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Morphology & Anatomy of Frog (at a glance)

Morphology

  • ovoid, slightly flattened body, generally 10-15 cm long, bilateral symmetry.
  • skin is naked, smooth, moist and slippery (due to mucus secreted by the cutaneous glands present in skin).
  • dorsal surface of skin is generally olive green and pigmented while ventral surface is uniformly pale yellow.
  • neck and tail are absent
  • body = head + trunk
  • Head = mouth + external nares + eyes + brow spot + ear-drums + throat
  • mouth = located below the snout (blunt apex, helps in digging the earth), extends from one ear drum to the other, upper jaw is armed with small, conical teeth while the lower jaw is toothless.
  • external nares = pair of small apertures, the nostrils or external nares, helps in breathing, situated dorsally at the tip of the snout.
  • eyes = pair of eyes one on either side of the median line, almost on the top of the head, position of the eyes enables to see in all directions, round and protected by three eye-lids (upper + lower + third), third eyelid is also called nictitating membrane. Nictitating membrane is thin transparent and freely movable eyelid that protect the eyes under water and on dry lands.
  • brow spot = represents a reduced third eye, sensitive to light of the lower wavelength, producing colour changes in the skin.
  • ear-drums = also called tympanic membrane or tympanum or ear-drum, receives sound waves.
  • Trunk = forelimbs + hindlimbs
  • forelimbs = brachium (upper arm) + antebrachium (fore arm) + manus (hand)
  • hands = wrist (carpal) + palm (metacarpal) + four, short, tapering fingers (digits)
  • the first digit (thumb or pollex) is rudimentary and invisible externally
  • nuptial or amplexusory or copulatory pads = present in males only at inner finger of each, become specially enlarged during the breeding season.
  • hind limbs = thigh + shank (crus) + foot (pes)
  • foot = ankle (tarsal) + instep (metatarsal) + five, long toes (digits)
  • toes are joined together by thin folds of skin called webs
  • Sexual dimorphism = separate sexes
  • male possess vocal sacs placed ventrolaterally on either side of the head behind the mouth, used to produce croaking sounds to call the females.

Anatomy

  • skin (naked) = epidermis + dermis
  • epidermis = outer thin, derived from ectoderm, keratinized, stratified squamous epithelium.
  • dermis = inner thick, developed from mesoderm, contain sac-like mucous glands and chromatophore.
  • true coelom = pericardial cavity (around the heart & very small) + pleuroperitoneal cavity (very large)
  • skeleton = axial skeleton (skull + vertebral column) + appendicular skeleton (limb-bones + girdles)

Digestive System

mouth –> buccopharyngeal cavity –> pharynx –> oesophagus –> stomach –> intestine (duodenum –> ileum –> rectum –> cloaca).

  • buccopharyngeal cavity contains numerous maxillary teeth arranged along the margin of the upper jaw and vomerine teeth, lower jaw is toothless
  • salivary glands are absent
  • muscular tongue = bilobed at the tip and free from behind, used to capture the prey.
  • small intestine = long coiled ileum + short straight duodenum
  • digestive glands = gastric glands + intestinal glands + liver + pancreas + gall bladder
  • gastric glands + intestinal glands = present in the walls of stomach and small intestine respectively, produce gastric juices
  • gastric juice = thin, strongly acidic (pH varying from 1 to 3), almost colorless liquid, essential constituents are the digestive enzymes pepsin, hydrochloric acid, and mucus.
  • gall bladder = stores bile secreted by liver, bile emulsifies fats, changes pH of food from acidic to alkaline and checks growth of bacteria.
  • pancreas = secretes pancreatic juice containing digestive enzymes (trypsin, amylase, lipases etc)
  • villi + microvilli = digested food is absorbed by the numerous finger-like folds in the inner wall of intestines called villi and microvilli.

