The genetic code is composed of triplets of nitrogen bases (codons) along mRNA that specify the sequence of amino acids in proteins. Evidence from experiments on mutations in bacteria and viruses showed that the genetic code is read in triplets, with one codon corresponding to one amino acid (except for some amino acids with multiple codons). The genetic code is nearly universal across all life and is non-overlapping, with each codon only being read once without influencing adjacent codons. The genetic code is composed of 64 possible codons that specify 20 standard amino acids.
Sequencing DNA means determining the order of the four chemical building blocks - called "bases" - that make up the DNA molecule. The sequence tells scientists the kind of genetic information that is carried in a particular DNA segment. For example, scientists can use sequence information to determine which stretches of DNA contain genes and which stretches carry regulatory instructions, turning genes on or off. In addition, and importantly, sequence data can highlight changes in a gene that may cause disease.
description of mechanism of transcription in prokaryotes and eukaryotes with clear explanation and clear pictures and also mentiong of different promotors and enhancers and silencers
Genetic code, Deciphering of genetic code, properties of genetic code, Initiation & termination of codons, Gene Mutation, non sense codon, release factors, Transition , Trans versions
Gene regulation in eukaryotes in a nutshell covering all the important stages of gene regulation in eukaryotes at transcriptional level, translation level and post-translational level.
Protein glycosylation and its associated disordersSaranya Sankar
Protein glycosylation and its associate disorders. Glycosylation is one of the post translational modifications important for the normal function of the protein such as cell adhesion, signalling etc.. defect in this process leads to fatal disorder such as cancer, PNH....
it describes transcription with simple diagram and animation. its steps and inhibitors are described for both eukaryotes and prokaryotes. it will be easily understood by UG students . post transcriptional modification of all the RNA are also described with diagrams.
Genetic information is stored in DNA by means of a triplet code that is nearly universal to all living things on Earth.
The genetic code is initially transferred from DNA to RNA, in the process of transcription.
Sequencing DNA means determining the order of the four chemical building blocks - called "bases" - that make up the DNA molecule. The sequence tells scientists the kind of genetic information that is carried in a particular DNA segment. For example, scientists can use sequence information to determine which stretches of DNA contain genes and which stretches carry regulatory instructions, turning genes on or off. In addition, and importantly, sequence data can highlight changes in a gene that may cause disease.
description of mechanism of transcription in prokaryotes and eukaryotes with clear explanation and clear pictures and also mentiong of different promotors and enhancers and silencers
Genetic code, Deciphering of genetic code, properties of genetic code, Initiation & termination of codons, Gene Mutation, non sense codon, release factors, Transition , Trans versions
Gene regulation in eukaryotes in a nutshell covering all the important stages of gene regulation in eukaryotes at transcriptional level, translation level and post-translational level.
Protein glycosylation and its associated disordersSaranya Sankar
Protein glycosylation and its associate disorders. Glycosylation is one of the post translational modifications important for the normal function of the protein such as cell adhesion, signalling etc.. defect in this process leads to fatal disorder such as cancer, PNH....
it describes transcription with simple diagram and animation. its steps and inhibitors are described for both eukaryotes and prokaryotes. it will be easily understood by UG students . post transcriptional modification of all the RNA are also described with diagrams.
Genetic information is stored in DNA by means of a triplet code that is nearly universal to all living things on Earth.
The genetic code is initially transferred from DNA to RNA, in the process of transcription.
Genetic information is stored in DNA by means of a triplet code that is nearly universal to all living things on Earth.
The genetic code is initially transferred from DNA to RNA, in the process of transcription.
• The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences) by living cells.
• The genetic code, once thought to be identical in all forms of life, has been found to diverge slightly in certain organisms and in the mitochondria of some eukaryotes.
• Nevertheless, these differences are rare, and the genetic code is identical in almost all species, with the same codons specifying the same amino acids.
