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Differentiate between Prokaryotic and Eukaryotic cells


Attempt any THREE questions:                                                                        (8 x 3 = 24)

       (a) Differentiate between Prokaryotic and Eukaryotic cells.


Biologists have divided cells into two types: prokaryotic and eukaryotic.
The differences between these two types of cells are mainly based upon the structure of their nuclei.

(i) Eukaryotes have a very well defined nucleus, in which nuclear material (chromosomes or DNA) is enclosed in a double nuclear membrane.

(ii) Those possessing eukaryotic cells are called eukaryotes.

(iii) Eukaryotes includes all other unicellular or multicellular organisms such as animals, plants, fungi and protista. (Containing all membrane bounded organelles)

(iv) Ribosomes compared of eukaryotes 80S.

(v) The cell wall of plants is generally made up of cellulose.

(vi) Eukaryotic cells are given in diagram.

(i) In prokaryotic cells, however the genetic material (DNA) is without any nuclear membrane covering and is directly submerged in the cytoplasm.

(ii) Organisms possessing prokaryotic cells are called prokaryotes.

(iii) Prokaryotes include bacterial and blue green algae. Prokaryotic cells generally lack many of the membrane bounded structures found in eukaryotic cells, For Example, mitochondria, endoplasmic reticulum, chloroplast and Golgi apparatus are absent in prokaryotic cells.
Since there is no nuclear membrane, a prokaryotic cell has no distinct nucleus and its DNA molecule is directly suspend in cytoplasm.

(iv) Prokaryotes have small sized ribosomes 70S.
In prokaryotes mitosis is missing and the cell divides by binary fission. Because of their simpler structure, it was widely accepted for a long time that prokaryotic cells represent a more primitive stage of evolution than eukaryotic cells Perhaps the most distinctive features of the prokaryotic cell is its cell wall, composed of polysaccharide chains bound covalently to shorter chains of amino acids forming.
Peptidoglycan and Murein.

(v) The entire cell wall is often regarded as a single huge molecule or molecular complex called Sacculus or murein and is differently structured than that of a bacterium.

(vi) Diagrams of prokaryotic.

        (b) Explain the nutrition in fungi.

  • All fungi lack chlorophyll and are heterotrophs (obtaining carbon and energy from organic matter).
  • They obtain their food by direct absorption from the intermediate environment and are thus absorptive heterotrophs.
(1) Saprotrophs:
  • Most fungi are saprotrophs (or saprobes), decomposers that obtain their food (energy, carbon and nitrogen) directly from dead organic matter.
  • They secrete out digestive enzyme which digest dead organic matter, and all organic molecules thus produced are absorbed back into the fungus.
  • Saprobic Fungi anchor to the substrate by modified hyphae, the rhizoids.
  • Fungi are the principal decomposers of cellulose and lignin, the main components of plant cell walls (most bacterial cannot break them).
  • Extensive system of fast growing hyphae provides enormous surface for absorptive mode of nutrition.
  • Saprobic fungi, along with bacteria, are the major decomposers of the biosphere, contributing to the recycling of the elements (C, N, P, O, H etc) used by living things.
(2) Parasites:

Some fungi are parasites.

  • Parasitic fungi absorb nutrient directly from the living host cytoplasm with the help of special hyphal tips called hustoria.
  • They may be obligate or facultative.
(i) Obligate Parasites:
  • Obligate parasites can grow only on their living hosts and cannot be grown on available defined growth cultures medium.
  • Various mildews and most rust species are obligate parasites.
(ii) Facultative Parasites:
  • Facultative parasites can grow parasitically on their host as well as by themselves on artificial growth media.
(3) Predators:
Some fungi are active predators.

(i) Oyster Mushroom (Pleurotus Ostreatus) is a carnivorous (predatory) fungus:
  • It paralyses the nematodes (that feed on this fungus), penetrate them, and absorb their nutritional-contents, primarily to fulfil its nitrogen requirements.
  • It fulfills its glucose requirements by breaking the wood.
(ii) Arthrobotrys:

Some species of arthrobotrys trap soil nematodes by forming contricting ring, their hyphae invading and digesting the unlucky victim. Other predators have other adaptations, such as secretion of sticky substances.
Mushroom Oyster ~200g/pkt – Gocer E-MartLee Davies🦑🍄 🏳️‍🌈 on Twitter: "Its #WormWednesday, so here's ...

