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Friday, September 25, 2009

Transcription in RNA Synthesis

Friday, September 25, 2009 - 0 Comments

It has been Commonly observed that:

(1) In EuKaryotes, DNA the genetic material is largely restricked to the nucleus and Proteins are synthesized in association with ribosomes formed of RNA in the cytoplasm. Therefore DNA does not participate in protein synthesis directly.

(2) The amount of Protein in a cell is generally proportional to the amount of RNA but not to the amount of DNA.

(3) In EyKaryotes the cellular RNA is synthesized in the nucleus where DNA is found.

It has been suggested that an intermediate RNA molecule is involved in Protein synthesis. The genetic information stored in DNA is transferred to RNA during the initial stage of gene expression. The process by which RNA molecules are synthesized on a DNA tempelate (blue print or guide) is called “Transcription”.

This RNA is called mRNA (messenger RNA) or transcript, because it contains message which directs ribosomes to assemble amino acids in a order which is specific for a given kind or Protein.

EVIDENCE OF EXISTENCE OF mRNA:

Two important evidences suggest the existence of mRNA. These are:

(a) DNA-RNA complementarity: It has been show that the base pairs of RNA produced by various organisms were present in the same ratios as in their DNA from where these are synthesized i.e. from template DNA.

(b) DNA-RNA hybridization: Hall, Spiegelman and other denatured DNA by heating. They found that two strands of double helix separate. When the solution is cooled certain proportion of DNA strands rejoined and rewound. This suggest that complementary strands recognised each other and joined to perform double helix. In another experiment they added RNA to the solution of denatured DNA and solution of DNA. These existence of complementarity between DNA and RNA indicate that DNA act as template on which complementary RNA is made.

The process of transcription occurs on a DNA template in the synthesis of complementary single stranded RNA molecule. Most evidence suggest that only one of the two strands of DNA duplex is transcribed. The strand that is transcribed is called sense strand and the complementary strand is called antisense strand. Transcription may be divided into three stages.

(1) Template binding (2) Elongation (3) Termination

(1) TEMPLATE BINDING AND IMITIATION

It involves RNA polymerase and DNA molecule. The initial binding of an RNA polymerase enzyme to DNA molecule must occur at a specific position just in front of the gene to be transcribed. These attachments points are called Promoters.

Promoter: Promoter is a short nucleotide sequence that is recognized by an RNA polymerase enzyme as a point at which it can bind to DNA in order to begin transcription. Promoters occur just lipstream of genes and no where else.

(2) Elongation:

During this stage RNA polymerase enzyme migrates along the DNA molecule, melting (breaking base pairings) and unwinding double helix as it moves along. A structure called ‘open promoter complex’ is formed in the region of -10 box where breakage of base pairing and unwinding of DNA double helix occurs. After the formation of open promoter complex, the sigma factor dissociates and the holoenzyme is converted into core enzyme. At the same time first two ribonucleotide are base paired to the template polynucleoptide.

One elongation begins the synthesized portion of RNA molecule gradually dissociated from the template (guide) strand allowing the double helix to return to its original state. Thus only a limited region of DNA molecule is melted at any one time. The open region also called transcription bubble contains between 12 and 17 RNA-DNA base pairs. The transcription is longer than the gene.

(3) TERMINATION

The termination of transcription occur only as suitable positions shortly after the ends of genes. The termination does not involve specific sequence analogous to the promoter but it is brought by a more complex signal. The termination signals are complementary palindromes.

TRANSCRIPTION IN EUKARYOTES:

Transcription in Eukaryotes occurs by a process very similar to that in Prokaryotes. Most important differences are thet in eukaryotes initiation of transcription is more complicated and termination does not involve stem loop structures. There are three different kinds of RNA polymerases in eukaryotes each have their own promoters, so each enzyme transcribes only its own set of genes. For example RNA polymerase II cannot transcribe transfer RNA genes as these have promoters for RNA polymerase III.

DNA Replication

Before Watson and Crick suggested model for DNA, it was known that DNA undergoes self duplication or replication but there was no idea how it takes place. One of the major achievement of Watson and Crick model is that it offered basis for DNA replication.

Mechanism of Replication

Watson and Crick proposed a possible mechanism of replication based on their model. Their observation stemmed from the property of complementarity.

According to them:

(a) The hydrogen bonds between the bases of two complementary chains dissolve and the two chains unwind.

(b) Each strand maintains its integrity in the process and does not break down.

(c) Each strand acts as a template (a guide or blue print) or pattern for assembly of another strand, one which is complementary to it.