Genetic Information Transfer (Biology for Engineers)Dr. Arun Sharma
Information Transfer: Purpose: The molecular basis of coding and
decoding genetic information is universal. Molecular basis of information
transfer. DNA as a genetic material. Hierarchy of DNA structure- from
single stranded to double helix to nucleosomes. Concept of genetic code.
Universality and degeneracy of genetic code. Define gene in terms of
complementation and recombination.
Genetic code is the term we use for the way that the four bases of DNA--the A, C, G, and Ts--are strung together in a way that the cellular machinery, the ribosome, can read them and turn them into a protein. In the genetic code, each three nucleotides in a row count as a triplet and code for a single amino acid.
The sequence of nucleotides in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) that determines the amino acid sequence of proteins. Though the linear sequence of nucleotides in DNA contains the information for protein sequences, proteins are not made directly from DNA. Instead, a messenger RNA (mRNA) molecule is synthesized from the DNA and directs the formation of the protein. RNA is composed of four nucleotides: adenine (A), guanine (G), cytosine (C), and uracil."(U)."
Genetic Code. A comprehensive overview..pdfmughalgumar440
The genetic code serves as nature's instruction manual, dictating how genetic information is translated into proteins essential for life. Comprised of codons which code for specific amino acid or signaling the start or end of protein synthesis. This code exhibits redundancy and universality across organisms, In essence, the genetic code is the foundation of biological diversity and functionality, shaping the characteristics and functions of all living beings.
The genetic code is a nonoverlapping code, with each amino acid plus polypeptide initiation and termination specified by RNA codons composed of three nucleotides.
Genes code for gene products. What does this statement mean Briefly.pdfjibinsh
Genes code for gene products. What does this statement mean? Briefly describe the structure of
DNA by using the following terms: nucleotide, strand, complementary, deoxyribose, phosphate,
anti-parallel, base pairing, adenine, cytosine. How many DNA molecules are in a chromosome?
How many genes are in an average bacterial chromosome? What is the purpose of DNA
replication? (\"To make more DNA\" would not be a complete answer.) Summarize the process
of DNA replication by using the following terms: replication fork, template, nucleotide, primer,
DNA polymerase, DNA ligase. (Know what each term means.) Summarize the process of RNA
synthesis (transcription) by using the following terms: template, promo RNA polymerase,
rRNA, mRNA, tRNA, terminator. (Know what each term means.) Where are operons found and
what is the advantage of organization of genes within operons? What are some fundamental
differences in organization of genes between prokaryotes and eukaryotes? How do eukaryotes
produce mRNA that they can use for translation? Summarize the process of protein synthesis
(translation) by using the following terms: genetic code, ribosome, mRNA, protein, amino acid,
tRNA, codon, anticodon, start codon, stop codons, polypeptide. (Know what each term means.)
Be able to predict the sequence of a complementary strand in both DNA and RNA synthesis, w
hen the template sequence is given. Be able to use genetic code table to predict amino-acid
sequence of a encoded by a nucleic acid, when the nucleic acid sequence is given. Classify
mutations by type and briefly describe how mutations arise, are prevented or repaired. Explain
why mutations are important by giving at least three different examples. Why do bacteria and
viruses mutate so much faster than eukaryotes? What are some consequences of Briefly explain
(and be able to compare and contrast) three different mechanisms of horizontal gene in bacteria:
transformation, conjugation and transduction. What are some practical implications of these
phenomena?
Solution
3. Genes code for gene products
Gene is a portion of DNA. It is made up of nucleotide sequences. It expresses itself and transfer
from one generation to next generation.
Gene expression means the nucleotide sequence is used for synthesizing a biomolecules-
generally it is protein but it may be RNA molecules. Here gene products are those proteins and
RNAs.
4. Structure of DNA
DNA or deoxyribonucleic acid is the molecule that contains all genetic information of an
organism.
