Genetic code.pptx
- 2. GENETIC CODE • Genetic Code refers to the relationship between the sequence of nitrogenous bases (UCAG) in mRNA and the sequence of amino acids in a polypeptide chain. • In other words, the relationship between the 4 letters language of nucleotides and twenty letters language of amino acids is known as genetic code. • 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. • The genetic code consists of 64 triplets of nucleotides. These triplets are called codons. • With three exceptions, each codon encodes for one of the 20 amino acids used in the synthesis of proteins. • This produces some redundancy in the code; most of the amino acids being encoded by more than one codon.
- 3. • DNA (or RNA) carries all the genetic information and it is expressed in the form of proteins. • Proteins are made of 20 different amino acids. • The information about the number and sequence of these amino acids forming protein is present in DNA, and during transcription is passed over to mRNA. • The form in which it is transferred was not understood for long. • Pentose sugar and phosphate of DNA could not perform this job of passing on the genetic message to mRNA because sugar is only of one type and so also the phosphate. • This leaves only four nucleotides to form the message for 20 amino acids, but 4 nucleotides are too few for twenty amino acids. • This difficult problem was solved with the discovery that a codon containing coded information for one amino acid consists three nucleotides (a triplet code). • Thus for twenty amino acids, 64 (4 x 4 x 4 or 43 = 64) possible permutation are available. This break through resulted into 64 codons dictionary - the Genetic Code.
- 4. • According to Bark (1970) the genetic code is a code for amino acids, specifically it is concerned with as to what codons specify what amino acids. • Genetic code is the outcome of experiments performed by M. Nirenberg, S. Ochoa, H. Khorana, F. Crick and Mathaei, Professor M. Nirenberg was awarded Nobel Prize in 1961 for this outstanding work. • The dictionary of genetic code employs the letters in RNA (U, C, A, G, i.e., A = Adenine, U = Uracil, C = Cytosine, G = Guanine) • The codon for the amino acids, which are the same in all known life forms, have been determined experimentally.
- 5. PROPERTIES OF GENETIC CODE 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 Base substitutions. 1
- 6. (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 inter- crossed, 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.
- 7. (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 mis-sence codon AGA which codes for arginine. • Mis-sence 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.
- 8. The code is non-overlapping • In translating mRNA molecules the codons do not overlap but are “read” sequentially. • Thus, a non-overlapping code means that a base in a mRNA is not used for different codons. • 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 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 synthetase, TMV coat protein, alkaline phosphatase, haemoglobin, etc. 2
- 9. 3 The code is comma-less • The genetic code is comma-less, 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. 4 The code has polarity • Each triplet is read from 5’ → 3’ direction and the beginning base is 5’ followed by the base in the middle then the last base which is 3’. • This implies that the codons have a fixed polarity and if the codon is read in the reverse direction, the base sequence of the codon would reverse and would specify two different proteins.
- 10. 5 The code is degenerate • The code is degenerate which means that the same amino acid is coded by more than one base triplet. • Genetic code is degenerate i.e., more than one codon can code for same amino acid. • For example, the three amino acids arginine, alanine and leucine each have six synonymous codons. • There are a total of 64 codons and 20 known amino acids. • Hence, if you are asked to assign one amino acid to each codon then it is obvious that same amino acids will be assigned too many amino acids. • Hence a single amino acid can be coded by many codons except tryptophan and methionine which are coded by one codon each. • Degeneracy of genetic code is also known as redundancy.
