genetic material in organization, Central dogma,transcription in prokaryotes vs eukaryotes and genetic codon

College of Agriculture,
NAU, Bharuch Campus
• Submitted to:
DR.S.R PATEL
Head & Associate professor,
Department of genetics and plant breeding,
College of agriculture,NAU, Bharuch campus
Submitted by
Patel Rushi
1st Sem M.sc. Agri. (Genetics & pl.breeding)
College of agriculture,NAU , Bharuch campus

Year Scientist Contribution
1929 Phoebus Levene Discovered ribose sugar in 1909
deoxyribose sugar in 1929, suggested the structure of nucleic
acid as a repeating tetramer.
he showed that the components of DNA were linked in the order
phosphate-sugar-base
1941 George Beadle and Edward Tatum
Got noble prize in (1958). First experimental linkage between gene and phenotype – One
gene one enzyme hypothesis.
1944 O. Avery, Colin McLeod and Maclyn McCarty Demonstrated that genes are made up of DNA and DNA is the
genetic material.
1952 Alfred Hershey and Martha Chase Proved that genetic material of the bacteriophage(virus) is
made up of DNA.
1953 Rosalind Franklin and Maurice Wilkins Determined X-ray diffraction patterns of DNA which greatly
helped in understanding the structure of DNA
1953 James Watson and Francis Crick
Got Noble price in (1962).
Discovered the double helical structure of the DNA molecule
Molecular biology originated in the 1930s and 1940s, and picked up momentum in the 1950s and 1960s.
Although its direct predecessor was classical genetics, the emergence of molecular biology represented a
convergence of work by geneticists, physicists, and structural chemists.

1957 Crick Laid out the “Central Dogma
Relationship between DNA → RNA → protein
1958 Mathew Meselson-Franklin Stahl Proves that DNA replication was semiconservative, a critical
confirmation of the replication mechanism that was implied by
the double-helical structure.
1961 Francois Jacob and Jacques Monod
Got noble prize in(1965).
Hypothesized the existence of an intermediary between DNA
and its protein products, which they called messenger RNA.
1961 – The genetic code was deciphered Crick and Brenner
Marshall Nirenberg and Heinrich J. Matthaei
identified the triplet codon pattern,
The NIH ( National institute of health) cracked the codes for
the first 54 out of the 64 codons.
1968 Har Govind Khorana and Robert Holley
Got Noble price in (1968)
Artificial synthesis of oligonucleotides
1970 Hamilton Smith and Danial Nathans
Got noble prize in (1978).
discovered restriction enzymes
1972 Paul Berg
Got noble prize in (1980).
recombinant DNA
1977 Fredrick Sanger
Got noble prize in (1980)
developed method to determine base sequence
1983 Kary Mullis
Got noble prize in (1993)
Polymerase Chain Reaction (PCR) Development
1995 Craig Venter Haemophilus influenza genome sequenced
Historical background of molecular genetics

 Genetic material:-
 Genetic information is defined as information contained in genes which passed to a new
generation, influences the form and characteristics of the offspring.
 Genetic material must be capable of replication, information storage, information expression
and variation (mutation).
 Some substance must be responsible for passage of traits from parents to offspring. For a substance
to do this it must be:
 a. Stable enough to store information for long periods.
 b. Able to replicate accurately
 c. Capable of change to allow evolution

 Griffith’s Transformation Experiment
 Frederick griffith (1928) used mice and streptococcus pneumoniae.
 Streptococcus pneumoniae two strains:
 Smooth(S) strain (virulant): has a polysaccharide mucus coat which case pneumonia
 Rough(R) strain (non-virulent): no mucus coat. Do not cause pneumonia.

 The Experiment
1. S-strain→inject into mice→Mice
die
2. R-strain → inject into mice→
Mice live
3. S train in (heat killed) →inject
into mice →mice live
4. S-train (heat killed) + R-strain
(live) →inject into mice → mice
die
GRIFFITH TRANSFORMATION EXPERIMENT
• He conducted that some transforming principle transferred from heat killed s-strain to R-strain.
• It is enable R-strain to synthesize smooth polysaccharide coat and become virulent. This is due to the
transfer of genetic material.

