Plus Two Zoology Notes Chapter 4 Molecular Basis of Inheritance is part of Plus Two Zoology Notes. Here we have given Plus Two Zoology Notes Chapter 4 Molecular Basis of Inheritance.
|Text Book||NCERT Based|
|Chapter Name||Molecular Basis of Inheritance|
|Category||Plus Two Kerala|
Kerala Plus Two Zoology Notes Chapter 4 Molecular Basis of Inheritance
Simplified Detailed Notes
Nucleic acids are long polymers of nucleotides. Nucleic acids are of two types they are
DNA is a long polymers of deoxyribonucleotides.
DNA acts as the genetic material in most of the organisms.
The length of DNA is usually defined as number of nucleotides (or a pair of nucleotide referred to as base pairs) present in it.
Examples are given below,
|A bacteriophage φ 174 phage||5386 nucleotides|
|Lambda||48502 base pairs|
|Escherichia coli||4.6 × 109 bp|
|human||3.3 × 109bp|
Structure of Polynucleotide Chain
A nucleotide has three components:
- Nitrogenous base
- Pentose sugar
- Phosphate group
There are two types of nitrogenous bases,
- Cytosine – Common in DNA and RNA.
- Thymine is present in DNA.
- Uracil is present in RNA at the ? place of Thymine.
A nitrogenous base is linked to the pentose sugar through a N-glycosidiclinkage to form a nucleoside.
Nitrogenous base + Pentose sugar = Nucleoside
Nucleosides are grouped into two according to the nature of pentose sugar,
A phosphate group is linked to 5’- OH of a nucleoside through phosphoester linkage to form a nucleotide.
Nucleoside + phosphoric acid = Nucleotide
- Two nucleotides are linked through 3’- 5’ phosphodiester linkage to form a dinucleotide.
Nucleotide + Nucleotide = Dinucleotide
- More nucleotides can be joined in such a manner to form a polynucleotide chain.
- A polymer thus formed has at one end a free phosphate group is referred to as 5’- end of DNA, and at the other end is a free 3′-OH group is present referred to as 3′ end.
- The back bone in a polynucleotide chain is formed by sugar and phosphates.
Sugar + Phosphate = Backbone of Polynucleotide chain
- The nitrogen bases linked to sugar moiety project from the backbone.
DNA double helix
- DNA in the nucleus was first identified by Friedrich Meischer in 1869. He named it as ‘Nuclein’.
- In 1953, double helix structure of DNA was given by James Watson and Francis Crick, based on X – ray diffraction data produced by Maurice Wilkins and Rosalind Franklin.
- One of the hallmark of their proposition was base pairing between two strands of polynucleotide chains.
- This was based on observation of Erwin Chargaff that for a double stranded DNA, the ratio between Adenine and Thymine, and Guanine and Cytosine are constant and equal one.
Salient features of the Double helix model of DNA
- It is made up of two polynucleotide chains, where the backbone is constituted by sugar – phosphate and the bases project inside.
- The two chains have anti-parallel polarity, one chain has the polar ity 5′- 3’ the other has 3’- 5’.
- The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp).
Between adenine and thymine there are 2 hydrogen bonds (A = T) and between guanine and cytosine there are 3 hydrogen bonds.
- The two chains in a right handed fashion. The pitch of the helix is 3.4 nm. The distence between a bp in a helix is approximately equal to 0.34 nm.
- The plane of one base pair stacks over the other in double helex.
Text book problem page no : 98
Find out why the distance between two polynucleotide chains in DNA remains almost constant ?
- The bases in two strands of DNA are paired through hydrogen bond forming base pairs (bp). Adenine forms two hydrogen bonds with Thy-mine from opposite strand. Similarly Guanine is bonded with Cytosine with three hydrogen bonds. As a result of these bonds, the distance between two polynucleotide chains in DNA remains almost constant.
Central Dogma Theory
- Francis Crick proposed the ‘Central dogma’ in molecular biology which states that genetic information flows from DNA → RNA → Protein.