(d) The complementary strands are constructed from the building material of the nucleotides present in the cell.

(e) As the sugar phosphate back bone is assembled each nitrogen base in the original strand attracts the complementary one. The purine adenine (A) in one chain attracts the Pyrimidine (T).

Similarly T in the other original chain attracts the free units of A present in the cell.

(f) This complementary attraction by all the bases in each of the two original strands along with the assembly of sugar phosphate back bone results in the construction of two new chains, each complementary to the original old ones.

(g) The enzyme DNA polymerase catalyzes the process of replication.

Schemes for DNA replication

There are three schemes.

(1) Semi conservative replication: The above mentioned mechanism for DNA molecule replication is called semi conservative replication because the entire double helix does not remain intact when new DNA is being formed, instead two strands come apart. However each strand is preserved intact. Every daughter DNA molecule has an intact template strand and an intact newly replicated strand.

(2) Conservative Replication: It is alternative method of replication. In this type of replication the original double helix stay together and a new double stranded molecule is built up next to them. The original double helix acts as template for a new one; one daughter molecule would consist of the original parent DNA and the other daughter would be totally new DNA.

(3) Dispersive Replication: The original strands breaks down end entirely new strands are constructed from these and other in the cell. Some parts of original double helix are conserved and some parts are not. Daughter molecules would consist of part template and part newly synthesized DNA.

EXPERIMENTAL EVIDENCE OF REPLICATION

Meselson and Stahl in 1958 provided a strong support for the concept of replication as suggested by Watson and Crick. They designed an experiment to determine the mode of DNA replication. They grew E. coli in medium containing a heavy isotope of nitrogen 15N (normal form of nitrogen is 14N). After growing for several generations on the 15N medium the DNA of E. coli becomes denser (heavy). They employed a technique called density gradient – centrifugation to determine the density of DNA strand. When extracted from cells this heavy DNA settled out in the density of DNA strands. When extracted from cells this heavy DNA settled out in the density gradient at characteristic position.

Replication is bidirectional: Carins established that replication initiates at a single site and it is bidirectional i.e. two replicating sites move in opposite directions around the circular DNA. He also determined the time of movement of this work around the circular DNA.

Replication in Eukaryotes: In Eukaryotes DNA molecules (chromosomes) are larger than in Prokaryotes and are not circular.

Linkage and Crossing over Linkage Groups and Linkage Maps

In four pairs of chromosome of Drosophila there are thousands of genes. Each chromosome contains large number of genes; same is the case with all organisms. The chromosomes behave as single units. All the genes in a given chromosome tend to remain together during inheritance. This tendency of genes in a chromosome to remain together is called ‘Linkage’. This Linkage is not absolute and the genes do not remain locked up in the same chromosome for ever. Otherwise the interitence of traits would also have remained constant. During Meiosis, the homologous chromosomes come together and form pairs, a process is called ‘Synapsis’. Soon after sometimes they exchange segments mutually and the process is called ‘Crossing over’. This exchange occurs at random along the length of chromosomes. After separation, the chromosomes carry some genes that were earlier located in different members of their pair of homologous chromosome. Such exchange of chromosomal segments or crossing over may occur at more than one point in homologous chromosomes in single meiotic division. In Drosophila the dominant gene V for normal wings and its recessive allele V for ventigeal wings and the dominant gene B for grey body colour and b for black body colour are located in the same pair of chromosomes. As they are linked, they tend to be inherited together i.e. V with B and v with b.

It implies that when a homozygous VVBB fly is crossed with a homozygous vvbb, all of offsping would be VvBb i.e. normal winged, grey body colour flies. When one of these heterozygous flies is crossed with a homozygous recessive vvbb and the genes remain completely linked and no crossing over takes place, then only two types of individual, would appear equally in the offspring that would resemble the two parents i.e. grey bodied normal winged (VvBb) and black bodied, vestigial winged (vvbb).

The tendency of the parental Combinations to remain together is called linkage, the genes show linkage because they are located on the same chromosome. The magnitude of linkage depends upon the strength of linkage.

Coupling and repulsion: If two dominant genes are located on the same chromosome, the linkage relationship is called coupling. If one dominant gene and one recessive gene are both located on the same chromosome, the linkage relationship is called repulsion.

Linkage cross: The members of linkage groups corresponds to the number of chromosome pairs because such pair represents on linkage group. In Drosophila, there are four pairs of linkage groups because there are 4 pairs of chromosomes. Similarly, in corn there are 10 pairs of Linkage groups because it has 10 pairs of chromosomes.

