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THE GENETIC CODE Dr. Sinan Bahjat M.B.Ch.B., M.Sc., F.I.B.M.S. Lec. 3 It is the sequence of nucleotides in DNA or RNA that determines the amino acid sequence in the proteins made. It is the biochemical basis of heredity and is nearly universal in all organisms. The genetic code is a triplet code (i.e. composed of 3 bases), these triplets are called codons Genetic code consists of 64 triplets of nucleotides. These codons encode for the 20 amino acids used in the synthesis of proteins. Each triplet codon specifies only one amino acid, but an individual amino acid may be specified by more than one codon. A start codon, AUG, sets the reading frame, and signals the start of translation of the genetic code. the code is read in a 5' to 3' direction. THE CODON, FUNCTIONS; it AUG SERVES TWO RELATED signals the start of translation. and it codes for the incorporation of the amino acid methionine (Met) into the growing polypeptide chain Translation continues in a nonoverlapping fashion until a stop codon (UAA, UAG, or UGA) is found in frame. The nucleotides between the start and stop codons encode for the subsequent protein produced. The genetic code can be expressed as either RNA codons or DNA codons. RNA codons occur in messenger RNA (mRNA) and are the codons that are actually "read" during the synthesis of polypeptides in the process of translation. PROPERTIES OF GENETIC CODES SEVERAL PROPERTIES OF THE GENETIC CODE WERE APPARENT, AS FOLLOWS: The genetic code is composed of nucleotide triplets. The code is non- overlapping. Each codon specifies a particular amino acid, and only one amino acid. Each amino acid can be specified by more than one codon. The code is nearly universal. DNA REPLICATION DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts when one double-stranded DNA molecule produces two identical copies of the molecule. Each strand of the original double-stranded DNA molecule serves as template for the production of the complementary strand. DNA replication begins at specific locations in the genome, called "origins". Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork. The DNA polymerase is an enzyme that is used by the cell to synthesize the new DNA by adding nucleotides matched to the template strand. A DNA polymerase can only extend an existing DNA strand paired with a template strand; it cannot begin the synthesis of a new strand. To begin synthesis, a short fragment of DNA or RNA, called a primer, must be created and paired with the template DNA strand. DNA REPLICATION PROCEEDS IN THREE COORDINATED STEPS: Initiation Elongation Termination Eukaryotic cells can only initiate DNA replication at a specific point in the cell cycle, the beginning of (S) phase. THANK YOU