Genetics
>> Thursday, December 16, 2010
An organism produces an organism of similar kind. A pea plant give rise the seeds of pea, an apple seed produce apple tree and rice plant produce rice grains. Now there arise a question that why an apple seed produce apple tree? Why not the rice plant? It is very simple and interesting. We know all the offspring are similar with their parents in many respects. This property of an individual to resemble with parents is called heredity. In other words heredity is the passing of traits to offspring (from its parent or ancestors). This is the process by which an offspring acquires or becomes predisposed to the characteristics of its parent. It may be defined as transmission of characters from parents to offspring.
Click here for The A-Z of DNA Exlanation With Flash Animation
Although offspring resemble with their parents in many respects, there are not exactly similar in sexual reproduction. For example, the human mother produces human child but it never resembles to either of the parents in all respects. The difference between parents and offspring or among the offspring of the same parents is called variation.
Click here for The A-Z of DNA Exlanation With Flash Animation Although offspring resemble with their parents in many respects, there are not exactly similar in sexual reproduction. For example, the human mother produces human child but it never resembles to either of the parents in all respects. The difference between parents and offspring or among the offspring of the same parents is called variation.
The characteristics are determined by the DNA present in any organism. The change in heredity brings variation. Organisms interact with environment. This interaction determines what an organism is like at a given moment and what it can develop in the future.
Types of Variation:
1. Variation based on the nature of cells it affects:
On the basis of nature of cells it affects, variations are divided into two types.
a) Somatic Variation: It occurs in vegetative cell of the organism. Such variations are acquired by organism during its own life time and are lost with the death. Hence, it is also called acquired variations. They are not transmitted to offspring.
It is occurs due to the environmental effect and use and disuses of organ.
b) Germinal Variation: It occurs due to the change in germ cells. It is inheritable i.e. transfer the character from parents to offspring. It arises due to crossing over, mutation, radiation, recombination of genes etc.
2. Variation based on degree of difference:
Click here for The A-Z of DNA Exlanation With Flash Animation On the degree of difference variation is classified into two types.
a) Continuous Variation: It is small and indistinct variation found on organism during a long course of time. It is gradual, something that is not so clear cut. They are non-inheritable so they are not important from evolutionary point of view. For eg: change in weight, size, colour of the organism etc.
b) Discontinuous Variation: It is large and distinct characters of offspring which are different from parents. It is occurs due to the mutation which seen suddenly and is stable. It is inheritable. For eg: Hairless variety of dogs and cats, polydactyly etc.
Early Views on Heredity (Pre-Mendalian Concept)
Mendel formulated different laws to explain heredity. Before him, there has been put forward different theory. Some of the theories are given below
1. Moist vapour theory: Pythagoras (550 – 500 BC) proposed moist vapour theory. According to the theory, moist vapour produced from different part of body during coitus induced the formation of embryo in uterus of female.
2. Fluid theory: It was given by Aristotle (384 – 322). According to the theory, highly purified blood which is produced by male coagulates in the body of female and form embryo. Female provides nutrition to the embryo.
3. Performation theory: It was proposed by Anton Van Leeuwenhoek (1632 – 1723). According to the theory hereditary characters are transmitted through egg or sperm or both. Gamete consists of entire organism in prefect miniature form (performationism) called homunculus. Development of an organism is a simple enlargement of homunculus.
4. Epigenesis theory: In the eighteenth century, Wolff replaced the performation theory with Epigenesis theory. According to which embryo develops gradually from a simple fertilized cell and becomes a fully developed baby in the later stage of pregnancy. Organism is not raised by the expansion of miniature form but arose by differentiation of homogenous embryonic tissue.
5. Pangeneesis theory: Pangenesis theory was proposed by Darwin (1809 – 1882) during explanation of inheritance of acquired characters. According to the theory, each organ of an organism produces very small, almost invisible identical copies of itself called pangenes or gemmules.
Gene: A gene is a sequence of nucleotides in a deoxyribonucleic acid (DNA) which codes for a particular character. It is a unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain an organism's cells and pass genetic traits to offspring.
