CONFIRM INFOZ

Echinococcus multilocularis

It is the causative agent of highly pathogenic Alveolar Hydatid disease in man and other mammals. The fox is the most important definitive host, although dogs, and occasionally cats, may also be infected with the adult parasite. Again there appears to be a number of sub-species of this organism, E. m. multilocularis in Europe and E. m. sibiricencis in North America.

 This is very similar to that of E. granulosus, but with more adaptations for colder climates. For example the eggs are highly resistant to cold temperatures, being able to survive at -20° for more than 2 weeks. In addition the pre patent period in the definitive host is much shorter, usually between 4 to 5 weeks.

 Morphology

Larvae - The larval E. multilocularis is very different from that of E. granulosus. In this case the 'cyst' grows invasively by external budding, forming a diffuse growth through the infected organ, replacing that organ’s tissues. The growth itself, (it cannot truly be called a cyst as there is no real cyst wall), is composed of numerous cavities containing a gelatinous matrix within which protoscolices and numerous brood capsules are produced, and which in its behaviour, most closely
resembles a malignant neoplasm. In contrast to E. granulosus this growth is also very rapid, infective protoscolesces being present after only 2 to 3 months, as compared to the 1 to 2 years in the related metacestode.

Adults - The adult parasite is very similar to E. granulosus, being slightly smaller,
with a maximum length of approximately 4mm, and consisting of 4 to 5
proglottids.

 Pathology of Infection

Larvae - The multilocular cyst is highly pathogenic due to its fast growth rate and
invasive nature, in extreme cases completely replacing liver tissue. As the cyst lacks the tough laminated layer seen in E. granulosus, and by its nature grows by budding, metastases of growth may also be seen, colonising other body organs. Due to this aspect of the parasite, it may also be transferred by transplantation. This parasite must be considered one of the most pathogenic of the parasitic helminths.

Adults - As with E. granulosa the adult tapeworm is usually non-pathogenic to its canine hosts

Genes

   The classical principles of genetics were deduced by Gregor Mendel in 1865 on the basis of the results of breeding experiments with peas. Mendel studied the inheritance of a number of welldefined traits such as plant height and was able to deduce general rules for their transmission. In all cases, Mendel could correctly interpret the observed patterns of inheritance by assuming that each trait is determined by a pair of inherited “factors”, which are now called genes. The term gene (Gr genos descent) was coined by W. Johannsen in 1909 to refer to the hereditary “factors” of Mendel.
The gene can be defined as a functional unit of heredity which occupies a specific place (locus) on a chromosome, is capable of  reproducing itself exactly at each cell division and directs the
formation of an enzyme or other protein.

 Cytologic and genetic studies show that genes are the fundamental units of inheritance regarded as indivisible units of the chromosomes on which they are located like “beads on a string”. Genes as the chief functional genetic unit, determine the basic architecture of every cell, the nature and life of the cell, the specific protein synthesis, the enzyme formation and the selfreproduction of the cell. Genes are molecular patterns that can maintain their identities for many generations, can be self-duplicated in each generation and can control cell processes by allowing their specificities to be occupied. Genes can mutate, can be assorted, can be shuffled in different combinations, therefore, genes are regarded as the basis for modern interpretation of evolution. On operational or functional basis as a cistron, a muton or a recon as determined or encountered by different approaches.

 
 A Cistron:  A gene can be referred to as a unit of function called a cistron which is the smallest functional region on a chromosome. This idea replaced the unitary concept of the physical gene with the concept of an operational gene composed of one or more functional components (mutant
alleles) that embraces an array of mutant sites, that behaves in a Mendelian fashion. The term cistron was coined by Seymour Benzer and is regarded as a portion of DNA specifying a single polypeptide chain. Each cistron is responsible for coding one messenger RNA molecule, which in turn participate in the formation of a polypeptide chain. It  has been discovered that hundreds of units of mutation
(mutons) and recombination (recons) exist within each cistron. Cistrons, therefore, occupy a much greater chromosomal length than mutons or recons.

 Muton: There are many positions or sites within a cistron where mutations can occur. A muton is the smallest genetic unit or length of DNA that can mutate i.e. a change in muton could result in mutation to produce a phenotypic effect. A muton

  may consist of a single nucleotide or many nucleotides.

