Friday, October 1, 2021
Thursday, May 27, 2021
Respiration in scoliodon or shark
Respiratory System of Scoliodon
🦈Scoliodon is an aquatic animal.
🦈It depends completely upon oxygen dissolved in sea water for respiration.
🦈Thus, respiration is aquatic and carried on entirely by vascular gills.
Respiratory Organs:
🦈There are five pairs of gill- pouches bearing gills, arranged in a series behind the hyoid arch in the lateral walls of the pharynx.
🦈 Each gill-pouch is compressed antero-posteriorly and communicates with the cavity of the pharynx through a large internal branchial aperture and with the exterior through a narrow external branchial
aperture also called gill- slit.
🦈Two adjacent gill-pouches are completely separated from each other by a vertical fibro-muscular partition, the interbranchial septum or gill septum.
🦈The inner or pharyngeal border of each gill septum is supported by a cartilaginous visceral arch or gill arch with its slender branchial rays.
🦈The septum is covered by an epithelium and contains blood vessels, nerves, etc.
🦈The mucous membranes of a septum are raised into numerous horizontal leaf-like folds, called gill lamellae or gill filaments.
🦈These constitute the gill proper and are richly supplied with bloodcapillaries.
🦈 Each septum bears two sets of gill-lamellae, one on its anterior face and the other on its posterior face.
🦈Each set makes a half gill called demibranch or hemibranch, while both the sets attached to a gill arch and its gill septum constitute a complete gill called holobranch.
🦈 In Scoliodon the interbranchial septa extend well beyond their gill lamellae to form flaps which protect the gills as well as external gill slits. Such gills are called lamelliform.
🦈In front of hyoid arch or the first internal gill slit, on either lateral side of pharynx, there is an oval pit of spiracle.
🦈It has no gill lamellae and 'no external opening and is regarded a vestigial gill pouch.
Mechanism of respiration
🦈During respiration, water taken into the mouth, passes through internal gill slits bathing gill lamellae and passes out of the external gill slits.
1. Inspiration:
🦈 The floor of the buccopharyngeal cavity is depressed by the contraction of hypobranchial (hypoglossal) muscles due to which the visceral arches expand the wall of the pharynx, so that sea-water containing dissolved oxygen rushes in through the opened mouth.
🦈Entry of water into external branchial apertures is prevented by an anterior fold of skin on each gill-pouch.
🦈 As the external gill slits are tightly closed, water enters the enlarged buccopharyngeal cavity through the open mouth
2. Expiration:
🦈During expiration the mouth becomes closed by the action of adductor muscle.
🦈 At the same time the constrictor and inter-branchial muscles contract raising the floor of pharynx and reducing its volume.
🦈As a result, water is forced into gill pouches, over the gill lamellae, and out through the open external gill slits.
🦈 The spiracles are occasionally used as accessory pathways for entry of water for respiration, instead of the mouth.
Physiology of respiration:
In the branchial lamellae the blood flows from the tip towards the base, that is in a direction opposite to that of the water current, so that the blood just before leaving the lamellae meets the highest concentration of oxygen and the lowest of carbon dioxide, thus, an efficient exchange of oxygen and carbon dioxide takes place between the blood and sea-water.Sea-water entering the gill- pouches with the respiratory current contains oxygen dissolved in it. This water is separated from the blood
contained within the capillaries of the gill-lamellae merely by the thin and permeable membranous walls of the capillaries. The oxygen of the water passes by endosmosis through the thin capillary walls into the blood, and at the same time the carbon dioxide of the blood passes into the water by a process of exosmosis.The oxygen is conveyed by the blood to all the parts of the body, while carbon dioxide brought to the gills in the venous blood is eliminated by the water of the outgoing respiratory current. As the blood makes a complete circuit in the capillaries of the gills in a very short time, it is evident that exchange of gases also takes place very quickly
Thursday, May 20, 2021
Digestive system of scoliodon
🦈The digestive system consists of the alimentary canal and the digestive glands.
🦈The alimentary canal
starts with the mouth and terminates in the cloaca.
🦈The mouth leads into a spacious buccal cavity which
is lined with mucous membrane.
🦈The floor of the buccal cavity becomes folded to form a non-muscular and non-granular ‘tongue’.