Click here for The A-Z of DNA Exlanation With Flash AnimationProperties of genes:
1) Each gene occupies a specific position in a specific chromosome. The position is called locus of the gene. Genes arrange in linear fashion.
2) Each gene has at least two alternative forms known as alleles or allelomorphs.
3) Few genes have more than two alleles called multiple alleles.
Click here for The A-Z of DNA Exlanation With Flash Animation
4) Genes are capable of being copied/duplication. This process is known as replication.
4) Genes are capable of being copied/duplication. This process is known as replication.
5) Genes can undergo modification to form a new gene with different character. This process is known as mutation. The altered gene is transmitted from one generation to another
Function of genes:
1) Genes code for special proteins which determine the various characters/functions of a cell of the organism. Character of the organism is determined by genes.
2) They are responsible for the passage of characters from parents to offspring.
3) Differential expression of gene contained in an organism's cell give rise to various specialized cells with different characters and responsible for different functions.
Some Terminology
1) Cistron: It is the "functional unit" of DNA. It is a gene in real sense consisting of number of nucleotides and which is capable of synthesizing a polypeptide chain. S. Benzer introduced the term "cistron".
2) Muton: It is the smallest unit of DNA which can undergo mutation. It represents a change in a pair of nucleotides.
3) Recon: It is the smallest unit of DNA that can undergo recombination and crossing over.
4) Central Dogma: It is a unidirectional flow of genetic information which is transferred from DNA to protein through DNA and RNA.
DNA-------------> DNA---------------->RNA------------>protein
5) Reverse Central Dogma: It is a unidirectional flow of genetic information which is transferred from RNA to protein through DNA and mRNA.
RNA------------> DNA--------------->mRNA------------>protein
Griffith Experiment of Bacterial Transformation:
In 1928 Frederick Griffith, in a series of experiments with Diplococcus pneumonia commonly called Pneumococcus (bacterium responsible for pneumonia), witnessed a miraculous transformation. During the course of his experiment, a living organism (bacteria) had changed in physical form.
The pneumococcus bacterium occurs naturally in two forms with distinctively different characteristics.
The virulent (S-strain) form has a smooth polysaccharide capsule that is essential for infection.
The non-virulent (R-strain) lacks the polysaccharide capsule, giving it a rough appearance.
Mice injected with S-strain of the pneumococcus bacteria die from pneumonic infection within a few days, while mice injected with the R-strain bacteria continue to live. Injection with heat-killed S-strain bacteria also results in the mice surviving.
Griffith was surprised to find in his experiments that mice injected with a mixture of heat-killed S-strain and live but non-virulent R-strain produced lethal results. In fact, Griffith discovered living forms of the S-strain bacteria in the infected mice!
His experiment is tabulated as
Living R-strain + Mice --------------------> No effect
Living S-strain + Mice --------------------> Mice died due to Pneumonia
Heat killed S-strain + Mice ---------------> No effect
Heat killed S-strain + Living R-strain + Mice ------------> Mice died due to Pneumonia
He hypothesize that the R-strain bacteria had something which has been transformed by the heat-killed S-strain bacteria. Some "transforming principle", transferred from the heat-killed S-strain, had enabled the R-strain to synthesize a smooth polysaccharide coat and become virulent.
Oswald Avery, Colin McLeod, and Maclyn McCarty (1944) at the Rockefeller Institute, building on Griffith's work, showed that only DNA could cause the transformation. They isolated a cell-free extract from the S-strain bacteria and were able to transform living R-strain into a culture containing both S-strain and R-strain cells.
They performed experiment in following ways.