- In some viruses the flow of information is in reverse direction that is from RNA → DNA, the process called reverse transcription.
Packaging of DNA Helix
The length of DNA double helix in a typical mammalian cell calculated is about 2.2 meters. It is a length that is far greater than the dimension of a typical nucleus.
Text book problem page no : 99
If the length of E.coli DNA is 1.36 mm, can you calculate the number of base pairs in E. coli ?
Length of the DNA =
Number of base pairs X distance between two adjacent base pairs.
Distance between 2 consecutive base pairs = 0.34 nm
= 0.34 × 10-9 m
In E.coli, length of DNA = 1.36 mm
= 1.36 × 10-3 m
1.36 × 10-3 m = No. of base pairs X 0.34 x 10-9 m
No. of base pairs = Length / Distance between two consecutive base pairs
= 1.36 × 10-3 m / 0.34 × 10-9 m
= 1.36 × 10 -3 × 109/ 0.34
= 4.6 × l06bp
There is no well defined nucleus. The DNA is not scattered through out the cell. DNA is being negatively charged, is held with some positively charged proteins and form nucleoid.
- There is a set of positively charged basic proteins called histones, which are rich in positively charged basic amino acid residues like lysines and arginines.
- Histones are organised to form a unit of eight molecules called histone octamer.
- The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome.
- A typical nucleosome contains 200 bp of DNA helix.
- Nucleosome constitute the repeating unit of a structure in nucleus called chromatin, thread like stained bodies seen in nucleus.
- Nucleosomes in chromatin are seen as ‘heeds on string’ when viewed under microscope.
- The chromatin fibers are further coiled and condensed at metaphase stage of cell division to form chromosomes.
- The packaging of chromatin at higher level requires additional set of proteins called Non – histone chromosomal proteins (NHC).
- The region of chromatin, which is loosely packed and stains light are Euchromatin, which are transcriptionally active.
- Densely packed and transcriptionally inactive chromatin, which stains dark are called as Heterochromatin.
Euchromatin – The region of chromatin, which is loosely packed and stains light and are transcriptionally active.
Heterochromatin – Densely packed and transcriptionally inactive chromatin, which stain dark.
The Search for Genetic Material
Transforming Principle (Griffith’s Experiment)
In 1928,Frederick Griffith conducted a series of experiments with streptocoecus pneumoniae (bacterium responsible for pneumonia).
- The bacterium has two strains he observed,
Smooth (S) strain or Virulent
- It has mucus polysaccharide coat and can cause pneumonia and has capsule covering.
- They are grown on a culture plate, produce smooth shiny colonies.
- They are pathogenic.
Rough (R) strain or Non-Virulent
- Absence of polysaccharide coat, they produce rough colonies.
- They are non-pathogenic.
- In his experiment, Griffith injected mice with mixes of the two strains.
- S strain → injected into mice → Mice die.
- R strain → injected into mice → Mice live
- Griffith was able to kill bacteria by heating them.
- S strain (heat killed) → injected into mice → Mice live
- S strain (heat killed) + R strain (live) → injected into mice → Mice die
- When heat-killed ‘S’ type cells are mixed with live R’ bacteria the mice died, he recovered living ‘S’ bacteria from the dead mice.
- He concluded that some ‘transforming principle’ transferred from heat killed S – strain to R – strain. It enable to synthesize smooth polysaccharide coat and become virulent. This must be due to the transfer of the genetic material.
- The biochemical nature of genetic material was not defined from his experiment.
Biochemical Characterisation of Transforming Princple
- In 1944, Avery, Colin MacLeod and Me Carty worked to determine the biochemical nature of “transforming principle” in Griffith’s experiment.
- They purified biochemicals (Proteins, DNA, RNA) from the heat-killed S cells to see which ones could trans form live R cells into S cells.
- DNA alone is transformed.
- They discovered that proteases and RNases did not affect transformation.
