Nucleic acids 3/4 -Gene Organisation & Transcription Flashcards
Describe what is meant by “Transcription”
The process in which nucleotide information in the DNA is copied into RNA
What protiens are involved with transcription (2)
- Gene transcription is carried out by enzymes called “RNA Polymerases”
- regulatory proteins called “Transcription Factors”
List the major functional classes of RNA
Functional classes of RNA
Transfer
Ribosomal
Messenger
Name the three Eukaryotic RNA Polymerases
RNA Polymerase I -Transcribes rRNA genes
RNA Polymerase II - Transcribes genes encoding proteins into mRNA
RNA Polymerase III- Transcribes tRNA and 5S RNA genes
Describe what is meant by a “Transcription factor”
The rate/level of transcription from a given gene is regulated by the activity of DNA binding proteins, or
Transcription Factors.
These can be transcriptional activators, or transcriptional repressors,
Describe the process involved with the basal transcription complex
Step 1
TF IID binds to TATA
• Partially unwinds the DNA helix, widening the minor grove to allow extensive contact with bases within the
DNA.
• This unwinding is asymmetric with respect to the TBP-TATA complex,thereby assuring transcription is
unidirectional
Step 2
• TFII A and TFII B bind
• TFII B is particularly important, as it is able to bind to TFII D and RNA Polymerase II
Step 3
• RNA polymerase binds to TFII B (with TF IIF bound to itself)
Step 4
• involve the binding of TFII E,TFII H and TFII J to RNA polymerase
• TFII H promotes further unwinding of the DNA helix to facilitate RNA synthesis by RNA Polymerase II.
Factors affecting Transcription Factor Expression
Cell lineage
Transcription factor expression is altered by signals outside the cell (e.g. Hormones, Growth Factors, Mechanical Stress)
Mutated transcription factors have been implicated in several human genetic disorders.
Abnormal transcription factor expression is found in several human factors.
Gene Regulation in Human Disease
- Mutated Transcription Factors have been implicated in several human hereditary disorders
- Abnormal Transcription Factor expression is found in several human cancers
- Mutations affecting the regulation of specific genes have been described in certain human diseases
Distinguish “sense” and “anti-sense” DNA template strands.
Sense strand: The sense DNA strand will read in the same direction as the RNA Strand formed. This will not be complementary to the RNA single strand, but rather will have the same sequence (with U replacing T)
Antisense strand: the DNA which the RNA is built up from, i.e. is complementary to the synthesised RNA, and will be read in the opposite direction
What is the basal transcription complex
The Basal Transcription Complex produces a low level of transcription in the absence of other transcription factors.
The Basal Transcription Complex allows RNA Polymerase II to be phosphorylated and then engage in transcription.
What does TF IID consist of
consists of TATA Binding Protein (TBP) and TBP Accessory Factors (TAFs).
What is RNA Processing
Turning pre-mRNA -> mRNA
• The initial RNA produced from a gene is known as a “primary transcript” or “Pre- mRNA” or “heterogenous
nuclear RNA” ( hn RNA)
- This needs to be processed to make mRNA before translation
- This occurs in the nucleus, and then the mRNA is exported to the cytoplasm for translation
The anatomy of gene
- The promoter lies at the 5’ end
- The sequence information contained in the final mRNA is encoded “discontinuously” in the DNA of the gene
- Segments of the gene which contain sequences that form part of the final RNA are called “exons”
- “Introns” are sequences in the gene which are transcribed but are edited out of the final mRNA
What is the sequence of events in mRNA splicing/processing (6)
- snRNP U1 binds to splice donor sequence
- snRNP U2, U4, U5 and U6 bind
- The end G of the intron bends round and an ‘A residue’ in the intron acts as a branchpoint in an intermediate step in splicing
- Phosphodiester bonds forms between G and A residue
- The phosphodiester bond beween end G of intron and the exon breaks so intron is removed as LARIAT structure
- Adjacent exons are ligated together
Define the function of the “Spliceosome”
The spliceosome is the complex formed by the binding or the small ribonuclear proteins with pre-mRNA during
mRNA processing that coordinates the splicing of pre-mRNA to form mature mRNA.
Describe the Cap added to the 5’ end of the mRNA in Post-transcriptional modification of mRNA. (3)
Cap is formed by hydrolysis of terminal triphosphate of mRNA to a diphosphate.
This then reacts with a phosphate of GTP to form 5’-5’phosphate linkage.
Cap is further modified by Methylation at the N7 position in the purine ring to form 7-Methylguanylate cap.
What does the CAP do?
CAP ACTS TO PROTECT mRNA AT 5’ END AND GREATLY ENHANCES THE TRANSLATION OF mRNA.
Addition of Poly-A tail
- Known as Polyadenylation
- Added one base at a time
- added 11-30 bases downstream (3’ end) of the sequence AAUAAA, which is found in all mRNA’s
- artificially engineered mRNA without the poly-A tail are more unstable and degenerate rapidly
describe polio
Interferes with recognition of cap during translation
Highly infectious disease
Virus can invade nervous system and cause total paralysis within hours
Mainly affects children under the age of three
Virus enters the body through the mouth, multiplies in the intestines.
Initial symptoms include fever, fatigue, headache, vomiting, stiffness in the neck and pain in limbs.
5-10% die because muscles involved in breathing become immobilised.
describe how mutations in splice sites feature in human disease.
A mutation is a heritable change in the sequence of a gene.
• ~ 13% of cases feature mutations in gene promoters
• ~ 6 % feature mutations of the polyadenylation sequence
• ~ 1.2 % feature mutations of RNA capping
• ~ 33% feature mutations in splice donor / acceptor sequences
• the remaining 47% of cases involve mutations that alter the structure / function of the protein being
produced e.g. in cancer
Describe thalassemia
- Example of mutations in splice sites in human disease
- very common in the Mediterranean, SE Asia, China
- Several types of b-thalassemia feature splice site mutations in b-Globin gene
What is the junction between exon and intron called?
SPLICE DONOR SITE
What is the junction between intron and exon ?
SPLICE ACCEPTOR SITE
What do introns and exons end with ?
AG
What type of protein does RNA processing use?
small Ribonuclear Proteins (snRNPs).
What are the features of Thalassemia (3)
Iron Overload (Hemosiderosis) - elevated absorption of Iron due to chronic anaemia.
Iron Overload results in: Hepatic fibrosis and cirrhosis, darkening of skin, cardiomyopathy.
Extramedullary hematopoiesis
Hepatomegaly/Hepatosplenomegaly
What is the gene promoter
The DNA sequence at which the transcription complex assembles is the gene promoter.