Polysaccharide of heat killed S-strain + Living R-strain + Mice------------> No infection in mice
Protein of heat killed S-strain + Living R-strain + Mice ---------------------> No infection in mice
DNA of heat killed S-strain + Living R-strain + Mice -----------------------> Infection in mice
DNA of heat killed S-strain + Living R-strain + DNAase + Mice ----------> No infection in mice
Nucleic Acid
Nucleic acids are non-protein, nitrogenous substances which is a macromolecule composed of chains of nucleotides. These molecules carry genetic information within cells. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are universal in living things. The nucleic acids DNA and RNA are made up of
a) Pentose sugar
b) Nitrogenous bases
c) Phosphoric acid
a) Pentose sugar: It is the type of sugar that contains 5 carbon atoms. It is of two types.
i) Ribose sugar: It is found in RNA. It is a water-soluble, pentose sugar (monosaccharide with five carbon atoms) that is an important component of nucleic acids, nucleotides, the vitamin riboflavin, and various co-enzymes. It has the chemical formula C5H10O5.
ii) Deoxyribose sugar: The deoxyribose sugar in DNA is a pentose, a five-carbon sugar. Four carbons and oxygen make up the five-membered ring. It has the chemical formula C5H10O4.
b) Nitrogenous bases: A nitrogenous (nitrogen-containing) base is an organic compound that owes its property as a base to the lone pair of electrons of a nitrogen atom. They are non-polar and due to their aromaticity, planar. In biological sciences, nitrogenous bases are typically classified as the derivatives of two parent compounds, pyrimidine and purine.
i) Pyrimidines: Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring.
It is of three types. They are Cytosine, Thymine and Uracil.
ii) Purine: A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines are the most widely distributed kind of nitrogen-containing heterocyclic in nature.
c) Phosphoric Acid: It contain phosphate group.
Nucleoside: It is formed by the combination of purine or pyrimidine base linked to pentose sugar.
Nucleotide: It is formed by the combination of nucleoside with phosphoric acid.
DNA (Deoxyribonucleic Acid): Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms. The main role of DNA molecules is storage of information. The DNA segments that carry this genetic information are called genes.
DNA consists of two long polymers of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds. These two strands run in opposite directions to each other and are therefore anti-parallel.
Within cells, DNA is organized into long structures called chromosomes, nucleolus, mitochondria and chloroplast.
Structure of DNA: Watson and Crick proposed the DNA model by using all the information that was available at that time. They used the data obtained from experiments carried out on DNA.
Based upon the previously obtaining facts, Watson and Crick proposed the famous DNA structure model. The important features of this model are:
1. The DNA molecule is a double helix with single polynucleotides (phosphates, sugar, base) running in opposite directions.
2. The double helix is right-handed.
3. The double helix has two different grooves.
4. The nitrogenous bases are stacked towards the inside of the helix.
5. Bases of the two polynucleotides interact by hydrogen bonding. An adenine residue in one of the polynucleotides is always adjacent to a thymine in the other strand; similarly guanine is always adjacent to cytosine.
6. Ten base pairs occur per turn of the helix. The double helix executes a turn every ten base pairs (abbreviated as 10 bp). The height or pitch of the helix is 34 Ao. The bases are stacked one on top of the other like a pile of plates. The space between the two base pairs is 8.4 Ao and has an angle of 36°.
7. The diameter of the helix is 20 Ao.
Types of DNA:
Based upon number of base pairs found in a turn, it is categorized under following types.
1. A- DNA: DNA that contains 11 base pairs per complete turns is called A- DNA. It is not found under normal physiological condition.
2. B- DNA: DNA that contains 10 base pairs per complete turns is called B- DNA.
3. C- DNA: DNA that contains 9 base pairs per complete turns is called C- DNA.
4. D- DNA: DNA that contains 8 base pairs per complete turns is called D- DNA.
5. Z- DNA: DNA that contains 12 base pairs per complete turns is called Z- DNA.
Function of DNA:
1. It acts as a carrier of genetic information from generation to generation.
2. It synthesizes ribonucleic acid (RNA) through the process of transcription.
3. It acts a prime molecule during protein synthesis.
4. It controls off the biological activities of cell.
5. DNA has autocatalytic function which directs the synthesis of its own copy.
DNA Replication:
DNA is the chemical basis of heredity, it should be able to synthesize its own replica. Watson and Crick hypothesized that the two strands of DNA untwist and separate from one another, starting from one end. During this process, covalent hydrogen bonds are broken. The separated single strands then act as templates for the synthesis of new strands. The new strands later polymerized to form complete polynucleotide chains. This process is called DNA replication.