DNA has a double helix shape, which is like a ladder twisted into a spiral. Each spiral is
composed of polynucleotides. Each nucleotide is made up of - deoxyribose, a kind of sugar with
5 carbon atoms; a phosphate group made of phosphorus and oxygen, and nitrogenous base. There
are four types of nucleotide: Adenine (A), Thymine (T), Cytosine (C), Guanine (G). Nucleotides
are joined to one another by covalent bonds between the sugar of one nucleotide and the
phosphate of the next, kn.
In botany · Fruits are the means by which flowering plants (also known as angiosperms) · In common language usage, "fruit" normally means the seed-associated
Rhynia is a single-species genus of Devonian vascular plants. Rhynia gwynne-vaughanii was the sporophyte generation of a vascular, axial, free-sporing diplohaplontic embryophytic land plant of the Early Devonian that had anatomical features more advanced than those of the bryophytes.
What is Meristematic Tissue? Carl Wilhelm von Nägeli coined the term “meristem.” Meristematic tissue contains undifferentiated cells, which are the building blocks of the specialized plant structures. Meristematic tissues contain living cells with varied shapes.
Stems of many plants are modified to perform different functions such as storage, protection, photosynthesis, support, propagation and perennation. Modifications help in better adaptation and survival.
Stems develop from the plumule of the germinating seed. It bears leaves, fruits, flowers, etc. The characteristic feature of a stem is nodes and internodes. The main function of the stem is to support other parts of the plant and conduction of food, water and minerals.
In some plants, stems are modified, which can be aerial, subaerial or underground modifications. They are modified to perform other functions, which are not normally associated with the stem.
Morphology and modifications of roots.pptxmanoj Joshi
The plants that we see today is the result of billions of years of evolution. Today, plants cover almost 30 per cent of the total landmass and account for the 50 per cent of the plant’s productivity (generation of biomass). Plants fulfil many roles in the ecosystem. They are a source of food, nutrition, shelter, maintain the integrity of soil (by preventing erosion) and most importantly, they are the main source for balancing the oxygen level in the atmosphere.
Plants are an essential part of the ecosystem. Every life on the earth is directly or indirectly dependent on plants. Among the different parts of a plant, the leaf is the most essential.
Characteristic form or bodily appearance of an organism.
(The habit of the plant can be understood only if the plant is provided with roots or seen growing in
nature.)
In plants some structures are already present to defend the attack while in others, the structures to defend the host develops after the infection. In this way, structural defense can be characterized as
Classification denotes the arrangement of a single plant or group of plants an distinct category following a system of nomenclature, and in accordance with a particular and well established plan.
A collection of dried and pressed plant arranged according to a classification system and available for study or reference is known as herbarium ( plural herbaria).
Taxonomy (or systematics) is basically concerned with the classification of organisms. Living organisms are placed in groups on the basis of similarities and differences at the organismic, cellular, and molecular levels.
Operation “Blue Star” is the only event in the history of Independent India where the state went into war with its own people. Even after about 40 years it is not clear if it was culmination of states anger over people of the region, a political game of power or start of dictatorial chapter in the democratic setup.
The people of Punjab felt alienated from main stream due to denial of their just demands during a long democratic struggle since independence. As it happen all over the word, it led to militant struggle with great loss of lives of military, police and civilian personnel. Killing of Indira Gandhi and massacre of innocent Sikhs in Delhi and other India cities was also associated with this movement.
Honest Reviews of Tim Han LMA Course Program.pptxtimhan337
Personal development courses are widely available today, with each one promising life-changing outcomes. Tim Han’s Life Mastery Achievers (LMA) Course has drawn a lot of interest. In addition to offering my frank assessment of Success Insider’s LMA Course, this piece examines the course’s effects via a variety of Tim Han LMA course reviews and Success Insider comments.