Crossing over: The genes show linkage because they are located on the same chromosome. The combination of the linked genes can arise by a process during which the chromatids of homologous chromosomes exchange parts, the process is known as Crossing over.

Some basic principles of genes are:

(1) The genes on the chromosome are arranged in a linear colour in a specific manner like the beads on a string.

(2) When a gene A and its allele ‘a’ both are present in different members of a pair of homologous chromosomes the genes and its alleles occupy corresponding locations on homologous chromosomes.

(3) In order to produce recombinations between two different allelic pairs which are situated on the same chromosome, the crossing over must occur.

(4) The crossing over characteristically occur during first meiotic division.

(5) Meiotic crossing over occur when four chromatids are present in each pair of chromosome.

Sometimes no crossing over occurs in male Drosophila. In Sinkworm no crossing over occurs in female. Reason of absence of crossing over is not known.

Linkage groups: If a certain gene A is linked to two other genes B and C and C as well are linked, if many genes are present in an organism, Crosses can be arranged to determine independently assorting genes or genes linked to one another in pairs or groups. The group of genes linked together and not showing independent assortment is called linkage groups.

Linkage Maps: Linkage maps have been prepared in Large number of animals and plants with the help of frequencies of recombinations. These maps are condensed graphic representations of the relative distances expressed in percentages of recombination among the genes in one linkage group, Consequently located in a single chromosome.

Sex Determination/Sex Determination in Drosophila and Human Beings

In earlier days the inheritance of sex was thought to be form factors unrelated to genes. The sex determination was attributed to phases of moon, time of days during fertilization, wind direction, whether, right or left testis was involved and other such causes. By the end of 17th century a French writer had recorded 262 such theories. With the discovery of Mendelism in 1900, the search for mechanism of sexual inheritance shifted to chromosomes and cytological studies. The discovery of sex chromosomes and their presence or absence shifted the attention to chromosome mechanism of determining sex.

MECHANISM OF SEX DETERMINATION

Basically four types of chromosomal sex determining mechanisms exist: the XO, XY, ZW and compound chromosome mechanism.

(i) The XO Mechanism: This system is sometimes referred to as an XO-XX system. It occurs in many species of injects, e.g. in grasshopper and Bug Protenor. There is no pairing partner in males, therefore heterogametic and termed XO. The females are homogametic and called XX. The sex is determined by males. Males produce gametes that contain either X chromosome or no sex chromosome, whereas all the gametes form a female which contains X chromosome.

(ii) The XY Mechanism: The XY situation occurs in human beings and Drosophila.

The XY mechanism in Humans: In human females there are 46 chromosomes arranged in 23 homologous pairs of chromosomes. Males have same number of chromosomes but 22 pairs of chromosomes are homologous where as 23rd is heterologous. One pairing partner is smaller in size as compared to the other. It is called Y chromosome. The larger chromosome is X chromosome. In female 23rd pair carries both X chromosomes. Male is heterogametic and produces sperms with other X or Y chromosomes. All the female eggs contain X chromosome, fertilization of egg by Y sperm results in male and by X sperm in a female, therefore the sex is determined by males. Thus father contributes his X to daughters and Y to sons, where as mother contributes her X to sons as well as daughters.

The XY mechanism in Drosophila: In Drosophila system is same but Y chromosome is J-shaped and X rod shaped. Male and female both possess two sets of autosomes, chromosomes II III and IV in their body cells. They differ in sex chromosomes. Male carries 1X + 1Y, where as the female carries 2X’s also called chromosome I. All of the eggs produced by female are alike in that they are X bearing. The male forms two classes of sperms and is designated the heterogametic sex. There an equal chance for any egg to be fertilized by X-bearing or Y bearing sperm. Consequently male and female offspring occur in equal number.

(iii) The ZW Mechanism: This system is found in moths, butterflies and birds. It is similar to XY system except in that the males are homogametic and females heterogametic and the sex is determined by females. The females carry heterologous sex chromosomes ZW and males homologous sex chromosomes WW. The eggs contain either Z or W where as all sperms contain W chromosomes.