Types of Replication:
It is mainly of three types. They are
1. Conservative replication: In this process, the parental strands are never completely separated. Hence after replication, first DNA strand contains only parental strands and the other DNA contains only daughter strand.
2. Semi- conservative replication: In this process, the two strands of DNA double helix are separated. Each of the separated strand acts as a template for the replication of new complementary strand. This produces two daughter molecules each having one new strand and one parental strand.
3. Dispersive replication: In this process, the original DNA strands break and synthesize their complementary part and recombines in a random fashion, so each strand may have old and new part.
Mechanism of Semi-Conservative mode of DNA replication:
The different steps are as follows:
1. Initiation of DNA replication: Replication starts at specific point of DNA known as origin or site of replication (Ori). In prokaryotic cell, there is one Ori where as in eukaryotic cell, number of Ori is more than one. There is formation of replication bubble due to multiple origin of DNA replication.
2. Unwinding of parental DNA: Double stranded DNA first start separating and uncoiling at least in a small region. The unwinding of DNA molecule takes place once in every ten nucleotide pairs in eukaryotic DNA. Helicase uses energy from ATP to break hydrogen bonds between base pairs. Each unwind parental DNA strand acts as a template DNA strand. When two strands unwind and separate incompletely they form a ‘Y’ shape where active synthesis occurs. This region is known as replication fork.
As the two strands are separated, a problem of positive supercoil is encountered. DNA topoisomerase has one strand cutting and strand resealing activities to prevent rewinding of DNA strand.
3. Initiation of DNA synthesis: It requires primer. The primer for DNA synthesis is a short piece of RNA that produce by RNA polymerase called primase.
4. Chain elongation: DNA polymerase III elongates a new DNA strand by adding activated deoxyribonucleotides one at a time to the 3’-OH end. Deoxyribonucleoside monophosphate is activated when they react with ATP to form deoxyribonucleoside triphosphates (dATP, dGTP, dTTP, dCTP).
During formation of phosphodiester bond between successive deoxynucleotides, two phosphate groups of deoxyribonucleotides are removed. Energy is liberated when phosphate bond is broken down, thus liberated energy is utilized for the formation of phosphodiester template. The continuous stretch of DNA in 5’-->3’ direction is called leading strand. On 5’-->3’ template there is discontinuous formation of DNA and produces short stretches of DNA which are called Okazaki fragments and the strand (3’-->5’) is called lagging strand.
5. Excision of RNA primer and their replacement by DNA: RNA primer is excised by exonuclease activity of DNA polymerase I and the gap is filled by DNA polymerase I.
6. Proof reading and DNA repair: In some cases, there is misinsertion of wrong base. The probability of misinsetion of wrong base is one in ten thousand. Correction is made during proof reading by repair enzymes.
RNA (Ribonucleic Acid): Ribonucleic acid (RNA) is a nucleic acid that contains a polymer of ribonucleotides of adenine, uracil which are joined together by phophodiester bond. It is found in nucleolus, ribosomes, mitochondria, chloroplast and cytoplasm.
Structure of RNA:
1. Primary structure of RNA: It is defined as the number of sequence of ribonucleotides in a chain. Ribonucleotides consist of ribose sugar, nitrogenous bases and phosphoric acid. It exists as single strand which are made up of ribonucleotide, bond to each other by phosphodiester bonds.
2. Secondary structure of RNA: It involves various coil formation of the polyribonucleotide chain. These coil structures are stabilized by hydrophobic interactions between purine and pyrimidine.
3. Tertiory structure of RNA: It involves the folding of the molecule into three dimensional structure.
Types of RNA:
On the basis of molecular size and functions, there are three major types of RNA
1. Ribosomal RNA (r-RNA): It is the most abundant and stable type of RNA comprising 70-80% of total cellular RNA. r-RNA is present in the ribosome. r-RNA is synthesized from the DNA of nucleolus.