 
 Recon: Sometimes crossing over or recombination occur in a cistron to provide another sub-divisional concept of the cistron, the recon. The recon is the smallest genetic unit that
can undergo crossing over (exchange of genetic material), or recombination i.e. the smallest unit within the DNA capable of being independently involved in recombination.

 CHROMOSOMES

Thomas Morgan after looking at Hugo De Vries theories of mutations, bred the fruit fly Drosophila melanogaster for a year with no mutations, but a fly appeared with white eye instead of red eyes, within another year. 40 different kinds of mutations had been noticed. In trying to explain how organisms inherit characteristics Morgan discovered that the various mutations of the flies were
associated with 4 pairs of chromosome possessed by Drosophila. Thomas Morgan’s work proved the chromosomal theory of inheritance which states that chromosome are the elements that transmits inheritable characteristics, also discovered that chromosome are the carriers of genes which cause the expression of individual characteristics.

  Chromosomes are filamentous rod-like or thread-like gene bearing bodies found in the nucleus during cell division. Each nucleus contain information coded in the form of DNA and organised into groups called genes. Genes are arranged on the chromosome and each gene contain enough information for the production of one protein which can have some effects on the individual chromosome vary widely between different organisms.

  The chromosome molecule may be circular or linear, typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have circular
chromosomes. Chromosomes are the essential unit for cellular division and must be replicated, divided and passed successfully to their daughter cells so as to ensure the genetic diversity and
survival of their progeny.

 Chromosome Structure

Chromosome within a cell occur in matched pairs called homologous xomes, joined at the centre by a centromse. Each chromosome contains many genes, and each gene is located at a
particular site on the chromosome, known as the locus. Like chromosome, genes typically occur in pairs. A gene found on one chromosome in a pair usually has the same locus as another gene
in the other chromosome of the pair, and these two genes are called alleles. Alleles are alternate forms of the same gene. In organisms that use sexual reproduction, offspring inherit
one-half of their genes from each parent and then mix the two sets  of genes together. This produces new combinations of genes; so that each individual is unique but still possesses the same genes
as its parents.

Chromosome Number

In the cells of most organisms that produce sexually, chromosomes occur in pairs; one chromosome is inherited from the female parent; and one is inherited from the male parent. The two chromosomes of each pair contain genes that correspond to the same inherited characteristics. Each pair of chromosomes is different from every other pair of chromosome in the same cell.
The number of chromosome pairs in an organism varies depending on the species. The number of chromosome characteristic of a particular organism is known as the diploid number. Dogs, for example, have 39 pairs of chromosomes and a diploid number of 78 while tomato plants have 12 pairs of chromosomes and a diploid number of 24.

Gametes or sex cells (eggs and sperm) contain only half the number of chromosomes found in the other cells of an organism. This reduced number of chromosomes in the gametes is known as the haploid number. During fertilisation the gametes unite to form a cell known as a zygote containing the diploid number of chromosomes characteristics of the species.

 Sex Chromosomes

Most organisms have complete sets of matching chromosomal pairs, known as autosomes. In mammals, birds and some other organisms, one pair of chromosomes is not identical.
Known as the sex chromosomes, this pair plays a dominant role in determining the sex of an organism. Females have two copies of the X chromosome while males have one Y chromosome and one X chromosome. Both males and females inherit one sex chromosome from the mother (always an X chromosome) and one sex chromosome from the father (an X chromosome in female
offspring and a Y chromosome in male offspring). The presence of the Y chromosome determines that a zygote will develop into a male.

 Human Chromosomes and Genetic Disorders

  Humans have 23 pairs of chromosomes, with a diploid number of 46 numbered according to their size. The largest is chromosome 1 and the smallest is chromosome 23. Physical and
chemical meiosis (formation of gametes) can damage chromosomes or alter their number in a cell, to give rise to embryos with more of less genetic material, sometimes resulting in developmental disabilities or health problems. In a process called non disjunction, paired members of chromosomes fail to separate from one another during meiosis.