SEX DETERMINATION IN DROSOPHILA

Calvin bridges suggested in 1922 that genes for sex in Drosophila are determined by balance (ratio) of autosomal alleles that favour maleness and alleles on X chromosomes that favour femaleness. The genes on autosomes have more tendencies towards maleness and genes on X chromosome towards femaleness. He crossed triploid (3x) Drosophila female with a normal male and observed many combinations of autosomes and sex chromosomes in the offspring. Autosomal set (A) in Drosophila consist of three chromosomes. Normally two sets of autosomes plus two X’s results in a female (2A + 2X), two sets of autosomes plus one X produce a male (2A + X). He calculated a ratio of X chromosomes a male (2A + X. He calculated a ratio of X chromosomes to autosomal sets in order to sec if this ratio would predict the sex of a fly. The results suggested that if X | A ratio is 1.00, the organisms is a female and when this is 0.50, the organism is a male. At 0.67 the organism is inter sex. Similarly if the ratio is greater than 1.00 the organisms are meta females (1.50) and if less than 0.50 they are meta males (0.33). The meta females and meta males are very weak and sterile. The meta females usually do not even emerge from their pupal case.

SEX DETERMINATION IN HUMAN BEINGS

Genic balance certainly plays an important role in Humans but there are distinct differences from Drosophila. The Y chromosome in Drosophjila is unnecessary for life and for male secondary sex characteristics. Thus X0 individual is male and XXY is female. Such is not the case in humans. The Y chromosome is essential for male attributes. X0 and XXY persons are found in about 1 in 3000 female births is usually of short stature, possesses a webbed neck, as undeveloped ovaries and an immature uterus plus cardio vascular defects and other somatic aberrations. Similarly K line felters, syndrome-an XXY male arises in about 1 out of 600 male births. These males typically show some breast development, small testes, sparse body hair and some mental deficiency.

Sex Linked Inheritance

A form of inheritance in which the transmission of the genetic material is correlated with the sex of the parents is called sex linked inheritance or any genetic trait which is transmitted through sex chromosomes is called sex linked inheritance.

Some genes of the body character are located on x chromosomes of the sex. Inheritance of such genes is known as sex linked inheritance e.g. red and white eye colour in Drosophila is a sex linked character. Similarly light yellow and bright yellow colour of the body of Drosophila is also sex linked character. In case of chickens the feathers may not be normal in structure and the condition is known as barred feathers. This is also a sex linked character. In human beings colour blindness (recessive) and haemophilia disease (recessive) are also sex linked.

Drosophila melanogaster (Black bellied dew lover) is a common fruit fly which can be seen hovering around over ripe fruits.

T. H. Morgan, a noble prize winner in 1933 first selected this fly as his experiments animal. He noted that the male and female Drosophila have differences in the chromosomes.

This shows that there are three pairs of chromosomes which are some in male as well as female fly.

These are called as autosomes but the differences lies in 4th pair. The female has both the chromosomes of the 4th pair similar and rod shaped. On the other hand male has both the chromosomes different from one another. One chromosome is rod shaped and other is hook shaped. This fourth pair of chromosomes has been designed as sex chromosomes because this pair is going to decide the sex in Drosophila. More over the rod shaped sex chromosomes, two of the female and one of the male, which are alike are called x chromosomes. The unlike sex chromosome is called Y. Drosophila individual getting XX will be a female and that receiving XY will be a female.

CROSS I

When red eyed female (XX) is crossed with white eyed male (XY), F1 generation shows all red flies, female as well as male. F2 generation shows red eyed and white eyed flies in ration of 3: 1 (all females are red but ½ male red and ½ male white eyed). Again female flies were of two types. One producing only red type offspring and another producing half red eyed and other half white eyed offsprings.

CROSS II

When red eyed male (XY) is crossed with white eyed female (XX) results are different from Cross I.

In F1 both types of flies were produced that is red eyed and white eyed. More over all the red eyed are females and all the white eyed males. In F2 generation again red and white eyed appeared in equal ratio and the result become half of male were red eyed and half white eyed. Similar in the case with female flies.

T. H. Morgan on the basis of results obtained from his experiments concluded that eye colour in Drosophila is present in X chromosome, and Y chromosome carries no allele for eye colour.

SIX LINKED INHERITANCE IN MAN

In human beings male has XY sex chromosomes while female has XX chromosomes. Total number of chromosomes is 46.

(a) Colour blindness: It is sex linked inheritance found in human beings. Person suffering from colour blindness have difficulty in distinguishing red from green. It is common trait and more common in males. It is because Y chromosome is inert for this trait, only one gene of this trait will render a colour blind man. While on the other hand a woman must have two genes for this trait to because colour blind. This trait of colour blindness can easily be detected by using special charts made up of a number of coloured dots so arranged what colour blind person see different pattern than other person do.

