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Transcription: Synthesizing RNA from DNA

Transcription: Synthesizing RNA from DNA. 6. 2. Transcription is the first step in gene expression and involves the production of an RNA molecule from a DNA template. Recall: The Central Dogma. DNA vs. RNA. Different RNA & their function (p.252).

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Transcription: Synthesizing RNA from DNA

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  1. Transcription: Synthesizing RNA from DNA 6. 2 Transcription is the first step in gene expression and involves the production of an RNA molecule from a DNA template.

  2. Recall: The Central Dogma

  3. DNA vs. RNA

  4. Different RNA & their function (p.252) There are different RNA molecules, which are classified according to the functions they perform.

  5. The Molecular Events of Transcription • As in the replication of DNA, there are three defined stages in transcription • Initiation • Elongation • Termination Transcription in prokaryotes and eukaryotes is similar. The main difference is that there are more proteins involved in eukaryotic transcription

  6. Stage 1: Initiation - The correct transcription start site is selected and the transcription machinery, composed of a large protein-DNA complex, is assembled For each gene, only one strand in a double stranded DNA is transcribed: • anti-sense strand or template strand The other strand that is not transcribed: - sense strand or coding strand -This strand has the same sequence as the mRNA that is produced, with Tinstead of U.

  7. RNA polymerases-a group of enzymes that catalyze the synthesis of RNA. Transcription begins when an RNA polymerase complex binds to a promoter region on the DNA. This region has a specific sequence of nucleotides that allows the RNA polymerase to bind to the correct strand in the correct orientation.   Once the RNA polymerase complex is bound to the DNA, it unwinds and opens a section of the double helix.

  8. Stage 2: Elongation mRNA is synthesized using template 5’-3’ direction Uracil (U) compliments adenine (A) RNA polymerase synthesizes mRNA along DNA Termination sequence ends

  9. The synthesis of many mRNA molecules can occur at one time.

  10. Stage 3: Termination • Specific nucleotide sequences in the DNA template serve as a signal to stop transcription • RNA synthesis ends • RNA polymerase and mRNA detach

  11. Transcription Animation http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf

  12. In prokaryotes, transcription and translation can occur simultaneously. In eukaryotes, mRNA must undergo modifications before it crosses the nuclear membrane to the cytoplasm. Once the modified mRNA enters the cytoplasm, it can undergo translation. There are three modifications that convert precursor mRNA (pre-mRNA) to mature mRNA.

  13. mRNA Modifications in Eukaryotes • Prokaryotes: mRNA is immediately ready for protein synthesis • Eukaryotes: mRNA requires some modifications • Convert precursor mRNA (pre-mRNA) to mature mRNA • There are 3 modifications…

  14. Modifications to Eukaryotic mRNA: 1)Addition of a 5′ cap of modified G nucleotides • The cap is recognized by protein synthesis machinery. 2)Addition of a 3′ poly-A tail • The tail is a series of A nucleotides that makes the mRNA more stable in the cytoplasm.

  15. mRNA Modifications in Eukaryotes All eukaryotic mRNAs undergo modification on their ends. (A) A derivative of guanine is added to the 5′ end. (B) A series of nucleotides containing the base adenine are added to the 3′ end.

  16. 3) Removal of Introns Introns (non-coding regions) are removed and exons (coding regions) are joined together. This process, called splicing, is performed by snRNA and snRNP proteins, which form a large spliceosome complex. In some cases, only certain exons are used to form a mature RNA, allowing for one gene to code for more than one protein.

  17. Removal of introns • Exons coding regions • Introns  non-coding regions • Introns removed via splicing • Spliceosomes (snRNA and protein) cut introns out of mRNA and join remaining coding regions

  18. Splicing Animation http://www.sumanasinc.com/webcontent/animations/content/mRNAsplicing.html

  19. mRNA is now ready to be translated by a ribosome into a protein • It is now called mRNA transcript • No quality control to check for errors • More errors in transcription than replication • b/c a single gene is transcribe repeatedly errors are not as detrimental • Proteins w/ error can be degraded

  20. Learning Expectations... • DNA vs. RNA • 3 steps in transcription • Initiation • Elongation • termination • Modifications to mRna • 5’ cap • poly-A tail • splicing

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