 Oswald Avery ,colin Macleod and
maclyn mcCarty (1944) worked to
determine the biochemical nature
of transforming principle in griffith
experiment.
 They Purified biochemical (protein
DNA RNA etc.) From heat killed s-
cells using suitable enzymes.

 They discovered that
 Digestion of protein and RNA (using proteases and
Rnases) did not affected transformation. So,
transforming substance was not a protein or RNA.
 Digestion of DNA with Dnase inhibited transformation. It
means that DNA caused transformation of R-cell to S-
cell. i.e. DNA was Following the transforming principle.
Avery, McCarty and MacLeod EXPERIMENT

 Hershey and chase (1952) grew some bacteriophage virus on medium containing radioactive
phosphatus (P32) and some Others on medium containing radioactive sulphur (S35).
 Virus grown in (P32) got radioactive DNA because only DNA contain Phosphorus.virus grown in
(S35) got readioactive protein because protein contain Sulphur.

 This preparations were used seperately to infect e.coli.
 After infection, the e.coli The culture was centrifuged two separate lighter virus
particle from heavier bacterial cell.
 The culture was centrifuged two separate lighter virus particle from heavier
bacterial cell.

 So, Bacteria infected with viruses having radioactive DNA were
radioactive i.e, DNA had passed from virus to bacteria.
 Bacteria infected with viruses having radioactive protein were not
radioactive. i.e. Protein did not enter the bacteria from the virus.

 All known cellular organisms have DNA as their genetic material. Some viruses, however, use
RNA instead of DNA.
 Tobacco mosaic virus (TMV) is composed of RNA and protein; it contains no DNA.
 In 1956 Gierer and Schramm showed that when purified RNA from TMV is applied
directly to tobacco leaves, they develop mosaic disease.
 In 1957 Fraenkel-Conrat and Singer showed that in TMV infections with viruses containing
RNA from one strain and protein from another, the progeny viruses were always of the type
specified by the RNA, not by the protein.
Typical tobacco mosaic virus
(TMV) particle

 Two types of genetic materials – DNA & RNA
 Nucleic acid found in the cells of all living organisms
 DNA found in chromosomes (Nucleus)
 RNA mostly found in the ribosomes (cytoplasm)
 Genetic material :
 DNA is genetic materials of all living organisms (Bacteria, animals, higher plants, human
being)
 RNA is also the genetic material of some viruses
 Nucleic acid – First isolated by Miescher (1871) and it was known as nuclein.
 Later on in 1899, Altmann used the term nucleic acid for nuclein.

Particulars DNA RNA
Strands Double stranded Single stranded
Sugar Dioxyribose Ribose
Base Adenine, Guanine, Cytosine and
Thymine
Adenine, Guanine, Cytosine and
uracil
Pairing AT & CG AU & CG
Location Mostly in chromosomes, some in
mitochondria and chloroplast
In chromosomes and ribosomes
Replication Self replicating Formed from the DNA. Self
replicating in some form of viruses.
Size Contain 4.3 M Nucleotides Contain 12,000 Nucleotide
Function Genetic code Protein synthesis and genetic
material in some viruses
Types Several types 3 types (m-RNA,r-RNA,t-RNA)

 Molecular model of the DNA structure was proposed by Watson and Crick in 1953 These
model was universally accepted.
 For of that contribution they got Noble prize in 1958.
 Main features of the model (Double helical model)
 The DNA has a double helical structure. Each DNA molecules consist of two strands of the
DNA which are spirally arranged in clock-wise direction.
 Each strand consists of deoxyribose sugar and phosphate group arranged in alternate
Patterns.
 Two strands are connected by purines and pyrimidine bases
 The width of the DNA molecules is 2 nm and in one turn of two strands is completed in 3.4
nm.