Respiratory System

cutaneous + buccopharyngeal + pulmonary

  • cutaneous respiration = occurs through naked skin, takes place in water as well as on land. During hibernation and aestivation frogs respires through this method only.
  • buccopharyngeal respiration = occurs only on land through moist lining of buccopharyngeal cavity.
  • pulmonary respiration = lungs are a pair of oval, pinkish, sacs located in the thorax, pulmonary respiration has a maximum frequency of 20/minutes. Occurs only when more energy is required.

Nervous System

  • Central nervous system + Peripheral nervous system + Autonomic nervous system.
  • CNS = brain + spinal cord.
  • Brain is contained in a bony structure known as brain box or cranium and protects it from the external shocks. Brain of frog consists of a pair of occipital condyles.
  • Brain = Fore-brain + Mid-brain + Hind-brain
  • Fore-brain = olfactory lobes + a pair of cerebral hemispheres + unpaired diencephalon.
  • Cerebrum = response to environment
  • Olfactory lobe = controls the sense of smell.
  • diencephalon = relays sensory information between brain regions and controls many autonomic functions of the peripheral nervous system.
  • Mid-brain = consists of a pair of optic lobes, involved in the frog’s vision.
  • Hind-brain = cerebellum + medulla oblongata
  • cerebellum = helps to maintain the balance and equilibrium of the frog, also controls the muscular coordination and posture.
  • medulla oblongata = passes out via the foramen magnum and continues into the spinal cord of the frog up to the tip of its trunk, which is contained in the vertebral column, helps in the regulation of respiration, digestion, and other automatic functions.
  • Peripheral nervous system = cranial + spinal nerves
  • Ten pair of cranial nerves are present in the frog (as compared to twelve pairs present in human) and it arises from the brain and innervate to the different parts of body. Cranial nerves are involved in passing the information from outside to the brain.
  • Frog contains 10 pairs of spinal nerves (as compared to 30 pairs present in humans) that rise from the spinal cord and gets distributed to the different parts of body. Spinal nerves of frog appear as white in color and are thread-like structures that emerge between the vertebrae and are located along the dorsal wall of body cavity. Spinal nerves functions in passing information from the extremities to brain through spinal cord.
  • Autonomic nervous system = the part of the nervous system responsible for control of the bodily functions not consciously directed, such as breathing, the heartbeat, and digestive processes.

 

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Natural Selection

Introduction

In the mid 19th century, Charles Darwin (1809 – 1882), formulated the scientific theory of evolution by natural selection, published in his book “On the origin of species” in 1859.

Darwin’s idea were inspired by the observations that he had made during a sea voyage in a sail ship called H.M.S Beagle round the world, from 1831 to 1836.

Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring. It is a key mechanism of evolution which involves the change in heritable traits of a population over time.

The concept of fitness is central to natural selection i.e., individuals that are more “fit” have better potentials for survival. Herbert Spencer (1820 –  1903) coined the phrase “survival of the fittest”, in his book “Principles of biology” in 1864, after reading Darwin’s book “On the origin of species”.

Alfred Russel Wallace (1823 – 1913) best known for independently conceiving the theory of evolution through natural selection by working in Malay Archepelago. The concept of natural selection was published by Darwin and Wallace in a “Joint presentation of papers” (1858).

 

Five basic concepts of Darwinism

  1. Rapid multiplication or overproduction
  2. Limited resources
  3. Variations
  4. Natural selection
  5. Speciation (new species formation)

 

Types of selections

  1. Stabilizing or balancing selection: It reduces the variations and maintains the mean value in a population, thereby preventing the evolutionary change. For example, selection against homozygous sickle-cell sufferers, and the selection against the standard HgbA homozygotes by malaria.
  2.  Directional or progressive selection: Population changes towards a particular direction, thereby disturbing the mean value in a population. For example, evolution of DDT resistant mosquitoes and industrial melanism (described below).
  3. Disruptive or diversifying selection: Favors both extremes in a population thereby eliminating most individuals of mean values. It leads to development of two different populations. For example, sexual dimorphism.