🦈The mucous membrane is very thick and rough due to the presence of dermal denticles or teeth.
🦈The teeth are very sharp and are obliquely placed .
🦈The teeth are homodont i.e., the teeth are similar in shape and polyphyodont ie posesses several sets of teeth functioning in succession.
🦈The buccal cavity leads into pharynx.
🦈On either side of the pharynx there lie the internal openings of the spiracles and five branchial clefts.
🦈The mucous membrane of the pharyngeal wall contains
numerous dermal denticles.
🦈The pharynx leads into a narrow oesophagus.
🦈The inner mucous membrane of the pharynx is raised to form longitudinal folds.
🦈 The oesophagus dilates posteriorly to form a large stomach.
🦈The stomach is highly muscular and is bent on itself to form a J-shaped configuration.
🦈The long limb of the stomach is continuous with the oesophagus and the shorter one passes into the intestine.
🦈The entrance of the oesophagus into the
stomach is provided with a crescentic fold which serves as the valve.
🦈The long anterior limb is called the cardiac stomach and the short posterior limb is designated as the pyloric stomach.
🦈A small outgrowth often called ‘blind sac’ is present at the junction of the cardiac and
pyloric limbs.
🦈The inner lining of the cardiac stomach is folded longitudinally like that of oesophagus
🦈The internal lining of the pyloric stomach is mostly smooth though slight foldings are observed at the distal end.
🦈The pyloric valve, at the end of the pylorus, guards the entrance of it into a thick-walled small chamber called the bursa entiana.
🦈The bursa entiana is immediately followed by wide tubular intestine which becomes narrowed posteriorly as the rectum.
🦈The rectum opens into the cloaca.
🦈A tubular caecal or rectal or digit form gland opens into the rectum.
🦈The inner surface of the intestine becomes folded to form an anticlockwise spiral of approximately two and a half turns.
🦈This is called the scroll valve which increases the absorptive surface of the intestine and also checks the rapid flow of digested food through the intestine.
🦈The major digestive gland is the liver which is a massive yellowish gland and consists of two lobes.
🦈The lobes are united anteriorly.
🦈A thin-walled V-shaped gall-bladder is present in the anterior part of the right lobe of liver.
🦈The bile duct receives a few smaller ducts from the two lobes of the liver and opens
into the anterior end of intestine near the commencement of the scroll valve.
🦈The pancreas is a pale compact irregular body and consists of a dorsal lobe situated parallel to the
posterior part of cardiac stomach and a ventral lobe which remains closely attached to the pyloric stomach.
🦈The pancreatic juice is poured into the intestine by pancreatic duct situated opposite to the aperture of the bile duct.
🦈The functional significance of rectal gland is not properly known.
🦈The *rectal gland* has a central cavity lined with cuboidal cells.
🦈It is highly vascular and composed of lymphoid tissue. 🦈It discharges a fluid
into the lumen of the intestine but its actual role is not known.
🦈The buccal cavity possesses no salivary glands.
Monday, May 10, 2021
Lecture notes on reproduction, fertilization and development in penaeus (prawn)
Penaeus Female Reproductive System:
The female has a pair of long ovaries extending the whole length of the thorax and the abdomen along the median line. The two ovaries are fused together posteriorly, but free anteriorly. Near the anterior end the ovaries produce finger-like outgrowths called diverticula. From each ovary arises an oviduct.The oviduct opens to the third walking leg.
Penaeus_Male Reproductive System:The male has a pair of tubular testes located in the thorax on either side of the middle line. The two testes are fused together anteriorly.Each testis has many finger-like outgrowts called diverticula. Posteriorly, the testis leads into a vas deferens. The terminal end of the vas deferens becomes dilated into an ejaculatory bulb. The ejaculatory bulb opensbto the outside by the male genital pore at thevbase of the last walking leg.
Fertilization and Development:In penaeus fertilization is external. During copulation, a male puts the female down on her back and deposits sperms (spermatophores) on the ventral surface in the thelycumnear the female genital pores. As the eggs come out of female genital pore, they are fertilized by the sperms already present there. The female lays eggs in large numbers. The fertilized eggs are attached to the pleopods by a sticky secretion of the tegumental glands. In Penaeus development is indirect. The fertilized egg hatches into a larva called nauplius. The nauplius is followed by a series of larval forms, namely metanauplius, protozoea, zoea and mysis. Finally the Mysis larva is transformed into a prawn.