 DNA contains two strands of nucleotides
 H bonds hold the two strands in a double-helix structure
 A helix structure is like a spiral stair case
 Bases are always paired as A–T and G-C
 Thus the bases along one strand complement the bases along the
other

 A combination of deoxyribose sugar and nitrogenous base is known as
nucleoside.
 A combination of nucleoside and phosphate is known as nucleotide.
 Nucleoside = sugar + nitrogenous base
 Nucleotide = sugar + nitrogenous base + phosphate
 Combination of several nucleotide leads to formation of
polynucleotide chain. Which consist thousands pairs of nucleotide
monomer.
 Total number of purines are always equal to number of pyrimidines
 A + G = C + T

 Nucleic acids consist of nucleotides, that have a sugar, nitrogen base, and phosphate
 Nucleoside = sugar + nitrogenous base
 Nucleotide = sugar + nitrogenous base + phosphate

 PHOSPHORIC ACID:-
 Phosphoric acid (H3PO4) has three reactive(-OH) groups of which two are
Involved in forming the sugar-phosphate backbone of DNA.
 A phosphate moiety binds to the 5”C of one and the 3”C of the other
neighbouring pentose molecule of DNA to produce the phosphodiester(5”C-
O-P-O-C3”) linkage.
 PENTOSE SUGAR:-
 The pentose present in RNA is called ribose & DNA contains 2’-deoxy-
ribose,which is the reason for the name deoxyribose nucleic acid.
 The oxygen atom present at the second carbon of ribose is missing in
deoxyribose, giving its name 2’-deoxyribose.
 The 5’ and 3’ carbons of pentoses participate in phosphodiester linkage.
while the 1’ carbon is always occupied by an organic base.
PHOSPHATE GROUP

Pyrimidines Purines
These are single ring structure These are double ring structure
They are three types : Cytosine,
Thymine and Uracil
They are two types : Adenine and
Guainine
It occupy less space in DNA
structure
occupy more space in DNA
structure
The pyrimidines is linked with
deoxyribose sugar at position 1
Deoxyribose is linked at position 9
of purines
Nitrogenous bases :
Two types of nitrogenous bases
Purines and Pyrimidines
Pyrimidines –
Single ring structure
Three types – Cytosine, Thymine and Uracil
Cytosine and Thymine founds in DNA and Cytosine
and Uracil in RNA
Purines –
Double ring structure
Both in DNA and RNA – Adenine and Guanine
• Adenine always pair with Thymine and Guanine with
cytosine
• Adenine are joined with Thymine by double hydrogen
bond
• Guanine are joined with cytosine by triple hydrogen
bond
• These bonds are weaker in nature, that will help
DNAin separation of strands during replication

 Polymers of four nucleotides
 Linked by alternating sugar-phosphate bonds
 RNA: ribose and A, G, C, U
 DNA: deoxyribose and A,G,C,T
Two H bonds for A-T
Three H bonds for G-C
Nucleotide chain
O
N
N
NH2
O
CH2
O
P
O
O-
O-
OH
O
N
N
NH2
CH2
O
P
O
O-
OH
O
N
N
AMP
CMP
3,5-phosphodiester bond
3
5
Nucleotides Nucleotides Nucleotides Nucleotides

Genetic material must have the following properties:
Ability to generate its replica(replication)
Chemical and structural stability
provide the mutation needed for evolution.
Ability to express as and mendelian characters.

Reasons for
stability of DNA
Reasons for
mutability of RNA
Double stranded Single stranded
Presence of
thymine
Presence of uracil
Absence of 2’OH in
sugar
presence of 2’OH in
sugar

 Thus RNA is unstable. so, RNA virus (e g. Bacteriophage, tobacco mosaic virus etc.) Mutate
and evolve faster.
 DNA strands are complementary. on hitting,it separate in appropriate condition, then come
together.
 in griffith experiment,some properties of DNA of the heat killed bacteria did not destroy. it
indicate stability of DNA.

 For the storage of genetic information, DNA is better due to its stability.but for the
transmission of genetic information, RNA is better.
 RNA can directly code for the protein synthesis, hence can easily express the character.
DNA is dependent on RNA for protein synthesis.