Possible results of various crosses are as under

(b) Haemophilia: It is sex linked inheritance found in human beings. It is a defect in which the blood fails to clot after external or internal injury, or it clots very slowly. Persons with extreme cases can be bleeding to death from even a small cut. Thus it is very serious or evens a lethal defect.Haemophilia is comparatively very rare in woman. The reason is the same. Y chromosome is inert for this trait. A man needs only one gene controlling this trait, while a woman needs two genes. This occurs very rarely because few haemophilic male survive and reproduce.

DNA and mRNA and their Co-Ordinated Role in Living Cells

They are important constituents of living cells. RNA occurs in cytoplasm and DNA occurs in the chromosomes of the nuclear reticulum. They consist of 5-carbon ribose sugar and phosphate but DNA has deoxyribose with one less oxygen atom in its molecule. DNA is double stranded molecule while RNA is single stranded one. Secrets of life are embodied in DNA or in other words it is the chemical basis of life. DNA is controlling centre of all vital activities of the cell. DNA is sole genetic (hereditary) material migrating from generation to generation through the reproductive units or gametes and is responsible for the development of specific characters of a plant. It also controls the biosynthetic processes of the cell including protein synthesis.

mRNA is chemical messenger and plays a key role in the process of protein synthesis. It is produced in nucleus from the coded instruction in the DNA and then passes into the cytoplasm where it becomes associated with the ribosomes. It carries chemical information from the DNA of the gene to ribosome for protein synthesis. Molecule of mRNA are 753000 nucleotides long and are not folded in any special way.

Spiegelman and his colleagues in 1961 infected E. coli bacteria with 32 p-labelled phages, and isolated labelled RNA. They used this labelled RNA in hybridization to DNA of both phages and bacteria in separate experiments. RNA hybridized only with phage DNA showing that it was complimentary in base sequence to the viral genetic information. The result of the experiment suggests that messenger RNA (mRNA) is made on a DNA template and it directs the synthesis of specific protein in association with ribosomes. The concept was formerly proposed by Francois Jacob and Jacques Monod in 1961.

Each messenger RNA molecule is a long single strand of RNA that passes from nucleus to the cytoplasm. During polypeptide synthesis mRNA molecules bring information from the chromosomes to the ribosomes to direct the assembly of amino acids into a polypeptide. These molecules together with ribosomal Proteins and certain enzymes constitute a system that carries out the task of reading the genetic message and producing the polypeptide that the particular message specifies. They are the principal components or an apparatus that a cell uses to translate its hereditary information. This information is a message written in the code specified by the sequence of nucleotides in DNA. The cell’s polypeptide producing apparatus reads this particular polypeptide. Biologists have also learnt to read this code and in so doing have learnt great deal about what genes are and how they work in dictating what a Protein will be like and when it will be made.

The codes are transmitted to different parts of the cell for their proper functioning.

mRNA acts as chemical messenger or carrier of such codes. The amount of DNA in the cells remain constant, no matter under what metabolic conditions the cell might be placed, once DNA of chromosomes is synthesized, it does not break down or undergo any metabolic change although DNA does participate in controlling various cellular events. The mitosis is characterized by exact replication and exact distribution of DNA and nuclear proteins become condensed to form chromatin threads of which the chromosomes are composed of obviously the initial event during mitosis is doubling DNA in the nucleus.

During transcription the genetic message coded in DNA is transcribed into mRNA which moves out of the nucleus and provides the information to the protein synthesizing organelles the ribosomes.

The sequence of nucleotide on mRNA directs the precise synthesis of amino acid sequence of a protein molecule. M. Nirenberg and H. G. Khorana said that the unit that codes for a given amino acid, consists of a group of three adjacent nucleotides on mRNA, the next three nucleotides on mRNA code fro the net amino acid.

Gene expression is the production of RNA, copy of the gene called messenger of RNA or mRNA. Like all classes of RNA that occur in cells mRNA is formed on DNA template. The production of RNA is called transcription mRNA molecule is said to have been transcribed from DNA. As the RNA polymerase moves along the strand into gene, encountering each DNA nucleotide in turn, it adds the corresponding complimentary RNA nucleotide to the growing RNA strand. This chain is complimentary to DNA strand from which the polymerase assembled it; thus, it is RNA transcript called Primary RNA transcript of DNA nucleotide sequence of the gene.