1. NaupliusThe larva hatched from the egg is called nauplius larva. It has the following features:It is a free swimming minute, microscopic and pelagic larva. It is oval in shape; the anterior end is broad and the posterior end is narrow.The body has three regions, namely the anterior head, the middle trunk and the posterior anal region.The head has a simple median eye on the dorsal side called nauplius eye.It has 3 pairs of appendages. They develop into the first 3 pairs of appendages of adult prawn. The first pair of appendages is uniramous and is called antennule. The second and third pairs of appendages are biramous and are called antenna and mandible. The mandible has no teeth.The posterior end has a caudal fork.The nauplius larva grows and undergos several moults and finally develops into the next larva called metanauplius.
| 1. 2. 3. The three pairs of appendages of the nauplius larva (the future first and second antennae and mandibles). |
| 3. Mandible |
| 2. Metanauplius:It is the second larva of Penaeus. It has an anterior cephalothorax and a posterior abdomen. The abdomen ends in a caudal fork.The anterior end has a pair of frontal sense organs. The larva has three pairs of appendages, the next three pairs of appendages develop as buds. |
3.Protozoea: Protozoea has an elongated body, divisible into cephalo-thorax and abdomen. Cephalo-thoracic region is covered by a dorsal shield or carapace. Theseven pairs of appendages of metanauplius become well developed. Alittle later, the 3rd pair of maxillipedes also develop. On the sides of the median eye a pair of lateral eyes appear.Protozoea grows in size, undergoes moulting and transforms to zoea.
4.Zoea:Zoea develops from the metanauplius.It is a free swimming pelagic, minute and microscopic.The body is divisible into an anterior cephalothorax and a posterior abdomen.The cephalothorax is covered by a carapace.Anteriorly the carapace is produced into median spine called rostrum.It has a pair of stalked compound eyes.All the 5 cephalic appendages are well developed.In the thorax, the 3 pairs of maxillipedesare slightly developed and the remaining 5 pairs of walking legs are in the form of buds.The abdomen has 6 segments. The last segment ends in a forked telson. The abdomen is without appendages.
5. Mysis:The mysis larva develops from zoea larva.It is elongated, laterally compressed and transparent.The body is divisible into a cephalothorax and an abdomen.The cephalothorax is covered by a carapace.Anteriorly the carapace is produced into a spine called rostrum.All the appendages are developed. They include 5 pairs of cephalic appendages, 8 pairs of thoracic appendages and 6 pairs of abdominal appendages.The thoracic legs are used for swimming. The abdomen ends in a telson.The eyes are stalked and compound.The mysis larva looks like the adult Mysis and hence the name. The larval mysis differs from the adult Mysis in the absence of a brood pouch beneath the thorax and a pair of statocysts.
Thursday, April 22, 2021
Thursday, April 1, 2021
Tuesday, February 16, 2021
Saturday, February 13, 2021
Friday, February 12, 2021
Thursday, February 11, 2021
Tuesday, February 9, 2021
Friday, February 5, 2021
Friday, January 29, 2021
ECONOMIC IMPORTANCE OF FISHES
🐡🐡🐡🐡🐡🐡🐡
*General essay on Economic importance of fishes*
🐠🐟🐠🐟🐠🐟🐠
Fishes are one of the most important group of vertebrates which possess a great economic, nutritional, medicinal, industrial, aesthetic and religious values. It provides job opportunities for millions of people around the world.They are both useful and also harmful to man.
*Uses fishes:*
1️⃣ *Fishes are major source of food with high nutritive value:* Nearly all fishes are edible and are important sources of proteins, fat, vitamins and minerals. They are rich in vitamin A and D and is a depot of calcium, phosphorus and iodine.
2️⃣ *Fish manure :* Low grade, unedible fish given out from the fish-canning factories, market and others are utilized to prepare the fish manure. These residues are dried, ground, mixed with ash and converted into manure, which contains a high percentage of nitrogen and phosphorous.