(4) Mutualistic Symbiotic:
  • Fungi form two key mutualistic symbiotic association (associations of benefit to both partners).
  • They are lichens and mycorrhizae.
(i) Lichens:
  • Lichens are mutualistic and have symbiotic associations between certain fungi mostly Ascomycetes and imperfect fungi, and few Basidiomycetes (about 20 out 15000 species of lichens) and certain photoautotrophes either green algae or a cyanobacterium, or some times both.
  • Most of the visible part of lichens consist of fungus and algal components are present within the hyphae.
  • Fungus protects the algal partner from strong light and desiccation and itself gets food through they courtesy of alga.
  • Lichens can grow at such place where neither of the components along can even at harsh places such as bare rocks etc.
  • Lichens vary in colour, shape, overall appearance, growth form.
Bioindicators: They are ecologically very important as bioindicators of air pollution.


(ii) Mycorrhizae:
  • Mycorrhizae are mutualistic association between certain fungi and roots of vascular plants (about 97% of all kinds of vascular plants).
  • The fungal hyphae dramatically increase amount of soil contact and total surface area for absorption and help in the direct absorption of phosphorus, zinc, copper and other nutrients from the soil into the roots. Such plants show better growth than those without this association.
  • The plant, on the other hand, supplies organic carbon to fungal hypahe.


There are two main types of mycorrhizae

(a) Endomycorrhizae:

Endomycorrhizae, in which the fungal hyphae penetrate the outer cells of the plant root, forming coils, swellings and minute branches, and also extend out into surrounding soil;

(b) Ectomycorrhizae:

Ectomycorrhizae in which the hyphae surround and extend between the cells but do not penetrate the cell walls of the roots. These are mostly formed with pines, firs etc.

However, the mycelium extend far out into the soil in both kinds of mycorrhizae.

Agronomy | Free Full-Text | A Review of Studies from the Last ...


Fungi grow best in moist habitats, but are found wherever organic matter is present.

Temperature and pH:
  • They survive in dry conditions in some resting stage or by producing resistant spores.
  • They can also tolerate a wide range of pH from 2 - 9, a wide temperature range, and high osmotic pressure such as in concentrated salt/sugar solutions as in jelly, jam etc.
  • These features also help them in their survival on land.

Reserve food Material:

Fungi store surplus food usually as lipid droplets or glycogen in the mycelium.
Differentiate between Prokaryotic and Eukaryotic cells Differentiate between Prokaryotic and Eukaryotic cells Reviewed by SaQLaiN HaShMi on 9:39 AM Rating: 5

Explain the structure and function of cell wall?


Attempt any THREE questions:                                                                        (8 x 3 = 24)

         (a)  Explain the structure and function of cell wall?

  • The outer most boundary in most of the plant cells is cell wall.
  • The cell wall of plant cell is different from that of prokaryotes, Both in structure and chemical composition.
  • It is secreted by protoplasm of the cell.
  • Its thickness varies in different cells of the plant.
(1) Composition and Structure:

It is composed of three main layers:
  • Primary wall
  • Secondary wall
  • Middle Lamella
(i) Middle Lamella:

The middle lamella is first to be formed in between the primary walls of the neighboring cells.

(ii) Primary Wall:

The primary wall is composed of :
  • Cellulose and some deposition of
  • Pectin (gelling agent) and
  • Hemicelluloses
Cellulose molecules are arranged in a criss-cross arrangement. The primary wall is a true wall and develops in newly growing cells.

(iii) Secondary Wall:

The secondary wall is formed on its inner surface and is comparatively thick and rigid.

Chemistry it is composed of:
  • Inorganic salts
  • Silica
  • Waxes
  • Cutin
  • Lignin (complex polymer) etc.

Cell Surface - Cell Structure and Function    Plant cell wall, SEM - Stock Image - C021/9825 - Science Photo Library

  • Prokaryotic cell wall lacks cellulose; its strengthening material is peptidoglycan or Murein.
  • Fungal cell wall contains chitin.
(2) Functions:
  • Cell wall is very important.
  • It provides a definite shape to the cell and keep it rigid.
  • It does not act as a barrier to the materials passing through it.

         (b) Write importance of pea family?


(i) High-Protein:

The family is of considerable importance as a source of high protein food, oil, and forage as well as ornamentals and other uses.