Watson Crick Model of DNA Structure

In 1953 Watson and Crick surprised the scientific world with a concise one page paper in the British Journal Nature. The paper reported their molecular model of DNA, the double helix, which has since become the symbol of molecular biology. The beauty of the model was that its structure suggested the basic mechanism of DNA replication. Watson and Crick suggested ladder type organization of DNA. Each molecule of DNA is made up of two poly nucleotide chains which are twisted around each other and form a double helix. The uprights of the ladder are made up of sugar and phosphate part of nucleotide and the rungs are made up of paired nitrogenous bases. The pairs are always as follows:

Adenine always pars with thymine and cytosine with guanine. There is no other alternate possible two polynucleotide chains which are complimentary to each other, are held together by hydrogen bonds. There are two hydrogen bonds between A = T, and three between C = G. Both polynucleotides strands remain separated by 2OA´´ distance. The coiling of double helix is right handed and complete turn occurs after 34A´´.

Since each nucleotide occupies 34A´´ distance along the length of a polynucleotide strand be 10 mononucleotide occur per complete turn. Watson Crick model explained chargaff’s rules. Wherever one strand of DNA molecule has an A, the partner strand has a T and G is one strand is always paired with a C in the complimentary strand. Therefore in DNA of any organism, the amount of academic equals the amount of thymine and the amount of guanine equals the amount of cytosine. Although the base-pairing rules dictate the combinations of nitrogenous bases that form the rungs of the double helix; they do not restrict the sequence of nucleotide along each DNA strand. Thus the linear sequence of four bases can vary in countless ways and each gene has unique order, or base sequence.

DNA REPLICATION

Watson Crick model suggested that the basis for occupying the genetic information is complimentary one chain of DNA molecule may have any conceivable base sequence but this sequence completely determines that of its partner in the duplex. If the sequence of one chain is ATTGCAT, the sequence of its partner in duplex must be TAAGGTA. Each chain in duplex is a complimentary mirror image of the other. To copy the DNA molecule one need only unzip it construct new complimentary chain along each naked strand.

Multiple Alleles & their Functioning

If a gene has many modified forms and each modified form of the same gene produces different phenotype in the same general character them all of them are known as multiple alleles.

The term locus is used to represent the position of a gene on the chromosome. The strand of DNA of each gene may itself be of varying lengths. During the curse of occurrence of mutations, the modifications might occur at different places along the gene strand, thus producing many types of mutations in the genes which will either disturb or block the normal function of the gene or will modify the function of the gene. Therefore there may be many sites of mutations within the strand of DNA of a single gene, each leading to the development of specific phenotype in the same basic character to be a number of mutant alleles in the same gene due to the above mentioned fact.

Example (1): Four types of body colour are found in rabbit (1) wild type (brownish grey) (2) Albino (entirely colourless) (3) Chinchilla (body colour silver type and is fully coloured) (4) Himalayan, it is another mutant which is white except at extremities i.e. nose, tail and limbs are black where as the rest of the body is white.

Homozygous stains of all four mutants are maintained by the breeders. If pure chinchilla albino or Himayan are crossed with pure wild type, all the F1 individuals are of wild colour, showing thereby that the wild colour is dominant over all the mutant types. If one mutant is crossed with another type of mutant, different results are obtained. Such crosses have revealed that chinchilla is dominant over Himalayan and albino mutants. However Himalayan is dominant over albino.

Above explanation indicates that there may be more than one units or sites of mutation within a single gene. The mutants with different sites of mutation within single gene leads to the development of different specific alterations in the functioning of the gene.

Example (2): Another example of multiple alleles is of the gene controlling the blood group series A, B, AB and O. Four blood groups have been distinguished due to three modified forms of the same gene. Four blood groups are based on the fact, when the blood from different individuals is mixed together, a clumping of blood cell might occur or not. If blood stream of blood group A is mixed with blood of B group, clumping of blood cells occur. Mixing of serum and blood cells of same group do not show any clumping. Blood group ‘O’ does not contain any antigens on its RBC’s but does contain antibodies of A and B type in its serum. Blood group A is under control of a gene ‘A’. The gene controls the synthesis of A-type antigens blood group B is under the control of a gene ‘B’. It controls the synthesis of B-type antigens on the R.BC and A type antigens in the serum. Blood group O is under the control of gene ‘L’. It controls the synthesis of only A and B type antigens in the serum but no antigens are synthesized on the R.B.C.

Different alleles have been found at the locus which is very closely linked together. Presence of many alleles at the locus suggests that a mutation can modify the genetic information in different ways.

Polyploidy, its Kinds and Significance

The term Polyploidy refers to an organism which contains more chromosomal sets than the normal diploid (2 haploid sets) number. Ploids refers to chromosome sets. Haploid means one set (also called genome). Diploid means two haploid sets (each chromosome is paired).

