Genetic code
- 1. GENETIC CODE GUIDED BY- DR. DHNANJAY SHUKLA PRESENTATION BY- CHANDRA SHEKHAR DHIRENDRA KUMAR KUSHAL KANT PANT UDYAN SHARMA
- 3. • The 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.
- 4. Translation The pathway of protein synthesis is called Translation because the language of nucleotide sequence on mRNA is translated in to the language of an amino acid sequence. The process of Translation requires a Genetic code, through which the information contained in nucleic acid sequence is expressed to produce a specific sequence of amino acids.
- 11. tRNA • Transfer RNAs or tRNAs are molecules that act as temporary carriers of amino acids, bringing the appropriate amino acids to the ribosome based on the messenger RNA (mRNA) nucleotide sequence. In this way, they act as the intermediaries between nucleotide and amino acid sequences. • tRNAs are ribonucleic acids and therefore capable of forming hydrogen bonds with mRNA. • They pair with mRNA in a complementary and antiparallel manner, and each tRNA can base pair with a stretch of three nucleotides on mRNA.
- 12. tRNA Structure and Function
- 13. • The anticodon loop, which pairs with mRNA, determines which amino acid is attached to the acceptor stem. The anticodon loop is recognized by aminoacyl tRNA synthetase (AATS), the enzyme that chemically links a tRNA to an amino acid through a high-energy bond. AATS ‘reads’ the anticodon and also recognizes the D-arm located downstream from the 5’ end of the tRNA. • The D-arm is made of a double-stranded stem region formed by internal base pairing as well as a loop structure of unpaired nucleotides. The D-arm is a highly variable region and plays an important role in stabilizing the RNA’s tertiary structure and also influences the kinetics and accuracy of translation at the ribosome. • T-arm, Similar to the D-arm, it contains a stretch of nucleotides that base pair with each other and a loop that is single stranded. The paired region is called the ‘stem’ and mostly contains 5 base pairs. The loop contains modified bases and is also called the TΨC arm.
- 15. GENETIC CODE Written in linear form of ribonucleotide bases (mRNA). Each word consists of 3 ribonucleotide letters which (triplet codon) specifies one amino acids. The code is unambiguous – each triplet specifies only a single amino acid. The code is degenerate , one amino acid can be specified by more than one triplet codon.
- 16. The code is comma less ;once translation beings the codons read one after the other with no breaks between them (until the stop signal is reached) The codon contain 1 start codon and 3 stop codons The code is non overlapping The code is (nearly) universal with only minor exception a single coding dictionary is used by all most all viruses, prokaryotes ,archaea and eukaryotes .
- 17. The initiation codon AUG is the most common signal for the beginning of a polypeptide in all cells in addition to coding for Met residues in internal positions of polypeptides. The termination codons (UAA, UAG, and UGA), also called stop codons or nonsense codons, normally signal the end of polypeptide synthesis and do not code for any known amino acids. In general, a reading frame without a termination codon among 50 or more codons is referred to as an open reading frame (ORF). When several different codons specify one amino acid, the difference between them usually lies at the third base position (at the 3 end). For example, alanine is coded by the triplets GCU, GCC, GCA, and GCG.
- 19. Wobble Allows Some tRNAs to Recognize More than One Codon • If the anticodon triplet of a tRNA recognized only one codon triplet through Watson-Crick base pairing at all three positions, cells would have a different tRNA for each amino acid codon. This is not the case, however, because the anticodons in some tRNAs include the nucleotide inosinate (designated I), which contains the uncommon base hypoxanthine . Inosinate can form hydrogen bonds with three different nucleotides.
- 20. The first two bases are identical (CG) and form strong Watson-Crick base pairs with the corresponding bases of the anticodon, but the third base (A, U, or C) forms rather weak hydrogen bonds with the I residue at the first position of the anticodon.
- 21. • Crick concluded that the third base of most codons pairs rather loosely with the corresponding base of its anticodon; to use his picturesque word, the third base of such codons (and the first base of their corresponding anticodons) “wobbles.” Crick proposed a setof four relationships called the wobble hypothesis: 1. The first two bases of an mRNA codon always form strong Watson-Crick base pairs with the corresponding bases of the tRNA anticodon and confer most of the coding specificity. 2. The first base of the anticodon (reading in the 5’-3’ direction; this pairs with the third base of the codon) determines the number of codons recognized by the tRNA. When the first base of the anticodon is C or A, base pairing is specificand only one codon is recognized by that tRNA. When the first base is U or G, binding is lesspecific and two different codons may be read.When inosine (I) is the first (wobble) nucleotide of an anticodon, three different codons canbe recognized—the maximum number for any tRNA.
- 22. • 3. When an amino acid is specified by several different codons, the codons that differ in either of the first two bases require different tRNAs. • 4. A minimum of 32 tRNAs are required to translate all 61 codons (31 to encode the amino acids and1 for initiation)
- 23. History • Paul Zamecnik discovered ribosome . • Robert Holley discovered transfer RNA (tRNA).
- 24. • In 1961 Marshall Nirenberg and Heinrich Matthaei found that the 3 codons of genetic code in mRNA codes for amino acids. • In 1964 Nirenberg and Philip Leder found that each amino acid codes for only specific amino acid.
- 25. • H. Gobind Khorana, developed chemical methods to synthesize polyribonucleotides with defined,repeating sequences of two to four bases. The polypeptides produced by these mRNAs had one or a few amino acids in repeating patterns. These patterns, when combined with information from the random polymers used by Nirenberg and colleagues, permitted unambiguous codon assignments.
- 26. Mutations and the genetic code There are two kinds of mutations which played a very significant role in the study of the genetic code in living systems. These are: (i) frame shift mutations and (ii) base substitutions.