2. Messenger RNA (m-RNA): It is the most heterogenous in size and stability. It carries the genetic information specifying the amino acids sequence in a protein to ribosome. m-RNA is the only type of RNA that is translated into protein. It comprises 5-10% of total RNA.
3. Transfer RNA (t-RNA): It is second most stable type of RNA. It comprises 10-15% of total RNA and it is the smallest of 3 major species of RNA. Each t-RNA serves as an “adaptor” molecule that carries its specific amino acids to the site of protein synthesis.
Heterogeneous nuclear RNA (Hn RNA or Pre-m RNA): It represents precursor of mRNA which is found only in the nucleus of eukaryotic cell.
Smaller nuclear RNA (Sn RNA): It is found in nucleus of eukaryotic cells. It participates in splicing m-RNA.
Function of RNA:
1. It plays a major role in protein biosynthesis. RNA is a working copy of DNA which helps in production of sequence of amino acids.
2. In some viruses, it acts as a hereditary material. Virus such as HIV, TMV etc. have RNA as genetic material.
Genetic Code:
The genetic code is the set of rules by which information encoded in genetic material (DNA or mRNA sequences) is translated into proteins (amino acid sequences) by living cells. The code defines from codons and amino acids. A triplet codon in a nucleic acid sequence specifies a single amino acid. Because the vast majority of genes are encoded with exactly the same code, this particular code is often referred to as the canonical or standard genetic code, or simply the genetic code, though in fact there are many variant codes.
Properties of Genetic Code:
1. Code is triplet.
2. Code is non-overlapping
3. Code is commaless
4. Code is universal
5. Code is degenerate
6. Unambiguity of code
7. Initiation codon
8. Nonsense codon
9. Colinearity
Gene Pool and Gene Frequency:
A gene pool is the collection of all the genes that exist for a given species. The bigger the gene pool, the more diverse the genetic information, and diversity as such helps populations withstand shifting, difficult environmental conditions.
For a given gene, this pool involves all the alleles of that particular gene which are present in a population. All genes or alleles are not found in equal proportion in a population.
Viral gene expression:
The meaning of virus is poisonous fluid. It is nucleoprotein which is made up of nucleic acid covered by protein coats. It was first reported by Adolf Mayer (1886) in mosaic disease in tobacco.
The term virus is first used by Beijernick (1898).
W. M. Stanley (1935) isolated a virus in crystalline form and reported that viruses were made upon exclusively of proteins.
N.W. Pirie and F.W. Bawden (1936) established that viruses were nucleoproteins.
Characteristics of viruses:
1. Viruses are the smallest and simplest of all known organisms. They can only seen with an electron microscope.
2. Viruses range in diameter from about 20 to 200 nm.
3. They are all obligate intracellular parasites and are incapable to carry out any of the typical life functions until they are inside a host cell.
4. They are parasitic on living cells but are not themselves cells.
5. They cannot be cultured on any synthetic medium.
6. They are highly specific for particular organisms.
7. Viruses exist as cubical, helical or tadpole shaped structures.
8. Viruses lack the enzymes necessary for the generation of energy.
9. They are capable of self reproduction inside the host cell.
10. They can be crystallized from suspensions.
Gene expression in Virus:
Virus consists of very few genes. The expression of viral gene takes place only inside the host.
The virus is going to follow a lytic or a lysogenic cycle. In a lytic cycle, it undergoes replication and produces more phages while in a lysogenic cycle, it may remain as temperate, it gives rise to a prophage as its DNA integrates with the bacterial DNA.
1. Lytic Cycle: The lytic cycle is typically considered the main method of viral replication, since it results in the destruction of the infected cell.
Viruses of the lytic cycle are called virulent viruses. The lytic cycle is a six-stage cycle. In the first stage, the virus injects its own nucleic acids into a host cell. Then the viral acids form a circle in the center of the cell. The cell then mistakenly copies the viral acids instead of its own nucleic acids. Then the viral DNA organizes themselves as viruses inside the cell.