 

Classical example of natural selection: Industrial melanism

The classical example of natural selection is provided by the response of a peppered moth Biston betularia, which is found in all parts of England.

Industrial melanism is an adaptation where the moths living in the industrial areas developed melanin pigment to hide themselves from their predators.

Before the industrial revolution, most of the peppered moths in the UK were white and very few were melanic (black). After their habitats become polluted with soot from the coal-fired industries, the white moths were selectively picked up predators.

On the other hand, dark-colored moths were camouflaged very well by the blackened trees and in turn their population rapidly increased.

Evidence from biogeographical distribution

Darwin studied the climatic conditions, flora and fauna of Galapagos islands, during his voyage around the world.

He noticed nearly 20 related varieties of small birds in these islands which differed mainly in the shape and size of their beaks. These birds are now called Darwin’s finches.

The first clear explanation for these different varieties is allopatric speciation or geographical speciation. It occurs when biological populations of the same species become isolated from each other to an extent that creates hindrance in genetic interchange.

 

Other observations of natural selection

  1. Resistance of insects to pesticides.
  2. Antibiotic resistance in bacteria.
  3. Heavy metal resistance in plants.
  4. Transient genetic polymorphism.
  5. Production of new varieties through artificial selection.

 

Limitations of natural selection

  1. Small variations which are not essential also inherited.
  2. According to this theory, vestigial organs should not be present.
  3. Evolution of complex internal structures and organs are not explained.
  4. No clear explanation for the causes and origin of variations.
  5. No explanation for evolution of terrestrial animals from aquatic animals.
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Chromosomal Mutations

The process that produces an alteration in DNA or chromosome’s structure or number is known as mutation.

The term ‘mutation’ was coined by Hugo Marie de Vries in 1901.

Any agent that cause mutation or increase the rate of mutation is known as mutagen.

Chromosomal Mutation

Chromosomal mutation is a missing, extra, or irregular portion of chromosome. It is also known as chromosomal anomaly, chromosomal abnormality, chromosomal aberration or chromosomal disorder.

It can be classified into following three categories:

  1. Polyploidy
  2. Aneuploidy
  3. Chromosomal translocation 

1. Polyploidy

‘Ploidy’ is the number of set of chromosomes in the nucleus of a cell.

When an organism or a cell contains more than two paired sets of chromosomes (>2n), then the condition is known as polyploidy. It may be tripolidy (3n), tetraploidy (4n), pentaploidy (5n) and so on.

This condition is mostly observe in plants.

Autopolyploids = Polyploids with chromosomes sets, derived from single species.

Allopolyploids = Polyploids with chromosome sets, derived from different species.

2. Aneuploidy

When an organism or a cell contains more than two paired sets of chromosomes, but not a complete set, then the condition is known as aneuploidy

Aneuploidy originates during anaphase of meiosis when the chromosome(s) do not separate properly between the two daughter cells, a condition known as non-disjunction.

Few examples of aneuploidy are given below:

Nullisomy (2n-2) The loss of both pairs of homologous chromosomes.

Monosomy (2n-1) – The loss of a single chromosome.

Example – Monosomy of sex chromosome (45, X) causes Turner syndrome or gonadal dysgenesis.

Trisomy (2n+1) – The gain of an extra copy of chromosome. 

Example – The presence of an extra chromosome 21 which is found in Down syndrome is called trisomy 21.

Tetrasomy (2n+2) – The gain of an extra pair of homologous chromosomes.

Example – Tetrasomy 9p is caused by the the presence of two extra copies of the short arm of chromosome 9.

3. Chromosomal Translocation

It is caused by rearrangement of parts between non-homologous chromosomes.