 RNA was the first genetic material.
 it act as a genetic material and
catalyst.
 essential life process like
(metabolism,translations,splicing
etc) evolved around RNA.
Types of RNA

 Central dogma is proposed by Francis Crick.
 it stand that the genetic information flow from DNA
to RNA to protein.
 In some viruses, flow of information is in reverse
condition (from RNA to DNA). it is called reverse
transcription.
Central Dogma

 DNA Replication is the copying of DNA from parental DNA.
 Watson and Crick proposed the semiconservative model of
replication.
 it suggested that the parental DNA strand act as a template for
the synthesis of new complementary strand.
 after replication, each DNA molecules would have one parental
and one new Strend.
 Matthew messelson and franklin stahl (1958) proved semi-
conservative model.

 They prepared to culture media of e coli.
 One Preparation content 15 NH4CL salt (15Nheavy
isotope of N). So 15N was incorporated into both
stand of bacterial DNA and the DNA become
heavier.
 Other preparation contain 14N salts.So14N was
incorporated into both strand of DNA and become
lighter.
 These two type of DNA can be separated by
centrifugation in a cscl density gradient.

 They took e.coli. Cell from 15N medium and transfered to 14N medium.
 After one generation (i.e.after 20 minutes), they isolated and centrifuged the DNA. Its
density was intermediate hybrid between 15N DNA and 14N DNA.
 This showed that in newly formed DNA, one stand is old (15N type) and one stand is new (14N
types)this confirm semiconservative replication.

Taylor and colleagues (1958)
performed similar experiments
on vicia faba (faba beans)
using radioactive thymidine
to detect distribution of
newly synthesized DNA in the
chromosomes.
It proved that the DNA in
chromosomes also replicate
semi-conservatively.

 DNA replication starts at a point called origin (ori)
 A unit of replication with one origin is called replicon.
 During replication, the 2 strands unwind and separate
by breaking H-bond in presence of an enzyme, helicase.
 Unwinding of DNA molecule at a point form a Y-shaped
structure called the replication fork.

 The separated strands act as a template for synthesis of new strands.
 DNA Replicates in the 5’ to 3’direction.
 Deoxyribonucleoside triphosphate (dATP,dGTP,dCTP, & dTTP) act as substrate and
provide energy for polymerization.

 Firstly, a small RNA primer is synthesized to the impression of an enzyme Primase.
 In presence of an enzyme, DNA dependent On DNA polymerase, many nucleotides join with
one another to prime stand and form a polynucleotide chain (new strend).

 During Replication one strand is formed as a
continuous stretch in 5’ to 3’ direction
(continuous synthesis).this trend is called a
leading strand.
 The other strand is formed in small straches
(okazaki fragments) in 5’ to 3’ direction
(discontinuous synthesis).
 The Okazaki fragments are then joined
together to form a new strend by an
enzyme,DNA ligase.this new stand is called
lagging strand.
 If a wrong base is introduced in a new
Strend,DNA polymerase can do proof reading.

 E.coli. Completes replication within 18 minutes.I.e. 2000 BP per second.
 In eukaryotes, the replication of DNA takes place at s-phase of the cell cycle.
 Failure in cell division after DNA replication result in polyploidy.

 Use of copying genetic information from one strand of the DNA into RNA.
 Here adanine forms base pair with uracil instead of thymine.
 DNA dependent RNA polymerase catalyse polymerisation only in 5’ to 3’ direction.
 3’ to 5’ act as a template strand and 5’ to 3‘ act as a coding stand.

 Both strand are not copied during transcription, because
 The code for protein is different in both stands. This complicates the translation.
 Two RNA molecules are produced simultaneously, this would be complementary to each
other. it form a double standard DNA and prevents translation.

 It is the segment of DNA between the site of inheritance and termination of transcription.
 It consists of three regions.
 A promoter: binding site for RNA polymerase. Located to world 5’ (upstream).
 Structural gene: reason between promoter and Terminator where transcription take place.
 A Terminator: the site where transcription stop.located toward 3’ end (downstream).

Gene is a functional unit of inheritance. It is the DNA sequence
coding for an RNA (mRNA,tRNA,rRNA).
Cistron is a segment of DNA coding for a polypeptide during
Protein synthesis. It is the largest element of a gene.