3️⃣ *Fish glue:* The gelatinous adhesive material obtained from the connective tissues of skin and bones of certain fish, principally cod fish, is called fish glue. It is used in gummed tape, letterpress printing plates, blueprint paper and adhering the wood, leather, glass, etc.
4️⃣ *Isinglass:* Isinglass is a substance obtained from the dried swim bladders of fish. It is a high-grade collagen used mainly for the clarification of wine, beer and vinegar. It can also be cooked into a paste for specialized gluing purposes. For making isinglass, the air bladder is removed from the fish, washed thoroughly and flattened by beating. The beaten pieces are then dried for the preparation of isinglass. It is in the form of shiny powder. Isinglass is generally made from sturgeon and cod fish.
5️⃣ *Fish leather:* The skins of several fishes (e.g., sharks and rays) are used for manufacturing, polishing and smoothing materials in place of sand paper. The dried and processed skin is also used for preparing high-quality shoes, moneybags, suitcases, belts, phone cases, etc. Fish leather is an eco-friendly alternative to the typical exotic leathers such as crocodile and snake that threaten endangered species.
6️⃣ *Fish fin:* The fins of the sharks and rays are used to make tasteful sauce and soups. It is a popular soup item of China.
7️⃣ *Medicinal value*
Certain fishes and their bye-products contribute to useful Ayurvedic and Unani medicines for treatment of duodenal ulcers, skin diseases, night blindness, general weakness, loss of appetite, colds, coughs, bronchitis, ashthma, tuberculosis, etc.
8️⃣ *Biological control*
Many species of carnivorous and larvivorous fishes prey upon insects and their larvae in water. These fishes can be used to control harmful insects, mosquito larvae, etc. Gambusia affinis is a well known fish for mosquito control, hence called mosquito fish.
9️⃣ *Decorative value*
Many species of colored fish are kept in aquarium, ocenarium, ponds and lakes for decoration or ornamentation. The common ornamental fish species kept in aquarium are gold fish, gourami , zebra fish , guppy , fighting fish, koi, etc.
🔟 *Sports and games*
Sport or recreation fishing is the fishing for pleasure or competition. Sport fishing can be done in a variety of ways. The most common form of sport fishing is done with a rod, line and hooks with baits, called angling.
1️⃣1️⃣ *Employment opportunity*
Fisheries and aquaculture sector provide, either directly or indirectly, a great employment opportunity for millions of people around the world. In 2012, about 500 million people were directly engaged in the world, part time or full time, in production of fish, either by fishing or in aquaculture.
*Harmful Fishes:*
1️⃣ *Some fishes are destructive:*
Majority of cartilaginous fishes are predaceous and feed chiefly on large quantities of crabs, lobsters, squids and other valuable marine animals. They also destroy eggs, young and even adults of food fishes.
2️⃣ *Some fishes are injurious:*
Some sharks are highly dangerous in the sea, injure fisherman and damage crafts and gears.
Some larger sharks and sword fish may capsize small boats and injure or even kill fisherman. Sharks are a menace to bathers and skin divers in shallow waters. The pirhanas of Central and South America with strong teeth are dangerous to man.
Electric ray (Torpedo) have electric organs which give powerful shocks to swimmers and fishermen.
3️⃣ *Some fishes are toxic:* Poisonous structures are found in some fishes like sting ray (Trygon). They can cause painful fatel wounds by the poisonous stings or spines.
Wednesday, January 27, 2021
SPLIT GENES(mosaic genes or interrupted genes)
Split genes (mosaic genes or interrupted genes) are the non-contiguous eukayotic genes with informative or coding sequences, interspaced with or interrupted by long stretches of non-informative or non-coding "silent" or "spacer" sequences. Thus, split genes appear to be complex and fragmented genes, split up into alternating coding and non-coding regions. The non- coding regions are commonly called junk DNA or introns .The coding units are called exons, and the intervening non-coding regions are called introns or inserts, or intervening sequences (the terms exons and introns were coined by Walter Gilbert in 1978). Introns and split genes are very rare, or absent, in prokaryotic, mitochondrial and chloroplast DNAs, but relatively numerous in most eukaryotic nuclear genomes.They are present in some animal viruses also. Intron size varies from less than 100 to over 10,000 base pairs.