The cells or organisms containing, more than two sets of chromosomes are called “Polyploids” e.g. triploid, with three sets, tetraploid with four sets. Polyploid condition is produced due to extra duplication of the chromosomes. In plants Polyploids can be produced by treating the plants with colchicines which inhibits cell division but do not inhibit chromosomal replication.

Polyploids are often large more healthy, more strong and more vigorous than the normal diploids. It is common in plant but is rare in animals, although mammalian tissue culture cells sometimes become polyploid.

EUPLOIDY

If every chromosome of the set is represented in the same number of times, the condition is called Euploidy.

Epiploids are of two types.

(1) Autoploids (2) Alloploids

(1) Autoploids: If the chromosome sets of the same specie are in any multiple condition the condition is called Autoploidy.

The antophloids with odd number of chromosomes of each type (3, 5, 7 etc) are sterile because they npaired chromosome would be unable to undergo pairing during metaphase of meiosis. A type of wheat called Einkorn (T. monococcum) contains 7 pairs (14) of chromosomes. Plants are very small and yield little grains.

Another group of wheat variety contains 28 chromosomes. Commonly cultivated wheat or bread wheat contain 42 chromosomes.

(2) Alloploids or Amphiploids: If the chromosome sets of two different species are in multiples in the hybrids of two different species, the condition is called alloploid. The hybrids of two different species are sterile because the non homologous chromosomes cannot pair at metaphase of meiosis. If the chromosomes are some how doubled the hybrids become fertile because the homologous chromosomes would thus be able to pair at metaphase of meiosis.

Karpecheake crossed Radish with cabbage, the hybrids were vigorous but sterile. He repeated his crossing again and again unless he was able to get a few fertile hybrids. Chromosomal analysis of fertile hybrids revealed that they contained two sets of chromosomes of both the species.

The hybrids were unable to cross with both of their parental species, thus the polyploid hybrids were named Raphanobrasica species.

Anenploidy: If all the chromosomes of the haploid set are not represented in the same number of times i.e. some chromosomes are represented more times than the others, such a condition is called aneuploidy. Aneuploids are less viable than euploids.

Polysomic condition: If only one or a few chromosomes are represented more number of times, while the rest of the chromosomes of the set are normal diploid, the condition is called Polysomic.

SIGNIFICANCE OF POLYPLOIDY:

(1) Poluploids are usually more vigorous as compared with the diploids e.g.: triploid aspen tree have very large leaves, large stomatal cells and xylem cells.

(2) It often reduces sexual fertility.

(3) It has been estimated that one third species of flowering plants are polyploids. Therefore polyploidy is important in evolution. Polyploidy leads to sudden development new species in few steps.

(4) Alloploids contain new combination of characters and they may adapt themselves in a better way in case of a sudden change in the environment as compared with the previous ancestors.

(5) Recessive nutation which are usually harmful have the least change of expressing themselves in polyploids.

(6) Inherited variability is important in evolution. As the polyploids inhibits the chances of inherited variability; therefore it is disadvantageous in the long term evolutionary process.

(7) Accumulation of diverse genomes in alloploids lead to the ability to adapt a wide range of environmental tolerance.

(8) Alternations in genome structure not only affect fertility but also affect synapsis and crossing over.

(9) Polyploidy also results in modification in functioning of the genes as well as alter the dominant recessive relationship of the genes. Thus the physiology of the organism is modified.

(10) Unbalanced genome structure e.g.: triploid, pentaploidy and polysomy mostly results in invariability or infertility. Such unbalanced genomes are not advantageous.

(11) Reduplication of complete genome do not usually unbalance the organisms and synapsis and meiosis can occur regularly to continue sexual reproduction normally e.g. tetraploidy, hexaplooidy.

RNA (RIBONUCLEIC ACID) (Short Note)

RNA occurs in Nucleoli, chromosomes and cytoplasm (about 90% of the cells RNA occurs in the cytoplasm). RNA chemically consists of 5 carbon ribose sugar. RNA is single stranded molecule/ RNA is chemical messenger and plays a key role in the process of Protein synthesis,

RNA consists of sugars, bases and Phosphoric acid. Sugar is ribose against Deoxyribose in DNA. In RNA the bases are adenine, guanine, uracil and cytosine i.e. they mine of DNA is replaced by uracil. Various types of RNA are found in plant cells. These are (a) messenger RNA (mRNA) which carries the information contained in DNA (b) transfer RNA (t RNA) also known as soluble RNA which work as adaptor molecules for carrying amino acids to the site of Protein synthesis (c) ribosomal RNA (r RNA) which is associated with ribosome. All these three types are monogenetic RNA.

