes and other particles. In the trachea for example ciliated epithelial cells sweep mucus and entrapped microbes upwards, preventing microbes from entering the lungs.

Beyond the physical role of inhibiting microbe entry, secretions also provide an environment that is hostile to microbes. Take for example secretions from sebaceous glands and sweat glands that give the skin a pH ranging from 3 to 5 which is acidic enough to inhibit microbe colonization. Secretions from the skin and mucous membranes also contain antimicrobial proteins like lysozyme which digests the cell walls of most bacteria.

Internal Defense

Microbes that penetrate the body’s external defense such as those that enter via a break in the skin would face the body’s internal innate defense. These defenses rely mainly upon phagocytosis, the ingestion of invading microorganisms by certain types of blood cells. Generically referred to as phagocytes, these cells produce certain antimicrobial proteins that help initiate inflammation which can limit the spread of microbes in the body. Non-phagocytic white blood cells, called natural killer cells also play a key role in innate defense. These various non-specific mechanisms help to limit the spread of microbes before the body can mount acquitted specific immune response.

 

Phagocytic cells

Phagocytes attach to their prey via surface receptors that bind to structures found on many micro-organisms. Among the structures bound by these receptors are certain polysaccharides on the surface of bacteria. After attaching to one or more microbes, a phagocyte engulfs the microbes, forming a vacuole that fuses with a lysosome. First, nitric oxide and other forms of oxygen contained in the lysosomes may poison the engulfed microbes. Second, lysozymes and other enzymes degrade microbial components. Some microorganisms have adaptations that enable them to evade destruction by phagocytic cells, by for example hiding their surface polysaccharides, thus preventing binding of phagocytes. Other bacteria like the one that causes tuberculosis are resistant to destruction within the lysosomes.

There are four different types of white blood cells which are phagocytic and they differ in abundance, average life span and phagocytic ability.

By far, the most abundant are neutrophils, which constitute about 60% to 70% of all white blood cells. Neutrophils are attracted to and then enter infected tissue, engulfing and destroying the microbes there. However, neutrophils tend to self-destruct in the process of phagocytosis, and their average life span is only a few days. Macrophages are more effective and they develop from monocytes which constitutes about 5% of the circulating white blood cells. New monocytes circulate the blood for a few hours before being transformed into macrophages. The other two forms of phagocytes are less abundant and play a more limited role in innate defense. Eosinophils have low phagocytic activity but are crucial to defense against multicellular parasitic invaders, such as blood fluke. Rather than engulfing the parasite, they position themselves against the parasite’s body and then discharge destructive enzymes that damage the invader. The last form of phagocyte is dendritic cells that ingest microbes.

Antimicrobial Proteins

Numerous proteins function in innate defense by attacking microbes directly attacking the microbes or by impeding their reproduction. Apart from lysozyme, there are about 30 other serum proteins that make up the complement system. In the absence of an infection, these proteins are inactive. Substances on the surface of many microbes, however, can trigger a cascade of steps that activate the complement, leading to lysis of invading cells.

Two types of interferon provide innate defense against viral infections. These proteins are secreted by virus-infected body cells and induce neighboring uninfected cells to produce other substances that inhibit viral production, thus limiting the cell-to-cell spread of virus in the body, helping to control viral infections such as colds and influenza. This innate defense mechanism is not virus specific and interferon produced in response to one virus might also confer short term resistance to unrelated viruses.

Inflammatory Response

Damage to tissue by physical injuries or the entry of pathogens leads to release of numerous chemical signals that trigger a localized inflammatory response. One of the most active chemical is histamine which is stored in mast cells found in connective tissues. When injured, mast cells release histamine that triggers dilation and increased permeability of nearby capillaries. These result in increased local blood supply, causing redness and heat typical of inflammation. The blood-engorged capillaries leak fluid into neighboring tissues, causing swelling. These vascular changes help deliver antimicrobial protein and clotting elements to the injured location. Blood clotting begins repair process and helps block the spread of microbes to other parts of the body. Increased blood flow and vessel permeability would also allow more neutrophils and monocyte macrophages to move from the blood into injured tissue. A minor injury causes a local inflammation, but the body may also mount a systemic (widespread) response to severe tissue damage or infection. In a severe infection such as meningitis or appendicitis the number of white blood cells would rapidly increase. Another systemic response is fever which may occur when certain toxins produced by pathogens and substances released by actuated macrophages set the body’s thermostat at a higher slightly higher temperature. A very high fever is dangerous, but a moderate fever can facilitate phagocytosis and, by speeding up body reactions, hasten the repair of tissues. Certain bacterial infection can induce an overwhelming systemic inflammatory response, leading to a condition called septic shock which is characterized by high fevers and low blood pressure.