Main importance lies in the pulses, belonging to this family, which are used as food, some important and common species of pulses yielding plants are:
  • Gram
  • Pea
  • Kidney bean
These pulses are rich in protein contents.

(ii) Forage Crop of Horses:
  • (Medicago sativa) Alfalfa is one of the worlds best forage crop for horses.
(iii) Fodder Crops:
  • Vicia. Melilotus and Trifolium are also cultivated as main fodder crops.
  • Many trees of this family provide excellent timber for building, furniture and fuel.
  • Main timber plants are Butea, Dilbergia etc.
(iv) Vegetable Oil:

Seeds of Arachis hypogea peanut are edible and also used for extraction of peanut oil which after hydrogenation is used as a vegetable oil.

(v) Indigo Dyes:

Indigo dyes are obtained from Indigofera tinctoria and Butea monosperma, yielding yellow dye from flowers.

(vi) Medicines:
  • Many plants of this family are important for medicines.
  • These include Glycyrrhiza glabra for cough and cold.
  • Clitoria ternatea is used against snake bite.
(vii) Weights:

The red and white seeds of Abrus precatorious are used by jewelers as wights called "ratti".

(viii) Ornamental:

Some important ornamental plants include Lathyrus, Lupinus, Clitoria, Butea etc.
Explain the structure and function of cell wall? Explain the structure and function of cell wall? Reviewed by SaQLaiN HaShMi on 9:09 AM Rating: 5

Biology Part 1 Intermediate - Solved Guess Paper No. 1 Short Questions

Biology Intermediate Part - 1 (11th Class)

Guess Paper No. 1

Total Marks: 83                                 (Subjective Type)                             Time Allowed: 3:10 hours

Q.2 Write answers of any Eight questions:                                                (8 x 2 = 16)

(i) Which process is most important at individual level organization?
Ans. Coordination between different organ and systems is most important at individual level organization. An individual cannot survive without it.

(ii) Differentiate between organic and inorganic molecules?
Ans. Organic: The molecules containing both carbon and hydrogen together are called organic molecules.
        Inorganic: The molecules without carbon and hydrogen together are called inorganic molecules.

(iii) Why are amino acids so reactive and form new peptide bonds?
Ans. Amino acids have two most reactive groups at both ends. Amine group at one end has Hydrogen atom and carboxylic group at the other end have OH group. So amino acids are reactive at both ends and can easily form new peptide bonds.

(iv) Give two functions of proteins?
Ans.  (i) They build many structures of the cells like cell membrane.
         (ii) All enzymes are protein in nature. These enzymes control the metabolism of cell.

(v) What is energy of activation? what is its relation with enzyme?
Ans. The minimum amount of energy required to start a reaction is called activation energy. Enzyme decreases the activation enzyme. So reaction takes place in less time.

(vi) What is induced fit model? Who did propose it?
Ans. This model was proposed Koshland in 1959. He describes that when a substrate combines with an enzyme, it induces changes in the enzyme structure. These changes enable the enzyme to perform its catalytic activity. This is also called "Hand Glove" model.

(vii) Give salient features of cell theory?
Ans.  (i) All organisms are composed of one or more cells.
         (ii) All cells arise from pre-existing cells.
         (iii) Cell is the basic structural and functional unit of all the organisms.

(viii) What is resolution of eye? How can the resolution of eye be improved?
Ans. The human eye can differentiate between two points, which are at least 1.0 mm apart. This is known as resolution of eye. The resolution of eye can be improved by the use of microscopes.

(ix) What is virion? Name its different parts?
Ans. The complete virus is called virion. It is composed of central core, Capsid, and envelop. Virion is outside the living organism.

(x) Why are retroviruses so dangerous and drugs do not kill them?
Ans. The retroviruses change their RNA into DNA by enzyme reverse transcriptase. This DNA is incorporated in the DNA of host in form of provirus. So it cannot destroy in this form.

(xi) What are three types of bacteria on the basis of shape? Give differences among them?
Ans. The three types of bacteria are cocci, bacilli and spirilla. Cocci are spherical or oval bacteria. Bacilli are rod shaped bacteria. The spiral shaped bacteria are spirally coiled.