  Non disjunction can lead to a condition known as Down Syndrome, in which a person inherits three copies of chromosome 21. Another condition that may result from non disjunction is Turner Syndrome, a disorder in which a female inherits only a single X chromosome. Breakage of a chromosome can lead to four types of changes in chromosome structure. A deletion occurs when a chromosome fragment lacking a centromere is lost during cell division. In some cases the fragment may join to the homologous chromosome to produce a duplication. It may reattach to the original chromosome but in a reverse orientation, producing an inversion; the fragment can join a non homologous chromosome a rearrangement called translocation.

Nucleic Acids

 

 

  Nucleic acids are extremely complex molecules produced by living cells and viruses, to pass on hereditary characteristics from one generation to the next, and to trigger the manufacture of
specific proteins. The name nucleic acids comes from their initial isolation from the nucleic of living cells. However, certain nucleic acids are found not in the cell nucleus but in cell cytoplasm.
Nucleic acid molecules are very large chains of repeating nucleotide units linked in many sequences. Thus, nucleic acids are high polymers with very high molecular weights. A nucleotide is a molecular unit or a nucleic acid

 • a phosphate group.
• a pentose sugar (ribose or deoxyribose).
• a nitrogen base (purine or pyrimidine).
A nucleoside is a compound consisting of 2 subunits – a pentose sugar and a nitrogen base. It is a precursor of a nucleotide. A summary of nucleic acid formation is:
• Pentose sugar + Nitrogen base = Nucleoside
• Nucleoside + Phosphate group = Nucleotide
• Nucleotide + Nucleotide + Nucleotide = Nucleic acid.

 TYPE OF NUCLEIC ACIDS

There are two types of nucleic acids – DNA (Dexyribonucleic acid) and RNA (Ribonucleic acid); both are chemical relatives that are universally present in all living cells and they form the chemical
basis of life. Both DNA and RNA contain the purines – Adenine (A) and Guanine (G) and the pyrimidine cytosine (C). The second kind of pyrimidine in DNA is Thymine (T) where as it is Uracil (U) in RNA. Therefore a unique pyrimidine distinguishes DNA from RNA.

 DNA

The DNA is a polymer made up of repeating units of mononucleotides carrying the genetic material of all cellular organisms and most viruses. DNA carries the information needed to direct protein synthesis ad replication. Protein synthesis is the production of the proteins needed by the cell or virus for its activities and development. Replication is the process of which DNA copies itself for each descendant cell or virus, passing on the information needed for protein synthesis. In most cellular organisms, DNA is organised on chromosomes located in the nucleus of the cell.

 DNA STRUCTURE

The DNA is a spiral ladder with the nucleotides forming the side pieces and the steps composed of a combination of purine and pyrimidine which join the deoxyribose sugars in the side
pieces to hold them together. The purines are of two types Adenine (A) and Guanine (G), the pyrimidines too are of two types Cytosine (C) and Thymine (T). In forming the steps of the ladder G
may join C, or C may join G, and T may join A, or A may join T, usually by hydrogen bond. With the pairings G–C, C–G, A–T or T– A occurring throughout the length of the DNA molecule. The
combination of the sugar molecule, phosphate group and a nitrogenous base completes the basic structure of a nucleotide. With purine linked to a pyrimidine precisely adenine (A) always
pairing with thymine (T) and guanine (G) pairing with cytosine (C). A mirror image of the nucleotide is added to produce a double nucleotide chain which will twist to produce the α – helix.

 RNA

Ribonucleic acid (RNA) - In cellular organisms is the molecule that directs the middle steps of protein production and the genetic material of certain viruses. In cellular organisms the
DNA, carries the information that determines protein structure. But DNA cannot act alone and relies upon RNA to transfer this crucial information (translate) during protein synthesis – production of the
proteins needed by the cell for its activities and development. In RNA viruses, the RNA directs two processes – protein synthesis (production of the virus’s protein coat) and replication (the process by which RNA copies itself).

 RNA STRUCTURE

The structure of RNA is similar to that of DNA and it is composed of a single string of ribonucleotides, each of which is composed of
• a pentose sugar (ribose sugar)
• a phosphate group
• a nitrogenous base (one of the two bases – adenine, guanine, uracil and cytosine)
These components are joined together in the same manner as in DNA molecule. But RNA differs chemically from DNA by being single stranded, having a D-ribose sugar instead of
Deoxyribose sugar and having uracil as nitrogenous base instead of thymine. These nitrogenous bases can occur in any sequence.

