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Genes and protein synthesis sla RNA Tr an sc DNA Tr an replication rip t tio n ion All genetic information flow from DNA into proteins Protein Genes and protein synthesis During the 1950s and 1960s, it became apparent that DNA is essential in the synthesis of proteins. Proteins are used as structural materials in cells and function in cellular activities. For example, in humans, the hormone insulin and the muscle cell filaments are composed of protein. The hair, skin, and nails of humans are composed of proteins, as are all the hundreds of thousands of enzymes in the body. Amino acids code The sequence of amino acids in a protein is a type of code that specifies the protein and distinguishes one protein from another. A genetic code in the DNA determines this amino acid code. A three-nucleotides codon (i.e. AUC) in a sequence specifies a single amino acid. With few exceptions, the genetic code is universal for all living organisms UCA = serine CCA = proline AAA = lysine Protein synthesis For protein synthesis to occur, several essential materials are needed: • • • • a supply of the 20 amino acids a series of enzymes that will function in the process DNA different types of RNA Types of RNA In the synthesis of protein, three types of RNA function: Ribosomal RNA (rRNA). This form of RNA is used, together with ribosomial proteins, to manufacture ribosomes, the chemical “workbenches” where amino acids are linked to one another to synthesize proteins. Transfer RNA (tRNA). Transfer RNA exists in the cytoplasm and carries amino acids (in a highly specific manner) to the ribosomes for protein synthesis. For example, tRNA molecule “X” will link only to amino acid “X”; tRNA “Y” will link only to amino acid “Y” Messenger RNA (mRNA). In the nucleus, mRNA reads the DNA and carries the genetic information to the ribosomes. RNA polymerase accomplishes mRNA, tRNA, and rRNA synthesis Protein synthesis The making of a protein requires two steps 1. Transcription: DNA is transcribed into mRNA. 2. Translation: the transcript mRNA directs the amino acid sequence Transcription A complementary strand of mRNA is synthesized according to the nitrogenous base code of a given DNA region. Only one DNA strand serves as a template for RNA synthesis. The other DNA strand remains dormant. RNA polymerase moves along the DNA strand and “reads” the nucleotides one by one. The mRNA strand elongates in a 5′-3′ direction. The enzyme selects complementary bases from available nucleotides and positions them in an mRNA molecule according to the principle of complementary base pairing. The chain of mRNA lengthens until a “stop” message is received DNA template strand RNA polymerase 3’ transcription direction noncoding strand mRNA transcript 5’ Transcription In eukaryotes, exons are DNA regions that will be expressed introns are DNA sequences that will not be expressed Both introns and exons are transcript to primary mRNA but are then removed into the final mRNA Translation The genetic code is transferred to an amino acid sequence (protein) through the translation process, which begins with the arrival of the mRNA molecule at the ribosome, which is composed of two subunits. Translation Amino acids are carried by tRNA to the ribosomes where mRNA is translated into proteins. Amino acid After it arrives at the ribosomes, mRNA exposes its bases in sets of three, the mRNA’s codon. Each codon has a complementary codon called a tRNA’s anticodon on a tRNA molecule. Polypeptide synthesis Polypeptide synthesis occurs as a ribosome moves along mRNA. Polypeptide synthesis Initiation begins the process of protein synthesis. The “start” codon AUG on the mRNA. AUG codon is the code for the amino acid methionine. The ribosome tRNA binding sites are: E (exit) site, P (peptide) site and A (amino acid) site. Initiation amino acid E mRNA P A Polypeptide synthesis Elongation A tRNA at the P site passes a peptide to an amino acid-carrying tRNA at the A site. elongation Translocation The ribosome moves forward and the peptide bearing tRNA is at the P site. The used tRNA exits from the E site translocation Gene expression At termination, the ribosome reaches a “stop codon” UAA, UAG or UGA and the polypeptide is released. The protein is now synthesized completely and removed from the ribosome for further processing and to perform its function. A protein may be used in the cell as a structural component, or it may be released as a hormone, such as insulin. After synthesis • The mRNA molecule breaks up and the nucleotides return to the nucleus. • The tRNA molecules return to the cytoplasm to unite with fresh amino acids. • The ribosome awaits the arrival of a new mRNA molecule Transcription and translation make the gene expression possible