So the transformingsubstance was not a protein or RNA.
- Digestion with DNase (DNA digesting enzyme) can inhibit transformation.
- They concluded that DNA is the hereditary material. But not all biologist were confirmed.
Difference between DNA and DNase
|Genetic material Deoxyribonucleic acid||DNA digesting enezyme Deoxribonuclease|
The Genetic Material is DNA
- In 1952 , Alfred Harshey and Martha Chase Performed experiment on bacteriophage (Viruses that infect bacteria) and E.coli showing that DNA is the genetic material.
- Bacteriophages has a protein coat which encloses DNA.
- Harshey and Chase worked to discover whether it was protein or the DNA from the viruses that entered the bacteria.
- They grew some bacteriophage on a medium have radioactive phosphorus and some others on medium that contained radioctive sulfur.
- Virus grown in the presence of radioactive phosphorous contained radioactive DNA but not radioactive protein because DNA contains phosphorus but protein does not.
- These phages were allowed to infect E.coli bacteria.
- After infecting, the protein coat of the bacteriophages was seperated from bacterial cell by blender and then subjected to the process of centrifugation.
- The bacteria which were infected with viruses that had radioactive DNA were radioactive and those with radioactive protein coat were not radioactive.
- DNA is the genetic material that is passed from virus to bacteria.
Properties of Genetic Material (DNA v/s RNA)
- It should be able to generate its replica (Replication).
- It should chemically and structurally be stable.
- It should provide the scope for slow changes (mutation) that are required for evolution.
- It should be able to express itself in the form of ‘Mendelian Characters’.
- RNA was the first genetic material.
- DNA was believed to be evolved from mRNA.
- DNA is chemically and structurally more stable than RNA.
- DNA is the better genetic material, it is more stable than RNA.
Difference between DNA and RNA
|It is double stranded||Single standard|
|Presence of thymine||Presence of uracil|
|It is quite stable and not very reactive||It is more reactive and structurally very stable|
|Genetic material in almost all organisms||Genetic material in some viruses|
RNA was the first genetic material. There is no enough evidence to suggest that essential life processes. RNA used to act as a genetic material as well as a catalyst.
DNA replication is the formation of DNA from pre existing DNA molecule.
Semiconservative mode of replication
- Watson and Crick proposed semiconservative method of DNA replication.
- They observed that the two strands of DNA are anti-parallel and complimentary to each other with respect to base sequences.
- The scheme suggested that the two strands would separate and act as a template for the synthesis of new complementary strands.
- After the completion of replication, each DNA molecule would have one parental and one newly synthesised strand. This scheme was termed as semiconservative DNA replication.
The Experimental Proof
Matthew Meselson and Franklin Stahl in 1958 performed experiments on E.coli to prove that DNA replication is semiconservative.
They grew E.coli in a medium contaning heavy nitrogen (15NH4CI).
- The result was that 15 N was also incoperated into both strands of bacterial DNA and the DNA became heavier.
- This heavy DNA can be differentiated from normal DNA by centrifugation in Caesium chloride (CsCl) density gradient.
- Then they transferred the cells into medium with normal 14NH Cl and took the samples at various definite time intervals.
The extracted DNAs were centrifuged and measured to get their densities.
- The DNA that was extracted from the culture one generation after the transfer from 15 N to 14 N medium had a hybrid or intermediate density.
- The DNA extracted from culture after two generations showed equal amount of light DNA and hybrid DNA.
- In 1958, similar experiments were also conducted by Taylor and collegues, by using radioactive thymidine on the root tip cells of faba beans (faba beans). They proved that DNA replicates in a semiconservative manner.
The Mechinery and the Enzymes
- RNA polymerase
- DNA polymerase
|Helicase||It unwind strands of DNA double helix|
|DNA ligase||It joins the okazaki fragments to form lagging strand|
|Topoisomerases||It breaks and reseal one strand of DNA|
|RNA Polymerase / primase||A short stretch of RNA formed on the DNA template called RNA primer, which is synthesised with the help of RNA polymerase.|
|DNA polymerase||It synthesise new DNA strand by adding nucleotide from 5′ → 3′ direction.