Natural Killer Cells

Natural killer cells patrol the body and attack virus-infected body cells and cancer cells. Surface receptors on natural killer cells recognize general features on the surface of its targets. Once it is attached to a virus infected cell or cancer cell, the natural killer cell releases chemicals that lead to the death of the stricken cell by apoptosis, or programmed cell death.

Invertebrate Immune Mechanism

Invertebrates also have highly effective innate defense, for example, sea stars possess amoeboid cells that ingest foreign matter via phagocytosis and secrete molecules that enhance the animal’s defensive response. The insects’ equivalent to blood, hemolymph, contains circulating cells called hemocytes. Some of them ingest bacteria and foreign substances while others would form a cellular capsule around large parasites.

 

Acquired Immunity

Pathogens would inevitably come into contact with lymphocytes while under the assault by one’s innate defense. Lymphocytes are the key cells of acquired immunity – the body’s second major form of defense. Direct contact with microbes and signals from active innate defenses will cause lymphocytes to join the battle. Any foreign molecule that is specifically recognized by lymphocytes and elicits response from them is called an antigen. Most antigens are large molecules, either polysaccharide. Some antigens, such as toxins secreted by bacteria, are dissolved in extracellular fluid, but many protrude from the surface of pathogens or transplanted cells. A lymphocyte actually recognizes and binds to just a small, accessible portion of an antigen, called an epitope (antigenic determinant). A single antigen usually has several epitopes, each capable of inducing a response from lymphocytes that recognize that epitope. Antibodies which are secreted by certain lymphocytes in response to antigens, likewise bind to specific epitopes.

Antigen recognition by Lymphocytes

The body is populated by two main types of lymphocytes – T cells and B cells.  Both types are circulated through the blood and lymph and are concentrated in the spleen, lymph nodes and other lymphoid tissues. B cells and T cells recognize antigens by means of antigen-specific receptors embedded in their plasma membranes, a single B or T cell bears about 100,000 of these antigen receptors, and all the receptors on a single cell are identical – that is, they all recognize the same epitope. Each lymphocyte displays specificity for a particular epitope on an antigen and defends against that antigen or a small set of closely related antigens. A great diversity of T cells and B cells are present, however, only a tiny fraction of the lymphocytes would ever be used.

B cell Receptors for Antigens

Each B cell receptor for an antigen is a Y-shaped molecule consisting of four pol-peptide chains: two identical heavy chains and two identical light chains linked by disulfide bridges. A region in the tail portion of the molecules, the trans-membrane region anchors the receptor in the cell’s plasma membrane and a short region at the end of the tail extends into the cytoplasm. At the tips of the Y are the light and heavy chain variables which vary extensively from one B cell to another. The remainder of the molecule is the constant regions whose amino acids vary little from cell to cell. The unique contour of each binding site is formed from part of a light-chain V region. The interaction between an antigen binding site and its corresponding antigen is stabilized by multiple non-covalent bonds between chemical groups on the respective molecules. The antigens bound by the B cell receptors in this way include molecules that are on the surface of, or are released from, all types of infectious agents, thus meaning that it can recognize an intact antigen in its native state.

 

T cell receptors for antigens and role of the MHC

Each T cell receptor for an antigen consists of two different polypeptide chains, a  cell and a  chain, linked by a disulfide bridge. Near the base of the molecule is a trans-membrane region that anchors the molecule in the cell’s plasma membrane. At the outer tip of the molecule, the  cell and a  chain variable regions form a single antigen binding site. The remainder of the molecule is made up of constant regions.

T cell receptors recognize and bind with antigens just as specifically as a B cell receptors. However, while the receptors on B cells recognize intact antigens, the receptors on T cells recognize small fragments of antigens that are bound to normal cell surface proteins called major histocompatibility complex (MHC) molecules. As a newly synthesizes MHC molecule is transported towards the plasma membrane, it bind with a fragment of protein antigen within the cell and brings it to the cell surface via a process called antigen presentation. A nearby T cell can detect the antigen fragment thus displayed on the cell surface. There are two ways in which foreign antigens can end up inside cells of the body. Depending on their source, these peptide antigens are handled by a different class of MHC molecule and recognized by a particular subgroup of T cells.

