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Gene and genetic code Gene and genetic code Presentation Transcript

  • MOLECULAR BIOLOGY GENE, GENETIC CODE AND GENETIC CONTROL DR.SRINIVAS REDDY PATIL M.Sc (Zoology).,Ph.D (Reproductive Physiology)., M.Ed., PGDBA., FMSPI Monday, May 9, 2011
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  • SYNOPSIS   1.Introduction. 2.One gene - 0ne enzyme concept. 3.Cistrons,mutons and recons. 4.Genetic code and characteristics. 5.Central dogma of molecular biology. 6.Transcription and translation. 7.Regulation of gene expression and lac-operon theory of regulation. Monday, May 9, 2011
  • The term gene was coined by Johannsen(1909). He defined gene as an elementary unit of inheritance.Morgan(1910) defined gene to be a part of chromosome which functions as unit of recombination. Gene is therefore, a unit of inheritance which carries coded information associated with a specific function.   Monday, May 9, 2011
  • Characteristics. 1.It is a component of genetic material which contains coded information. 2.It is able to express its effect by regulating metabolism. 3.It is capable of replication. 4.It can undergo mutation. It is basic to biological variations that are essential for adaptation and evolution. 5.New genes develop due to reshuffling of components of existing genes. 6.It occupies a specific locus over the chromosome.   Monday, May 9, 2011
  • Genes and Enzymes Archibald Garrod (father of biochemical genetics) found that black urine disease alkaptonuria is a genetic disorder caused by a recessive allele. The disorder is a result of an inborn error of metabolism wherein homogentisic acid or alkapton accumulates in the body.The acid is formed during metabolism of 2 amino acids,phenylalanine and tyrosine.Normally, alkapton or homogentisic acid is oxidized to CO2 and H2O with the help of alkapton oxidase. Phenylalanine +Tyrosine / Alkapton / oxidase CO2 +H2O Monday, May 9, 2011
    • One Gene One Enzyme Hypothesis .
    • George Beadle and Edward Tatum (1944)
    • Bread Mould (Neurospora crassa) they obtained a number of mutants called auxotrophs.Auxotrophs are nutritional mutants which cannot grow on normal or minimal medium.For their survival they require an organic nutrient in their growing medium.In contrast,the wild type fungus can grow on minimal medium(ammonia+sugar +salts +biotin) and synthesise all the organic nutrients required by it.Beadle and Tatum selected three auxotrophs for their study-ornithine, citrulline and arginine requiring.
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    • Cistrons,Mutons and Recons
    • They are functional units of DNA recognized by Benzer (1955).
    • Cistron. It is a unit of a gene or genetic material which is equivalent to a gene. A cistron is a structural gene which takes part is synthesis of one polypeptide.
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    • Muton. It is a unit of mutation.A muton may be as small as a nucleotide pair.Gene mutation is caused by replacement of one purine or pyrimidine, or vice versa,addition or deletion of nucleotides. Mutations bring about change in structure and expression of cistron.
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    • Recon. It is a unit of recombination or rearrangement due to crossing over.Usually recon is a large segment of cistron but it can be as small as a single nucleotide pair.
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    • Genetic code
    • It is genetic information present in the nucleotide sequences of mRNA obtained from template or sense strand of DNA with three consecutive nucleotides functioning as a codon in a non-overlapping fashion. The genetic vocabulary or dictionary consists of 64 codon has a message for construction of one amino acid in a polypeptide chain  
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    • Gamow (1955), theoretically proposed a triplet genetic code.
    • The genetic code was deciphered by the efforts of a number of workers like Crick,Ochoa,Nitenberg,Khorana and Nirenberg et al(1961). They deduced that the codon UUU to amino acid phenylalanine, codon CCC to amino acid proline and AAA signifies amino acid lysine.