The reproductive process of virulent phage is called lytic cycle because the host is lysed at the end and virus particles release. Lytic cycle is common in T2, T4 bacteriophage. The process is divided into following stages.
a) Adsorption: The bacteriophage gets attached to the wall of bacterium by its tail fibres. It brings tip of the tail in contact with host cell wall.
b) Penetration: This stage is the injection of nucleic acid of the virus into the host cell. The tail sheath contracts and the enzyme lysozyme helps in creating a hole in the cell wall so, as to inject phage DNA into the bacterial cell. The protein coat remains outside, attached to the cell wall.
c) Eclipse stage: The phage DNA now codes for the phage enzymes using the host machinery. The enzyme nuclease produced by the expression of viral gene breaks down the host DNA. Enzyme nuclease fails to harm phage DNA as it contains modified cytosine residues. Cytosine residues make phage DNA resistant to nuclease attack. The phage DNA replicates and codes for new coat proteins.
d) Maturation stage: In this stage, the new phage particles are made by the assembly of protein coats surrounding the phage DNA.
e) Lysis of host and release of virus particles: The lysozyme made by phage DNA brings about the lysis of the bacterial cell releasing the phages, ready to infect more bacteria. Such a cycle, where phage bring about the disintegration or lysis of the bacterium, is called as lytic cycle.
2. Lysogenic Cycle: Lysogeny, or the lysogenic cycle, is methods of viral reproduction (the lytic cycle is the other). Lysogeny is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome. Lysogenic cycles can also occur in eukaryotes, although the method of incorporation of DNA is not fully understood. Certain types of viruses replicate by the lysogenic cycle, but also partly by the lytic cycle (mixed cycles).
The adsorption and the penetration phases are same as in lytic cycle. In the process, DNA integrates with the bacterial DNA and does not exert any influence over the bacterial cell. DNA which is intregrated with bacterial DNA is called prophage or provirus.
The phage DNA also replicates along with the host DNA. A repressor protein produced by the prophage keeps the phage genes in repressed stage. In this way, phage DNA may keep multiplying the prophage generation after generation without causing any damage to host. Such a bacterial cell that carries the potential seed of destruction by carrying prophage is called as lysogenic cell. The phenomenon by which phage DNA becomes a part of host cell is called as lysogeny.
Reverse Transcription: A reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into double-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse transcribed into a single strand DNA. Normal transcription involves the synthesis of RNA from DNA.
Well studied reverse transcriptases include:
# M-MLV reverse transcriptase from the Moloney murine leukemia virus
# AMV reverse transcriptase from the avian myeloblastosis virus
Process of reverse transcription
Reverse transcriptase creates single stranded DNA from an RNA template.
In virus species with reverse transcriptase lacking DNA-dependent DNA polymerase activity, creation of double-stranded DNA can possibly be done by host-encoded DNA polymerase δ, mistaking the viral DNA-RNA for a primer and synthesizing a double-stranded DNA by similar mechanism as in primer removal, where the newly synthesized DNA displaces the original RNA template.
Gene Expression in Prokaryotes:
Bacteria consist of DNA as genetic material and it is found in two separate genomes. The first type is the double stranded circular DNA which is relatively larger in size. It is called nucleoid or chromosomal DNA. In addition to nucleoid, bacterial cytoplasm normally contains many small, separate, circular, self replicating, extra chromosomal DNA which are called plasmids. Plasmids which can temporarily integrate into nucleoid and replicate with it are called episomes. Plasmid carry gene as fertility factor (F+ plasmid), colicinogen factor (Col plasmid), R factor (R plasmid) etc. Genes of nucleoid express for the synthesis of chemicals for the living and survival of bacteria.
Genetic Recombination in Bacteria:
Bacteria have no sexual reproduction in the sense that eukaryotes do. The have
But the essence of sex is genetic recombination, and bacteria do have three mechanisms to accomplish. They are:
· transformation
Transformation: Transformation is the genetic alteration of a cell resulting from the uptake, incorporation and expression of exogenous genetic material (DNA) that is taken up through the cell wall(s). Transformation occurs most commonly in bacteria and in some species. Transformation can also be effected by artificial means. Bacteria that are capable of being transformed, whether naturally or artificially, are called competent.