Embracing GenAI - A Strategic ImperativePeter Windle
Artificial Intelligence (AI) technologies such as Generative AI, Image Generators and Large Language Models have had a dramatic impact on teaching, learning and assessment over the past 18 months. The most immediate threat AI posed was to Academic Integrity with Higher Education Institutes (HEIs) focusing their efforts on combating the use of GenAI in assessment. Guidelines were developed for staff and students, policies put in place too. Innovative educators have forged paths in the use of Generative AI for teaching, learning and assessments leading to pockets of transformation springing up across HEIs, often with little or no top-down guidance, support or direction.
This Gasta posits a strategic approach to integrating AI into HEIs to prepare staff, students and the curriculum for an evolving world and workplace. We will highlight the advantages of working with these technologies beyond the realm of teaching, learning and assessment by considering prompt engineering skills, industry impact, curriculum changes, and the need for staff upskilling. In contrast, not engaging strategically with Generative AI poses risks, including falling behind peers, missed opportunities and failing to ensure our graduates remain employable. The rapid evolution of AI technologies necessitates a proactive and strategic approach if we are to remain relevant.
June 3, 2024 Anti-Semitism Letter Sent to MIT President Kornbluth and MIT Cor...Levi Shapiro
Letter from the Congress of the United States regarding Anti-Semitism sent June 3rd to MIT President Sally Kornbluth, MIT Corp Chair, Mark Gorenberg
Dear Dr. Kornbluth and Mr. Gorenberg,
The US House of Representatives is deeply concerned by ongoing and pervasive acts of antisemitic
harassment and intimidation at the Massachusetts Institute of Technology (MIT). Failing to act decisively to ensure a safe learning environment for all students would be a grave dereliction of your responsibilities as President of MIT and Chair of the MIT Corporation.
This Congress will not stand idly by and allow an environment hostile to Jewish students to persist. The House believes that your institution is in violation of Title VI of the Civil Rights Act, and the inability or
unwillingness to rectify this violation through action requires accountability.
Postsecondary education is a unique opportunity for students to learn and have their ideas and beliefs challenged. However, universities receiving hundreds of millions of federal funds annually have denied
students that opportunity and have been hijacked to become venues for the promotion of terrorism, antisemitic harassment and intimidation, unlawful encampments, and in some cases, assaults and riots.
The House of Representatives will not countenance the use of federal funds to indoctrinate students into hateful, antisemitic, anti-American supporters of terrorism. Investigations into campus antisemitism by the Committee on Education and the Workforce and the Committee on Ways and Means have been expanded into a Congress-wide probe across all relevant jurisdictions to address this national crisis. The undersigned Committees will conduct oversight into the use of federal funds at MIT and its learning environment under authorities granted to each Committee.
• The Committee on Education and the Workforce has been investigating your institution since December 7, 2023. The Committee has broad jurisdiction over postsecondary education, including its compliance with Title VI of the Civil Rights Act, campus safety concerns over disruptions to the learning environment, and the awarding of federal student aid under the Higher Education Act.
• The Committee on Oversight and Accountability is investigating the sources of funding and other support flowing to groups espousing pro-Hamas propaganda and engaged in antisemitic harassment and intimidation of students. The Committee on Oversight and Accountability is the principal oversight committee of the US House of Representatives and has broad authority to investigate “any matter” at “any time” under House Rule X.
• The Committee on Ways and Means has been investigating several universities since November 15, 2023, when the Committee held a hearing entitled From Ivory Towers to Dark Corners: Investigating the Nexus Between Antisemitism, Tax-Exempt Universities, and Terror Financing. The Committee followed the hearing with letters to those institutions on January 10, 202
The Roman Empire A Historical Colossus.pdfkaushalkr1407
The Roman Empire, a vast and enduring power, stands as one of history's most remarkable civilizations, leaving an indelible imprint on the world. It emerged from the Roman Republic, transitioning into an imperial powerhouse under the leadership of Augustus Caesar in 27 BCE. This transformation marked the beginning of an era defined by unprecedented territorial expansion, architectural marvels, and profound cultural influence.