The two new strands are synthesised along the replication fork. These are,
1. leading strand: The long stretch of DNA formed is called leading strand in the 5′ → 3′ direction
2. Lading strand: The other strand is formed in small stretches in 5′ → 3′ direction.
Mechanism of DNA Replication
- The process of DNA replication begins at a point called origin of replication or Ori.
- The enzyme helicase unwinds the DNA double helix and seperates it into two strands.
- This leads to the formation of ‘Y’ shaped replication fork.
- The enzyme, topoisomerase causes over winding strain ahead of replicaition fork by breaking and rejoining DNA strands.
- The enzyme primase synthesis RNA primer on which polymerisation of complementary DNA strand begins with the help of the enzyme DNA polymerase.
- The DNA depenedent DNA polymerase catalyse the polymerisation of deoxyribonucleotides only in 5′ → 3′.
- The new strand formed on DNA ternplate 3′ → 5’(continous) called leading strand.
- In the second parental strand.Small pieces of DNA formed in 5’→ 3′. Here, the replication is discontinuous and the DNA strand is known as lagging strand.
- The small fragments of DNA formed are called ‘Okazaki fragments’ are synthesised discontinuously in 5′ → 3′ direction.
- These fragments are joined by the enzyme DNA ligase.
The process of copying genetic information from one strand of the DNA into RNA is termed as transcription.
- The principle of complementary governs the process, except that adenosine now base pairs with uracil instead of thymine, as in replication.
- Unlike replication, only a single strand of DNA gets copied into RNA.
- Both strands are not copied during transcription, because
- If both strands of DNA acts as a template, they would code for RNA molecules with different sequences and inturn if they code for proteins, the sequence of amino acids in the proteins would be different. Hence one segment of DNA would be coded for two different proteins.
- The two RNA molecules, if they produced simultaneously, would be complementary to each other, and hence form a double stranded RNA. This would prevent the process of translation into protein.
Transcription unit of DNA consists of:
- The Structural gene
- A Terminator
- It is the binding site for RNA polymerase. It is located towards 5′- end of the structural gene.
- The presence of promoter defines the template and coding strands.
The Structural gene
- It is the region between promoter and terminator where transcription takes place.
- It is located towards 3′- end of coding strand.
- It terminates the process of transcription.
Transcription Unit and Gene
- Gene is defined as the functional unit of inheritance
- Cistron – A segment of DNA coding for polypeptide
Monociistronic – They code for single polypeptide
Polycistronic – they code more than one polypeptide
- Structural genes in a transcription unit is monocistronic (eukaryotes) or polycistronic (prokaryotes).
- In Eukaryotes gene containing two types of segments, exons and introns. They collectively called split genes.
- Exons- Coding sequences or expressed sequences that appear in mature or processed mRNA.
- Introns – They are intervening sequences. Exons are interrupted by intrones.
Types of RNA and the Process of Transcription
- mRNA(messenger RNA)
- tRNA (transfer RNA)
- rRNA (ribosomal RNA)
It provides the template for protein synthesis. Found in nucleus and cytoplasm.
It is acts as an adapter molecule for translation.
It plays a structural and catalytic role during translation.
All these three RNAs are needed to synthesise a protein in a cell.
In prokaryotes transcription involves 3 main procesess,
It takes place in nucleus.
The enzyme RNA polymerase binnds at the promoter site of DNA, and initiates the process of transcription. It moves along the DNA unwinds and one of the two strands acts as a template for RNA synthesis.
- The RNA chain is synthesized in the 5′- 3′ direction.
- The RNA polymerase after initiation of RNA transcription loses the – factor but continues the polymerisation of ribonucleotides to form RNA.