- 11. Partial Degeneracy • When the first two nitrogenous bases are same but the third base is different, the degeneracy is called partial degeneracy, e.g. CUU and CUC (codes for amino acid leucine) Complete Degeneracy • Complete degeneracy occurs when the third position in the genetic code can be taken by any of the four bases and the codon in each case codes for same amino acid. i.e., UCU, UCA, UCC, UCG (codons codes for serine)
- 12. 6 Co – linearity • DNA is a linear polynucleotide chain and a protein is a linear polypeptide chain. • The sequence of amino acids in a polypeptide chain corresponds to the sequence of nucleotide bases in the gene (DNA) that code for it. • Change in a specific codon in DNA produces a change of amino acid in the corresponding position in the polypeptide. • The gene and the polypeptide it codes for are said to be co-linear. Non - ambiguity • A particular codon will always code for the same amino acid, (i.e. genetic code is specific ) for e.g. UUU codes for amino acid Phenylalanine, it cannot code for any other amino acid. • While the same amino acid can be coded by more than one codon (the code is degenerate), the same codon shall not code for two or more different amino acids. • This property of genetic code makes them non ambiguous. However, there are some exceptions. AUG and GUG both may code for methionine although GUG codes for valine. GGA is another codon which codes for two amino acid glycine and glutamic acid. 7
- 13. 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- formyl methionine (prokaryotes). • Methionyl or N-formyl methionyl-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. 9 8 Gene – polypeptide pairity • A specific gene transcribes a specific mRNA that produces a specific polypeptide. • On this basis, a cell can have only as many types of polypeptides as it has types of genes. However, this does not apply to certain viruses which have overlapping genes.
- 14. 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. • The existence of more than one stop codon might be a safety measure, in case the first codon fails to function. 10
- 15. 11 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.
- 16. Codon Mammalian mitocondrial code Yeast mitochondrial code” “Universal Code 1. UGA Trp * Trp Stop 2. AUA Met Met Lie 3. CUA Leu Thr Leu 4. AGA Stop Arg Arg Differences between ‘universal genetic code’ & two mitochondrial genetic codes * Italic type indicates that the code differs from the ‘universal’ code
- 17. REFERENCES https://www.yourarticlelibrary.com/biology/9-most-important-properties- of-genetic-code-biology/6393 https://www.google.com/search?q=genetic+code+and+its+properties&rlz= 1C1CHBF_enIN775IN775&oq=genetic+code+and+its+properties&aqs=chro me..69i57j0j0i390.11817j0j7&sourceid=chrome&ie=UTF-8 https://byjus.com/biology/genetic-code/ https://www.biologydiscussion.com/genetics/genetic-code/genetic-code-8- important-properties-of-genetic-code/15550
Editor's Notes
- #4: Once you find your sources, you will want to evaluate your sources using the following questions: Author: Who is the author? Why should I believe what he or she has to say on the topic? Is the author seen as an expert on the topic? How do you know? Current: How current is the information in the source? When was the source published? Is the information out-of-date? Accuracy: Is the content accurate? Is the information presented objectively? Do they share the pros and cons?
- #5: When conducting research, it is easy to go to one source: Wikipedia. However, you need to include a variety of sources in your research. Consider the following sources: Who can I interview to get more information on the topic? Is the topic current and will it be relevant to my audience? What articles, blogs, and magazines may have something related to my topic? Is there a YouTube video on the topic? If so, what is it about? What images can I find related to the topic?
- #7: After consulting a variety of sources, you will need to narrow your topic. For example, the topic of internet safety is huge, but you could narrow that topic to include internet safety in regards to social media apps that teenagers are using heavily. A topic like that is more specific and will be relevant to your peers. Some questions to think about to help you narrow your topic: What topics of the research interest me the most? What topics of the research will interest my audience the most? What topics will the audience find more engaging? Shocking? Inspiring?
- #8: Now, that you have narrowed your topic, you will want to organize your research in a structure that works. There are some common organizational patterns based on the kind of research you are doing. Organizational Structures: Cause and Effect- this kind of structure is great for explaining the causes and effects of a topic Compare and Contrast- in this pattern you highlight the similarities and differences of the topic Explain process- this structure is great for outlining a series of steps to follow; Definition- if you want to make sure your audience understands what something is using illustrations, meanings, clarifying misconceptions, you may want to use this structure Classification- a common organizational structure is grouping like topics or facts from the research together. For instance, in the internet safety about social media apps, you may organize the research where you look at each social media app one at a time