SUB BOTANYHONS SEM IV PAPERC8TUNIT 3 TOPICS CENTRAL DOGMA AND GENETIC CODE Dr Arati Malakar The central dogma of molecular biology describes the twostep process transcription and translation by which the information in genes flows into proteins DNA RNA protein ID: 913922
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TAMRALIPTA MAHAVIDYALAYA
SUB: BOTANY(HONS)SEM: IVPAPER:C8T(UNIT 3)TOPICS: CENTRAL DOGMA AND GENETIC CODE
Dr. Arati Malakar
Slide2The central dogma of molecular biology describes the two-step process, transcription and translation, by which the information in genes flows into proteins: DNA → RNA → protein. Transcription is the synthesis of an RNA copy of a segment of DNA. RNA is synthesized by the enzyme RNA polymerase
Central DogmaFig.1: Central Dogma
Slide3What is Genetic Code?
DiscovaryGeorge Gamow conclude that a three letter code must be employed to encode the 20 amino acids used by living cells to build proteins. Genetic code is a dictionary that corresponds with sequence of nucleotides and sequences of amino acids. Genetic code is a set of rules by which information encoded in genetic in material (DNA or RNA sequences) is translated into proteins by living cells.
Slide4Introduction
There are A, G, C and T assemble to the nucleotides found in DNA. They are organized into codon. The collection of codons is called Genetic code. For each amino acids there should be 20 codons. Each codon should have 3 nucleotides to impart specificity to each of the amino acid for a specific codon. 1 nucleotide- 4 combinations. 2 nucleotides- 16 combinations 3 nucleotides- 64 combinations most suited for 20 amino acids
Fig.2: The genetic code.
Slide5Genetic code can be expressed in a simple table with 64 entries
Fig.3: Genetic code and indicating Amino Acids Table
Slide6Codon and its type
Genetic code is a dictionary consists of “Genetic words” called CODONS. Each codon consists of three bases(triplet) There are 64 codons. 61 codons code for 20 amino acids in proteins. 3 conons do not code for any amino acids. They are called non sense codon.
Slide7Types of codon
Sense codon: The codon that code for amino acid called sense codon. Signal codon: Those codons that code for signal during protein synthesis are called signal codons. For example: AUG, UAA, UAG & UGAThere are two types of signal codons: Terminating codon Initiating codonStart codon. Stop codon.
Slide8AUG is the initiating codon. It codes for the first amino acid is all proteins. At the starting point it codes for methionine in eukaryotic & formylmethionine in prokaryotes.
UAA, UAG & UGA are terminating codons or nonsense codons & are often referred to as amber, ochre & opal codos.Terminating codon: Initiating codons:
Slide9Anticodon:
The base sequences of t RNA which pairs with codon of mRNA during translation is called anticodon.Fig.4: Anticodon on tRNA
Slide10CodonAnticodon
Codon is found on the coding strand of ds DNA & ss mRNAanticodon is found always in tRNA Codons are written in 5 to 3 direction anticodons are usually written in 3 to 5 direction.It is complementary to the nucleotide triplet on the DNA
It is complementary to the codonIt determines the position of amino acidsIt brings the specific amino acid by the codon
One mRNA contains many codons
One tRNA contains one anticodon.
Difference between codon and anticodon
Slide11The codons and anticodons have complementary nitrogenous bases, allowing them to base pair. Because the kind of amino acid attached to a tRNA depends on the tRNA's anticodon, the base pairing between the anticodons and codons brings a specific sequence of amino acids to the ribosomes.
The relationship between codon and anticodon
Slide12Characteristics of the genetic code
Triplet code. Comma less. Non overlapping code. The coding dictionary. Degenerative code. Universally code. Non ambiguous code. Chain initiation code. Chain termination code.
Slide13A single amino acid is specified by a sequence of three nucleotides in the mRNA. Due to this triplet nature, it consists of 64 codon. There are 20 amino acid Genetic code is tripletThe genetic code is triplet. There are 64 codons and they are coded by 64 codons
The genetic code is triplet. There are 64 codons.Genetic code is tripletFig.4: Genetic code is triplet
Slide14The genetic code is universal AUG is the codon for methionine in mitochondria. The same codon codes for isoleucine in cytoplasm.
Non ambiguous: Universality: The genetic code is non ambiguous. Thus one codon can not specify more than one amino acid
Slide15One base Cannot participate in the formation of more than one codon.This means that the code is non overlapping.
Continuopus Translation: The gene is transcribed & translated continuously from a fixed starting point to a fixed stop codon. Punctuations are not present between the codons.Non overlapping:
Slide16Degeneracy of genetic code
An amino acid can be coded for by more than one codon. This is called degeneracy of genetic code.Fig.6: Degeneracy of genetic code
Slide17Wobble Hypothesis
Crick postulated the Wobble Hypothesis to account for the degeneracy of genetic code. According this hypothesis, the first two bases of a codon pair accordingly to the normal base pairing rules with the last two bases of anticodon. Base pairing at the third position of a codon is wobble.Fig.7: Wobble Hypothesis
Slide18Wobble hypothesis explains degeracy
Wobble hypothesis explains the degeneracy of the genetic code i.e. existence of multiple codons for a single amino acids. Although there are 61 codons for amino acids, the number of tRNA is far less( around 40) high is due to wobbling.
Slide19Biological significance of degenaracy of the genetic code
If the code were not degenerate, 20 codons would designated amino acids and 44 would lead to chain termination The probability of mutating to chain termination could therefore be much higher with a non degenerate code.
Slide20Flow of genetic information
Fig.8: Flow chart of genetic information.
Slide21Clinical Significance
Mutation can be well explained using the genetic code Point mutations A. Silent B. Misense C.NonsenseFrame shift Mutation.
Slide22Silent mutation
Single nucleotide change A to G, same amino acid is incorporated. Mutation goes unnoticed.Missense MutationSingle nucleotide change A to C different amino acid incorporated. Loss of functional capacity of protein.Non sense Mutation
Single nucleotide change from C to T, stop codon is generated( In m RNA represented by UAG), premature termination of chain, may be incompatible with life.Frame shift MutationInsertion or removal of a bases can alter the reading frame with the resultant incorporation of different amino acids.
Slide23THANK YOU