The empire's roots lie in the city of Rome, founded, according to legend, by Romulus in 753 BCE. Over centuries, Rome evolved from a small settlement to a formidable republic, characterized by a complex political system with elected officials and checks on power. However, internal strife, class conflicts, and military ambitions paved the way for the end of the Republic. Julius Caesar’s dictatorship and subsequent assassination in 44 BCE created a power vacuum, leading to a civil war. Octavian, later Augustus, emerged victorious, heralding the Roman Empire’s birth.
Under Augustus, the empire experienced the Pax Romana, a 200-year period of relative peace and stability. Augustus reformed the military, established efficient administrative systems, and initiated grand construction projects. The empire's borders expanded, encompassing territories from Britain to Egypt and from Spain to the Euphrates. Roman legions, renowned for their discipline and engineering prowess, secured and maintained these vast territories, building roads, fortifications, and cities that facilitated control and integration.
The Roman Empire’s society was hierarchical, with a rigid class system. At the top were the patricians, wealthy elites who held significant political power. Below them were the plebeians, free citizens with limited political influence, and the vast numbers of slaves who formed the backbone of the economy. The family unit was central, governed by the paterfamilias, the male head who held absolute authority.
Culturally, the Romans were eclectic, absorbing and adapting elements from the civilizations they encountered, particularly the Greeks. Roman art, literature, and philosophy reflected this synthesis, creating a rich cultural tapestry. Latin, the Roman language, became the lingua franca of the Western world, influencing numerous modern languages.
Roman architecture and engineering achievements were monumental. They perfected the arch, vault, and dome, constructing enduring structures like the Colosseum, Pantheon, and aqueducts. These engineering marvels not only showcased Roman ingenuity but also served practical purposes, from public entertainment to water supply.
Acetabularia Information For Class 9 .docxvaibhavrinwa19
Acetabularia acetabulum is a single-celled green alga that in its vegetative state is morphologically differentiated into a basal rhizoid and an axially elongated stalk, which bears whorls of branching hairs. The single diploid nucleus resides in the rhizoid.
2. Are they written in articulated or
coded language on DNA molecule
As DNA is a genetic material, it carries genetic
information's from cell to cell and from generation to
generation. At this stage, an attempt will be made to
determine that in what manner the genetic information's
are existed in DNA molecule ? Are they written in
articulated or coded language on DNA molecule? I the
language of codes, what is the nature of genetic code ? f in
3. A DNA molecule is composed of three kinds of moieties:
(i) Phosphoric acid,
(ii) Deoxyribose sugar, and
(iii) Nitrogen bases.
4. The genetic informations may be written in any one of the three
moieties of DNA.
But the poly-sugarphosphate backbone is always the same, and
it is therefore unlikely that these moiteies of DNA molecule
carry the genetic informations.
The nitrogen bases, however, vary from one segment of DNA
to another, so the informations might well depend on their
sequences.
The sequences of nitrogen bases of a given
segment of DNA molecule, actually has been
found to be identical to linear sequence of
amino acids in a protein molecule
5. The proof of such a colinearity between DNA nitrogen base sequence
and amino acid sequence in protein molecules was first obtained from an
analysis of mutants of head protein of bacteriophage T4 (Sarabhai et al,
1964) and the A protein of tryptophan synthetase of Escherichia coli
(Yanofski et al, 1964).
The colinearity of protein molecules and DNA polynucleotides has given the clue
that the specific arrangement of four nitrogen bases (e.g., A, T, C and G) in DNA
polynucleotide chains somehow determines the sequence of amino acids in
protein molecules. Therefore, these four DNA bases can be considered as
four alphabets of DNA molecule.
All the genetic information, therefore, should be written by these four
alphabets of DNA.