 Structural gene in a transcription unit is 2 types:
 Monocistronic structural gene (splits gene):
 it is seen in eukaryotes.
 Here, coding sequence ( exons or expressed sequence) are interrupted by
introns (intervening sequences).
 Exons appeared in processed mRNA. Introns do not appear in process mRNA.
 Polycistronic structural gene:
 It is seen in prokaryotes. Here,There are no split genes.

 Introduction:
 The process of transcription is the first stage of gene expression resulting in the production of a primary RNA
transcript from the DNA of a particular gene.
 This step of gene expression which is followed by a number of post-transcriptional processes such as RNA splicing
and translation.
 These lead ultimately to the production of a functional protein and this process is highly regulated.
 Both basal transcription and its regulation are dependent upon specific protein factor known as transcription factors.
 These highly specific protein bind to the specific regulatory gene of DNA sequence and control the transcription
process and regulate it.
 For example
 Enzyme RNA polymerase catalyzes the chemical reaction that synthesize RNA, using the DNA gene as a template, the
transcription factor control when, where, and how efficiency RNA polymerase function.
 Play a important role in the normal development and routine of cellular function.

 Transcription factors are commonly classified into families on the basis of the precise protein
structure which they use to mediate binding to DNA or to cause factor dimerization which is
often essential for DNA binding.
Domain Role Factor containing domein
Homeobos DNA binding Numerous drosophile homeotic genes.
Cysteine- histidine zinc finger DNA binding Related genes in other organisms TFIIIA, Kruppel,sp1 etc.
Cysteine- cysteine zinc finger DNA binding Steroid-thyroid hormone receptor family
Basic element DNA binding C/EBP,c-ios,c-jun,GCN4
Leucine zipper Protein dimerization C/EBP,c-ios,c-jun,GCN4,e-mye
Helix – loop- helix Protein dimerization C-mye, drosophila doughterless MyoD,E12,E47.

1.ACTIVATION
2.REPRESSION

 Some transcription factors activate transcription. For instance, they may help
the general transcription factors and/or RNA polymerase bind to the promoter,
as shown in the diagram.

 Other transcription factors repress transcription. This repression can work in a
variety of ways. As one example, a repressor may get in the way of the basal
transcription factors or RNA polymerase, making it so they can’t bind to the
promoter or begin transcription.

1.competetion for binding
2.Sequencing in Solution
3.Quenching of Activity
4.Direct Repression

 In Bacteria (prokaryotes), synthesis of all type of RNA are catalysed by single
RNA polymerase.
 Transcription has three steps:
 1.Initination
 2.Elongation
 3.termination

 RNA polymerase synthesize RNA in the direction of 5’-
3’ that means DNA template is read in 3’-5’ direction.
 Ribonucleotides required – ATP, GTP, CTP & UTP.
 The prokaryotic RNA polymerase is a multimeric
enzyme consisting of six subunits, two identical a-
subunits, similar but not identical β and β’ and σ sixth is
σ factor.
 2α,β,β’,ω --- core enzyme(without σ )
 2α,β,β’,ω + σ --- Holoenzyme (Active)


 A single RNA polymerase performs multiple functions in transcription process.
 1- search & binds to promoter site
 2- unwinds a short stretch of double helical DNA.
 3- selects correct ribonucleotide & catalyze the formation of phosphodiester bond
(polymerization according to base pair rule)
 (RNA)n + NTP (RNA)n+1 + PPi
 4- detects termination signals
 5- interacts with activator & repressor proteins that regulate the rate of transcription.

 Here, RNA polymerase enzyme blind as a promoter site of DNA.
 This causes the local unwinding of DNA double helix.
 An initation factor (σ factor) present in RNA polymerase initiate the RNA synthesis.

 Two general types of sequence elements are found.
 One sequence element is believed to promote initial binding of the enzyme RNAP.
 Other element usually has high content of adenine & thymine.
 These sequences are 6 to 8 nt in length and located about -35 & -10 bp upstream of the start
point of transcription.
 These are on coding strand indicates duplex DNA required for transcription.
 Change in only one base pair in promoter region decrease the rate of transcription.