(xii) Name a parasitic amoeba what disease does it cause?
Ans. Entamoeba histolytica is an intestinal parasite. It causes amoebic dysentery in humans

Q.3 Write answers of any Eight questions:                                                (8 x 2 = 16)

(i) How are Foraminifer source of lime stone?
Ans. Dead Foraminiferans sink to the bottom of the ocean. They form grey mud, this grey mud gradually transformed into chalk. The old and dead Foraminiferans are changed into limestone deposits.

(ii) Differentiate between Ectomycorrhlizae and Endomycorrhlizae?
Ans. In case of Endomycorrhizea, the fungal hyphae penetrate into the cell wall of the plant root. In case of Ectomycorrhlizae, the hyphae surround and extend between the cells. But they do not penetrate into the cell wall of the root cells.

(iii) Give two terrestrial adaptations in fungi?
Ans. Fungi develop extensive system of fast spreading hyphae. The hyphae penetrate into the substrate. Thus they increase the contact and the surface area for absorption. The cell wall of hyphae
is made up of chitin.

(iv) Why is adiantum called as maiden hair ferm?
Ans. The stipe (stalk) and rachis of leaf of adiantum are black, smooth and shiny. Hence it is called maiden hair fern.

(v) Why did the gymnospers have naked seed and it failed to form fruit?
Ans. the megasporophlls  cover ovule. These sporophylls are not folded or joined at the margins to from an ovary. That is why, seed are naked on megasporophyll. So it could not develop fruit.

(vi) What are protozoa and metazoa ?
Ans. Metazoa includes all multicellular animals like man.Protozoa includes all unicellular animals like amoeba amoeba.  

(vii) What is pseudocoelom and pseudocoelomate?
Ans. The animals with false coelom developed from blastocoel are called pseudocoelomate. The space between the body wall and the digestive tube is called pseudocoelom.

(viii) What is Ark-e-Gulab? Give its uses?
Ans. The petal of rose give Ark-e-Gulab on distillation with water. This Ark-e-Gulab is used for curing eye disease and for many other purposes.

(ix) Differentiate between polyp and medusa?
Ans. Polyp is a tube like structure. It reproduces asexually. While medusa is an umbrella like structure. It reproduce sexually.

(x) What is principal function of xylem?
Ans. The principal function of xylem in the plants is the upward transportation of water and solutes.

(xi) What is NADP? Give its significance?
Ans. NADP is nicotine amide dinucleotide phosphate. Hydrogen is produced by the splitting of water. It reduces NADP* (Nicotine Amide Dinucleotide phosphate) to NADPH + H+. The NADPH2 has reducing power.

(xii) What is photophosphorylation?
Ans. The synthesis of ATP due to light energy is called photophosphorylation. Its mechanism is chemiosmosis.

Q.4 Write answers of any SIX questions:                                                (6 x 2 = 12)

(i) What is peristalsis?
Ans. Peristalsis is wave of contraction of the circular and longitudinal muscle which is preceded by wave of relaxation to squeeze the food down into the digestive canal.

(ii) What is constipation?
Ans. It is other extreme condition. In this case, excessive water is absorbed.

(iii) What is common thing between photorespiration and photosynthesis?
Ans. Both the photorespiration and photosynthesis occurs in the presence of light. Both uses RuBP as their reactant.

(iv) What are air sacs in birds? Give their function?
Ans. The lungs of birds have developed several extensions called air sacs. These airs sacs help in the movement of air.

(v) What are lenticels? What is lenticular transpiration?
Ans. The loss of water vapours through the lenticels is called lenticular transpiration. Lenticels are aerating pores in the bark of some plants.

(vi) What are haparin and histamine?
Ans. The Basophils produce heparin. It is substance that inhibits the clotting of blood. These cells also produce histamine. The histamine participates in the allergic reactions.

(vii) What is haemolymph? Can you call it blood?
Ans. The haemolymph is a colourless fluid flow in the haemocoel of the insects. It lacks RBC or haemoglobin, so it is colourless. Blood the insects. Blood means red colour. But still it may be called blood.

(viii) What is Histoplasmosis? How does its infection spread?
Ans. It is a serious infection of lungs. It is caused by inhaling spores of a fungus. This fungus is common in soil contaminated with bird's feaces.

