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A segment of DNA that codes for the synthesis of a protein is called a
gene.
The synthesis of protein in eukaryotic cells happens outside the nucleus.
But the DNA is inside the nucleus!
RNA carries the information from the
DNA to the ribosomes in the cytoplasm.
How is RNA different to DNA?
RNA is…
• normally single stranded
•Has a ribose sugar instead of deoxyribose
•Uracil (U) replaces Thymine (T) as a base
There are 3 types of RNA in a cell…
• messenger (mRNA)
Carries the code from genes to ribosomes
•transfer (tRNA)
Carries amino acids to ribosome
•ribosomal (rRNA)
Combines with protein to form ribosome
Protein synthesis is divided
into 2 processes.
•Transcription
(inside the nucleus
for eukaryotes)
•Translation
(outside the nucleus)
Information in DNA is copied into mRNA
Only the section of DNA that codes for
the selected gene is copied
Only one strand of the DNA is copied
This strand is called the
template stand
• RNA Polymerase unwinds and unzips DNA
• RNA Polymerase begins transcribing the
DNA at a specific point (promoter region)
• Complementary nucleotide chain is created
from template DNA strand in the 5’ to 3’
• RNA strand is identical to the non-coded DNA
EXCEPT FOR...
mRNA is different from DNA in that..
•Single stranded
•Thymine is replaced by uracil
•Sugar in the nucleotide is a ribose sugar instead of a deoxyribose sugar
NTP = nucleotriphosphate
ATP, GTP, UTP, CTP
used in protein synthesis
dNTP = nucleotriphosphate
dATP, dGTP, dTTP, dCTP
Complementary NTP are attached
to the template strand.
The two extra phosphate groups are
clipped off by the RNA polymerase
• Transcription is the Same process as Prokaryotes!
• But after mRNA is transcribed from DNA
in eukaryotic cells it has to be processed.
• Prokaryotes immediately begin
translating the mRNA.
Prokaryotes
No mRNA Processing
Eukaryotes:
mRNA Processing:
• intron/exon
• methyl cap
• poly-A tail
• Viral DNA injected into cells
• Cells evolved nucleases in cytoplasm that chomp up
any RNA or DNA out there
• Nucleases can’t get through the nuclear envelope so
DNA is safe
• mRNA sent out into the cytoplasm must be protected
– Methyl cap is a block
– Poly A tail is a fuse
• mRNA is still chomped up into NTP’s and recycled,
but the Poly A tail gives it some time
• Eukaryotic DNA is composed mostly of “non-coding
DNA” (or “junk DNA”)
– We’re still not entirely sure what it does
– Was probably inserted by different viruses over time
– The ultimate selfish gene just hitching a ride on a successful
group of genes…
• The introns are the sections of DNA not expressed, the
exons are the sections that are expressed (ex-ons are
ex-pressed, get it?)
• The introns are sniped out
• So now we’ve got some mRNA that codes for a protein
Spliceosome loops out introns and snips them out
Introns
exons
It then joins the exons together
Spliceosome
The mRNA is now ready to leave the nucleus for translation
RNA transcription in action
Animations
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Transcription
http://www.biostudio.com/d_%20Transcription.htm
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http://www.sumanasinc.com/webcontent/animations/content/lifecyclemrna.html
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Translation
http://www.sumanasinc.com/webcontent/animations/content/translation.html
http://www.sumanasinc.com/webcontent/animations/content/polyribosomes.html