When RNA polymerase reaches the terminator sequence, the synthesis stops the RNA polymerase is separated from DNA-RNA hybrid, the RNA separates. This process is called termination, the process is facilitated by p (rho) factor.
In Bacteria, mRNA requires no processing to become active.
Transcription and translation takes place in the same compartment (no separation ofcytoplasm andnucleus). Translation begin before mRNA is fully transcribed.
Process of transcription in eukaryotes
In eukaryotes , there are two additional complexites :
There are 3 RNA polymerases sent in the nucleus.
RNA Polymerase I
It transcribes rRNAs (28 S, 18 S and 5.8 S).
RNA Polymerase II
It transcribes precursor of mRNA, the heterogenous nuclear RNA.
RNA Polymerase III
It responsible for the transcription
of tRNA, 5 srRNA, and snRNAs (Small nuclear RNAs).
Post transcriptional processing are
The -primary transcripts are non functional, containing both the coding region, exon and non co-ding region intron. It is subjected to a process called splicing.
In capping methyl guanosine triphosphate is added to the 5′ end ofhnRNA. .
- In this adenylate residues (200¬300) are added at 3′ end ofhnRNA in a template independent manner.
- The processed hnRNA is now called mRNA and transported out of the nucleus for translation.
It is defined as the sequence of nucleotides in mRNA that contain information for protein synthesis.
Salient features of genetic code
- The codon is triplet. 61 codons code for amino acids and 3 codons do not code for any amino acids, hence they function as stop codons.
- One codon codes for only one amino acid, hence, it is unambiguous and specific.
- Some amino acids are coded by more than one codon, hence the code is degenerate.
- The codon is read in mRNA in a contigous fashion. There is no punctuation.
- The code is nearly universal.
e.g., From bacteria to human UUU codes for phenylalanine.
- Initiation codon: AUG is the initiation codon, it codes for methionine and has dual functions.
Genetic codes of Amino acids
1. Alanine (Ala)
2. Arginine (Arg)
3. Asparagine (Asn)
4. Aspartic acid (Asp)
5. Cysteine (Cys)
6. Glutamine (Gin)
7. Glutamic acid (Glu)
8. Glyine (Gly)
9. Histidine ( His)
10. Isoleucine (He)
11. Leucine (Leu)
12. Lysine (Lys)
13. Methionine (Met)
14 Phenylalanine (Phe)
15. Proline (Pro)
16. Serine (Ser)h
17. Threonine (Thr)
18. Tryptophan (Trp)
19. Tyrosine (Tyr)
20. Valine (Val)
Mutations and Genetic code
Relationship between DNA and genes are best understood by mutation.
Mutation : Sudden heritable change in the genetic material. It is of two types,
It is the mutation in a single base pair, which is replaced by another base pair.
Eg., Sickle-cell anaemia. It involves mutation in a single base pair in the beta globulin chain of hemoglobin pigment in the blood.
- It occurs due to deletion or insertion of a DNA segment.
- There is a change in whole sequence of amino acid from the point of insertion or deletion.
tRNA – The adapter Molecule
Francis Crick proposed the presence of an adapter molecule which would read the code on one end and on the other end would bind to the specific amino acids.
- The tRNA is called sRNA (soluable RNA).
- It role as an adapter molecule.
- tRNA has an
- Anticodon loop, that has bases complimentary to the code.
- Amino acid acceptor end to which amino acid binds.
- Each tRNA is specific for each amino acid.
- Another specific tRNA for initiation is called initiator tRNA.
- There is no tRNA for stop codons.
- The secondary structure of tRNA looks like a clover leaf.
- In actual structure, the tRNA look like inverted L
Translation refers to the process of polymerisation of amino acid to form a polypeptide.
- The order of sequence of amino acids are defined by the sequence bases in the mRNA.
- The amino acids are joined by a bond which is known as a peptide bond. It’s formation requires energy.
- Charging of tRNA
In the first phase of translation, amino acids are activated in the presence of aminoacyl tRNA synthetase this process is commonly called as charging of tRNA.
