gene expression and protein synthesis Flashcards
central dogma of molecular biology
DNA-mRNA via transcription
mRNA-protein via translation
what are the potential products of mRNA?
rRNA, tRNA, ribozymes, microRNAs and proteins
what are ribozymes?
RNA that act as enzymes
how does the double helix provide a copying mechanism?
A-T and C-G for DNA replication via complementary base pairing with hydrogen bonds
antiparallel strands 5 prime to 3 prime
nucleotides held in place by phosphodiester bonds
sense strands via antisense strands
sense strands code, (5’ to 3’) antisense is the opposite to sense (3’ to 5’)
what does ATG code for?
methionine- start codon
what do TAA, TAG and TGA code for?
stop codons
gene structure
5’ to 3’ direction
promoter initiates gene expression
coding sequence starts with ATG and finishes TAG (e.g)
either side are untranslated regions (5’ UTR and 3’UTR)
function of UTRs
regulation of mRNA stability
eukaryotic vs prokaryotic gene structure
eukaryotic DNA has introns and exons, prokaryotic is only exons
introns make sequence non-continuous
TATA box
sequence in some eukaryotic genes (30%)
part of promoter region- initiation of gene expression
TBP (TATA binding protein) binds to the TATA box and causes a unwinding of the DNA, allowing the transcription machinery to access the template strand for RNA synthesis
what are enhancers?
regulatory DNA sequences that play a critical role in controlling the expression of specific genes
enhance expression of molecules to modulate gene expression
turn up or turn down gene expression (+ve or -ve)
where do initial transcription factors bind?
TATA box
how do we make mRNA from DNA?
RNA polymerase II makes mRNA from DNA and binds to promoter region
what makes up the transcription initiation complex?
regulatory enhancers/silencers, RNA polymerase II and TFs
what strand is used for transcription?
primary RNA transcript made from antisense strand (template)
template strand complementary ro primary RNA transcript
post-transcriptional mRNA processing (5’ capping)
during transcription, guanosine monophosphate (GMP) added to the 5’ end via 5’-5’ link
after guanylation, the guanosine cap is methylated at the 7th position of its purine ring by a methyltransferase enzyme (m7GpppG)
prevents degradation of mRNA by 5’-3’ exonucleases and promotes nuclear export (transport of the mRNA from the nucleus to the cytoplasm)
how is post-transcriptional mRNA processing regulated?
5’ capping and 3’ polyadenylation
how does 5’ capping prevent degradation?
5’ exonucleases cannot bind to 5’-5’ link initially
steps of 3’ polyadenylation
- initiated immediately transcription is terminated- poly(A) polymerase adds a string of adenine residues to the 3’ end of the pre-mRNA
- poly A tail can be VERY long – hundreds of bases
- enhances stability of mRNA – 3’-5’ nucleases are common
- required for translocation out of the nucleus
how are introns removed?
splicing (pre-mRNA to mature mRNA)
steps of translation
- takes place on 80S ribosomes in the rough endoplasmic reticulum
- ribosomes have two subunits - rRNA and protein complexes
- mRNA is moved through a groove at the junction of the two RNA molecules
- tRNA with complementary codon sequences bring the
relevant amino acids - translation terminates at the STOP codon (e.g. UAG)
tRNA is a copy of what strand?
antisense strand because it is complementary to mRNA
how do you get selective expression of a gene in a neuron?
some genes have cell-type-specific promoters, meaning that these promoters are recognized and activated only in certain types of cells, such as neurons
neurons express a unique set of transcription factors that activate or repress gene expression in a cell-type-specific manner
what do all primary proteins start with?
methionine
how do TF control suites of genes?
expression of TF in different cells
genes can be common across cells (e.g cytoskeleton and mitochondria)
how do you get selective expression of a gene in a neuron?
pluripotent stem cells can diffrentiate into any type of cell
certain TF can turn this into a neuron
this can determine the type of neuron (e.g spinal motor and cranial motor)
elements of cranial and motor neurons
Hb9=spinal motor TF
Phox2b=cranial motor TF (accessory nerve)
Tuβ3=common to all neurons
what are microRNAs (miRNA)?
post-transcriptional regulators
small non-coding RNA sequences
produced as long hairpin structures
processed to produce short sections of RNA <30 bases in
length
bind to RISC (RNA-induced silencing complex)
cleaves target mRNA
leads to down-regulation of protein expression
what is a sense strand?
refers to one of the two strands of DNA that carries the genetic information for protein synthesis (DNA same sequence as mRNA)
the sense strand is often referred to as the coding strand since it contains the same sequence as the RNA transcript
what is an antisense strand?
antisense strand is used by the enzyme RNA polymerase to synthesize mRNA during transcription
what is the alternative to the TATA box?
initiator (INR)
acts as a secondary promoter element that can drive transcription initiation in the absence of a TATA box
what is a transcription initation complex?
large assembly of proteins and RNA that form at the promoter region of a gene to initiate the process of transcription
this complex is essential for the synthesis of mRNA
how does 3’ polyadenylation enhance the stability of mRNA?
3’ polyadenylation enhances the stability of mRNA by adding a poly-A tail to the 3’ end of the transcript
poly-A tail serves as a buffer, protecting the mRNA from exonucleases (less exposed)
effects of microRNA-128 knockout
causes hyperactivity and spontaneous seizures in mice
this leads to premature death
can be rescued by treatment with anti-convulsant valproate
