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Gene Expression الجيني الجيني Gene Expressionالتعبير التعبير

CHAPTER 17 FROM GENE TO PROTEIN

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Gene expression الجيني التعبير• Formation of protein from DNA من البروتينات -:in two steps تكوين

• 1. transcription االنسالخ (mRNA synthesis تخليق from DNA in the

nucleus)

• 2. translation الترجمة (protein synthesis at ribosomes in the

cytoplasm).• Because bacteria lack nuclei نواة على تحتوي transcription and ,الtranslation are coupled مُـتالزمان.

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mRNADNATranscription

ProteinTranslation

• The DNA inherited المتوارث by an

organism leads to الى specific traits يؤدي

الخصائص من by the synthesis of مجموعة

proteins البروتينات تخليق طريق .عن

• Proteins are the links روابط between

genotype and phenotype.

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• Triplets ثالثية اصغر of nucleotide bases are the smallest units مجموعات.that can code for all the amino acid الوحدات

In the genetic code الـچينية nucleotide triplets ,الشفرةspecify amino acids ثالثى

• 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.

• During translation, blocks of three nucleotide bases (codons ,(شفرةمن ) ثالثية مجموعة عن عبارة الشفرةالشفرة are decoded (القواعد into a فكsequence تتابع of amino acids.

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• 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 UAA, UAG, UGA are called stop codons for translation

Fig. 17.4, Page 308

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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 .at the beginning of the transcription unit (gene) ,الم*َح)فـز

– The terminator النهاية .ends the transcription منطقة

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• Transcriptioncan be separatedinto three stages:1- initiation البدء 2- elongation االستطالة, 3- termination االنتهاء.

Fig. 17.6a, Page 309

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1- initiation البدء • RNA polymerase moves along the DNA, it untwists the double

helix, 10 to 20 bases at time until it reaches promoter.

2- elongation االستطالة • The enzyme adds

nucleotides to the3’ end of thegrowing strand.

3- termination االنتهاء • Transcription proceeds

until after the RNA polymerase transcribes a terminator sequence in the DNA.

Fig. 17.6b, 309

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• 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:

1) protect mRNA from hydrolytic م*َحلل enzymes.

2) a translation start point for ribosomes.

• 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.

Post transcription modification االنسالخ بعد ما of تغيراتmRNA

Fig. 17.8, Page 313

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• Translation occurs in three stages:

1- initiation البدء of translation

2- elongation البنائية of polypeptide chain االستطالة

3- termination اإليقاف of translation at stop codon

B)- The Synthesis of Protein (Translation)

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G C TA AT G

GT ATA CC

G

C

A

T

T

A

G

C

A

T

Bubble

G C UA AU G

3 5

G A

TC

G C TA AT G

GT ATA CC

Ribosome

Protein

RNA transcription & translation

RNA polumerase

Promoter

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Definitions

Codons ( الـچينية or triplet code: it is a block of three consecutive (الـشفرة.a particular amino acid تَحدد nucleotide bases that specify متتالى

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 الم*َح)فـز: a specific short sequence on DNA at which RNA polymerase attaches and initiates transcription at the beginning of the transcription unit.

Terminator النهاية a specific short sequence on DNA at which RNA :منطقةtranscription ends (the end of the gene).