 Genetic information, stored in the chromosomes and transmitted to the daughter cells through DNA replication is expressed through transcription to RNA and, in the case of mRNA, subsequent translation into polypeptide chains.  This flow of information from DNA to RNA to protein is termed the "central dogma“  The process of translation requires a genetic code, through which the information contained in the nucleic acid sequence is expressed to produce a specific sequence of amino acids.  Any alternation in the nucleic acid sequence may result in an improper amino acid being inserted into the polypeptide chain, potentially causing diseases or even death.

central dogma

 The genetic code is a dictionary that identifies the correspondence between a sequence of nucleotide bases and a sequence of amino acids.  Each individual word in the code is composed of three nucleotide bases. These genetic words are called codons

 The codons are usually presented in the messenger RNA language of A, G, C, and U.  Their nucleotide sequence are always written from the 5'-end to the 3'end.  The four nucleotide bases are used to produce the three-base codons. There are therefore 64 different combinations of bases, taken three at a time (4 3 ).

 Three of the codons, UAG, UGA, and UAA do not code for the amino acids, but rather are termination codons.  when one of these codons appears in an mRNA sequence, it signals that synthesis of the peptide chain coded for by that mRNA is completed.  AUG is a start codon.

1. Specificity: a specific codon always codes for the same amino acid. 2. Redundancy or Degeneracy: Although each codon corresponds to a single amino acid, a given amino acid may have more than one triplet coding for it. (61 sense codes for 20 a. a… i.e. more than 3 sense codes for 1 amino acid). 3. Triplicity: Triplet of nucleotides for each a.a., 4=64, 61 sense code, 3 non-sense.

4. Non overlapping and comma less: the code is read from a fixed starting point as a continuous sequence of bases, taken three at a time. For example, ABCDEFGHIJKL…is read as ABC/DEF/GHI/JKL…without any "punctuation" between the codons. 5. Co-linearity: correspondence between the linear seq. of the codon in mRNA+ a.a in the protein. 6. Universality: T he genetic code is universal. The specificity of the genetic codes has been conserved from very early stages of evolution.

Amino AcidSLCDNA codons Isoleucine IATT, ATC, ATA Leucine LCTT, CTC, CTA, CTG, TTA, TTG ValineVGTT, GTC, GTA, GTG Phenylalanine FTTT, TTC MethionineMATG Cysteine CTGT, TGC Alanine AGCT, GCC, GCA, GCG Glycine GGGT, GGC, GGA, GGG Proline PCCT, CCC, CCA, CCG Threonine TACT, ACC, ACA, ACG Serine STCT, TCC, TCA, TCG, AGT, AGC Tyrosine YTAT, TAC Tryptophan WTGG Glutamine QCAA, CAG Asparagine NAAT, AAC Histidine HCAT, CAC Glutamic acid EGAA, GAG Aspartic acid DGAT, GAC Lysine KAAA, AAG Arginine RCGT, CGC, CGA, CGG, AGA, AGG

o Some aspects of gene structure, such as the existence of the introns and exons. o Most human genes are divided into exons and introns. o The exons are the sections that are found in the mature transcript (messenger RNA), o while the introns are removed from the primary transcript by the process called splicing.

 While DNA is formed and replicated in the cell nucleus, protein synthesis takes place in the cytoplasm.The information contained in DNA must be transported to the cytoplasm and then used to dictate the composition of proteins.  This involves two processes, transcription and translation. Briefly, the DNA code is transcribed into mRNA, which then leaves the nucleus to be translated into proteins.  There is an important difference between replication and transcription. During replication the entire chromosome is copied to yield daughter DNA identical to the parent DNA.But in the transcription process not all of the cell DNA is necessarily transcribed, usually only individual genes are transcribed.

 Is the process by which an m-RNA sequence is formed from a DNA template.  This process is divided into three phases: 1. Initiation. 2. Elongation. 3.Termination.

 Involves the binding of the RNA polymerase to a region on the DNA known as promoter region (a promoter is a nucleotide sequence that lies up stream of the gene).  The RNA polymerase then pulls a portion of the DNA strands apart from one another, exposing unattached DNA bases. One of the two DNA strands provides the template for the sequence of mRNA nucleotides.

 RNA polymerase begins to synthesize a transcript of the DNA sequence by adding complementary ribonucleotide to the new RNA.  Because the m RNA molecule can be synthesized only in the 5' to 3' direction, the promoter, by specifying directionality, determines which DNA strand serves as the template. This template DNA strand is also known as the antisense strand.

 RNA polymerase moves in the 3' to 5' directions along the DNA template strand, assembling the complementary m RNA strand from the 5' to 3'.  Because of complementary base paring, the m RNA nucleotide sequence is identical to that of the DNA strand that does not serve as the template- the sense strand- except, of course, for the substitution of uracil for thymine.

 The process of elongation of the RNA chain continues until a termination signal is reached.  Then the enzyme releases the completed RNA and detaches from DNA.

 A primary transcript is a linear copy of a transcriptional unit, the segment of DNA between specific initiation and termination sequences. The primary transcript is modified after transcription.  These Modifications include: 1. 5' "Capping“. 2. Addition of a poly-A tail. 3. Removal of introns.

 After RNA synthesis begins, the 5' end of the growing RNA molecule is capped by the addition of a guanine nucleotide. To prevent the RNA molecule from being degraded during synthesis, and late it helps to indicate the starting position for translation of m RNA molecule into protein.

 A series of 40 to 200 adenine bases are added to the 3' end of the RNA molecule.  This structure involves in stabilizing the mRNA molecule so that it is not degraded when it reaches the cytoplasm.

 Maturation of m RNA may involve in the removal of RNA sequences(introns) that do not code for protein by nuclear enzymes.  and the remaining coding sequences, the (exons )are spliced together to form the mature m RNA that will migrate to the cytoplasm.

Post-Transcriptional Modification of RNA 1) 5' "Capping“. 2)Addition of a poly-A tail. 3)Removal of introns.