Transformation was first demonstrated in 1928 by Frederick Griffith, an English bacteriologist searching for a vaccine against bacterial pneumonia. Griffith discovered that a harmless strain of Streptococcus pneumoniae. DNA of donor bacterium is incorporated into the DNA of recipient bacterium forming recombinant DNA. Genes present in donor DNA express and change the characteristics of recipient.
Stages in Transformation:
Transformation goes through three stages:
1. In the first stage, cell becomes competent. Competence is the ability of cells to take up DNA through changes in the cell wall. This involves the formation or activation of special DNA receptor protein.
2. The second stage is DNA binding and uptake. This involves interaction between the cell wall receptors and the donor DNA.
3. The third step involves intracellular transport of transforming DNA to recipient. A single strand of donor DNA is incorporated into the recipient DNA by displacing a homologous section of one of the recipient DNA strand. The integrated donor strand then replicates forming double helix.
Transduction: Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector.
When bacteriophages (viruses that infect bacteria) infect a bacterial cell, their normal mode of reproduction is to harness the replicational, transcriptional, and translation machinery of the host bacterial cell to make numerous virions, or complete viral particles, including the viral DNA or RNA and the protein coat.
Methods of Transduction
The packaging of bacteriophage DNA has low fidelity and small pieces of bacterial DNA, together with the bacteriophage genome, may become packaged into the bacteriophage genome. At the same time, some phage genes are left behind in the bacterial chromosome.
Generalized transduction
Generalized transduction may occur in two main ways, recombination and headful packaging.
If bacteriophages undertake the lytic cycle of infection upon entering a bacterium, the virus will take control of the cell’s machinery for use in replicating its own viral DNA. If by chance bacterial chromosomal DNA is inserted into the viral capsid used to encapsulate the viral DNA, the mistake will lead to generalized transduction.
The new virus capsule now loaded with part bacterial DNA continues to infect another bacterial cell. This bacterial material may become recombined into another bacterium upon infection.
When the new DNA is inserted into this recipient cell it can fall to one of three fates
a) The DNA will be absorbed by the cell and be recycled for spare parts.
b) If the DNA was originally a plasmid, it will re-circularize inside the new cell and become a plasmid again.
c) If the new DNA matches with a homologous region of the recipient cell’s chromosome, it will exchange DNA material similar to the actions in conjugation.
This type of recombination is random and the amount recombined depends on the size of the virus being used.
Specialized transduction
The second type of recombination event is called specialized transduction and occurs as a result of mistakes in the transition from a virus' lysogenic to lytic cycle. If a virus incorrectly removes itself from the bacterial chromosome, bacterial DNA from either end of the phage DNA may be packaged into the viral capsid. Specialized transduction leads to three possible outcomes:
a) DNA can be absorbed and recycled for spare parts.
b) The bacterial DNA can match up with a homologous DNA in the recipient cell and exchange it. The recipient cell now has DNA from both itself and the other bacterial cell.
c) DNA can insert itself into the genome of the recipient cell as if still acting like a virus resulting in a double copy of the bacterial genes.
Conjugation: Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. It is discovered in 1946 by Joshua Lederberg and Edward Tatum.
Bacterial conjugation is often incorrectly regarded as the bacterial equivalent of sexual reproduction or mating since it involves the exchange of genetic material. During conjugation the donor cell provides a conjugative is most often a plasmid. Most conjugative plasmids have systems ensuring that the recipient cell does not already contain a similar element.
The genetic information transferred is often beneficial to the recipient. Benefits may include antibiotic resistance, xenobiotic tolerance or the ability to use new metabolites. Such beneficial plasmids may be considered bacterial endosymbionts.
Differences between Transformation and Transduction
Transformation | Transduction |
1. It is the transfer of DNA from donor to recipient. It does not require virus. 2. Many genes may be transferred. 3. It involves absorption of genes. 4. Calcium chloride is necessary. | 1. It is the transfer of DNA from donor to recipient bacterium by virus. 2. Only 1-2 genes are transferred. 3. Transduction involves carrying genes from a living host. 4. It does not require calcium chloride. |
Gene expression in Eukaryotes:
Genetic expression and regulation in eukaryotes is very much complicated than that of the prokaryotes. The DNA of eukaryotes is packed in chromosome where it is associated with different molecules including histone proteins.
