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

The two extra phosphate groups are clipped off by the RNA polymerase Complementary NTP are attached to the template strand.

• 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

• • • Transcription http://www.biostudio.com/d_%20Transcription.htm

http://www.sumanasinc.com/webcontent/animations/content/lifecyclemrna.html

• • • Translation http://www.sumanasinc.com/webcontent/animations/content/translation.html

http://www.sumanasinc.com/webcontent/animations/content/polyribosomes.html