Example – Chronic Myelogenous Leukemia (CML) 


 

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Chromosome Abnormality: Down Syndrome

  • A chromosome abnormality or a chromosomal disorder is a missing or irregular or extra portion of chromosomal DNA.
  • usually occur when there is an error in cell division following meiosis or mitosis.
  • are generally not inherited but occur as random events during the formation of reproductive cell.
  • an error in cell division called non-disjunction results in the reproductive cells with an abnormal number of chromosome.
  • Non-disjunction – the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during anaphase (meiosis), usually resulting in  abnormal distribution of chromosomes in the daughter nuclei.
  • examples of chromosomal disorders = Down syndrome, Edwards syndrome, Patau syndrome, Cri du Chat syndrome, Turner syndrome etc

Mitotic_nondisjunction
Fig: Non-disjunction (wikipedia)

Down Syndrome:

  • also called trisomy 21 or 47, + 21 or DS or DNS
  • chromosomal disorder caused by the presence of whole or a part of third copy of chromosome 21.
  • first characterized by English physician John Langdon Down in 1862.
  • occurs in about one per 1000 babies born each year.
  • sign & symptoms = mental illness, stunted growth, shortened hands, slanted eyes, abnormal teeth, short neck, flat head, large and protruding tongue, poor muscle tone (hypotonia), extra space between big toe and second toe etc
  • prone to respiratory disease, heart malformation, keratoconus and leukemia.
  • most individuals with Down syndrome have mild IQ (50-70) or moderate IQ (35-50).
  • characteristic cause = non-disjunction of chromosome 21 during meiosis – paired homologous chromosome failed to disjoin either during anaphase – I or II that may lead to gametes with n +1 chromosome composition.
  • main source (about 95%) of trisomy 21 is the ovum.
  • other cause = chances dramatically increase as the age of mother increases.

At 30 –> 1 in 1000
At 40 –> 1 in 100
At 45 –> 1 in 5o

  • parental diagnosis = Amniocentesis (AFT) or Chorionic Villus Sampling (CVS)
  • therapeutic abortion is one option currently available to parents with number of religious and ethical issues.
karyotype-down-syndrome
Fig: Karyotype – Down Syndrome

 

 


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Principles of One-Gene Inheritance

  • The study and analysis of inheritance as a result of monohybrid crosses is called one-gene inheritance.
  • The cross between true-breeding pea plants with tall stems and dwarf stems is represent Mendel’s monohybrid crosses.
  • When Mendel crossed true breeding tall plants with dwarf plants, the resulting F1 generation consisted of only tall plants.
  • When members of the F1 generation were selfed cross, Mendel observed that 787 0f 1064 F2 plants were tall while 277 of 1064 were dwarf – a ratio of about 3:1.
  • Mendel made similar crosses between pea plants, exhibiting each of the other pairs of contrasting traits. In every case, the outcome was similar to the tall/dwarf cross.
  • Mendel proposed the existence of what he called ‘unit factors’ for each trait. He suggested that these factors serve as the basic unit of heredity and are passed unchanged from generation to generation.
  • Using the results of monohybrid crosses, Mendel derived the following three postulates, also known as ‘Principles of Inheritance’:
  1. Principle of paired factor:- Genetic characters are controlled by unit factors that exist in pairs in individual organisms.
  2. Principle of Dominance:- Out of the two contrasting traits, only one is able to express its effect in the individual. It is called dominant factor while the other is called recessive factor which does not show its effect.
  3. Principle of Segregation:- During the formation of gametes, the paired unit factors separate or segregate randomly so that each gamete receives one or the other with equal possibility.

 