 TYPES OF RNA

There are three types of RNA classified based on their molecular size. The smallest type of RNA is called transfer – RNA (tRNA) which carries amino acids to the ribosomes for
incorporation into a protein. Each amino acid has different classes of tRNA that read the codes of mRNA, therefore involved in protein synthesis. The tRNA receives information from mRNA, through pairing of their bases and accordingly selects particular amino
acids and pass to the ribosome.

The second type of RNA is the ribosomal – RNA (rRNA), thisis larger than tRNA and composes the ribosomes in the cytoplasm, the specialised structures that are the sites of protein synthesis.

Transfer – RNA are the most abundant type of RNA and they coordinate the sequential coupling of tRNA molecules to the series of mRNA codons.

The largest type of RNA is the messenger – RNA (mRNA). Messenger – RNA is a strand of RNA that is complementary to the DNA sequence for a gene and carries the genetic blueprint copied
from the sequence of bases in a cell’s DNA. This blue print specifies the sequence of amino acids in a protein. All the types of RNA are formed as needed, using specific sections of the cell’s
DNA as template.

Escherichia coli as Experimental Model

   Because of their comparative simplicity, prokaryotic cells(bacteria) are ideal models for studying many fundamental aspects of biochemistry and molecular biology. The most thoroughly studied species of bacteria is E. coli, which is the most favoured organism for investigation of the underlying mechanisms of molecular genetics. Most of the present concepts of molecular biology –
 DNA replication, the genetic code, gene expression and protein synthesis – are derived from studies of E. coli.

 E. coli is useful to molecular biologists because of its relative simplicity and the ease of its propagation and study in the laboratory. For example, the genome of E. coli consists of
approximately 4.6 million base pairs and encodes about 4000 different proteins. While the human genome is more complicated, with nearly 3 billion base pairs and encodes about 100,000  different proteins. The small size of the E.coli genome provides advantages for genetic analysis, and the sequence of the entire E.coli genome has been determined.

Molecular genetic experiments are further facilitated by the rapid growth of E.coli under well-defined laboratory conditions. E. coli can divide every 20-60 minutes, depending on culture conditions and a clonal population of E.coli all cells derived by the division of a single cell of origin – can be isolated as a colony grown on agar – containing a medium. Bacterial colonies contain many cells, and selecting and analysing genetic variants of E.coli strain is straightforward and rapid. This generally contributes to the success
of experiments in molecular genetics.

E.coli can divide rapidly in nutrient mixtures like glucose, salts, amino acids, vitamins and nucleic acid precursors. However, E. coli can also grow in much simpler media consisting of only
salts as a source of nitrogen (such as ammonia) and a source of carbon and energy (such as glucose). But in such simple medium, the bacteria grow a little slowly (a division time of about 40
minutes) because they must synthesise all their own amino acids, nucleotides and other organic compounds.

The ability of E. coli to carry out these biosynthetic reactions in simple defined media has made them extremely useful in elucidating the biochemical pathways involved. Thus, the rapid growth and simple nutritional requirements of E. coli have greatly facilitated fundamental experiments in both molecular biology and biochemistry.
Although bacteria are models for studies of cell properties, they cannot be used to study aspects of cell structure and function that are unique to eukaryotes. Yeasts, the simplest eukaryotes, have several experimental advantages similar to those of E. coli and have provided a model for studies of many aspects of eukaryotic cell biology.

The genome of the most studied yeasts, Saccharomyces cerevisiae, consists of 12 million base pairs of DNA and contains about 6000 genes; and is about 3 times larger than that of E. coli it is much more manageable than the genomes of more complex eukaryotes, such as humans. Yeasts can be readily grown in the laboratory and can be studied by many of the same molecular genetic approaches that have proved successful with E. coli. Although yeasts do not replicate as rapidly as bacteria, they still
divide as frequently as every 2 hours and can quickly be grown as colonies from a single cell. Yeasts can be used for a variety of genetic manipulations similar to those that can be performed using
bacteria. Yeast mutants have been influential in understanding many fundamental processes in eukaryotes, including DNA replication, transcription, RNA processing, protein sorting and
regulation of cell division.