-        Class 1 MHC molecules, found on almost all nucleated cells of the body bind peptides derived from foreign antigens that have been synthesized within the cell. Any body cell that becomes infected or cancerous can display such peptide antigens by virtue of its Class 1 MHC molecules. Class 1 molecules displaying bound peptide antigens are recognized by a subgroup of T cells, called cytotoxic T cells.

-        Class 2 MHC molecules are made by just a few cell types, mainly dendritic cells, macrophages and B cells. In these cells, class 2 MHC molecules bind peptides derived from foreign materials that have been internalized and fragmented through phagocytosis or endocytosis. Dendritic cells, macrophages and B cells are known as antigen presenting cells because of their key role in displaying such internalized antigens to another subgroup of T cells called T helper cells.

Each vertebrate species possesses numerous different alleles for each class 1 and 2 MHC gene. Because of the large number of different MHC alleles in the human population, most of us are heterozygous for every one of our MHC genes and produce a broad array of MHC molecules. Collectively these molecules are capable of binding to and presenting a large number of peptide antigens. Thus MHC produces a biochemical fingerprint unique to virtually every individual. That marks body cells as ‘self’.

Humoral and Cell Mediated Immunity

There are two separate branches of acquired immunity, humoral immune response which involves the activation and clonal selection of B cells, resulting in production of secreted antibodies that circulate in the blood and lymph. The cell mediated immune response involves the activation and clonal selection of cytotoxic T cells, which directly destroy certain target cells. Central to the network of the humoral and cell mediated immune response are the helper T cells which responds to peptide antigens displayed on antigen presenting cells and in turn stimulates the activation of nearby B cells and cytotoxic T cells.

Helper T cells: A response to antigens

When a helper T cell encounters and recognizes a class 2 MHC molecule-antigen complex on an antigen presenting cell, the helper T cell proliferates and differentiate into a clone of activated helper T cells and memory helper T cells. Activated helper T cells secrete several different cytokine that would stimulate other lymphocytes, thereby promoting humoral and cell mediated response. The helper T cell itself is also subject to regulation by cytokines.

Cytotoxic T cells: A response to infected cells and cancer cells

Cytotoxic T cells, the effectors of cell-mediated immunity eliminate body cells infected by viruses or other intracellular pathogens as well as cancer cells and transplanted cells. Fragments of non-self-proteins synthesized in such target cells associate with class 1 MHC molecules and are displayed on the cell surface. When a cytotoxic cell is selected by binding to class 1 MHC molecule-antigen complexes on an infected body cell, the cytotoxic T cell is activated and differentiates into an active killer. Cytokines secreted from nearby helper T cells promote this activation. The activated cytotoxic T cell then secretes proteins that act on the bound infected cell, leading to its destruction. The death of the infected cell not only deprives the pathogen of a place to reproduce but also exposes it to circulating antibodies, which marks it for disposal. After destroying an infected cell, the cytotoxic T cell may move on and kill other cells infected with the same pathogen. In the same way, cytotoxic T cells defend against malignant tumors. Because tumor cells carry distinctive molecules not found on normal body cells, they are identified as foreign by the immune system. Class 1 MHC molecules of a tumor cell display fragments of tumor antigens to cytotoxic T cells.

B Cells: A response to extracellular Pathogens

Antigens that elicit a humoral immune response are typically proteins and polysaccharides present on the surface of bacteria or incompatible transplanted tissue or transfused blood cells. The activation of B cells is aided by cytokines secreted from helper T cells activated by the same antigen. Stimulated by both an antigen and cytokines, the cell proliferates and differentiates into a clone of antibody-secreting plasma cells and a clone of memory B cells. When an antigen first binds to receptors on the surface of memory B cells, the cell takes in a few of the foreign molecules by receptor mediated endocytosis. In process similar to antigen presentation by macrophages and dendritic cells, the B cell then presents antigen fragments to the helper T cells. However, a macrophage or dendritic cell can present peptide fragments from a wide variety of antigens, whereas a B cell internalizes and presents only the antigen to which is specifically binds.

Antigens that induce antibody production only with assistance from helper T cells are known as T-dependent antigens. Some antigens however, can evoke a B cell response without involvement of helper T cells. Such t-independent antigens include polysaccharide of many bacterial capsules and proteins that make up the bacteria flagella. However, this response is generally weaker than the response to T-dependent antigens and through this process; no memory B cells are generated.