- A translation unit in mRNA is the sequence of RNA that is flanked by the start codon (AUG) and the stop codon and codes for protein.
- The mRNA with nucleus migrates to the cytoplasm for translation.
- An mRNA also has some additional sequences that are not translated and are referred as untranslated regions (UTR).
- UTRs are present at both 5′ – end and at 3′ – end, they are required for efficient translation
Initiation of translation
The ribosomes binds to the mRNA at the start codon (AUG), recognised by initiator tRNA.
- Ribosome proceeds to the elongation phase of protein synthesis.
- During this stage amino acid-tRNA complex, sequentially binds to the appropriate codon in mRNA by forming complementary base pairs with the tRNA codon.
- The ribosome moves from one codon to the next along the mRNA.
- Amino acids are added one by one, translated into polypeptide sequences dictated by DNA and represented by mRNA.
A release factor binds, to the stop codon, terminating translation and releasing the complete polypeptide to from the ribosome.
Importance of Ribosome during translation
- It is the cellular factory responsible for synthesising proteins.
- It consists of structural RNAs + 80 different proteins.
- It exists as 2 subunit (in inactive states).
Act as amino acid binding site.
It encounters mRNA and forms protein synthesising complex.
- Ribosome act as a catalyst for the formation of peptide bond, eg., 23s rRNA in bacteria act as a ribozyme.
Regulation of Gene Expression
Gene expression results in the formation of polypeptide.
In eukaryotes, the gene regulation includes four levels.
- Transcriptional level (formation of primary transcript).
- Processing level (regulation of splicing).
- Transport of mRNA from nucleus to the cytoplasm.
- The metabolic, physiological or environmental conditions that regulate the expression of genes, eg., In E.coli the enzyme, beta-galactosidase hydrolyses lactose into galactose and glucose. If the bacteria do not have lactose the synthesis of beta-galactosidase stops.
- The development and differenciation of embryo into adult organisms are also a result of the co – ordinated regulation of expression of several sets of genes.
- In prokaryotes, control of the rate of transcription initiation is the sites for control of gene expression.
The Lac operon
Geneticis, Francois Jacob and Jacque Monod were the first to describe a transcriptionally regulated system of gene expression.
In bacteria, Polycistronic structural gene is regulated by a common promoter and regulatory genes called operon.
Eg., lac operon, trp operon, ara operon, his operon, val operon.
Components of lac operon
It consists of-
- Regulatory genes (i – gene)
It codes for repressor.
- Structural genes (z, y, a)
- z-gene – Codes for beta-galactosidase ( β – gal).
It responsible for the hydrolysis of the disaccharide, lactose into it’s monomeric units, galactose and glucose.
- y – gene – Codes for permease. Which increases permeability of the cell to beta – galactosidases ( β – gal).
- a – gene – It encodes a transacetylase.
- z-gene – Codes for beta-galactosidase ( β – gal).
- All the three gene products in lac operon are required for metabolism of lactose.
- Lactose is the substrate for the enzyme beta-galactosidase and it is termed as j inducer.
- In the absence of a preferred carbon source such as glucose, if lactose is provided in the growth medium of the bacteria, the lactose is transported into the cells through the action of permease.
- The lactose induces the operon in,
- The repressor of the operon is synthesised from the i gene.
- The repressor protein binds to the operator region of the operon and RNA polymerase from transcribing the operon.
- In the presence of an inducer, such as lactose or allolactose, the repressor is inactivated by interaction with inducer. This allows RNA polymerase access to the promoter and transcription proceeds.
In the absence of lactose inducer
- The level of inducer is completely metabolised by enzymes, it causes synthesis of repressor from repressor gene.
- The repressor binds to the operator gene and blocks RNA polymerase from transcribing the operon. The result, transcription is stopped. This type of regulation is called negative regulation.
Human Genome Project
Human Genome Project (HGP) was called mega project, because its main goal to find out the complete DNA sequence of human genome.
