GENE EXPRESSION.pptxhhhhhhhhhhhhhhhhhhhh

nill

1. GENE EXPRESSION Dr. Emmanuel Olal ( MBChB ) Makeni School of Clinical Sciences 21/Nov/2017

2. QUESTIONS What are two major differences between transcription in prokaryotic and eukaryotic cells? RNA polymerase and DNA polymerase enzymes catalyze the “same” reaction, but there are some distinct differences in what is required to make them begin catalysis and end catalysis. What are these differences ? Which is more accurate, DNA replication or RNA transcription? Explain the proteins and mechanisms involved in the initiation of transcription What determines how many copies of a transcript (mRNA) are made? How are elongation and termination of the transcript regulated?

3. Overview Gene expression: is the transformation of the information in the DNA into functional molecules Genetic code: is the relationship between the base sequence in the DNA or mRNA and that of the amino acids; the genetic code is non overlapping In humans, the nucleus of each cell contains 3 × 10 9  base pairs of DNA distributed over 23 pairs of chromosomes, This is known collectively as the human genome. The human genome contains around 30 000 genes, each of which codes for one protein.

4. Types of RNA 1. Ribosomal RNA ( rRNA ) Used as a structural component of a ribosome 2. Transfer RNA ( tRNA ) Carry activated amino acids to the ribosomes during protein synthesis 3. Messenger RNA (mRNA) Carries information to the ribosome on the amino acid sequence of protein during protein synthesis 4. Heterogeneous nuclear RNA ( hnRNA , pre-mRNA) Represents precursors of mRNA during its posttranscriptional processing 5. Small nuclear RNA ( snRNA ) Participates in the splicing (removal of introns) mRNA 6. Ribozymes RNA molecules with enzymatic activity

5. RNA Polymerases Prokaryotes Eukaryotes All RNA are synthesized by one RNA polymerase ( α 2 ββ ’) RNA 1: rRNA (nucleolus) except 5S rRNA RNA 2: hnRNA /mRNA and some snRNA RNA 3: tRNA , 5S rRNA Requires sigma ( σ ) to initiate at a promoter No sigma, but transcription factors (TFIID) bind before RNA polymerase Sometimes requires rho ( ρ ) to terminate No rho required Inhibited by rifampicin and Actinomycin D RNAP 2 inhibited by α - amantin (mushrooms), Actinomycin D

6. Concepts and terminology RNA is synthesized by DNA-dependent RNA polymerase Transcription begins when RNA polymerase locates promoter genes on the DNA template RNA Polymerase moves along the template in the 3’-5’ direction as it synthesizes RNA in 5’-3’ direction using NTPs (ATP, GTP, CTP, UTP) The coding strand ( antitemplate ) is not used during transcription; has identical sequence to RNA except Thymine is substituted by Uracil

7. The first RNA base transcribed is defined as +1 base of that gene region -to the left (5’ or upstream) the bases are -1, -2, -3, etc. -to the right (3’, or downstream) of the +base are +2,+3 etc. Transcription ends when the RNA polymerase reaches the termination signal

8. Transcription of Prokaryotic mRNA transcription begins with recognition and binding to the promoter region with the aid of sigma factor Consensus sequence of promoter region, -10 sequence also called Pribnow box or TATA box , and -35 sequence RNA polymerase separates the DNA double strand Transcription begins at +1 base pair, sigma factor is release upon initiation of transcription Core polymerase continues moving in the 3’-5’ direction, synthesizing in the 5’-3’ direction Transcription continues until polymerase reaches the termination signal, transcription stops and the new mRNA molecule is released

9. Transcription terminators in prokaryotic genes Prokaryotic genes commonly have two types of transcription terminators: Rho-independent termination: RNA dissociates when the newly formed molecule folds back in the GC-rich region into a hairpin loop, that is followed by 6-8 U residues Rho-dependent termination: rho( ρ ) factor binds to the newly formed RNA, it then dislodges the RNA polymerase paused at the termination site from the 3’ end of the RNA

10. In bacteria transcription and translation occurs simultaneously because their mRNA needs no further processing Ribosomes binds to the Shine- Dalgarno sequence of the UTR mRNA region Protein synthesis begin at an AUG codon, and continues to the end Ribosomes translate mRNA in the 5’-3’ direction: amino terminus to carboxyl terminus Some bacterial operons have polycistronic messages which are translated separately

11. Eukaryotic mRNA transcription Transcription factors aid the RNA Polymerase II to recognize and bind to the promoter regions TATA box ( Hogness ) and CAAT box RNA Polymerase II separates a short region of the DNA and transcription begins DNA template strand is read in 3’-5’ direction and pre-mRNA is synthesized in 5’-3’ direction transcribing both introns and exons RNA Polymerase II transcription ends when the termination signal is reached

12. A eukaryotic transcription unit

13. Processing of pre-mRNA the primary transcript undergoes further processing into mature mRNA in the nucleus The pre-mRNA formed contains introns which are not required for protein synthesis . The pre-mRNA is chopped up to remove the introns and create messenger RNA (mRNA) in a process called RNA splicing Splicing is accomplished by spliceosomes also known as snRNA or snurp Mature mRNA is transported to the cytoplasm for translation

14. Other RNAs RIBOSOMAL RNA ( rRNA ) Eukaryotic rRNA is transcribed in the nucleolus by RNA polymerase I They are transcribed in different subunits which combine with ribosomal proteins The eukaryotic subunits are 60S and 40S, these join into 80S

15. 2. Transfer RNA ( tRNA ) There are different specific tRNAs Each tRNA carries only one type of activated amino acid for making proteins during translation tRNAs are transcribed for by RNA polymerase III

16. Exercise: group 1 Write short notes on the following Mutations (definition, types, consequences) Translation (protein synthesis: initiation, elongation, termination)

17. Bridge Pharmacology