Now the question arises that whether the genetic informations are written
in articulated language or coded language
6. Articulated language:
If genetic informations might have occurred in an
articulated language, the DNA molecule might require
various alphabets, a complex system of grammer and
ample amount of space on it. All of which might be
practically impossible and troublesome too for the DNA.
7. coded language
Therefore, it was safe to conclude for molecular
biologists that genetic informations were existed in DNA
molecule in the form of certain special language of code
words which might utilize the four nitrogen bases of
DNA for its symbols. Any coded message is commonly
called cryptogram.
8. NATURE OF THE GENETIC CODE
Earlier, Gamow, the well-known nuclear physicist, proposed that the genetic
code consists of three nitogenous (N) bases and the adjacent triplets
overlap. This meant that at any particular point the same N-base occurs
three times in a vertical manner instead of one which is expected on the
basis of colinear model. This hypothesis, however, was not accepted on the
following grounds :
1. In the overlapping model only certain amino acids can follow certain others. After
the first amino acid in a protein is coded, the next two and for that matter the
remaining amino acids in the protein are partially predetermined. If the first code is
CAG, then the next must begin with AG and the third one with G.
2. Mutation involving a change in one base, according to this hypothesis, must involve
three amino acids.
9. In order to find the arrangement of codons, in later experiments,
it was found that when a change occurs due to a mutation, it is
confined only to one amino acid. For instance, when sickle cell
anemia occurs, only one amino acid, namely glutamic acid is changed
into valine, the two adjacent amino acids remaining unaffected.
Further research showed that the codons are arranged in a linear
order.
This explains as to why the change in one involves only one amino
acid and not three; if Gamow‟s hypothesis were correct, change of
one nitrogenous base would have involved 3 amino acids.
The sequence of bases that encodes a functional protein molecule is
called a gene. And the genetic code is the relation between the
base sequence of a gene and the amino acid sequence of the
polypeptide whose synthesis the gene directs. In other words, the
specific correspondence between a set of 3 bases and 1 of the 20
amino acids is called the genetic code.
10. J.D. Burke (1970) defined genetic code in the following words,
“The genetic code for protein synthesis is contained in the base sequence of DNA.
... The genetic code is a code for amino acids. Specifically, it is concerned with what
codons specify what amino acids.”
The genetic code is the key that relates, in Crick‟s words, “...the two
great polymer languages, the nucleic acid language and the protein
language.”
The “letters” in the “language” were found to be the bases; the
“words” (codons) are groups of bases; and the “sentences” and
“paragraphs” equate with groups of codons (Eldon J. Gardner, 1968).
11. Is singlet, doublet or triplet code
The basic problem of such a genetic code is to indicate how information
written in a four-letter language (four nucleotides or nitrogen bases of
DNA) can be translated into a twenty-letter-language (twenty amino acids
of proteins). The group of nucleotides that specifies one amino acid is a code
word or codon.
The simplest possible code is a singlet code (a code of single letter) in
which one nucleotide codes for one amino acid. Such a code is inadequate
for only four amino acids could be specified.
A doublet code (a code of two letters) is also inadequate because it
could specify only sixteen (4 × 4) amino acids,
Whereas a triplet code (a code of three letters) could specify sixty four
(4 × 4 × 4) amino acids. Therefore, it is likely that there may be 64 triplet
codes for 20 amino acids.
12.
13. first experimental evidence
The first experimental evidence in support to the concept of triplet
codes is provided by Crick and coworkers in 1961.
During their experiment, when they added or deleted single, or
double base pairs in a particular region of DNA of T4 bacteriophages
of E. coli, they found that such bacteriophages ceased to perform their
normal functions.
However, bacteriophages with addition or deletion of three base pairs
in DNA molecule, had performed normal functions. From this
experiment, they concluded that a genetic code is in triplet form because
the addition of one or two nucleotides has put the reading of the code out
of order, while the addition of third nucleotide resulted in a return to
the proper reading of the message.