(ix) Define Cardiac Cycle?
Ans. It is the sequence of events, which take place during the completion of one heartbeat. Heartbeat involves three distinct stages.
      (i) Relaxation phase-diastole.
      (ii) Atria contract-atrial systole.
      (iii) Ventricles contract-ventricular systole.
Biology Part 1 Intermediate - Solved Guess Paper No. 1 Short Questions Biology Part 1 Intermediate - Solved Guess Paper No. 1 Short Questions Reviewed by SaQLaiN HaShMi on 9:20 AM Rating: 5

Biology Part 1 Intermediate - Solved Guess Paper No. 1 MCQS

Biology Intermediate Part - 1 (11th Class)

Guess Paper No. 1

Total Marks: 17                                (Objective Type)                             Time Allowed: 20 minutes

Q. 1 You have four choices for each objective type question as A, B, C and D. The choice which you think is correct; fill that circle in front of that question number. User marker or pen to fill the circles. Cutting or filling two or more circles will result in zero mark in that questions.

(i) Following structure is not concerned with lipid metabolism:
(A)   SER                                        (B)   Glyoxisome
(C)   Mitochondria                          (D)   None of these

(ii) Nucleoli are not:
(A)   Darkly stained                        (B)   Membranous
(C)   Variable in number                 (D)   Factory of ribosome

(iii) Total animals, which have been discovered so far are:
(A)   0.5 million                              (B)   1.5 million
(C)   2.5 million                              (D)   3.5 million

(iv) Word vaccination was first used by:
(A)   Edward Jenner                         (B)   Louis Pasteur
(C)   Robert Koch                             (D)   C. Linnacus

(v) Which of the following is not an infectious disease?
(A)   Polio                                        (B)   Typhoid
(C)   Rabies                                      (D)   Cancer

(vi) Which is considered as small virus?
(A)   Polio virus                                (B)   Retrovirus
(C)   Small pox                                 (D)   HIV

(vii) Feature not related to lytic cycle:
(A)   Adsorption                               (B)   Penetration
(C)   Injection                                   (D)   Induction

(viii) Polio virus most commonly affects:
(A)   Peripheral nerves                     (B)   Spinal cord
(C)   Brain                                        (D)   None of these

(ix) In small pox, areas which contain pus are called:
(A)   Pustules                                    (B)   Scars
(C)   Pocks                                        (D)   None of these

(x) Which of the following disease can develop autoimmunity?
(A)   Influenza                                  (B)   Hepatitis
(C)   Measles                                    (D)   Herpes simplex

(xi) Following statement is incorrect about HIV:
(A)   Enveloped virus                               (B)   Attached to WBC through receptor
(C)   Transcriptase converts RNA into DNA        (D)   None of these

(xii) Hepatitis is caused by:
(A)   Virus                                         (B)   Toxic agents
(C)   Drugs                                        (D)   All of these

(xiii) HAV is:
(A)   DNA enveloped                       (B)   DNA non-enveloped
(C)   RNA enveloped                       (D)   RNA non-enveloped

(xiv) Type of hepatitis, which causes chronic liver disease is:
(A)   Hepatitis A                               (B)   Hepatitis B
(C)   Hepatitis C                               (D)   Hepatitis D

(xv) Antibiotics cannot kill:
(A)   Streptococcus pneumonia        (B)   Staphylococcus
(C)   Hepatitis virus                          (D)   Tuberculosis bacteria

(xvi) Which one of the following is not related to cloning?
(A)   Replacement of the nucleus of zygote, by another nucleus of the same organism
(B)   Separation of an embryo to form more embryos
(C)   The individual resulting have the similar biological make up
(D)   Removal of the piece if DNA or gene from the cell and incorporating another gene or piece of DNA in its place

(xvii) Human beings can't digest cellulose because:
(A)   Cellulose is hard to chew
(B)   Cellulose can't be absorbed from small intestine after its digestion
(C)   Cellulose can't be digested because of its composition
(D)   Cellulose can't be digested because we lack enzyme to digest it


(i)        (D)                    (ii)      (A)                    (iii)      (C)                     (iv)        (B)
(v)       (D)                    (vi)     (A)                    (vii)      (D)                    (viii)      (C)
(ix)      (A)                    (x)      (C)                    (xi)       (D)                    (xii)       (D)
(xiii)    (D)                    (xiv)   (C)                    (xv)      (C)                    (xvi)      (A)
(xvii)   (D)
Biology Part 1 Intermediate - Solved Guess Paper No. 1 MCQS Biology Part 1 Intermediate - Solved Guess Paper No. 1 MCQS Reviewed by SaQLaiN HaShMi on 9:45 AM Rating: 5

How to get a gene of interest?


