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

unwindsunzips RNA Polymerase unwinds and unzips DNA Complementary nucleotide chain is created from template DNA strand in the 5’ to 3’ RNA Polymerase begins transcribing the DNA at a specific point (promoter region) 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

Same process Transcription is the Same process as Prokaryotes! But after mRNA is transcribed from DNA in eukaryotic cells it has to be processed. translating Prokaryotes immediately begin translating the mRNA.

Eukaryotes: mRNA Processing: intron/exon methyl cap poly-A tail Prokaryotes No mRNA Processing

Viral DNA injected into cells nucleasesCells 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

“non-coding DNA”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… introns exonsThe 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 Spliceosome Introns exons It then joins the exons together The mRNA is now ready to leave the nucleus for translation

RNA transcription in action

Animations Transcription Translation