All exons may undergo transcription and translation. So, they find their expression in the pimary, intermediate and final products (i.e. in pre-mRNA, mature mRNA and proteins).On the other hand, introns are only transcribed to the pre-mRNA and then sliced out during the post-transcriptional chemical processing of immature mRNA. This is called mRNA splicing
Thus, introns appear only in the primary mRNA transcript and never in mature mRNA and proteins. On the whole, eukaryotic genes appear to be fragmented or interrupted, and they are expressed only after the splicing or excision of their introns during mRNA processing Split genes were discovered in 1977 by Richard. J. Roberts and Phillip Sharp (1993 Nobel Prize recipients), while studying the genetic material of adenoviruses (viruses which cause common cold, conjunctivitis, etc.). This discovery was a surprise jolt to the colinearity of genes and their products.
Features of split
genes
Some of the features, common to all split genes, are the following:
(i) The organization of split genes would be consistently the same in all tissues of an organism.
(ii) Exons have precisely the same order in the split gene and also in its primary and secondary mRNA transcripts.
(iii) Introns, all by themselves, do not have any coding function. But, exon-intron combination codes for maturases. Maturases (Lazowska et al 1980, Cech 1985) are the proteins encoded by exon-intron combination. They catalyse the excision of introns from the primary transcript and the subsequent splicing of exons.
(iv) The presence of introns is neither essential nor deleterious to the organism.
(v) All split genes begin with an exon, and also end with an exon. The primary mRNA, transcribed from a split gene, is known as heterogeneous nuclear RNA (hnRNA) or precursor mRNA (pre-mRNA). t might be an exact replica of the split gene, with altemating exons and introns. Before its transformation to mature mRNA, it undergoes a chemical tailoring". Its introns are very carefully sliced away by excision enzymes (exo and endonucleases). Soon, the exons are spliced together by maturases (RNA ligases). If mutanon takes place in introns, maturases will not be produced, or modified maturases without splicing powers will be produced.
Significance of split
genes
The significance of split genes is not definitely understood. Still, they are obviously advantageous to organisms in the following ways:
(I) Split genes provide an ingenious device for accommodating more DNA than what is actually required for the functioning of genes.
(ii) They enable the storage of the genetic information within minimum DNA.
(iii) The different exons of the same split gene can code for the different active sitesot a protein molecule (e.g., antibody genes).
(iv) Several proteins with the same amino acids can be formed from the same split gene that has many exons.
(v) Exons and introns allow flexibility in the synthesis of a variety of gene products.
The Concept of Gene
Genes are regarded as the biological units, which design development, control metabolism, govern heredity, generate mutations and produce heritable variations.
They are the molecules which govern the morphological, anatomical, physiological, behavioural, reproductive and developmental aspects of organisms.
In 1903, Sutton and Bouveri put forweard the "chromosome theory of heredity" and established that heredi tary factors are located in chromosomes and so chromosomes are the physical basis of heredity. In 1909, Johannsen coined the term "gene" to designate the hereditary determiner.
Classical concepts of
gene:The concepts about genes, formulated before the birth of molecular
genetics, are often
described as classical concepts. Their major generalizations are the following:
(i) Genes are independent, self-duplicating, hereditary units which determine the physical ,physiological and behavioral aspects of organisms.
(ii) Genes are the ultimate units of function that express themselves through the production of proteins.
(iii) Genes are the fundamental units of recombination, mutation and variation.
(iv) Genes are the units of hereditary transmission, and they can be transmitted from one
generation to the next, without considerable changes in them.
(v)Genes have linear and non-overlapping arrangement in the specific points or loci of chromosomes.
(vi )A single chromosome contains hundreds of thousands of genes. A single gene may exist in alternative forms, known as alleles, which occur in pairs. Most genes have only two alleles, dominant and recessive. But, some may have multiple alleles
(viii)two or more genes often interact to produce a single effect (interaction of genes)
Modern concepts of
gene
(i) Genes are complex hereditary determiners, composed of nucleic acid which is sequences of nucleotides, which contain the information necessary for all biological functions.
(ii) The nucleotide sequence of a gene encodes the amino acid sequence of a polypeptide or protein.
(iii) Genes are mostly sequential and non-overlapping.