mendel monohybrid cross


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Liver: At a glance

Overview

  • It is the heaviest internal organ and largest gland in the body, weighing about 1.2 – 1.5 kg in an adult human.
  • It is located in the upper right side of the abdominal cavity and is reddish brown in colour.
  • It has two main lobes (larger right lobe & smaller left lobe) and two small lobes (quadrate lobe & caudate lobe) present behind the main lobes.
  • The two main right and left lobes are separated by the falciform ligament.
  • The liver consists of small structural and functional units called hepatic lobules.
  • The lobules are roughly hexagonal, and consist millions of glycogen-rich cells, the hepatocytes (hepatic cells).
  • Each lobule is covered by a thin connective tissue sheath known as Glisson’s capsule.
  • The mammalian liver also contains phagocytic cells known as Kupffer cells. They engulf dead WBCs, RBCs and pathogens.
  • A thin-walled, sac like structure, the gall bladder, lies in the lower surface of the right lobe. It stores and concentrates bile secreted by the liver cells.
  • The duct of gall bladder is called cystic duct.
  • The cystic duct is connected with common hepatic duct to form bile duct which passes downward and is joined by the pancreatic duct to form hepatopancreatic duct.
  • The hepatopancreatic duct is connected with duodenum through a hepatopancreatic ampulla which is guarded by a sphincter called the sphincter of Oddi.
  • There are two distinct sources that supply blood to the liver: oxygenated blood flows in, from the hepatic artery and nutrient-rich (deoxygenated) blood flows in, from the hepatic portal vein.

 

Liver
Fig: Liver and pancreas with their ducts.

Vital Functions

  • Filtration of blood coming from the digestive tract, before passing it to the rest of the body.
  • Regulation of blood sugar level through glycogenesis (conversion of excess glucose into glycogen) and glycogenolysis (conversion of glycogen into glucose).
  • Performs gluconeogenesis (formation of glucose from non-carbohydrate source).
  • Controls lipogenesis (conversion of excess glucose and amino acids into fats).
  • Acts as haemolytic organ (breaks old RBCs).
  • Secretes angiotensinogen, that later forms angiotensin – a peptide hormone and a potent dipsogen (agent that causes thirst). Angiotensin is also a part of renin-angiotensin system (a hormone system that regulates blood pressure and fluid balance).
  • Detoxifies chemicals and metabolizes drugs.
  • Secretes bile – an alkaline fluid having several organic and inorganic salts that helps in the emulsification of fats.
  • Storage of glucose in the form of glycogen, mineral like copper, iron etc as well storage of vitamins like vitamin A, D, E, K and B12.

 

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Histology of Human Gut

The wall of the human alimentary canal consists of four distinct layers: the mucosa, sub-mucosa, muscularis and serosa.

shayan

Fig: Transverse section of gut (diagrammatic representation)

Mucosa The mucosa is the innermost layer of the gastrointestinal tract that is surrounding the lumen (open space within the tube). This layer comes in direct contact with digested food. This layer forms rugae in the stomach and villi in the small intestine. The mucosa is made up of epithelium – innermost layer, responsible for most digestive, absorptive and secretory processes. Mucosa also contains goblet cells which produces mucus that protects the epithelial surface.

Sub-mucosa The sub-mucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymph vessels and nerves. In duodenum, glands are also present in it. Sub-mucosa supports the mucosa.

Muscularis/Muscular layer – Muscularis is a thin layer of smooth muscles arranged into an outer longitudinal layer and an inner circular layer. The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract. The coordinated contraction of these layers is called peristalsis (alternate contraction and relaxation, which pushes ingested food). At a number of points along the gut the circular muscle thickens into structures called sphincters.

Serosa Serosa is the outermost layer of the human alimentary canal. It is made up of a thin layer of secretory epithelial cells, with some connective tissues underneath. The epithelial layer, produce the lubricating serous fluid. This fluid has a consistency similar to thin mucus. These cells are bound tightly to the underlying connective tissue. The connective tissue layer provides the blood vessels and nerves for the overlying secretory cells, and also serves as the binding layer which allows the serosa to adhere to organs and other structures.


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The Tryptophan Operon – A Repressible Operon System

The tryptophan (trp) operon system is a type of repressible operon system. It was worked out by Jacob and Monod in 1953.

The 20 amino acids are required in large amounts for protein synthesis and E.coli can synthesize all of them. The genes for the enzymes needed to synthesize tryptophan are generally clustered in trp operon and are expressed whenever existing supplies are limiting.

When tryptophan is present, it binds the trp repressor and induces a conformational change in that protein, enabling it to bind the trp operator and prevent transcription (operon is repressed).