DNA molecules are surrounded by octamer of histone complex which is formed by four kinds of histone proteins namely H1A, H2B, H3 and H4. Each histone protein has two molecules in octamer of histone to form beaded structures called nucleosomes. Several nucleosomes are gathered in certain areas called chromomeres. Uncoiling of DNA is necessary for transcription. In addition to histone proteins, RNA and non-histone proteins are also associated with DNA and they may also take part in regulation of gene expression in higher organisms.
All genes present in chromosome are not functional. Some genes of certain areas are not functional. Such areas are called heterochromatin whereas other part contains functional genes where DNA is relatively uncoiled. Such area is called euchromatin. So, higher the uncoiling of DNA, higher will be the gene expression.
Some Important Terms:
Capsid: It is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic material of the virus.
Co-repressor: A protein that decreases gene expression by binding to a transcription factor which contains a DNA binding domain. The co-repressor is unable to bind DNA by itself. This increases the positive charge on histones which strengthens in the interaction between the histones and DNA, making the latter less accessible to transcription.
Exon: A nucleic acid sequence that is represented in the mature form of an RNA molecule after either portions of a precursor RNA (introns) have been removed by cis-splicing or when two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA or a functional form of a non-coding RNA such as rRNA or tRNA. Depending on the context, exon can refer to the sequence in the DNA or its RNA transcript.
Intron: A DNA region within a gene that is not translated into protein. These non-coding sections are transcribed to precursor mRNA (pre-mRNA) and some other RNAs (such as long noncoding RNAs), and subsequently removed by a process called splicing during the processing to mature RNA. After intron splicing (ie. removal), the mRNA consists only of exon derived sequences, which are translated into a protein.
The word intron is derived from the term intragenic region and also called intervening sequence (IVS)
Inducer: The DNA sequence must be copied to make a smaller, mobile molecule called messenger RNA (mRNA), which carries the instructions for making a protein to the site where the protein is manufactured. Many different types of proteins can affect the level of gene expression by promoting or preventing transcription. In prokaryotes (such as bacteria), these proteins often act on a portion of DNA known as the operator at the beginning of the gene. The operator is where RNA polymerase, the enzyme which copies the genetic sequence and synthesizes the mRNA, attaches to the DNA strand.
Inducers function by disabling repressor proteins. Repressor proteins bind to the DNA strand and prevent RNA polymerase from being able to attach to the DNA and synthesize mRNA. Inducers bind to repressors, causing them to change shape and preventing them from binding to DNA. Therefore, they allow transcription, and thus gene expression, to take place. Some inducers are modulated by activators, which have the opposite effect on gene expression as repressors. Inducers bind to activator proteins, allowing them to bind to the DNA strand where they promote RNA transcription.
Oncogene: An oncogene is a gene that is mutated or expressed at high levels, and thus helps turn a normal cell into a tumor cell.
Many abnormal cells normally undergo a programmed form of death (apoptosis). Activated oncogenes can cause those cells to survive and proliferate instead. Most oncogenes require an additional step, such as mutations in another gene, or environmental factors, such as viral infection, to cause cancer. Since the 1970s, dozens of oncogenes have been identified in human cancer. Many cancer drugs target those DNA sequences and their products.
Operon: An operon is a functioning unit of genomic material containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create monocistronic mRNAs that are translated separately, i.e. several strands of mRNA that each encode a single gene product. The result of this is that the genes contained in the operon are either expressed together or not at all. Originally operons were thought to exist solely in prokaryotes but since the discovery of the first operons in eukaryotes in the early 1990s, more evidence has arisen to suggest they are more common than previously assumed. Several genes must be both co-transcribed and co-regulated to define an operon.
Lysogeny: Lysogeny, or the lysogenic cycle, is one of two methods of viral reproduction (the lytic cycle is the other). Lysogeny is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome.