 RNA chain is synthesized in 5’ to 3’ direction.
 In This process, activated ribonucleoside triphosphatase(ATP,GTP,UTP,&
CTP) Added this is Complementary To The Base sequence In The DNA
Templet.

 The transcribed region of DNA template contain stop signals.
 Prokaryotes have two classes of termination signals.
 1. relies on protein factor called rho (ρ-Factor) rho-dependent termination.
 2 other is rho-independent termination.
 Termination factor (ρ) blind to the RNA polymerase and terminate the transcription.

 1.Rho-dependent termination
 Signalled by a sequence in the template strand of the DNA molecule
 Which are 40bp in length & are inverted repeat or hyphenated. These signals recognized by a
termination protein, the rho (p) factor.
 Rho is an ATP-dependent RNA-stimulated helicase which binds to the signals. Thus RNAP
cannot move further, so it dissociates from DNA that disrupts the nascent RNA-DNA
complex., release nascent RNA.
 2.Rho independent
 Most –independent terminators have two distinguishing features.
 1. One is palindromic G-C rich region which is followed by an A-T rich region.
 Thus RNA transcript of this palindrome is self complementary sequences, permitting the
formation of a hairpin structure.
 2. The second feature is a highly conserved string of A residues in the template strand that are
transcribed nto U residues. The RNA transcript ends within or just after them.

 In bacteria, transcription and translation can be coupled (translation begin before mRNA is
fully transcribed) because
 mRNA Requires no processing to become active.
 Transcription and translation take place in same compartment (no separation of cytosol and
nucleus).

 Primary transcript hnrnp contain Exons and introns are
non-functional. Hence introns must be removed.

 There are three types of RNA polymerase in eukaryotes:
 RNA polymerase 1: transcribe rRNA(28s,18s,58s).
 RNA polymerase 2: transcribe the heterogeneous nuclear RNA(hnRNA).it is the precursor of
mRNA.
 RNA polymerase 3: transcribe tRNA,5S rRNA,and An RNAs( small nuclear RNAs).

Prokaryotes Eukaryotes
Occurs in the cell Cytoplasm Occurs in the cell Nucleus
Transcription and translation happen
simultaneously
Transcription and translation Differ in time
and place
M-RNA is transcribed directly from template
DNA molecule.
Initially a pre-m-RNAmoleculeis formed And
then procesed to yield a mature m-RNA
RNA polymerase consists of five subunits. RNA polymerase consists of 10-17 sub-units.
Holoenzyme (RNA+Polymerase+sigma
factor) recognise and bind directly to the
pramoter.
Pramotor recognition cannot be carried out by
RNA polymerase alone.

1. Splicing
2. Addition of 5' cap
3. Creation of poly A tail
4. RNA editing

 Processing of primary transcript (hnRNA)
 1.Splicing: from hnRNA,intron are removed (by
spliceosome) and exons are spliced together.
 2. Capping: here, a nucleotid methyl guanosine
triphosphate (cap) is added to 5’ end of hnRNA.
 Phosphate group removes by phosphatase enzyme.
 GTP is added by releasing pyrophosphate.
 7th N of guanine is methylated by methyl transferase
enzyme. Methyl group donor is S-adenosyl methionine.
 3.Tailling (polyadenylation): here, adenylate residue
(200-300) are added at 3’- end.
 Adenine nucleotides are added by enzymeadenylate
transferase.

 It is the sequence of nucleoside (nitrogen bases) in mRNA
that contain information for protein synthesis (translation).
 The sequence of three bases determining a single amino acid
is called codon.

 George gamow suggested that for coding 20 amino acid,the code should be
made up of 3 nucleotides does there are 64 codon. (64= 4×4×4).
 Har Govind Khurana developed the chemical method is synthesizing RNA
molecules with defined combination of bases (homopolymers and
copolymers).
 Marshall Narenberg developed a cell-free system for protein synthesis.
 Savero ochoa enzyme (polynucleotide phosphorylase) is used to polymerize
RNA with define sequence in template independent manner.

