1 / 13

Transcription

Transcription. Initiation. RNA polymerase binds to the promoter region, which is upstream of the gene that is being copied, and opens the double helix. Promoter region is high in A and T bases. Elongation. Begins building the single stranded mRNA in the 5’-3’ direction.

sienna
Télécharger la présentation

Transcription

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Transcription

  2. Initiation • RNA polymerase binds to the promoter region, which is upstream of the gene that is being copied, and opens the double helix. • Promoter region is high in A and T bases

  3. Elongation • Begins building the single stranded mRNA in the 5’-3’ direction. • No primer required. • One strand of the double stranded DNA acts as a template strand for RNA polymerase. • The strand not being used is the coding strand. • mRNA is a complement of the template strand, and an identical copy of the coding strand (with uracil).

  4. Termination • Occurs when RNA polymerase reaches a terminator sequence at the end of the gene. • mRNA is released from the template.

  5. Post-transcriptional Modifications • At this point, the mRNA (primary transcript) is not ready to leave the nucleus. A process known as capping and tailing must occur. • 5’ cap (7- methyl gunanosine) is added to the start of the ‘transcript’. • This protects the mRNA from digestion by nucleases and phosphatases as it exits the nucleus. • 3’ end gains approx. 200 adenine ribonucleotidescalled a poly-A tail by poly-A polymerase.

  6. Further Modifications • DNA genes consist of coding regions: EXONS and non-coding regions: INTRONS • If the introns are translated, the protein will not fold properly and will be useless for the cell. • Spliceosomes remove intronsand join the remaining exons together so that coding regions are continuous. • Introns remain in the nucleus and are recycled. • Now it is mRNA transcript which is ready to be TRANSLATED by the ribosome.

  7. Introns and Exons

  8. Translation • mRNA has now exited the nucleus and is in the cytoplasm. • Initiation: the 5’ cap of the mRNA is recognized by ribosomes • Ribosomes: consist of 2 subunits; a large subunit (60s) and a small subunit (40s)

  9. Elongation • The ribosome reads the mRNA 5’-3’ adding a new amino acid for every 3 nucleotides (codon). • Depending on where the ribosome starts reading, the reading frame will change. If the reading frame changes, the amino acids being coded for may also change.

  10. tRNA • The molecule that delivers the amino acid is the transfer RNA. • tRNA is a small, single stranded nucleic acid resembling a clover. • The anti-codon (a series of three bases) recognizes the codon of the mRNA. • The opposite arm carries the corresponding amino acid. • Ex: mRNA codon = UAUanticodon arm= AUA

  11. tRNA continued... • Only ONE amino acid is carried PER tRNA, therefore at least 20 tRNA’s are required (up to approx. 45). • With it’s amino acid tRNA is called aminoacyl-tRNA. • There are multiple codons for some amino acids: Serine: UCU, UCC, UCA, UCG. If the anticodon is: AGA, it can still bind to the codons UCC, UCA and/or UCG. • The third position (3’ on the mRNA and 5’ on the anticodon) can have non-standard base pairing: this is the ‘wobble hypothesis.’

  12. Elongation • First codon recognized by the ribosome is the START codon (AUG)- methionine. Two sites: A (accepter site) P (peptide site) • The tRNA that carries methionine first enters the P site. • The next tRNA enters the A site. • Once the amino acids have been bonded (peptide bond), the tRNA exits the P site, and the tRNA from the A site takes its place. • The ribosome then shifts over one codon, and the new tRNA enters the A site.

  13. Termination • The ribosome reaches a STOP codon • UGA, UAG, or UAA. • Protein called release factor aids in the release of the polypeptide chain from the ribosome. • The two subunits of the ribosome fall off of the mRNA. • Modifications to the polypeptide chain occur: Glycosylation- addition of a sugar group Phosphorylation- addition of a phosphate group

More Related