RNA synthesis Flashcards
The Human Genome
how many pairs of chromosomes? sex?
what does telomere do?
what does centromere do?
what does chrosomes contain? what do these do?
Contains 3.2 x 10^9 nucleotides
Contains 22 autosomal pairs of chromosomes
Contains 1 pair of sex chromosomes
each chromosome contains a maternal and a paternal homolog
they also contain a telomere
- these protect the chromosome
they also contain a centromere
- these hold the chromosomes
Each chromosome carries genes genes are made of DNA DNA function is to store genetic information of the cell - genes are transcribed to a mRNA - mRNA is translated into a polypeptide
What is a gene
where does transcription of a gene begin?
what 4 key regions do genes contain? what are their roles?
why do genes differ in size?
A gene is a DNA segment containing instructions for making a particular product, including the regulatory elements.
Transcription of a gene begins at the 5’ end
Genes contain UTRs (untranslated regions)
They contain introns: which do not code for any amino acid or protein, these are later removed after transcription
They contain exons: these code for proteins
They have a promoter region which contains elements to initiate transcription
Genes also differ in size
genes coding for histones are small as many of them are needed (so the process needs to be efficient)
Genes differ in number of introns and exons
Genes cluster into families
What is the RNA
strands?
thymine?
sugar?
stability? why?
what is RNA used as?
RNA is single stranded
RNA uses the nucleotide Uracil instead of Thymine
RNA consists of a ribose sugar
RNA is much less stable than DNA
the ribose sugar can attack and react with the backbone
RNA is used as a temporary genetic store
RNA can act as a temporary messenger, allowing the cell to regulate gene expression in a short- term manner
RNA Synthesis (Basic)
what transcribes dna into rna?
what are the 3 types of this and their function?
why do lots of polymerases work on the same gene? (2)
DNA is transcribed into RNA by RNA Polymerase
• there are 3 types of RNA polymerases
- RNA polymerase I: used for most ribosomal RNA
- RNA polymerase II: used for Protein-coding mRNA, microRNA (miRNA), non-coding RNA
- RNA polymerase III: used for Transfer RNA (tRNA), 5S rRNA, other small RNAs
dozens of polymerases work on the same gene
- this helps form many transcripts from a gene simultaneously
- this also makes the process quick at 1500 nucleotides/50s
• The general transcription process involves the
following
• 1) phosphodiester bond forming
• 2) uridine-triphosphate
• 3) Watson-Crick base pairing
• 4) 5’→ 3’ synthesis
• 5) single stranded RNA formation
• 6) unwinding small portion of DNA
• The RNA formed is complementary to the DNA template
strand
• No primers are required in this process
RNA Synthesis (Detailed)
where does the rna polymerase attach? where is this found? what does this region contain? What recognises this?
once this molecule binds to this region, what two molecules bind to the other region? why? what else binds?
which rna polymerase assembles here? what pulls apart the DNA helix and what does it do to the polymerase?
what happens now?
RNA polymerase first attaches to the promoter
the promoter is found upstream on the 5’ end of the gene
the promoter region contains an element called the TATA box
the TATA box is recognized by a TATA binding protein which is a subunit of a protein called TFIID (transcription factor 2D)
TFIID now attaches to the TATA box
the DNA molecule now gets distorted
TFIIA and TFIIB now bind to the promoter region (TFIIA stabilizes the complex)
other general transcription factors (E & H) bind
then RNA polymerase II assembles at the promoter, forming the transcription initiation complex
TFIIH pulls apart the DNA helix and phosphorylates RNA Pol II
phosphorylated RNA Pol II is released from the complex and begins transcription
Phosphorylated RNA polymerase is recruited to the gene
DNA helix is locally unwound
RNA synthesis now begins
Elongation takes place as the RNA polymerase moves forward
Termination takes place as the RNA polymerase comes to the end of the gene
The RNA polymerase then dissociates forming a primary RNA transcript
mRNA Processing
what are the 3 processes? explain the 3 processes and why they take place
Eukaryotic mRNA is extensively modified after transcription
This involves 3 processes:
• 5’ capping
- 5’ to 5’ triphosphate bridge and methyl guanosine are attached at the 5’ end
- this gives the RNA stability
- capping occurs as the transcription is going on
• 3’ polyadenylation
- this involves adding A nucleotides to the 3’ end of the RNA
- this process uses ATP
- this provides stability and protection to the RNA
- this also tells the cell that the mRNA is ready to leave the nucleus
- a longer polyA tail tends to mean that the mRNA molecule is more stable: ie will remain in the cell for long
• splicing
- this involves removal of introns from the primary transcript
- small ribonuclear proteins recognize the ends of the exons
- these proteins (snRNPs) then attach to either sides on introns
- the site where these proteins attach are called splice site junctions
- snRNPs then push down thus removing the introns
- the exons are now side by side
- then introns then degrade later in the cell
• PolyA and cap provide stability and protection to the mRNA
• the mRNA will not leave the nucleus unless it
has been capped, spliced and has a polyA chain
• The cap and polyA are also important for translation
Regulation of Gene Expression
3 different gen expressions
give examples
Some “housekeeping genes” code for proteins needed by all cells and are always ON
example: beta actin
Some genes are “inducible”
for instance genes induced by steroid hormones (only turn on when needed)
Some genes are tissue-specific
example: alpha and beta globin genes are only expressed in red cell precursors
What stops RNA Polymerase from transcription of all genes
what else apart from tata box regulates this? where?
how does rna polymerase bind to dna? how is this overcome? what are key proteins for this?
how can they prevent this?
what are enhancer regions?
- TATA box is not the only regulatory element required for transcription
- Elements proximal to the gene also regulate transcription
- The sequence immediately 5’ to the region to be transcribed is called a promoter, the job of which is to recruit RNA polymerase to the DNA
• RNA polymerase binds very poorly to DNA on its own
• promoters contain DNA sequences that other proteins can come in and bind to, to help the
RNA polymerase to bind
• these proteins that bind, are called transcription factors
• they are DNA specific binding proteins, this means they recognize a particular sequence on the DNA molecule, usually fairly short
• they bind in order for the DNA to be opened up and then RNA polymerase can get on with its job
• as well as activating polymerase binding, they can also repress it, meaning they can prevent RNA polymerase binding and hence prevent transcription.
Depending, on the cell type and what the gene happens to be
• Enhancer regions signal the promoter region to allow or disallow other transcription factors to
bind
Splicing (more details)
what are snRNPs? where do they bind? what does this form and effect?
there are protein complexes that recognize DNA sequences at the ends of the exons, snRNPs (small nuclear ribonucleic proteins) bind to these regions on either side of the intron, this forms a splicosome which then pushes out and removes the intron. The exons are now side by side.
Tissue-Specific Gene Expression - what does it depend upon>
?
Human cells all contain the same genes (genotype), the type of genes expressed will depend on the type of cell
Untranslated Regions - types and roles?
- UTRs are transcribed but not translated
- 5’ UTR: regulation of translation
- 3’ UTR: mRNA stability & miRNAbinding
Why Cap & Polyadenylate? (3)
- Stability
- Transport to cytoplasm
- Integrity prior to translation
Export from the Nucleus
what three things regulate this? roles?
CBC = cap-binding complex -> which will recognised that mRNA has been capped
TREX = transcription-coupled export complex -> to help export out of the nuclear pore
EJC = exon-junction complex -> checkpoint for correctly spliced to be exported