 Codon is triplet (three letter code).
 61 codon for amino acid. 3 codons (UAA,UGA, & UGA) do not code for any amino acids. They Act as a
stop codon (termination codon and nonsense codon).
 AUG has dual functions. It code for methionine and acts as a inhibitor codon. In eukaryotes, methionine
is the first amino acid and formyl methionine in prokaryotes.
 An amino acid is coded by many codon (except AUG for methionine & UGG for tryptophan).
 Such codons are called degenerate codons.

 No punctuation between adjucent codons (comma less code). The Codon is read in mRNA in
a contiguous fashion.
 Genetic code is universal. e.g. From bacteria to human UUU codes for phenylalanine.some
expeptions are found in mitochondrial codons, and in some protozoans.
 Genetic-Code is non-overlapping.
 Genetic code is unambiguous & specific. i.e. One Codon specify only one amino acid.

 Relationship between gene and DNA are best understood by mutation studies. Delicious and
rearrangements in DNA may case loss or gain of gene and so a function.
 Insertion or deletion of one gene or two bases exchange the reading frame from the point of
insertion or deletion. It called frame-shift insertion or delatation mutations.
 Insertion or deletion of three or its multiple base is insert or delete one or multiple codon.
The reading frame remain unaltered from that point onwords. Hence one or multiple amino
acids Inserted or deleted.
 It true that codon is a triplet and is read contaguously.

 Sense Codons
 Signal Codons
 Start codons
 Stop codons
 Sense codon:- The codon that code for amino Acid are called sense codon.
 Signal codon:- Those codons that code for Signal during protein synthesis are called signal codons.
 For Example:- AUG, UAA, UAG & UGA
 There are Two types of signal codons
 1.Terminating Codon
 2.Initiating Codon.

 Terminating Codons”
 UAA, UAG & UGA are termination codons or
 Nonsense codons & are often referred to as amber, ochre & opal codons.
 “Initiating codon”
 AUG is the initiation codon. It codes for the first amino acid in all proteins.
 At the starting point it codes for methionine in eukaryotes & formyl methionine in
prokaryotes.

 Anti-codon
 The base sequence of t RNA which pairs with codon of mRNA during translation is called
anticodon.

 Triplet code
 Comma less
 Nonoverlapping code
 The coding dictionary
 Degenerate code
 Universality of code
 Non ambiguous code
 Chain inition code
 Chain termination codons

 Crick postulated the ‘wobble hypothesis’ to account for the degeneracy of genetic code. According this hypothesis,
the first two bases of a codon pair according to the normal base pairing rules with the last two bases of the
anticodon. Base-pairing at the third position of a codon is wobble.
 Wobble hypothesis explains the degeneracy of the genetic code, i.e, existence of multiple codons for a single
amino acid. Although there are 61 codons for amino acids, the number of tRNA is far less (around 40)which is due
to wobbling.
• Biological significance of degeneracy 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 would
therefore be much higher with a non degenerate code.

 Mutation can be well explained using the genetic code.
 A) Point Mutations
 Silent
 Misense
 Nonsense
 B) Frame shift mutations

 Single nucleotide change-A to G, same amino acid is incorporated. Mutation goes unnoticed.

 Single nucleotide change A to C- different amino acid incorporated. Loss of functional
capacity of protein.

 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.

 Insertion or removal of a bases can alter the reading frame with the resultant incorporation of
different amino acids.

genetic material in organization, Central dogma,transcription in prokaryotes vs eukaryotes and genetic codon

genetic material in organization, Central dogma,transcription in prokaryotes vs eukaryotes and genetic codon

Recommended

Recommended

More Related Content

Similar to genetic material in organization, Central dogma,transcription in prokaryotes vs eukaryotes and genetic codon

Similar to genetic material in organization, Central dogma,transcription in prokaryotes vs eukaryotes and genetic codon (20)

Recently uploaded

Recently uploaded (20)

genetic material in organization, Central dogma,transcription in prokaryotes vs eukaryotes and genetic codon