"The cloning in which identical copies of genes are produced is called gene cloning".

PCR (Polymerase Chain Reaction):

"The reaction which is used for production of lesser number of gene copies within test tube".

While, in case of formation of large gene copies recombinant DNA technology is used.


Recombinant DNA means a DNA with two different combination of genetic materials. (It is also called Chimeric DNA).

Method for Production of Recombinant DNA:

(i) Interest: Gene of interest (OR) selection, which is used to be cloned.

(ii) Cut out: Scissors enzymes (restriction endonuclease) to cut out the gene of interest.

(iii) Placement: Molecular carrier or VECTOR, on which gene of interest could be placed

(iv) Introduction: The gene of interest along with the vector is then introduced into an expression system, as a result of which a specific product is made.

Three ways to get the gene of interest:

(i) Isolation of gene from the chromosomes.

(ii) Synthesis of gene chemically.

(iii) Making of gene from mRNA.


(i) Isolation by restriction enzyme: The gene of interest can be isolated from the chromosomes by cutting restriction endonuclease is used to cut on the flanking sites of the gene.

(ii) Synthesis of small genes: In case of small genes they can also be synthesized in the laboratory.

(iii) Use of reverse transcriptase: Genes may be synthesized from mRNA by using the reverse transcriptase (an enzyme). This kind of DNA is called complementary DNA i.e, cDNA 
How to get a gene of interest? How to get a gene of interest? Reviewed by SaQLaiN HaShMi on 1:54 AM Rating: 5

Differentiate between Restriction endonuclease and ligase?


A technique for developing large numbers of genetically identical cells or organisms is known as cloning.

Formation of a new association of DNA molecules or part of DNA molecules is termed as recombination.

A plasmid that carries an inserted pieces of DNA into a host cell in recombinant DNA technology


Any DNA molecule, such as plasmid, which serves to carry foreign DNA into host cells, where it may be replicated and expressed.

An unit of inheritance is called gene.

The total genetic constitution of an organism is known as genome.

Q No.2 Differentiate between restriction endonuclease and ligase?

Restriction Endonuclease (Scissor):
(i) It is the enzyme which can cut a DNA molecule within the strand.
(ii) It is also termed as "scissor".
(iii) "Restriction endonuclease" recognizes specific nucleotide sequence in DNA and then cut both strands in specific manner.

Ligase (Glue):
(i) The enzyme which has ability to seal up the DNA molecule.
(ii) Is is also termed as "glue".
(iii) "Enzyme that creates bonds between the ends of DNA molecules and form a large polynucleotide".

Differentiate between Restriction endonuclease and ligase? Differentiate between Restriction endonuclease and ligase? Reviewed by SaQLaiN HaShMi on 2:04 AM Rating: 5

Biotechnology and Gene Therapy


"The molecular genetics which enables us to manipulate genetic materials for the welfare of mankind".

Desired varieties are formed by gene recombinations. These recombinant genes are made for the production of substance such as enzymes, antibiotics, and hormones needed for human use.


"The process by which faulty genes are replaced by normal genes is known as gene therapy".

Genotype and then Phenotype of organisms may be changed for important and good results by gene therapy. "Genetic engineering means manipulation of genes by man"

★ Gene Therapy in Bacteria

Many kinds of useful bacteria have been reproduced by genetic engineering:

(i) Clean up Pollutants: Some genetically engineered bacteria are used to clean up environmental pollutants.

(ii) Increase the Fertility of Soil: Certain bacteria have been engineered which increase the fertility of soil.

(iii) Kill Insects Pests: Bacteria are also used to kill insect pests.

★ Gene Therapy in Man

(i) Medical Applications: These include the production of hormones, vaccines, enzymes, antibodies, antibiotics and vitamins, and the gene therapy for some hereditary diseases.

(ii) Human insulin has been prepared by this method. It plays an important role in treating the diabetic patients.