Active and Passive Immunization

Immunity conferred by natural exposure to an infectious agent is called active immunity because it depends on the actions of a person’s own lymphocytes and the resulting memory cells specific for the invading pathogen. Active immunity also can develop following immunization, or vaccination. Modern vaccines include inactivated bacterial toxins, killed microbes or parts of microbes, viable but weakened microbes that generally do not cause illness. All these agents would induce an immediate immune response and long lasting immunological memory.

Immunity can also be conferred by transferring antibodies from an individual who is immune to a particular infectious agent to someone who is not. This is known as passive immunity because it does not result from the action of the recipient’s own B and T cells. Instead, the transferred antibodies are poised to immediately help destroy any microbes for which they are specific. Passive immunity provides immediate protection, but it only persists for as long as the transferred antibodies last.

Allergies

Allergies are exaggerated response to certain antigens called allergens. One hypothesis to explain the origin of allergies is that they are evolutionary remnants of the immune system’s response to parasitic worms. The humoral mechanism that combats worms is similar to the allergic response that causes such disorders.

The most common allergies involve antibodies of the IgE class. Hay fever; for instance, occur when plasma cells secrete IgE antibodies specific for antigens on the surface of pollen grains. Some of the antibodies attach by their tails to mast cells present in the connective tissues. Later when pollen grains enter the body, it induces the mast cell to release histamine and other inflammatory agents from their granules, a process called degranulation. Such vascular changes lead to typical allergy symptoms: sneezing, runny nose, teary eyes or difficulties in breathing. Antihistamines diminish allergy symptoms by blocking receptors for histamine.

An acute allergic response can sometimes lead to anaphylactic shock, a whole body, life threatening reaction that can occur within seconds of exposure to an allergen. Anaphylactic shock develops when widespread mast cell degranulation triggers abrupt dilation of peripheral blood vessels, causing a precarious drop in blood pressure.

DNA Profiling

Blood Typing

  • Very fast and straight forward à Can only be used to exclude suspects
  • Not very specific

RFLP

  • Restriction Fragment Length Polymorphism
  • Tests regions in the DNA sequence that have Variable Tandem Repeats (VNTR)
    • No. of repeats vary for person to person and coded by alleles
    • Highly variable à Over 50 mutant alleles
    • Each person can only have 2 alleles à Inherited from parents à Large no. of possibilities
    • 14 – 100 base pairs
  • Procedure
    • Restriction enzymes used to cut out the target DNA sequence (VNTR region)
    • The DNA fragments are denatured into single strands
    • The fragments ran through gel electrophoresis
      • The fragments separated by length
    • Blotting
      • Nitro – Cellulose Paper put on top of results
      • The DNA fragments would be drawn up along with the solvent by capillary action
      • A copy of the gel electrophoresis would be on the nitro – cellulose paper
    • The DNA fragments are made back to double strands by adding complimentary primers (attached with radio – nuclides)
    • The radiation emitted by the radio – nuclides are captured on photography paper
  • Advantages
    • High discriminating potential
  • Disadvantages
    • Laborious and not easily automated
    • DNA fragments are of large size and therefore cannot undergo PCR
    • DNA requires to be of reasonable quality
    • Large amounts of DNA required as it cannot undergo PCR
    • Time consuming

STR

  • Short Tandem Repeats
  • Test polymorphic regions in the DNA which contains Short Tandem Repeats
    • No. of repeats vary for person to person and coded by alleles
    • Highly variable à Over 50 mutant alleles
    • Each person can only have 2 alleles à Inherited from parents à Large no. of possibilities
    • Just 2 – 10 base pairs
  • Procedure
    • The polymorphic regions (13 loci) are amplified by PCR
    • The results are run through gel electrophoresis
    • The fragments ran through gel electrophoresis
      • The fragments separated by length
    • Blotting / Staining
      • Nitro – Cellulose Paper put on top of results
      • The DNA fragments would be drawn up along with the solvent by capillary action
      • A copy of the gel electrophoresis would be on the nitro – cellulose paper
      • The DNA fragments are made back to double strands by adding complimentary primers (attached with radio – nuclides)
      • The radiation emitted by the radio – nuclides are captured on photography paper
      • The DNA fragments are stained with dyes that make them visible
  • Advantages
    • High discriminating potential
    • Short DNA fragments required and therefore can undergo PCR
    • PCR can amplify DNA, therefore, minute samples are required
    • Can be carried out speedily and cheaply

