Transcription Eukaryotic Cells

Transcription

1 Transcription Eukaryotic Cells Packet #20 1

2 Introduction Transcription is the process in which genetic information, stored in a strand of DNA (gene), is copied into a strand of RNA. Protein-encoding genes (structural genes) carry the information FOR the amino acid sequence of a polypeptide. When a protein-encoding gene is transcribed, the product produced is RNA. There are different types of RNA. 2

3 Introduction II 3 4/11/2016

4 RNA 4

5 RNA Reminder Ribonucleic Acid Composed of RNA nucleotides Phosphate Sugar Ribose Nitrogenous Bases Purines (2 rings) Adenine Guanine Pyrimidines (1 ring) Cytosine Uracil 5

6 Types of RNA Messenger RNA Contains information that specifies the amino acid sequence of polypeptide chains Serves as the template for protein synthesis 6 4/11/2016 2:47 PM

7 Types of RNA II Transfer RNA Class of RNA molecules that transport amino acids to ribosomes for incorporation into a polypeptide undergoing synthesis. Transporter 7

8 Types of RNA III Ribosomal RNA RNA that make up ribosomes. Provides a mechanism for decoding mrna into amino acids and to interact with the trna during translation by providing peptidyl transferase activity. 8

9 Types of Eukaryotic RNA 9

10 Genes & Transcription 10

11 Genes The Review Where are genes located? On the chromosome What is the name of the location on the chromosome where genes are located? The Locus What are genes composed of? DNA 11

12 Genes Review II The beginning and the ending of genes play an important role in the regulation of RNA synthesis. Regulatory Region Beginning Coding Region Middle Terminator Ending Area of gene where mrna is transcribed from.

13 Eukaryotic Genes III Eukaryotic Gene Regulatory Region Promoter Region Coding Region Terminator bp b/f transcription start site TATA box (or Goldberg- Hogness Box) ) Transcriptional Start Site Enhancers 25 base pairs from Transcription start site Silencers

14 Regulatory Region Receives signals from other genes or cellular environment The binding site for regulatory proteins. Regulatory proteins influence the rate of transcription. This region lies 100 to 50 base pairs before the transcription start site. The regulatory region affects the ability of RNA polymerase to recognize the promoter region (core promoter). 14

15 Eukaryotes Transcription Factors 15

16 Transcription Factors Term loosely applied to any protein required to initiate or regulate transcription in eukaryotes. Control transcription There are two classes of transcription factors General transcription factors. Gene regulatory proteins Regulate transcription General Transcription Factors Transcription Factors Gene Regulatory Proteins 16

17 General Transcription Factors In eukaryotes, general transcription factors will bind to RNA polymerase to form a pre-initiation complex that is necessary to begin transcription. 17

18 General Transcription Factors II General Transcription Factors TFIID TFIIB TFIIE TFIIH 18

19 Gene Regulatory Proteins I Gene Regulatory Proteins Activators Repressors Basal Transcription Factors Coactivators There are four* types of gene regulatory proteins.

20 Gene Regulatory Proteins II Activators Proteins bind to genes at sites known as enhancers and speed up the rate of transcription Repressors Proteins that bind to selected sets of genes at sites known as silencers and slow down transcription 20

21 Gene Regulatory Proteins III Basal Transcription Factors* In response to injunctions (command) from activators, these factors position RNA polymerase at the start of transcription and initiate the transcription process 21

22 Gene Regulatory Proteins IV Coactivators{Mediator} Adapter molecules that integrate signals from activators and perhaps repressors. Allows communication between activators, possibly repressors, and basal transcription factors. 22

23 Eukaryotes RNA Polymerase 23

24 RNA Polymerase Only enzyme used during transcription. However, in eukaryotic organisms, there are three variations of the enzyme depending on the type of RNA being produced Eukaryotes Classes of RNA Polymerase RNA Polymerase I Transcribes rrna genes RNA Polymerase II RNA Polymerase I RNA Polymerase II RNA Polymerase III Transcribes protein coding (mrna) genes RNA Polymerase III rrna mrna trna Transcribes other functional RNA genes trna 24

25 Transcription Step by Step 25

26 Transcription at a Glance There are three general stages of transcription. Initiation Elongation Termination Initiation Elongation Termination

27 Initiation

28 Transcription {Eukaryotes} Initiation I RNA Polymerase II & General Transcription Factors TFIID binds at the TATA box TFIID includes a TATA binding protein (TBP) Has a role in the recognition of the core promoter. After binding, TFIID associates with TFIIB. TFIIB is attracted (drawn in ) to TFIID and they bind together at the TATA box. 28

29 Transcription {Eukaryotes} Initiation II RNA Polymerase II & General Transcription Factors TFIIB Promotes the binding of (attracts/draws in) RNA polymerase II Promotes the binding of (attracts/draws in) TFIIF RNA polymerase II and TFIIF bind to complex of TFIID & TFIIB 29

30 Transcription {Eukaryotes} Initiation III RNA Polymerase II & General Transcription Factors TFIIE and TFIIH bind to the complex This completes the formation of the preinitiation complex. TFIIH Contain helicases that break hydrogen bonds 30

31 Transcription {Eukaryotes} Initiation IV RNA Polymerase II & General Transcription Factors TFIIH breaks the hydrogen bonds of the DNA molecule TFIIH phosphorylates a domain (part) of RNA polymerase II called CTD (carboxyl tail domain). TFIIB is released from complex. TFIIH initiates the breaking apart of the complex. 31

32 Transcription {Eukaryotes} Initiation V RNA Polymerase II & General Transcription Factors TFIIB, TFIIE and TFIIH dissociate (break apart). RNA polymerase II proceeds forward with transcription starting at the transcription start site. 32

33 Elongation

34 Transcription {Eukaryotes} II Elongation I Elongation occurs similar to what is seen within prokaryotes. RNA Polymerase works downstream adding RNA nucleotides to the 3 end of the newly formed RNA strand The new RNA strand grows in the 5 to 3 direction. 34

35 Transcription {Eukaryotes} III Elongation II As the first bit of mrna emerges from the RNA polymerase, guanyltransferase, guided by the CTD, attaches a 7- methylguanosine triphosphate cap to the 5' end. 5 cap At the 3 end, adenines are attached to the cut end, forming a poly-a tail. 35

36 Termination

37 Transcription Termination {Eukaryotes} After the poly A tail is completed, RNA polymerase typically terminates at about base pairs downstream. 37

38 Transcription Termination {Eukaryotes} There are two proposed mechanisms as to how termination is completed. 1. RNA polymerase is destabilized after completion of the poly A tail causing it to dissociate from the DNA strand. 2. An exonuclease physically removes the RNA polymerase II from the DNA. 38

39 Transcription Termination {Eukaryotes} Once RNA polymerase has been removed from the DNA strand, the newly made RNA is released If the RNA synthesized is mrna, the new RNA strand is called premrna Why? Because the premrna needs to be groomed for the moment of translation. 39

40 Conversion of pre-mrna to mrna

41 Sections of pre-mrna Contains introns and exons. Introns Regions of the RNA strand that must be REMOVED, via splicing, and will not be used during Translation Exons Regions of RNA that WILL be used and translated into amino acids during translation. 41

42 Splicing Eukaryotic Cells Splicing, occurring in the nucleus, is the removal of introns and the joining together of exons. One way of completing splicing is via the use of the enzyme called spliceosome. Removes the introns via cut & paste After splicing, the pre-mrna will be officially known as mrna mrna leaves the nucleus, through the nuclear pores, and enters the cytoplasm in preparation for translation. 42

43 mrna Transcription & Translation 43

44 Review