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genetic code.pptx
- 1. GENETIC CODE Presentation By: Nageen Talib Department: IBAHS
- 2. Outlines What is genetic code? Genetic code table Wobble Hypothesis Properties of genetic code Exceptions to the code
- 3. GENETIC CODE The genetic code can be defined as the set of certain rules using which the living cells translate the information encoded within genetic material (DNA or mRNA sequences). The ribosomes are responsible to accomplish the process of translation. They link the amino acids in an mRNA-specified (messenger RNA) order using tRNA (transfer RNA ) molecules to carry amino acids and to read the mRNA three nucleotides at a time.
- 4. The complete set of relationships among amino acids and codons is said to be a genetic code which is often summarized in a table.
- 5. Genetic Code Table It can be seen that many amino acids are shown in the table by more than one codon. For example, there are six ways to write leucine in mRNA language. Note: A codon is a sequence of three nucleotides which together form a unit of genetic code in a DNA or RNA molecule. A key point of the genetic code is its universal nature. This indicates that virtually all species with minor exceptions use the genetic code for protein synthesis. In other words, genetic code is defined as the nucleotide sequence of the base on DNA which is translated into a sequence of amino acids of the protein to be synthesized.
- 6. The Wobble Hypothesis There are more than one codon for one amino acid. This is called degeneracy of genetic code. To explain the possible cause of degeneracy of codons, in 1966, Francis Crick proposed the Wobble hypothesis. According to this hypothesis, only the first two bases of the codon have a precise pairing with the bases of the anticodon of tRNA, while the pairing between the third bases of codon and anticodon may Wobble (wobble means to sway or move unsteadily). The phenomenon permits a single tRNA to recognize more than one codon. Therefore, although there are 61 codons for amino acids, the number of tRNA is far less (around 40) which is due to wobbling.
- 7. The first two bases of the codon make normal (canonical) H-bond pairs with the 2nd and 3rd bases of the anticodon. At the remaining position, less stringent rules apply and non- canonical pairing may occur. The wobble hypothesis thus proposes a more flexible set of base-pairing rules at the third position of the codon. The relaxed base-pairing requirement, or wobble, allows the anticodon of a single form of tRNA to pair with more than one triplet in mRNA. The rules: first base U can recognize A or G, first base G can recognize U or C, and first base I can recognize U, C or A.
- 10. Properties of Genetic Code Triplet code Non-ambiguous and Universal Degenerate code Non-overlapping code Commaless Start and Stop Codons Polarity
- 11. Triplet code A codon or a code word is defined as a group of bases that specify an amino acid. There is strong evidence, which proves that a sequence of three nucleotides codes for an amino acid in the protein, i.e., the code is a triplet. The four bases of nucleotide i.e, (A, G, C, and U) are used to produce three-base codons. The 64 codons involve sense codons (that specify amino acids). Hence, there are 64 codons for 20 amino acids since every codon for one amino acid means that there exist more than one code for the same amino acid. Commaless code No room for punctuation in between which indicates that every codon is adjacent to the previous one without any nucleotides between them.
- 12. Nonoverlapping code The code is read sequentially in a group of three and a nucleotide which becomes a part of triplet never becomes part of the next triplet. For example 5-UCU-3 codes for Serine 5-AUG-3 codes for methionine 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. Degenerate code Every amino acid except tryptophan (UGG) and methionine (AUG) is coded by various codons, i.e, a few codons are synonyms and this aspect is known as the degeneracy of genetic code.
- 13. Start and Stop Codons Generally, AUG codon is the initiating or start codon. The polypeptide chain starts either with eukaryotes (methionine) or prokaryotes (N- formylmethionine). On the other hand, UAG, UAA and UGA are called as termination codons or stop codons. These are not read by any tRNA molecules and they never code for any amino acids. Non-ambiguous and Universal The genetic code is non-ambiguous which means a specific codon will only code for a particular amino acid. Also, the same genetic code is seen valid for all the organisms i.e. they are universal.
- 14. Exceptions to the Code The genetic code is universal since similar codons are assigned to identical amino acids along with similar START and STOP signals in the majority of genes in microorganisms and plants. However, a few exceptions have been discovered and most of these include assigning one or two of the STOP codons to an amino acid. Apart from this, both the codons GUG and AUG may code for methionine as a starting codon, although GUG is meant for valine. This breaks the property of non-ambiguousness. Thus, it can be said that few codes often differs from the universal code or non- ambiguous code.
- 15. Codon Universal code Human mitochondrial code AGA Arg Stop AGG Arg Stop AUA Ile Met