The E.coli trp operon includes five trp genes (trp E, D, C, B, A) that encode enzymes required to convert chorismate to tryptophan.

The gene products are:

trpE – anthranilate synthetase

trpD – phosphoribosyl anthranilate transferase

trpC – phosphoribosyl anthranilate isomerase-indole glycerol phosphate synthetase

trpB – tryptophan synthetase β

trpA – tryptophan synthetase α

Mechanisms:

Transcription is initiated at the beginning of the 162 nucleotide mRNA leader encoded by a DNA region called trpL. Once repression is lifted and transcription begins, the rate of transcription is controlled by a second regulatory process, called transcription attenuation. This regulatory process determines whether transcription is attenuated (terminated) at the end of the leader or continues into the structural genes.

The trp operon attenuation mechanism uses signals encoded in four sequences within a 162 nucleotide leader region at the 5’-end of the mRNA, before the initiation codon of the first gene (trpE). Within the leader lies a region known as the attenuator, made up of sequences 3 and 4. The attenuator structure forms by the pairing of sequences 3 and 4. The attenuator structure acts as a transcription terminator.

Sequence 2 is an alternate complement for sequence 3. If sequences 2 and 3 base-pair, the attenuator structure cannot form and transcription continues into the trp genes. The 2:3 structure, unlike the 3:4 attenuator, does not prevent transcription.

The sequence encoding the leader peptide has two  tryptophan codons in a row. When tryptophan concentrations are high, concentrations of charged trp tRNA are also high. This allow ribosome to quickly translates sequence 1 and block sequence 2. Ribosome blocking sequence 2 allows formation of the 3:4 attenuator, aborting transcription at the end  of the  leader RNA. The leader peptide has no other known cellular function, its  synthesis is simply an operon regulatory device.

trp 1When tryptophan levels are low, there is very little charged tryptophan tRNA available, and the ribosome stuck when it reaches the tryptophan  codons. A ribosome caught at the tryptophan codons, masks region 1, leaving sequence 2 free to pair with sequence 3, thus the 3:4 attenuator hair-pin structure cannot be made. In this way, RNA polymerase passes the attenuator and moves on into the operon, allowing trp enzymes  expression.

trp 2


 

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The Human Genome Project

Francis Collins

The genome is the ultimate source of information about an organism. Advances in genetic engineering techniques made it possible for the scientists to isolate and clone DNA pieces and determine nucleotide sequences of these genome. After the development of practical DNA sequencing methods, serious discussions began about the prospects for sequencing the entire 3 billion base pairs of the human genome. The international Human Genome Project got underway with extensive funding in the late 1980s. The effort eventually included significant contributions from 20 sequencing centers distributed among six nations: the United States, Great Britain, Japan, France, China, and Germany. General coordination was provided by the Office of Genome Research at the National Institutes of Health, led first by James Watson and after 1992 by Francis Collins.

At the outset, the task of sequencing a 3 X 109 bp genome seemed to be a massive job, but it gradually yielded to advances in technology. The completed sequence of the human genome was published in April 2003, several years ahead of schedule. The sequence of chromosome 1 was completed only in May 2006. The Human Genome Project (HGP) was a 13-year project coordinated by the U.S. Department of Energy and National Institute of Health. Decades-old estimates that humans possessed about 100,000 genes within the approximately 3 X 109 bp in the human genome have been supplanted by the discovery that we have only 30,000 to 35,000 genes.

The Human Genome Project marks the culmination of twentieth-century biology and promises a vastly changed scientific landscape for the new century. The human genome is only part of the story, as the genomes of many other species are also being sequenced, including the yeasts Saccharomyces cerevisiae (completed in 1996) and Schizosaccharomyces pombe (2002), the nematode Caenorhabditis elegans (1998), the fruit fly Drosophila melanogaster (2000), the plant Arabidopsis thaliana (2000), the mouse Mus musculus (2002), zebrafish, and dozens of bacterial and archaebacterial species.