RNA Synthesis Flashcards
What is transcription?
Synthesis of mRNA transcript from DNA
What is Translation?
Production of proteins from the mRNA transcript
How many nucleotides does the human genome contain?
3.2 x 10^9
What are the 2 types of chromosome?
Autosomes and Sex Chromosomes
How many pairs of autosomes are present within the human genome?
22 pairs
How many pairs of sex chromosomes are present within the human genome?
1 pair
What is a centromere and what is its function?
Structure made up of lots of repeats of DNA that holds chromatids of a chromosome together
During mitosis it keeps chromosomes attached to mitotic spindle during mitosis
What is a telomere and what is its function?
Long repetitive piece of DNA (6 base pair repeat) at each end of the chromosome.
Its function is to Protect genes at end of chromosome from being lost and prevents cell from “sticking together” the ends of chromosomes with each other.
What is the intergenic region of a chromosome?
region between genes on a chromosome
How much of the human genome do intergenic regions make up?
98%
What other functions can the regions that don’t code for proteins have?
Pseudo genes (duplicates of genes that are non-functional) and repetitive DNA
What are the 2 different definitions of a gene?
- Unit of heredity; contains instructions for an organism’s phenotype
- DNA segment containing instructions for making a particular product
What is the transcription start site?
Location on a chromosome where the transcription of a particular gene occurs. Always at the 5’ end of a gene.
What is an exon?
Part of gene that get expressed as proteins
What is an intron?
Part of the gene that gets removed before mRNA is released from the cell
What is the promoter region?
Region 20 or 30 base pairs Upstream of transcription start site. Makes sure that RNA polymerase II binds to the gene to start transcription
Key properties of RNA polymerase
Synthesises RNA molecule in the 5’ to 3’ direction
Uridine-triphosphate (U) Replaces (T) in RNA compared with DNA
Unwinds small portion of DNA
Catalyses production of phosphodiester bonds
What are the 3 types of RNA polymerase present within human cells and what do they each transcribe?
RNA Polymerase I - Transcribes most ribosomal RNA
RNA polymerase II - Transcribes mRNA, miRNA and non-coding RNA
RNA polymerase III - Transcribes tRNA and other small RNA’s
Why does more than one RNA polymerase work on the same gene?
Allows multiple mRNA transcripts to be produced at the same time meaning more than one protein will eventually be produced. Speeds up the process of translation
What are the 5 things required for transcription?
- RNA polymerase
- rNTPs - the building blocks of RNA
- Promoter region - acts as a ‘bookmark’ so RNA polymerase knows where to transcribe
- Transcription factors
- Enhancers and silencers - elements which enhance and suppress the transcription of a gene.
What are transcription factors and what function do they serve?
They are proteins required to initiate or regulate transcription in eukaryotes
They assemble on the promoter to position RNA polymerase II and pull apart DNA helix and expose template strand
Describe the events that occur during the initiation stage of transcription (production of transcription initiation complex).
A) General transcription factor TFIID recognises and then binds to promoter region
TFIID is made of multiple sub-units and one of the subunits is called TBP (TATA binding protein) TBP is responsible for allowing TFIID to bind to the TATA box. This then forms a complex which allows other proteins to bind
B) TFIIA and TFIIB then bind (TFIIA stabilises the complex while TFIIB is responsible for linking RNA polymerase to the TFIID and TBD)
C) Other general transcription factors, e.g. TFIIE and TFIIH bind. This causes conformational change which allows RNA polymerase II to recognise the complex and bind to it. This causes the formation of the Transcription initiation complex
D) TFIIH pulls apart the DNA helix around the initiation site to form a cistron. TFIIH also phosphorylates RNA polymerase II which requires ATP.
E) Phosphorylation of RNA polymerase II allows it to be released from the transcription initiation complex and gives it the energy needed to translocate along DNA and begin transcription
What are untranslated regions?
regions of a gene that are transcribed into mRNA but not translated into proteins
What are the 2 types of untranslated region and what do they do?
5’ UTR - involved in regulation of transcription
3’ UTR - involved in mRNA stability and miRNA binding
Describe the events that occur during the elongation stage of Transcription
RNA polymerase translocates along the anti-sense strand of the DNA in the 5’ to 3’ direction. As it does this it catalyses the reaction that joins the next nucleotide of the newly synthesised mRNA strand to the 3’OH end of the previous nucleotide. (Formation of phosphodiester bond between them). Complementary mRNA strand produced is exact copy of sense strand (coding strand) except that uracil replaces thymine.
During elongation capping also occurs
Describe the process of capping?
Capping occurs during the elongation phase of Transcription.
After 25 nucleotides of RNA are synthesized the 5’ end becomes modified.
A guanine nucleotide is added to the 5’ end of the mRNA strand and an enzyme called methyl transferase methylates the guanine at position 7 to give rise to 7 methylguanine cap.
The polymerase II continues along the DNA making pre-mRNA until it reaches the termination sequence.
What purpose does the 5’ cap serve?
Protects the 5’ end from the activity of 5’ exonucleases which digest the RNA.
Also acts as a checkpoint for the ribosome as if it doesn’t recognise a 5’ cap then the mRNA will not be exported out of the nucleus
Describe the process of polyadenylation
After the stop codon there is a sequence called AAUAAA which is the Polyadenylation signal or termination signal.
A specific endonuclease recognizes the termination signal. The CPSF (cleavage and polyadenylation specificity factor) is the enzyme that cleaves the mRNA strand after the polyadenylation signal.
Polyadenylate polymerase, Poly(A) polymerase, then adds 250 adenosine monophosphates to the 3’ end of pre-mRNA to form the poly (A) tail.
This process is called Polyadenylation.
What is the purpose of polyadenylation?
The poly a tail protects the pre-mRNA from degradation. It protects it from RNAses which digest the RNA in the cytoplasm as it moves out of the nucleus. (The longer the poly A tail the more stable the RNA is).
It also promotes the nuclear transport of the mRNA
What is splicing?
Splicing is the removal of introns and joining of exons together within the pre-mRNA to produce mature mRNA.
Describe the process of splicing
snRNPs bind to the pre-mRNA at the GU site and the AG site which marks where the intron is within the pre-mRNA
Spliceosome then cleaves the GU site (5’ splice site) of intron
GU sequence then loops around and joins with the branch point (A) to form lariat – like intermediate
The bonding of the guanine and adenine takes place via transesterification in which a hydroxyl group (OH) on the carbon atom of the adenine ‘attacks’ the bond of the guanine nucleotide at the splice site
Spliceosome then cleaves at AG site (3’ splice site) of the intron
Ligase then joins the 2 exons together
What is the idea of alternative splicing?
Idea that not all of the exons within a pre-mRNA molecule are needed to produce mature mRNA so many different mRNA molecules can be produced from one gene.
What are the 3 proteins that need to bind to mRNA in order to guide it to the nuclear membrane for export out of the nucleus?
Poly-A tail binding protein
Exon junction complex
Cap binding protein
What are exon junctions?
Exon junctions are the gaps between exons which are filled with introns if they are still present within the mRNA
