RNA, Transcription, and Translation

1. RNA, Transcription, and Translation How we get from DNA to proteins

2. RNA (Ribonucleic Acid) • RNA is very similar to DNA, but with three main differences: • The backbone of RNA uses a ribose sugar molecule in place of the deoxyribose sugar molecule that DNA uses • RNA is typically single stranded • RNA uses the nucleotide uracil in place of thymine

3. RNA • There are three main types of RNA • messenger RNA (mRNA) – this carries messages contained in the DNA outside the nucleus to the ribosomes • ribosomal RNA (rRNA) – along with specific proteins it makes up the ribosomes, which make proteins • transfer RNA (tRNA) – carries amino acids to the ribosome and transfers them to the protein that is being built

4. Transcription • Transcription is the process which copies a piece of a strand of DNA into a complimentary piece of RNA to make messenger RNA • Just like in replication transcription requires enzymes for the process to occur • The main enzyme used is RNA polymerase. • RNA polymerase separates the two strands of DNA, uses one strand as a template to assemble nucleotides into a piece of RNA • This is very similar to replication, but instead of matching an A with a T, RNA uses a U (uracil) to match up with A

5. Transcription • So, what would the RNA transcribed from this piece of DNA say? G G A T C CC G A A T G T C 3’ 5’

6. Transcription • How do you think RNA polymerase knows which piece of DNA to copy?

7. Transcription • Your DNA contains specific sequences of DNA know as promoters • Promoters are signals in the DNA that indicate to enzymes where to bind and begin making RNA • Similar signals tell the RNA polymerase that the RNA is complete and transcription stops. • You might think that transcription is now complete, but there is still some editing that needs to be done

8. RNA Editing • After transcription we are left with pre-mRNA because it still needs to be cut. • Your cells take that large strand of RNA and cut out pieces called introns • The pieces that are left, exons, are spliced back together to make the final copy of mRNA • Then a cap and tail are added.

9. Translation • Translation occurs when ribosomes use mRNA as instructions to attach amino acids together to make a protein. • The mRNA strand exits the nucleus and enters the endoplasmic reticulum where it can locate and bind to a ribosome • Every three nucleotides in the mRNA codes for an amino acid, which is brought to the ribosome by tRNA • Every three nucleotides are called a codon

10. Translation • Scientists have figured out how to read this code. • We use an amino acid codon wheel to easily figure out what each codon means. • AUG means start as well as methionine

11. Translation • After the mRNA binds to the ribosome it moves into different sites. • The first site matches the anti-codon on the tRNA with the codon on the mRNA • Each tRNA molecule has three nucleotides at the bottom of it called the anti-codon which will form a compliment to some codon along the mRNA. • Attached to the top of the tRNA molecule is the amino acid matching the mRNA codon

12. Translation • When the tRNA comes into the first binding site it binds and recognizes the codon on the mRNA in that site • Then the ribosome moves along the mRNA attaching the amino acid to the previous one creating a polypeptide chain and the now empty tRNA is released • This continues elongating the polypeptide chain until the ribosome reaches a stop codon • An enzyme binds to the mRNA in place of tRNA and tells the two parts of the ribosome to break apart releasing the mRNA and the polypeptide chain

14. Protein Synthesis • After the polypeptide chain is released it must fold up to become a protein • The interactions create different bonds between amino acids that give a protein it’s specific shape • A protein’s shape is very important because that determine it’s function • If a protein does not fold up properly it will not be able to do its job

15. Transcription and Translation