DNA (DEOXYRIBO NUCLEIC ACID) (Short Note)

They are important constituents of cell. They occur in nuclear reticulum (chromosome). DNA has deoxyribose with one less oxygen atom in ints molecule. DNA is double strended molecule Biologists view that all secrets of life are embodied in DNA. It is the chemical basis of life. DNA is the controlling centre of all vital activities of the cell. DNA is sole genetic (hereditary) material migrating intact from generation to generation through the reproductive units or gametes and is responsible for the development of specific characters of a plant. It also controls biosynthetic process of cell including Protein synthesis. In 1953 Watson and Crick gave the model of DNA.

Each pair is made of two distinct nitrogenous bases Purines and Pyramidines. Altogether there are two Purnes adnine and guanine and two Pyrimidines thyamine and cytosine. It is a rule that a specific purine always pairs with a specific pyrimidine.

MICROBODIES (GLYOXYSOMES AND PEROXYSOMES) (Short Note)

In plant cell they are of following two types:

(1) Peroxysome: They are single membrane bounded microbodies that contain enzymes for transferring hydrogen atom to oxygen forming hydrogen peroxide (H2O2), a toxic molecule that is immediately broken down t water by the enzyme catalase. Peroxysome are abundant in cells that are metabolizing alcohol. Peroxysome are believed to help in detoxification of alcohol.

(2) Glyoxysome: They are another type of microbodes. Each glyoxysome has single layered bounding membrane enclosing fine granular stroma. Glyoxysome contain enzymes that can metabolize some of the molecules involved in Photosynthesis process and respiration through oxidation of fatty acid.

VACUOLE (Short Note)

Vacuoles are non protoplasmic liquid filled cavities in the cytoplasm and are surrounded by a membrane called the tonoplast. Tonoplast is selectively permeable; it allows certain substancesto enter in the vacuole. They are clear in plant cells. They are prominent in nature cells. They are filled with cell sap and act as store house, which often plays role in plant defence, which is necessary for plant cell enlargement. Plant vacuoles sometimes act as lysosome as they contain hydrolytic enzymes and after death of cells tonoplast lose its differential permeability and its enzyme causes lysis of the cell. Vacuole is filled with a fluid called cell sap which is water containing large number of soluble chemical substances such as inorganic salts, organic acids, soluble carbohydrates e.g. sugar, soluble proteins, and amino acids and in certain cells mucilage, anthocyamins, tannins, latex and alkaloids in varying proportions. The vacuole is this a tiny reservoir of the cell from which the cytoplasm draws water and other material according to its need.

EUPLOIDY (Short Note)

It is a condition where one or more full sets of chromosomes are present in an organism. The euploids may e Monoploids, Diploids or Polyploids. Monoploids can be distinguished from haploids as they have a single basic set of chromosomes are in Barley 2n = x = 7 or in corn 2n = x = 10 while the Haploids have the half the somatic number of chromosomes found in normal individual i.e. each chromosome is represented once. In some cases as in male insects the haploids are produced due to parthenogenesis. In these insects the queen and drones are diploid females. The haploids also originate due to development of egg parthenogenetically in following plants such as Tomatoes and Cotton. The haploids also originate from pollen tube rather than from egg, synergids or antipodals of the embryo sac. Such haploids are known as Androgenic Haploids.

Significance: Haploids are characterised by a reduction in size of all vegetative and floral parts than a diploid. Haploids are used in production of homozygous diploids as haploids can be doubled by colchicines treatment. These homologous diploids are used for cultivation e.g. rice, wheat and tobacco.

ANAEOPLOIDY (Short Note)

It is change in number of chromosome which can be either due to loss of one or more chromosomes or due to addition or deletion of one or more chromosomes. It leads to variations in chromosome number and do not involve the whole of the Karyothype. The nuclei of the aneuploids contain chromosomes whose number is not true mullyple of the basic number (n). The aneuploidy arises due to non disjunction. The loss of one chromosome produces a Monosomic (2n—1) and the condition is termed as Monosomy. The gain of one chromosome produces Trisomic (2n—1) and the condition is known as Trisomy. In the same way the addition of two or more chromosomes is respectively known as Tetrasomy and Pentasomy, the individuals are known as Tetrasomic and Pentasomic. In some cases a pair of homologous chromosomes is lost (2n—2), such indivividuals are termed as Nullisomic and the condition is called Nullisomy.

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