1 Gene Expression التعبير الجيني CHAPTER 17 FROM GENE TO PROTEIN

2 Gene expression التعبير الجيني Formation of protein from DNA تكوين البروتينات من in two steps:-Formation of protein from DNA تكوين البروتينات من in two steps:- 1. transcription الانسلاخ (mRNA synthesis تخليق from DNA in the nucleus)1. transcription الانسلاخ (mRNA synthesis تخليق from DNA in the nucleus) 2. translation الترجمة (protein synthesis at ribosomes in the cytoplasm).2. translation الترجمة (protein synthesis at ribosomes in the cytoplasm). Because bacteria lack nuclei لا تحتوي على نواة, transcription and translation are coupled.Because bacteria lack nuclei لا تحتوي على نواة, transcription and translation are coupled مـُتلازمان. 2 mRNA DNATranscription ProteinTranslation The DNA inherited المتوارث by an organism leads to يؤدي الى specific traits مجموعة من الخصائص by the synthesis of proteins عن طريق تخليق البروتينات.The DNA inherited المتوارث by an organism leads to يؤدي الى specific traits مجموعة من الخصائص by the synthesis of proteins عن طريق تخليق البروتينات. Proteins are the links روابط between genotype and phenotype.Proteins are the links روابط between genotype and phenotype.

3 Triplets of nucleotide bases are the smallest units اصغر الوحدات that can code for all the amino acid.Triplets مجموعات ثلاثية of nucleotide bases are the smallest units اصغر الوحدات that can code for all the amino acid. In the genetic code الشفرة الـﭽينية, nucleotide triplets ثلاثى specify amino acids During transcription, one DNA strand (the template strand) provides an RNA template.During transcription, one DNA strand (the template strand) provides an RNA template. The complementary RNA molecule is synthesized according to base-pairing rules, except that uracil is the complementary base to adenine.The complementary RNA molecule is synthesized according to base-pairing rules, except that uracil is the complementary base to adenine. During translation, blocks of three nucleotide bases (codons, () are decoded into a sequence of amino acids.During translation, blocks of three nucleotide bases (codons شفرة), (الشفرة عبارة عن مجموعة ثلاثية من القواعد) are decoded فك الشفرة into a sequence تتابع of amino acids.

4 The codon UUU codes for the amino acid phenylalanine.The codon UUU codes for the amino acid phenylalanine. The codon AUG not only codes for the amino acid methionine but also indicates the start of translation.The codon AUG not only codes for the amino acid methionine but also indicates the start of translation. The codon UAA, UAG, UGA are called stop codons for translationThe codon UAA, UAG, UGA are called stop codons for translation Fig. 17.4, Page 308

5 A)- The Transcription and Processing of mRNA RNA polymerase separates the DNA strands and bonds يربط the RNA nucleotides as they base-pair along the DNA template. Like DNA polymerases, RNA polymerases can add nucleotides only to the 3’ end. Specific sequences of nucleotides along the DNA mark where gene transcription begins and ends. –RNA polymerase attaches and initiates transcription at the promotor المُحَفـز, at the beginning of the transcription unit (gene). –The terminator منطقة النهاية ends the transcription.

6 Transcription can be separated into three stages: 1- initiation البدء 2- elongation الاستطالة, 3- termination الانتهاء. Fig. 17.6a, Page 309

7 1- initiation البدء RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at time until it reaches promoter.RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at time until it reaches promoter. 2- elongation الاستطالة The enzyme adds nucleotides to the 3’ end of the growing strand.The enzyme adds nucleotides to the 3’ end of the growing strand. 3- termination الانتهاء Transcription proceeds until after the RNA polymerase transcribes a terminator sequence in the DNA.Transcription proceeds until after the RNA polymerase transcribes a terminator sequence in the DNA. Fig. 17.6b, 309

8 Enzymes in the eukaryotic nucleus modify pre-mRNA before moving to the cytoplasm.Enzymes in the eukaryotic nucleus modify pre-mRNA before moving to the cytoplasm. At the 5’ end of the pre-mRNA molecule, a modified form of guanine is added, the 5’ cap which function as:At the 5’ end of the pre-mRNA molecule, a modified form of guanine is added, the 5’ cap which function as: 1)protect mRNA from hydrolytic enzymes. 1)protect mRNA from hydrolytic مُحلل enzymes. 2)a translation start point for ribosomes. At the 3’ end, an enzyme adds 50 to 250 adenine nucleotides, the poly(A) tail.At the 3’ end, an enzyme adds 50 to 250 adenine nucleotides, the poly(A) tail. The poly(A) tail facilitate the export of mRNA from the nucleus.The poly(A) tail facilitate the export of mRNA from the nucleus. Post transcription modification تغيرات ما بعد الانسلاخ of mRNA Fig. 17.8, Page 313

9 Translation occurs in three stages: 1- initiation 2- elongation 3- terminationTranslation occurs in three stages: 1- initiation البدء of translation 2- elongation الاستطالة البنائية of polypeptide chain 3- termination الإيقاف of translation at stop codon B)- The Synthesis of Protein (Translation)

10 G CT A A T G G TATAC C G C A T T A G C A T Bubble G C U A A U G 3 5 G A T C G C T A A T G G T A T A C C Ribosome Protein RNA transcription & translation RNApolumerase Promoter

11 Definitions  Codons ( الـشفرة الـﭽينية ) or triplet code: it is a block of three consecutive متتالى nucleotide bases that specify تحدد a particular amino acid.  Start codon: a codon that specifies the start of RNA translation.  Stop codon: a codon that specifies the end of RNA translation.  RNA polymerases: RNA transcription enzyme that first separates the DNA strands at the suitable point then start to add nucleotides to the 3’ end of the growing RNA polymer until completed.  Promotor المُحَفـز : a specific short sequence on DNA at which RNA polymerase attaches and initiates transcription at the beginning of the transcription unit.  Terminator منطقة النهاية: a specific short sequence on DNA at which RNA transcription ends (the end of the gene).