- It was a 13 year project cordinated by the US Department of energy and the National Institute of Health.
- It was completed in the year 2003.
- Human genome contains about 3 × 109 bp.
- HGP was closely associated with the rapid development of a new area in biology called Bioinformatics.
Goals of HGP
- To identify all the approximately 20,000 – 25,000 genes in human DNA.
- Determine the sequences of the 3 billion chemical base pairs that make up human DNA.
- Store this information in databases.
- Improve tools for data analysis.
- Transfer related technologies to other sectors, such as industries.
- Address the ethical, legal and social issues (ELSI) that may arise from the project.
There are two approaches:
- Expressed sequence tags
This focused on identifying all the genes that is expressed as RNA.
- Sequence annotation
It is the blind approach of sequencing the whole set of genome, that included all the coding and non-coding sequences and later assigning different regions in the sequence with functions.
The commonly used hosts for sequencing were bacteria and yeast and vectors,
BAC-Bacterial artificial chromosome
YAC -Yeast artificial chromosome
Salient features of Human Genome
- The human genome contains 3164.7 million nucleotides bases
- The average gene consists of 3000 bases, but sizes vary greatly, with the largest known human gene being dystrophin at 2.4 million bases.
- The total number of genes is estimated at 30,000. Almost all nucleotide bases are exactly the same in all people.
- The functions are unknown for over 50 percent of the discovered genes.
- Less than 2 percent of the genome codes for proteins.
- Repeated sequences makeup a very large portion of the human genome.
- Repetitive sequences are stretches of DNA sequences that are repeated many times, sometimes a hundred to a thousand times. They are thought to have no direct coding functions. But they help to reveal chromosome structure, dyanamics and evolution.
- Chromosome 1 has most genes (2968), and the Y has the fewest (231).
- About 1.4 million locations where single base DNA differences known as single nucleotide differences SNPs (Single Nucleotide Polymorphism) occur in humans.
Applications of HGP
- Having the complete sequences of human genome, will enable to develop radically new approaches to biological research.
- All genes in a genome or all the transcripts in a particular tissue or organ can be studied.
- It enable us to understand how enormous number of genes and proteins
It is the technique to identify the similarities or variations of DNA fragments of two individuals or among in dividuals of a population.
Developed by Alec Jeffreys.
- It involves identifying differences in some specific regions n DNA called repetitive DNA.
- The short nucleotide repeats called variable number of tandem reprates (VNTR).
- These repetitive DNA are seperated from bulk genomic DNA as different peaks during density gradient centrifugation.
- The bulk DNA forms major peaks and the other forms small peaks are called satellite DNA.
- Satellite DNA is classified into many categories: minisatellites and microsatelltes, based on:
- Base composition
- Length of segment
- Number of repetitive units
Features of these sequences
- Do not codes for any proteins.
- Shows high degree of polymorphism and forms the basis of DNA fingerprinting.
- Polymorphism in DNA sequence is the basis of DNA fingerprinting.
- Polymorphism arises due to mutations.
- If an Inheritable mutation is observed in a population at high frequency it is referred as DNA polymorphism.
- Polymorphism is higher in non coding DNA sequence.
- These mutations accumulates generation after generation and cause polymorphism.
|Satellite DNA||Repetitive DNA|
|DNA sequences that contain highly repetitive DNA||DNA sequence that contain small segments|
|They are not transcribed||They are transcribed|
Methods of DNA fingerprinting
- Isolation of DNA.
- Digestion of DNA by restriction endonucleases.
- Seperation of DNA fragments by electrophoresis.
- Transferring of seperated DNA fragments to synthetic membranes, such as nitrocellulose or nylon.
- Hybridisation using labelled VNTR probe.
- Detection of hybridised DNA fragments by autoradiography.
Applications of DNA fingerprinting
- It is used to establish paternity and family relationships.
- To identify criminals.
- It is used to determine population and genetic diversities to study evolution.
- Very useful identification tool in forensic applications.
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