14. THE GENETIC CODE
The genetic language consists of only four letters contained in the word “GACU”. These four letters
can be combined to form 64 genetic words, each consisting of 3 letters.
Each triplet word (codon ) has a specific meaning which the cell understands. It codes for a
particular amino acid.
It shows the base sequences of the various codons (triplets) and against each codon is given
the amino acid that it codes. Just as different combinations of different words make different
sentences, each having a specific meaning, similarly different sequences of codons on mRNA
specify different proteins, each with a specific sequence of amino acids.
Although the genetic information resides in DNA, the terms genetic code and codon are used
with reference to mRNA because mRNA is the nucleic acid which directly determines the
sequence of amino acids in a protein.
This expression of genetic information in the amino acid sequence of proteins by mRNA is
called translation. The DNA-RNA-Protein code may be expressed as under :
15.
16. The synthesis of cellular proteins takes place in the
joining together of several amino acids to form a linear
polypeptide chain of variable length. There are 20 different
amino acids which are commonly found in protenis
(hence called protein amino acids) and which take part in
their synthesis.
The mRNA codons for these 20 protein amino acids, as
can be deduced from
17.
18. 1. The code is a triplet codon:
The nucleotides of mRNA are arranged as a linear
sequence of codons, each codon consisting of three
successive nitrogenous bases, i.e., the code is a triplet
codon. The concept of triplet codon has been supported
by two types of point mutations: frame shift mutations
and base substitutions.
19. (i) Frameshift mutations:
Evidently, the genetic message once initiated at a fixed
point is read in a definite frame in a series of three letter
words. The framework would be disturbed as soon as
there is a deletion or addition of one or more bases.
When such frame shift mutations were intercrossed, then
in certain combinations they produce wild type normal
gene. It was concluded that one of them was deletion and
the other an addition, so that the disturbed order of the
frame due to mutation will be restored by the other
20. (ii) Base substitution:
If in a mRNA molecule at a particular point, one base pair is replaced by
another without any deletion or addition, the meaning of one codon
containing such an altered base will be changed. In consequence, in place
of a particular amino acid at a particular position in a polypeptide, another
amino acid will be incorporated.
For example, due to substitution mutation, in the gene for tryptophan
synthetase enzyme in E. coli, the GGA codon for glycine becomes a
missence codon AGA which codes for arginine. Missence codon is a codon
which undergoes an alteration to specify another amino acid.
A more direct evidence for a triplet code came from the finding that a
piece of mRNA containing 90 nucleotides, corresponded to a polypeptide
chain of 30 amino acids of a growing haemoglobin molecule. Similarly, 1200
nucleotides of “satellite” tobacco necrosis virus direct the synthesis of
coat protein molecules which have 372 amino acids.
21. 2. The code is non-overlapping:
In translating mRNA molecules the codons do not overlap but are “read”
sequentially (Fig. 38.27). Thus, a non-overlapping code means that a base in
a mRNA is not used for different codons. In Figure 38.28, it has been
shown that an overlapping code can mean coding for four amino acids
from six bases.
However, in actual practice six bases code for not more than two amino
acids. For example, in case of an overlapping code, a single change (of
substitution type) in the base sequence will be reflected in substitutions of
more than one amino acid in corresponding protein. Many examples have
accumulated since 1956 in which a single base substitution results into a
single amino acid change in insulin, tryptophan synthelase, TMV coat
protein, alkaline phosphatase, haemoglobin, etc.
However, it has been shown that in the bacteriophage ɸ × l74 there is a
possibility of overlapping the genes and codons
22. 3. The code is commaless:
The genetic code is commaless, which means that no
codon is reserved for punctuations. It means that after
one amino acid is coded, the second amino acid will be
automatically, coded by the next three letters and that no
letters are wasted as the punctuation marks
23. The code is non-ambiguous:
(express in more then one mean)
Non-ambiguous code means that a particular codon will
always code for the same amino acid. In case of
ambiguous code, the same codon could have different
meanings or in other words, the same codon could code
two or more than two different amino acids. Generally, as
a rule, the same codon shall never code for two different
amino acids.