(iv) Genes are not the units of either function or recombination or mutation, but these are all the functional aspects or properties of genes.
(v) In some cases (e.g, bacteria), some closely associated genes behave as a single functional unit, known as operon. An operon governs the production of the enzymes of a metabolic pathway. It may include four kinds of genes, namely structural gene, operator gene, regulator gene and promoter gene. Structural gene codes for an enzyme.
Cistron:Cistron
(Benzer -1957) is the fundamental genic unit, which governs the synthesis of a polypeptide
chain or protein molecule (now-a-days the term cistron is used as a synonym of gene).
A DNA molecule contains many cistrons. The mRNA of eukaryotes is monocistronic, while that of
prokaryotes is polycistronic. A cistron, in turn, has several functional units,
called codons. Each codon codes for a specific amino acid. It is formed of a
set of three nucleotides and hence called triplet codon.
Muton:Muton
(Benzer 1957) is the basic unit of gene mutation. In other words, it is the
smallest unit of DNA, that can be altered in the formation of a mutation. Since
mutation can occur even by a change in a single base, a single nucleotide pair
can serve as a muton.
Recon:Recon
(Benzer 1958) is the smallest genetic unit, capable of recombination . It may
be formed of one or more nucleotide pairs. It is exchangeable but not divisible
by genetic recombination. A cistron may contain several recons .
Complon :Complon
is the unit of allelic complementation. It is almost similar to a cistron.
Complementation is the process in which two mutant alleles together perform a
function, which cannot be carried
out by any one of them alone.
Transcripton:Transcripton
is the unit of transcription, and it govems the synthesis of an RNA molecule. It
is usually monocistronic in eukaryotes, and polycistronic in prokaryotes.
SPLIT GENES:Split
genes (mosaic genes or interrupted genes) are the non-contiguous eukayotic
genes with informative or coding
sequences, interspaced with or interrupted by long stretches of non-informative
or non-coding "silent" or "spacer" sequences. Thus, split
genes appear to be complex and fragmented genes, split up into alternating
coding and non-coding regions. The non- coding regions are commonly called junk
DNA or introns .The coding units are called exons, and the intervening
non-coding regions are called introns or inserts, or intervening sequences (the
terms exons and introns were coined by Walter
Gilbert in 1978). Introns and split genes are very rare, or absent, in
prokaryotic, mitochondrial and chloroplast DNAs, but relatively numerous in
most eukaryotic nuclear genomes.They are present in some animal viruses also.
Intron size varies from less than 100 to over 10,000 base pairs.
All exons may undergo transcription and translation. So, they find their expression in the pimary, intermediate and final products (i.e. in pre-mRNA, mature mRNA and proteins).On the other hand, introns are only transcribed to the pre-mRNA and then sliced out during the post-transcriptional chemical processing of immature mRNA. This is called mRNA splicing
Thus, introns appear only in the primary mRNA transcript and never in mature mRNA and proteins. On the whole, eukaryotic genes appear to be fragmented or interrupted, and they are expressed only after the splicing or excision of their introns during mRNA processing Split genes were discovered in 1977 by Richard. J. Roberts and Phillip Sharp (1993 Nobel Prize recipients), while studying the genetic material of adenoviruses (viruses which cause common cold, conjunctivitis, etc.). This discovery was a surprise jolt to the colinearity of genes and their products.
Features of split
genes
Some of the features, common to all split genes, are the following:
(i) The organization of split genes would be consistently the same in all tissues of an organism.
(ii) Exons have precisely the same order in the split gene and also in its primary and secondary mRNA transcripts.
(iii) Introns, all by themselves, do not have any coding function. But, exon-intron combination codes for maturases. Maturases (Lazowska et al 1980, Cech 1985) are the proteins encoded by exon-intron combination. They catalyse the excision of introns from the primary transcript and the subsequent splicing of exons.
(iv) The presence of introns is neither essential nor deleterious to the organism.
(v) All split genes begin with an exon, and also end with an exon. The primary mRNA, transcribed from a split gene, is known as heterogeneous nuclear RNA (hnRNA) or precursor mRNA (pre-mRNA). t might be an exact replica of the split gene, with altemating exons and introns. Before its transformation to mature mRNA, it undergoes a chemical tailoring". Its introns are very carefully sliced away by excision enzymes (exo and endonucleases). Soon, the exons are spliced together by maturases (RNA ligases). If mutanon takes place in introns, maturases will not be produced, or modified maturases without splicing powers will be produced.