Mitochondrial DNA Analysis

  • mtDNA is more stable (occur in a plasmid) than chromosomal DNA
  • More copies of mtDNA are present in every cell, allowing molecular analysis even when material is limited
  • Hypervariable (polymorphic) region allows for discrimination
  • Can be used when DNA of reasonable quality is not available
  • However, this method is not highly discriminating


 

Genetic Technology

Many different organisms have been genetically modified by humans in order for them to possess the desired traits. Genetic modification has specific applications in the bacterial industry in order to produce drugs. Genetic engineering has many different potential benefits for humans, but there may be unknown hazards which may arise in the future. Genetic engineering began with the discovery of the restrictive endonucleases. These enzymes occur naturally in bacteria where they protect the organism against DNA injected by virus by cutting it into small pieces, thereby inactivating it. Virus DNA otherwise would take over the host cell. Restriction enzymes were named so due to their ability to restrict the multiplication of viruses. Many different restriction enzymes have been discovered and purified, and today they are used widely in genetic engineering experiments.

A bacterium contains two different type of genetic material. One is a single strand of long double strand DNA in the form of a ring while the other is a single circular chromosome – the plasmid. Plasmids are easily isolated from the bacteria and then can be re-introduced to the bacteria cell relatively easily. They can therefore be extremely useful as vectors. In the bacterium, plasmids replicate themselves independently from the bacteria so the gene introduced to the plasmid would also be replicated when the bacteria reproduces.

To prepare plasmids as vectors, a restriction enzyme would have to be used which would form the ‘sticky ends’ which would have to be used to cut open the plasmid. The restriction enzymes would also be required to cut open the DNA and isolate the required gene. The open plasmid and the gene would then be combined in the presence of an enzyme called ligase. Ligase occurs naturally in the nuclei where it will ‘repair damaged’ DNA during the replication. The ligase would therefore catalyze the combination of the complimentary strand of DNA and the plasmid after their sticky ends have paired through a process called ‘annealing’. These plasmids are then reintroduced to the bacterium my mixing bacterium and the plasmids in the presence of Calcium Chloride which would render the cytoplasm permeable to the plasmids

An alternative technique would be to create a genetically modified bacteriophage which contains the gene. The bacteriophage would then inject that strand of DNA into the bacteria and following that, the gene would be incorporated into the bacteria plasmid and it would be expressed in the bacterium’s metabolism.

Selection

The vector that is most commonly used as a plasmid that carries two genes for antibiotic resistance is known as the R-plasmid has a gene for ampicillin resistance and a gene for tetracycline resistance. It happens that the tetracycline resistance gene is cut in half by the BamHI restriction enzyme whilst the ampicillin gene is uncut. When a gene is cut, it becomes deactivated; the resistance against tetracycline is therefore lost when the gene is cut. In order to isolate bacteria which are resistant to the respective disease, the bacteria colonies are first placed in the agar plate which kills are bacteria which are not resistant to ampicillin. Following, these bacteria would then be placed on an agar plate with tetracycline and all bacterium with damaged genes would fail to grow. A comparism between the different colonies which were placed in the ampicillin and the tetracycline plate would then be made and a conclusion would be drawn the colony which is found in the ampicillin plate, however, failed to grow in the tetracycline plate would be selected for use while the others would be disposed.

Bacteria are used because of its fast replication rate and that it is easy to culture and grown. They are also easy to observe due to their relatively simple structure. Ideal conditions for replication like the sufficient amount of nutrients, optimal temperature and pH are required to maximize production.


 

Pregnancy Prevention Methods

Mechanical Methods

CONDOM

It is a flexible sheath that is designed to cover the man’s penis during sexual intercourse. The condom is to be put on the penis before it touches the vulva. Condoms prevent sperms from entering the woman’s vagina by trapping the semen inside the condom, and not let it go into the vagina. They are easily available in drugstores, family planning clinics, some supermarkets, and from vending machines. (Do not require a prescription). They are lightweight and disposable and can protect user against many sexually transmitted diseases—called STDs for short. It is also easier to use than the other methods. No side effects unless you are allergic to latex. However, condoms may break and cannot be used for people who are allergic to latex. They also dull the sex sensation. Of 100 women whose partners use condoms, about 15 will become pregnant during the first year of typical use. Only two women will become pregnant with perfect use.