However, there are some reported exceptions to this
rule: the codons AUG and GUG both may code for
methionine as initiating or starting codon, although GUG
is meant for valine. Likewise, GGA codon codes for two
amino acids glycine and glutamic acid.
24. The code has polarity:
The code is always read in a fixed direction, i.e., in the
5‟→3′ direction. In other words, the codon has a polarity.
It is apparent that if the code is read in opposite
directions, it would specify two different proteins, since
the codon would have reversed base sequence:
25. The code is degenerate:
More than one codon may specify the same amino acid; this is called degeneracy of
the code. For example, except for tryptophan and methionine, which have a single
codon each, all other 18 amino acids have more than one codon. Thus, nine amino
acids, namely phenylalanine, tyrosine, histidine, glutamine, asparagine, lysine, aspartic
acid, glutamic acid and cysteine, have two codons each. Isoleucine has three
codons. Five amino acids, namely valine, proline, threonine, alanine and glycine, have
four codons each. Three amino acids, namely leucine, arginine and serine, have six
codons each .
The code degeneracy is basically of two types: partial and complete. Partial
degeneracy occurs when first two nucleotides are identical but the third (i.e., 3′
base) nucleotide of the degenerate codons differs, e.g., CUU and CUC code for
leucine, Complete degeneracy occurs when any of the four bases can take third
position and still code for the same amino acid (e.g., UCU, UCC, UCA and UCG
code for serine).
Degeneracy of genetic code has certain biological advantages. For example, it
permits essentially the same complement of enzymes and other proteins to be
specified by microorganisms varying widely in their DNA base composition.
Degeneracy also provides a mechanism of minimising mutational lethality.
26. Some codes act as start codons:
In most organisms, AUG codon is the start or initiation codon,
i.e., the polypeptide chain starts either with methionine
(eukaryotes) or N- formylmethionine (prokaryotes). Methionyl
or N-formylmethionyl-tRNA specifically binds to the initiation
site of mRNA containing the AUG initiation codon. In rare cases,
GUG also serves as the initiation codon, e.g., bacterial protein
synthesis. Normally, GUG codes for valine, but when normal
AUG codon is lost by deletion, only then GUG is used as
initiation codon.
27. Some codes act as stop codons:
Three codons UAG, UAA and UGA are the chain stop or
termination codons. They do not code for any of the amino acids.
These codons are not read by any tRNA molecules (via their
anticodons), but are read by some specific proteins, called release
factors (e.g., RF-1, RF-2, RF-3 in prokaryotes and RF in eukaryotes).
These codons are also called nonsense codons, since they do not
specify any amino acid.
The UAG was the first termination codon to be discovered by
Sidney Brenner (1965). It was named amber after a graduate student
named Bernstein (= the German word for „amber‟ and amber means
brownish yellow) who help in the discovery of a class of mutations.
Apparently, to give uniformity the other two termination codons
were also named after colours such as ochre for UAA and opal or
umber for UGA. (Ochre means yellow red or pale yellow; opal
means milky white and umber means brown). The existence of more
than one stop codon might be a safety measure, in case the first
codon fails to function.
28. The code is universal:
Same genetic code is found valid for all organisms ranging from
bacteria to man. Such universality of the code was
demonstrated by Marshall, Caskey and Nirenberg (1967) who
found that E. coli (Bacterium), Xenopus laevis (Amphibian) and
guinea pig (mammal) amino acyl-tRNA use almost the same
code. Nirenberg has also stated that the genetic code may have
developed 3 billion years ago with the first bacteria, and it has
changed very little throughout the evolution of living organisms.
Recently, some differences have been discovered between the
universal genetic code and mitochondrial genetic code