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    • Khorana(1964) , synthesized a chain of alternate nucleotide GUGUGUGUGU. He found that it stimulated synthesis of a peptide having alternate valine-cysteine-valine-cysteine. For his contribution to deciphering of genetic code, Khorana was awarded Nobel Prize in 1968 along with Nirenberg and Holley. He was awarded Padma Vibhushan in 1969.
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    • Ochoa et al(1965,67), prepared a number of nucleotide co-polymers and deciphered several codons.
    • Brenner(1965) , discovered nonsense codon UAG which was named amber by him.Later other nonsense codons and initiation codons were found out.
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  • Codons of Genetic Code and Their Assignments U C A G UUU UCU UAU UGU U U UUC UCC UAC UGC C UUA UCA UAA UGA A UUG UCG UAG UGG G CUU CCU CAU CGU U C CUC CCC CAC CGC C CUA CCA CAA CGA A CUG CCG CAG CGG G A AUU ACU AAU AGU U AUC ACC AAC AGC C AUA ACA AAA AGA A AUG ACG AAG AGG G G GUU GCU GAU GGU U GUC GCC GAC GGC C GUA GCA GAA GGA A GUG GCG GAG GGG G
    • Characteristies of Genetic Code
    • 1. Triple Nature.
    • 2. Non-Overlapping.
    • 3. Commaless.
    • 4. Universality.
    • 5. Number.
    • 6. Initiation Codons.
    • 7. Termination Codons.
    • 8. Polarity.The code is read in 5’-3’ direction
    • 9. Degeneracy.Out of 64 codons only 3 signify stop signals.
    • 10. Colinearity.Both DNA(or mRNA)and polypeptide have linear arrangements of components.
    • 11. Cistron-Polypeptide Parity. A cistron or gene takes part in synthesis of single polypeptide.The number of cistrons must.therefore,be equal to the number of polypeptides.
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    • Wobble Hypothesis
    • It is hypothesis proposed by Crick(1966), which states that tRNA anticodon has the ability to wobble at its 5’ end of mRNA codon. As a result tRNA can get associated with more than one codon. It corresponds to third base degeneracy of the codons.The position of the third nitrogen base is also known as wobble position. Ex. GGA,GGU,GGC and GGG for glycine, UUU and UUC for phenylalanine, CAA and CAG for glutamine.Because of wobbling,the 5’ end base G of anticodon can establish linkage with 3’ end base C or U of a codon while 5’ end anticodon base U can pair with 3’ end codon base A or G.In many cases inosine(I) has been found to occur at 5’ end position of anitcodon.It is base derived fom adenine.It can pair with A,U and C.
    • Importance. (i) Wobbling reduces the number of tRNAs required for polypeptide synthesis. (ii) It overcomes the effect of code degeneracy.
    Monday, May 9, 2011
    • Central Dogma of Molecular Biology
    • Central dogma or basic formulation of molecular biology states that there is one way or unidirectional flow of information from master copy DNA to working copy RNA(transcription) and from working copy RNA to building plan polypeptide(translation).
    • T ranscription Translation
    • DNA ---------------- mRNA ----------------------- Polypeptide.
    • Central dogma of molecular biology was proposed by Crick(1958). As DNA is synthesized from DNA,the central dogma is also written as follows.
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    • Replication Transcription Translation
    • DNA------------DNA -------------mRNA-----------Polypeptide
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    • H.M.Temin and D.Baltimore (1970), discovered retroviruses which contain RNA as genetic material.The viruses produce an enzyme reverse transcriptase.The enzyme performs the central dogma reverse by synthesizing DNA over template of genetic RNA.The newly synthesized DNA then functions as master copy producing RNAs through transcription and RNAs controlling translation to synthesise polypeptides.
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    • Reverse Transcription Translation
    • RNA----------DNA-----------------mRNA------------------Polypeptide.
    • Transcriptase
    • The process of formation of DNA from RNA is called reverse transcription. AIDS virus HIV(human immuno-deficiency virus) is a retrovirus where reverse transcription is accomplished. The enzyme reverse transcriptase is now used commonly for synthesizing processed genes from their mRNAs.