Significance of split
genes
The significance of split genes is not definitely understood. Still, they are obviously advantageous to organisms in the following ways:
(I) Split genes provide an ingenious device for accommodating more DNA than what is actually required for the functioning of genes.
(ii) They enable the storage of the genetic information within minimum DNA.
(iii) The different exons of the same split gene can code for the different active sitesot a protein molecule (e.g., antibody genes).
(iv) Several proteins with the same amino acids can be formed from the same split gene that has many exons.
(v) Exons and introns allow flexibility in the synthesis of a variety of gene products.
PSEUDOGENES:Pseudogenes
(Jacqet al-1977) are the mutant,
imperfect and non-functional genes, which are strikingly homologous to their
functional counterparts at different
loci. In fact, they are the divergent members of gene families, which have
become non-functional through mutations in the course of evölution. They have
sequences coresponding to the exons and introns of functional genes. Still,
they have no transcriptional and translational powers for want of promoters.
Ihis is due to mutational changes. So, pseudogenes are only the defective and
non-functional copies of functional genes, without powers for expression. They
are produced by the mutation of active ancestral genes. The functional genes,
which are homologous to the pseudogenes, are called productive genes.
Pseudogene-producing mutations are mostly nonsense mutations, which generate
stop codons and result in premature temination of translation. Some pseudogenes
differ from their parent genes in having lost their introns. Curently,
pseudogenes are regarded as a kind of "debris or evolutionary
relics", accumulated in the genome as the "dead ends of evolutionary processes
Pseudogenes area common feature of many eukaryotic gene families. The gene
families of human actin genes, alpha and. beta globin genes, immunogobulin
genes, etc. contain pseudogenes.
Features of pseudogenes
i) Most pseudogenes out number their normal counterparts. So, they are mostly repetitive sequences
(i) Most pseudogenes are flanked by short repetitive sequences. This possibly suggests their insertion to a target site.
CRYPTIC GENES :These are the inactive or phenotypically silent prokaryotic genes which have been silenced by a single nucleotide substitution. They remain unexpressed during the normal life cycle of the organism.
However, they may be reactivated by mutation, recombination,transposition and other genetic events or sometimes by changes in environmental conditions. Thus, the cryptic state of these genes is only a transient feature.
Oscillations between cryptic
and active states do occur under the influence of genetic and environmental
factors. Consequently, they are
believed to confer some selective advantage on the organisms which carry them.
Tuesday, January 26, 2021
Biological Effects of Radiation
Radiation is energy which travels in the form of waves or high-speed particles. Radiation can occur naturally or be man-made. There are two types:
Non-ionizing radiation, which includes radio waves, cell phones, microwaves, infrared radiation and visible light
Ionizing radiation, which includes ultraviolet radiation, radon, x-rays, and gamma rays
Sources of radiation exposure:
Background radiation is all around us all the time. Most of it forms naturally from minerals. These radioactive minerals are in the ground, soil, water, and even our bodies. Background radiation can also come from outer space and the sun.
Other sources are man-made, such as x-rays, radiation therapy to treat cancer, and electrical power lines.
Radiation is one of the best-investigated hazardous agents.Biological effects of radiation are typically divided into two categories.
1️⃣The first category consists of exposure to high doses of radiation over short periods of time producing acute or short term effects.
2️⃣The second category represents exposure to low doses of radiation over an extended period of time producing chronic or long term effects.
High doses tend to kill cells, while low doses tend to damage or change them.
High doses can kill so many cells that tissues and organs are damaged. This in turn may cause a rapid whole body response often called the *Acute Radiation Syndrome (ARS).*
The effects of low doses of radiation occur at the level of the cell, and the results may not be observed for many years.
Acute RadiationSyndrome (ARS)
ARS is having three types syndrome,
1. Haematopoietic Syndrome
2. Gastrointestinal Syndrome
3. Central Nervous System syndrome
As the dose increases above 1.5Gy these three systems manifest it depending on the dose levels.