Genetic engineering is also playing excellent role in industrial applications, environmental applications, agricultural applications and biological researches.
Biotechnology and Gene Therapy Biotechnology and Gene Therapy Reviewed by SaQLaiN HaShMi on 1:29 AM Rating: 5

Continuous Variation, Molecular Basis Of Allelic Variation

CONTINUOUS VARIATION A character showing continuous variation has an unbroken range of phenotypes in a population (see Figure 1-10b). Measurable characters such as height, weight, and skin or hair color are good examples of such variation. Intermediate phenotypes are
generally more common than extreme phenotypes. In some cases, all the variation is environmental and has no genetic basis, as in the case of the different languages spoken by different human groups. In other cases, such as that of the various shades of human eye color, the differences are caused by allelic variation in one or many genes. For most continuously variable characters, both genetic and environmental variation contribute to differences in phenotype. In continuous variation, there is no one-to-one correspondence of genotype and phenotype. For this reason, little is known about the types of genes underlying continuous variation, and only recently have techniques become available for identifying and characterizing them.

Continuous variation is encountered more commonly than discontinuous variation in everyday life. We can all identify examples of continuous variation, such as variation in size or shape, in plant or animal populations that we have observed—many examples exist in human populations. One area of genetics in which continuous variation is important is in plant and animal breeding. Many of the characters that are under selection in breeding programs, such as seed weight or milk production, arise from many gene differences interacting with environmental variation, and the phenotypes show continuous variation in populations. We shall return to the specialized techniques for analyzing continuous variation in Chapter 20, but for the greater part of the book, we shall be dealing with the genes underlying discontinuous variation.

Molecular basis of allelic variation

Consider the difference between the pigmented and the albino phenotypes in humans. The dark pigment
melanin has a complex structure that is the end product of a biochemical synthetic pathway. Each step in the pathway is a conversion of one molecule into another, with the progressive formation of melanin in a step-by-step manner. Each step is catalyzed by a separate enzyme protein encoded by a specific gene. Most cases of albinism result from changes in one of these enzymes—tyrosinase. The enzyme tyrosinase catalyzes the last step of the pathway, the conversion of tyrosine into melanin.

To perform this task, tyrosinase binds to its substrate, a molecule of tyrosine, and facilitates the molecular changes necessary to produce the pigment melanin. There is a specific “lock-and-key” fit between tyrosine and the active site of the enzyme. The active site is a pocket formed by several crucial amino acids in the polypeptide. If the DNA of the tyrosinase-encoding gene changes in such a way that one of these crucial amino acids is replaced by another amino acid or is lost, then there are several possible consequences. First, the enzyme might still be able to perform its functions but in a less efficient manner. Such a change may have only a small effect at the phenotypic level, so small as to be difficult to observe, but it might lead to a reduction in the amount of melanin formed and, consequently, a lighter skin coloration. Note that the protein is still present more or less intact, but its ability to convert tyrosine into melanin has been compromised. Second, the enzyme might be incapable of any function, in which case the mutational event in the DNA of the gene would have produced an albinism allele, referred to earlier as an a allele. Hence a person of genotype a/a is an albino.
The genotype A/a is interesting. It results in normal pigmentation because transcription of one copy of the wild type allele (A) can provide enough tyrosinase for synthesis of normal amounts of melanin. Genes are termed haplosufficient if roughly normal function is obtained when there is only a single copy of the normal gene. Wild-type alleles commonly appear to be haplosufficient, in part because small reductions in function are not vital to the organism. Alleles that fail to code for a functional protein are called null (“nothing”) alleles and are generally not expressed in combination with func-
tional alleles (in individuals of genotype A/a). The molecular basis of albinism is represented in Figure 1-13. Third, more rarely, the altered protein may perform its function more efficiently and thus be the basis for future evolution by natural selection.

The mutational site in the DNA can be of a number of types. The simplest and most common type is
nucleotide-pair substitution, which can lead to amino acid substitution or to premature stop codons. Small deletions and duplications also are common. Even a single base deletion or insertion produces widespread damage at the protein level; because mRNA is read from one end “in frame” in groups of three, a loss or gain of one nucleotide pair shifts the reading frame, and all the amino acids translationally downstream will be incorrect. Such mutations are called frameshift mutations.

At the protein level, mutation changes the amino acid composition of the protein. The most important
outcomes are change in protein shape and size. Such change in shape or size can result in an absence of biological function (which would be the basis of a null allele) or reduced function. More rarely, mutation can lead to new function of the protein product.