    Monday, May 9, 2011
    • Transcription : Transcription is the synthesis of RNA over the template of DNA. The strand of DNA which functions as template is called sense, template or coding strand. Segment of template strand that takes part in transcriptions is called transcription unit. It may consist of one or more cistrons.Each transcription unit has a promoter for recognition,an initiation point for beginning of RNA polymerase activity, a coding region for transcription and a terminator region.
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    • Translation or Protein Synthesis : It is the process in which the coded genetic message brought by mRNA from DNA is changed into a polypeptide chain with a seuqece of amino acids similar to the sequence of codons present over mRNA.Translation occurs in the cytoplasm in both eukaryotes and prokaryotes.However,in eukaryotes,a part of translation also take inside the two types of semi-autonomous orgnelles(mitochondria and plastids).
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    • Translation Machinery
    • 1. Ribosomes. They are ribonucleoprotein particles which function as sites for polypeptide synthesis.Therefore,they are called protein factories.
    • 2. Amino Acids. Twenty amino acids take part in building proteins.
    • 3. tRNAs. They are the smallest of RNAs which possess a loop for attachement to ribosome, a loop for attachment to enzyme aminoacyl synthetase, an anticodon for attachment to codon of mRNA and-CCA containing 3’ end for picking up specific amino acid.Some amino acids have 2-6 tRNAs while others have a single tRNA. tRNAs take part in carrying amino acids from cellular pool to the region of polypeptide synthesis over the ribosome. They are therefore called adapter molecules.
    • 4. Amino-Acyl tRNA Synthetase. It is an enzyme specific for combining a particular tRNA to its particular amino acid. There is a separate synthetase enzyme for each type of tRNA.
    • 5. mRNA. Messenger RNA carries coded information from one or more cistons of DNA for translation over ribosomes.
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    • Steps in Translation
    • 1. Activation of Amino Acids. In the presence of Mg2+and ATP an amino acid gets attached to a specific enzyme amino acyl tRNA synthetase.
    • 2.Charging of tRNA. tRNA specific for the activated amino acid attaches itself with the help of its loop to amino acyl AMP enzyme complex.
    • 3. Initiation. The attachment brings the initiation codon of mRNA over the P-site.The initiation codon is AUG or GUG. AUG is specific for methionine whileGUG specifies valine.
    • 4. Elongation. After the formation of template ribosome-mRNA-tRNA complex an aminoacyl acceptor or A-Site is established next to the P-site.
    • 5.Termination. Polypeptide synthesis stops when a termination codon reaches the A-site.There are three termination codons-UGA,UAG and UAA.They are also called nonsense codons.Nonsense codons do not attract any aminoacyl tRNA.
    • 6. Modification. Formylated methionine present at the beginning of prokaryotic polypeptide is either removed or deformylated.The newly released polypeptide has only primary structure.
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    • OPERON CONCEPT
    • From their study in bacterial genetics Jacob and Monod(1961) proposed that genetic material has a number of functional units called operons. Each functional unit has a few genes which are regulated and operated simultaneously. An operon consists of minimum four types of genes-regulator,operator, promoter and structural.
    • Regulator Gene. It is gene which forms a biochemical for suppressing the activity of operator gene.
    • Operator Gene. It is a gene which receives the product of regulator gene.It allows the functioning of the operon.
    • Promoter Gene. It is the gene which provides point of attachment to RNA polymerase required for transcription of structural genes.
    • Structural Genes. They are genes which transcribe mRNA for polypeptide synthesis. The polypeptides may become component of structural proteins, enzymes,transport proteins, hormones, anthibodies,etc. Some structural genes also form non-coding RNAs. An operon may have one or more structural genes,e.g.,3 in lac operon 5 in tryptophan operon,9 in histidine operon.
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  • JACOB AND MONAD Monday, May 9, 2011
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