CHAPTER 17
FROM GENE TO PROTEIN
Gene Expression ‫التعبير الجيني‬
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Gene expression ‫التعبير الجيني‬
• Formation of protein from DNA ‫تكوين البروتينات من‬in two steps:• 1. transcription ‫( االنسالخ‬mRNA synthesis ‫ تخليق‬from DNA in the nucleus)
• 2. translation ‫( الترجمة‬protein synthesis at ribosomes in the cytoplasm).
• Because bacteria lack nuclei ‫ال تحتوي على نواة‬, transcription and
translation are coupled ‫مـُتالزمان‬.
•
The DNA inherited ‫ المتوارث‬by an organism
leads to ‫ يؤدي الى‬specific traits ‫مجموعة من‬
‫ الخصائص‬by the synthesis of proteins ‫عن طريق‬
‫تخليق البروتينات‬.
•
Proteins are the links ‫ روابط‬between
genotype and phenotype.
DNA
Transcription
mRNA
Translation
Protein2
In the genetic code ‫الشفرة الـﭽينية‬, nucleotide triplets ‫ثالثى‬
specify amino acids
•
Triplets ‫ مجموعات ثالثية‬of nucleotide bases are the smallest units ‫ اصغر الوحدات‬that
can code for all the amino acid.
•
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 ‫)شفرة‬,
(‫ )الشفرة عبارة عن مجموعة ثالثية من القواعد‬are
decoded ‫ فك الشفرة‬into a sequence ‫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 ‫ يربط‬the 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
‫ال ُم َحفـز‬, at the beginning of the transcription unit (gene).
– The terminator ‫ منطقة النهاية‬ends the transcription.
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• Transcription
can be separated
into three stages:
1- initiation ‫البدء‬
2- elongation ‫االستطالة‬,
3- termination ‫االنتهاء‬.
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Fig. 17.6a, Page 309
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 the
3’ end of the
growing strand.
3- termination ‫االنتهاء‬
• Transcription proceeds
until after the RNA
polymerase transcribes
a terminator sequence
in the DNA.
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Fig. 17.6b, 309
Post transcription modification ‫ تغيرات ما بعد االنسالخ‬of mRNA
•
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)
2)
protect mRNA from hydrolytic ‫ ُمحلل‬enzymes.
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.
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Fig. 17.8, Page 313
B)- The Synthesis of Protein
(Translation)
•
Translation occurs in three stages:
1- initiation ‫ البدء‬of translation
2- elongation ‫ االستطالة البنائية‬of polypeptide chain
3- termination ‫ اإليقاف‬of translation at stop codon
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RNA transcription & translation
Bubble
5
3
C
G
T
T
A
C
G
A
C
T
G
A
C
A TRNA UT
G
A
A
C
CT
UG
G
A
C
A
C
T
T
G
A
C
T
G
T
G
A
G
A
C
polumerase
A
A
Promoter
A
T
T
G
Ribosome
Protein
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Definitions
 Codons (‫ )الـشفرة الـﭽينية‬or triplet code: it is a block of three consecutive ‫متتالى‬
nucleotide bases that specify ‫ تحدد‬a 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 ‫ال ُم َحفـز‬: 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).
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