Acute Effect
A single accidental exposure to a high dose of radiation during a short period of time is referred to as an acute exposure and may produce biological effects within a short period after exposure. The signs and symptoms of the ARS are nausea, vomiting, fatigue and loss of appetite.
🌐Skin damage
🌐Nausea and vomiting
🌐Malaise and fatigue
🌐Increased temperature
🌐Blood changes
🌐Bone marrow damage
🌐Damage to cells lining the small intestine
🌐Damage to blood vessels in the brain
🌐Hematopoietic orBlood forming organs are most sensitive
🌀Gastrointestinal system is very sensitive
🌀Central Nervous System with brain and muscles are least sensitive
Effects of Low Doses of Radiation:
There are three general categories of effects resulting from exposure to low doses of radiation. These are:
1️⃣Genetic - The effect is suffered by the offspring of the individual exposed (mutations).
2️⃣Somatic - The effect is primarily suffered by the individual exposed. Since cancer is the Primary result, it is sometimes called the Carcinogenic Effect.
3️⃣In utero effect-the effect, suffered by a developing embryo/fetus, is seen after birth. However, this is actually a special case of the somatic effect, since the embryo/foetus is the one exposed to the radiation.
Risks of prenatal radiation exposure:
1. Mortality : This depends on when the exposure occurs.
2. Malformations:Malformations may be caused in organs developing at the time of the exposure.
3. Severe Mental Retardation:The developing human brain is very vulnerable to radiation damage between the 8th and 15th weeks of pregnancy.
4. Reduced Intelligence
5.Childhood Cancer.
Effect of radiation on DNA
Direct action can lead to either DNA damage or DNA mutations.
DNA damage includes changes to the chemical structure of the DNA molecule .
DNA mutations involve changes to the sequence of base pairs .If DNA damage is not repaired, it can lead to mutations. Mutations can prevent genes from making correct proteins. This can be very harmful to an organism.
Ionizing radiation can also affect important molecules other than DNA. For example, it can break the bonds holding water molecules together. This creates hydrogen (H+) and hydroxyls (OH-) ions. These are called free radicals.
Free radicals are highly reactive. They easily combine with other ions inside cells. For example, hydroxyl ions (OH-) can react with hydrogen atoms inside a DNA molecule to form hydrogen peroxide (H2O2). This can cause DNA damage.
Over time, damage from free radicals can build up. It is believed that free radical damage contributes to aging and diseases like cancer, Alzheimer’s and Parkinson’s disease.
Sunday, January 17, 2021
The one gene–one enzyme hypothesis
The one gene–one enzyme hypothesis was proposed by Beadle and Tautam in 1948. It states that single gene produces a single enzyme, which later affects an individual step in a metabolic pathway and they were awarded Nobel Prize for this work in 1958.
Beadle and Tatum later confirmed this hypothesis using genetic and biochemical studies on the bread mold Neurospora crassa.
Beadle and Tatum treated Neurospora crassa with X-rays and obtained a number of nutritional mutants called auxotroph’s. An auxotroph or nutritional mutant is that mutant which is not able to prepare its own metabolites from the raw materials obtained from outside. Therefore, it cannot live in normal environment but can be maintained in culture by providing the required metabolites. The wild type is called prototroph, is the normal individual which can synthesize all the complex metabolites required for its growth from raw materials obtained from outside. They can grow in the laboratory on minimal medium consisting of ammonia, sugar, salts and biotin.
They found three types of auxotroph’s requiring amino acids ornithine, citrulline and arginine. The prototrophs were found to have amino acid arginine in their body and has been synthesized from ammonia and sugar of the minimal medium. Auxotroph requiring ornithine for its growth does not contain arginine and dies due to protein deficiency. When supplied with ornithine, it is found to synthesise arginine.
Auxotroph requiring citrulline synthesize ornithine but no arginine. When citrulline is supplied, the auxotroph possesses arginine. The nutritional mutant requiring arginine contains both ornithine and citrulline. It seems that arginine is synthesized from ammonia and sugar of the minimal medium through at least three steps each requiring its own enzyme
Beadle and Tatum predicted that defective enzymes are due to defective or mutant genes. Hence, genes express their effect by controlling the synthesis of enzymes.
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