DIAPHRAGM

A small dome shaped rubber cap that is fitted in the female’s vagina. The diaphragm is placed in the female’s vagina before having sex. This prevents sperms from entering the woman’s vagina, and acts as a barrier. There are no side effects unless you are allergic to its ingredients and they are widely sold and easy to get, and some do not need prescriptions. This method is often not used correctly, thus increasing the chances of accidental pregnancy. It is not as easy to use as condoms, as it is more inconvenient to put in a woman’s vagina rather than a man’s penis. Similarly, it cannot be used for people who are allergic to latex. Sixteen out of 100 women who use the diaphragm will become pregnant during the first year of typical use. Six
will become pregnant with perfect use.

IUD (Intrauterine device)

It is a small, T- shaped plastic object with a fine copper wire around the device and a thread attached to the base. It is to be placed in the uterus to prevent the implantation of a fertilized egg.

The IUD must be inserted in the uterus by a doctor. It prevents sperm from fertilizing the egg, by changing the way it moves. It also changes the lining of the uterus which makes it very hard for the fertilized egg to attach itself to the wall of the uterus. It is the longest lasting method and you need not do anything before sexual intercourse. It is almost 100% effective and you can get pregnant once the IUD is taken out. However, it cannot protect you from STDs and it has to be prescribed by a doctor and it may cause an increase in menstrual flow

Chemical methods

Spermicide

It is a chemical that kills sperms and it is inserted into the vagina, or applied along with other contraceptive methods, such as condoms or diaphragms, so as to kill the sperms and prevent fertilization of the egg. It is easy to use and it is easily purchased with no prescriptions needed. It can be used with other contraceptive methods, improving the effectiveness. It may not be able to kill all the sperms, thus not effective if used on its own and 6 out of 100 might become pregnant for perfect use, while 21 of 100 might become pregnant for typical use

Ligation/Sterilization

Tubal occlusion (female) is an operation that blocks, seals or cuts the fallopian tubes; this means that your eggs can no longer be fertilized by your partner’s sperm through sexual intercourse. Vasectomy (males) is an operation that blocks, seals or cuts the vas deferens which carries sperm from your testicles to your penis. Although you will still be able to ejaculate, your semen will no longer contain any sperm. It is 100% effective and there are no hormonal side effects. However once sterilized, the couples may no longer give birth and there is a risk of getting infection through the surgery.

Oral Contraceptive Pills

It is a pill that contains hormones that prevent pregnancy and it prevents pregnancy primarily by preventing ovulation. It also has the side-effect of thickening the mucus over the cervix, which can prevent or slow sperm entry into the uterus. The Pill also thins the endometrium. It is simple, safe, and convenient and it does not interfere with having sex. There is also protection against developing cancer of the ovary or the lining of the uterus can last up to 30 years. There are however, side effects like headache, breast tenderness and many conditions to be fit to take the pill like 35 or older, smoke. Consultation at a clinician to tell whether you can take the pill and what dose is right for you. Of 100 women who use the pill, only eight will become pregnant during the first year of typical use. Less than one will become pregnant with perfect use.

 

Natural Methods

Outer course

It is not having sexual intercourse. It is any kind of sexual activity in which the penis does not enter the vagina, mouth, or anus. It allows a couple more intimate and even have an orgasm with one another without having sexual intercourse. With outer course, no semen, vaginal fluids, or blood is shared between partners. Abstinence is 100% effective at preventing pregnancy and STDs. However both you and your partner must be committed to not having vaginal, oral (involving the mouth), or anal sex with anyone. Abstinence is 100% effective at preventing pregnancy. Outer course is nearly 100% effective at preventing pregnancy. There is a small chance pregnancy could occur if sperm or pre-ejaculate (the fluid that is sometimes released from the penis before orgasm) is ejaculated (or released) close to the opening of the vagina.

Fertility Awareness

It is a way for a woman to find out what days during her menstrual (monthly) cycle she either is or is not likely to get pregnant. The days she is likely to get pregnant are called “fertile” days. It is done by keeping track of the changes that occur in her body during the menstrual cycle-the time between the first day of her period and the last day before her next one. To avoid getting pregnant, a woman should not have sexual intercourse on her fertile days. There are no side effects and they can be used in combination with barrier methods during the fertile time. They often are acceptable to couples who choose not to use other birth control methods for religious, cultural, health, or other reasons. The disadvantages however lead to low rates of effectiveness when not used effectively. If used exactly according to the directions, between 2 and 10 women out of 100 might become pregnant in one year. For typical users, between 12 and 25 women out of 100 might become pregnant when using FAB methods for one year.