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Genetic Variation, Types of Variation

1.2 Genetic variation

If all members of a species have the same set of genes, how can there be genetic variation? As indicated earlier, the answer is that genes come in different forms called alleles. In a population, for any given gene there can be from one to many different alleles; however, because most organisms carry only one or two chromosome sets per cell, any individual organism can carry only one or two alleles per gene. The alleles of one gene will always be found in the same position along the chromosome. Allelic variation is the basis for hereditary variation.

Types of variation
Because a great deal of genetics concerns the analysis of variants, it is important to understand the types of variation found in populations. A useful classification is into discontinuous and continuous variation (Figure 1-10). Allelic variation contributes to both.

DISCONTINUOUS VARIATION Most of the research in genetics in the past century has been on discontinuous variation because it is a simpler type of variation, and it is easier to analyze. In discontinuous variation, a character is found in a population in two or more distinct and separate forms called phenotypes. “Blue eyes” and “brown eyes” are phenotypes, as is “blood type A” or “blood type O.” Such alternative phenotypes are often found to be encoded by the alleles of one gene. A good example is albinism in humans, which concerns phenotypes of the character of skin pigmentation. In most people, the cells of the skin can make a dark-brown or black pigment called melanin, the substance that gives our skin its color ranging from tan color in people of European ancestry to brown or black in those of tropical and sub-tropical ancestry. Although always rare, albinos, who completely lack pigment in their skin and hair, are found in all races (Figure 1-11). The difference between pigmented and unpigmented skin is caused by different alleles of a gene that encodes an enzyme involved in melanin synthesis.

The alleles of a gene are conventionally designated by letters. The allele that codes for the normal form of the enzyme involved in making melanin is called A, and the allele that codes for an inactive form of that enzyme (resulting in albinism) is designated a, to show that they
are related. The allelic constitution of an organism is its genotype, which is the hereditary underpinning of the phenotype. Because humans have two sets of chromosomes in each cell, genotypes can be either A/A, A/a, or a/a (the slash shows that the two alleles are a pair). The phenotype of A/A is pigmented, that of a/a is albino, and that of A/a is pigmented. The ability to make pigment is expressed over inability (A is said to be dominant, as we shall see in Chapter 2).

Although allelic differences cause phenotypic differences such as pigmented and albino coloration, this does not mean that only one gene affects skin color. It is known that there are several, although the identity and number of these genes are currently unknown. However, the difference between pigmented, of whatever shade, and albinism is caused by the difference in the alleles of one gene—the gene that determines the ability to make melanin; the allelic composition of other genes is irrelevant.

In some cases of discontinuous variation, there is a predictable one-to-one relation between genotype and phenotype under most conditions. In other words, the two phenotypes (and their underlying genotypes) can almost always be distinguished. In the albinism example, the A allele always allows some pigment formation, whereas the a allele always results in albinism when present in two copies. For this reason, discontinuous variation has been successfully used by geneticists to identify the underlying alleles and their role in cellular functions.

Geneticists distinguish two categories of discontinuous variation. In a natural population, the existence of two or more common discontinuous variants is called polymorphism (Greek; many forms). The various forms are called morphs. It is often found that different morphs are determined by different alleles of a single gene. Why do populations show genetic polymorphism? Special types of natural selection can explain a few cases, but, in other cases, the morphs seem to be selectively neutral.

Rare, exceptional discontinuous variants are called mutants, whereas the more common “normal” phenotype is called the wild type. Figure 1-12 shows an example of a mutant phenotype. Again, in many cases, the wild-type and mutant phenotypes are determined by different alleles of one gene. Both mutants and polymorphisms originally arise from rare changes in DNA (mutations), but somehow the mutant alleles of a polymorphism become common. These rare changes in DNA may be nucleotide-pair substitutions or small deletions or duplications. Such mutations change the amino acid composition of the protein. In the case of albinism, for example, the DNA of a gene that encodes an enzyme involved in melanin synthesis is changed, such that a crucial amino acid is replaced by another amino acid or lost, yielding a nonfunctioning enzyme. Mutants (such as those that produce albinism) can occur spontaneously in nature, or they can be produced by treatment with mutagenic chemicals or radiation.
Geneticists regularly induce mutations artificially to carry out genetic analysis because mutations that affect some specific biological function under study identify the various genes that interact in that function.

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