transcripts from animals

1. Gene Expression

2. Eukaryotic Gene Expression
- Transcription
- RNA Splicing
- Capping 5’ , poly A tails at 3’
- Regulation

3. Molecular Control of Transcription in Eukaryotes
The transcription of eukaryotic genes is regulated
by interactions between proteins and DNA
sequences within or near the genes and some
times from distance.

4. Cis–acting factors (sequences)
Promoters are DNA sequences upstream of
transcripts that initiate transcription.
Promoter attracts RNA polymerase to the
transcription start site.

5. Enhancers and Silencers
• Enhancers stimulate transcription.
• Silencers inhibit transcription.
• Both are:
1- Orientation independent.
2- Position independent.
3- Can work at a distance from promoter.
4- Can bind to regulated transcription factors.
5- Position upstream or downstream sequences.
6- find in certain genes.

6. Structure of a typical eukaryotic gene, with its core promoter
and proximal control region.
control elements

7. Promoters
5’
Promoter 3’
7

8. Trans-acting factors (TAFs)
The pre-initiation complex (PIC): is a large complex of
proteins that is necessary for the transcription of protein-
coding genes in eukaryotic.
PIC function: helps position RNA polymerase II over gene
transcription start sites and denature the DNA.

9. Models for Gene Expression

11. Post-transcriptional Regulation
• Control of gene expression regulated by the
control of transcription initiation.
• But gene expression can be controlled after
transcription, with mechanisms such as:
– RNA editing.
– Alternative splicing.
– mRNA stabilization & degradation.
– Protein stabilization & degradation.

12. RNA Editing
• Definition
• RNA editing can be broadly defined as any site-specific
alteration in RNA sequence during or after transcription,
excluding changes due to processes such as RNA
splicing.
• RNA editing: is a process that changes the identity of an
RNA bases after it has been transcribed from a DNA
sequence.

13. RNA Editing | Mechanisms
RNA editing mechanisms:
Insertion or Deletion:
During Transcription Nucleotide.
Post-transcriptional Nucleotide.
Conversion or substitution nucleotide editing such as:
Editing (A-G, most prevalent in human)
Editing (C-U)
Editing (U-C)

14. RNA Editing | Functions

15. • RNA editing creates mature mRNA that are not
truly encoded by the genome.
For example
Apolipoprotein B exists in 2 isoforms;
1- Isoform one produced by editing the mRNA to create a
stop codon (CAA → UAA) and translated to create
Apolipoprotein B-48 in intestine tissue.
2- Isoform two produced by unedited mRNA to create
Apolipoprotein B-100 in liver tissue.

16. Post-transcriptional Regulation
(Alternative splicing)
• Introns are spliced out of pre-mRNAs to produce the mature
mRNA that is translated.
• Splicing:-removing introns-spliceosomes.
• Alternative splicing recognizes different splice sites
included some exons in different tissue types.
• The mature mRNAs in each tissue possess different exons,
resulting in different polypeptide products from the same
gene.
• Alternate splicing of transcripts makes it possible for a single
gene to encode several polypeptides.
• Alternate splicing mechanism generate protein diversity.

17. Post-transcriptional Regulation (Alternative splicing)