GCD3033:Cell Biology. Transcription

Transcription

1 Transcription

2 Transcription: DNA to RNA A) production of complementary strand of DNA B) RNA types C) transcription start/stop signals D) Initiation of eukaryotic gene expression E) transcription factors II) RNA processing A) intron/exons & splicing B) nuclear export C) mrna degradation

3 I) DNA Replication NUCLEUS CYTOPLASM protein (inactive) 1 Steps #2 & #3 TODAY DNA 2 1 RNA 3 mrna 4 protein 5 1) DNA replication 2) transcription 3) RNA processing (e.g. capping, splicing, etc) 4) translation 5) post-translational modification (e.g. phosphoylation, ubiquitination, etc) protein (active)

4 DNA directs the synthesis of proteins

5 Different Transcription Rates: A cell can express different genes at different rates.

6 Structural Differences: The chemical structure of RNA differs slightly from that of DNA. 1 2

7 Structural Differences: Uracil forms a base pair with adenine.

8 Secondary Structure: RNA molecules can form intramolecular base pairs and fold into specific structures.

9 Complementary Strand: Transcription of a gene produces an RNA complementary to one strand of DNA.

10 RNA Polymerase: DNA is transcribed into RNA by the enzyme RNA polymerase.

11 Visualization: Transcription can be visualized in the electron microscope.

12 RNA type Function mrna messenger RNA codes for proteins rrna ribosomal RNA form the ribosome core and catalyze protein synthesis trna transfer RNA serve as adaptors between mrna and amino acids mirna micro RNA regulate gene expression other non-coding used in RNA splicing, gene regulation, telomere maintenance, etc

13 Transcription Start/Stop: Signals in the nucleotide sequence of a gene tell bacterial RNA polymerase where to start and stop transcription.

14 DNA Recognition: Bacterial promoters and terminators have specific nucleotide sequences that are recognized by RNA polymerase.

15 Both DNA Stands Can be Used: On an individual chromosome, some genes are transcribe using one DNA strand as a template, and others are transcribed from the other DNA strand.

16 Polymerase type Function RNA polymerase I most rrna genes* RNA polymerase II all protein-coding genes, mirna & other noncoding genes RNA polymerase III trna genes, 5S rrna gene*, & many other small RNAs

17 Transcription Factors: To begin transcription, eukaryotic RNA polymerase II requires a set of general transcription factors.

18 Transcription Complex Formation

19 Sequence-Specific Recognition: TATA-binding protein (TBP) binds to the TATA box (indicated by letters) and bends the DNA double helix.

20 II) RNA processing Nuclear Export: Before they can be translated, mrna molecules made in the nucleus must be exported to the cytosol via pores in the nuclear envelope RNA processing occurs in the nucleus 1) RNA capping 2) splicing (pre-mrna to mrna) 3) poly-adenylation

21 II) RNA processing RNA Polymerase Modification: Phosphorylation of the tail of RNA polymerase II allows RNA-processing proteins to assemble there.

22 II) RNA processing RNA modification: Eukaryotic pre-mrna molecules are modified by capping and polyadenylation. Pre-RNA Capping functions: 1. Regulate nuclear export 2. Prevent RNA degradation 3. Facilitate splicing 4. Promote translation mrna polyadenylation functions: 1. protects the mrna from degradation 2. aids in transcription termination 3. facilitates nuclear export 4. helps translation

23 II) RNA processing Genome Organization: Eukaryotic and bacterial genes are organized differently.

24 II) RNA processing Intron/Exons: Most protein-coding human genes are broken into multiple exons and introns.

25 II) RNA processing Sequence-Specific Recognition: Special nucleotide sequences in a pre-mrna transcript signal the beginning and the end of an intron. Y = C or T R = A or G N = any base

26 II) RNA processing An intron in a pre-rna molecule forms a branched structure (lariat) during RNA splicing. Splicing is carried out by a collection of RNA protein complexes called snrnps.

27 Splicing

28 II) RNA processing Alternative Splicing: Some pre-mrnas undergo alternative RNA splicing to produce various mrnas and proteins from the same gene.

29 II) RNA processing Alternative Splicing: In action

30 II) RNA processing mrna Nuclear Export: A specialized set of RNA-binding proteins signals that a mature mrna is ready for export to the cytosol.

31 II) RNA processing Prokaryotes and eukaryotes handle their RNA transcripts differently.