Biomolecules - Gene Control Flashcards
DNA and chromatic regulation
- DNA is packed into chromosomes in the form of chromatin. There are repeating units in chromatin called nucleosomes (146 base pairs of double helical DNA wrapped around 8 histones)
- H2A, H2B, H3, H4 are acetylated by histone acetyltransferase leading to uncoiling so they can be transcribed. The opposite happens with histone deacetyltransferase
- heterochromatin: densely packed, transcriptionally inactive
- euchromatin: loosely packed, trannscriptionally active.
- DNA methylation is a more permanent regulatory mechanism. Adds a methyl group to the cytosine by methyltransferase. It stably silences the expression of genes. Methylation might physically inhibit the binding of transcription proteins. It might be bound by methyl CpG binding domain proteins that recruit other proteins like histone deacetyltransferases to tightly coil the genes and make it transcriptionally inactive.
Transcriptional regulation
Transcription factor protein binds to the operator.
- General transcription factors bind to start transcription. These, RNA polymerase and mediator multiple protein complex are the basic transcripts apparatus.
- activators enhance the interaction between the promoter and RNA polymerase e.g. CAP
- enhancers are sites on the DNA that are bound to by activators to bend the DNA in certain ways to bring promoters closer.
- repressor are proteins that bind to the operator, impeding RNA polymerase’s progress along the strand and so the expression of the gene
- inducer is a molecule that induces gene expression; it can remove repressor e.g.
- silencer is a region of the strand bound by a repressor protein to silence gene expression; can be several bases upstream or downstream from the promoter
- prokaryotes use this regulation to quickly adapt. Activators, repressor and rarely enhancers are used. Eukaryotes are more sophisticated. They have a nuclear envelope that adds more spacial and temporal control of gene expression.
Post-transcriptional regulation
Only in eukaryotes. Stabilises the mRNA to prevent premature degradation
- splicing of introns by a spliceosome. A spliceosome binds on either side of the intron, loops it into a circle and then cleaves it and ligase the remaining two ends of the exons.
- the pre-mRNA also gets a 5’ cap and a poly-A tail. The cap protects from exonuclease, promotes ribosome binding and regulates nuclear export. The tail protects from degradation, promotes translation, regulates nuclear export and helps with transcription termination. Adenylation is catalysed by an enzyme. It’s about 250 nucleotides long
- RNA editing leads to sequence variation and its catalysed by many enzymes. It includes insertion, deletion and substitute. ADAR enzymes convert adenosine to inosine. CDAR converts cytidine to uridine.
Non coding RNA (ncRNA)
Isn’t translated to a protein.
-MicroRNA (miRNA) - base pair with complementary sequences in RNA or mRNA leading to gene silencing. They do this by preventing translation or causing degradation
- rRNA, tRNA and small nucleolar RNA (snoRNA - guide other RNA through methylation and pseudouridylation) are involved in translation
- small nuclear RNA (snRNA); about 150 nucleotides; they process pre-mRNA, maintain telomeres and work with snRNPs to form complexes e.g. spliceosome (binds to certain sequences in the pre-mRNA)
Oncogenes
-Code for genes that normally regulate cell growth. Start off as proto-oncogene before being converted oncogenes.
-Proto-oncogenes code for genes that regulate cell growth and differentiation. The products of these genes are often involved in signal transduction and execution of mitogenic (chemical substance that encourages a cell to start cell division) signals
- proto-oncogenes become oncogenes through
*point mutations/deletion: gene itself or promoter region can lead to a hyper-active gene or over expression
*gene amplification/ increased mRNA stability: might lead to the prolonged action of mRNA in the cell and can lead to overexpression
*chromosomal rearrangements: translocation of a gene to a nearby regulatory sequence leading to over expression or fusion to an actively transcribed gene.
Oncogenes:
*Src: codes for a non receptor tyrosine kinase
*Ras: codes for a small GTPase. Functions as an on/off switch and a downstream effector is MAO kinase and it mediates that regulate cell division
*Myc: codes for a transcription factor that induces cell proliferation
*RTK: receptor tyrosine kinase which add phosphate groups to genes to turn them off or on e.eg VEGF, EGFR, PDGF
*CTK: cytoplasmic tyrosine kinase which mediates responses to the activation of receptors of cell proliferation, survival, differentiation e.g. the Philadelphia chromosome (fusion of 9 and 22)
Tumour suppressors
Genes whose products halt the regulation of the cell cycle or promote apoptosis.
- DNA repair proteins
- cell cycle repressors
*The two hit hypothesis says that both alleles must be damaged for the effect to occur. First discovered in the pRb. Prevents progression of cell cycle form G1 to S phase. pRb is a growth repressor and also attracts histone deacetylase which reduces transcription
*p53 is a tumour suppressor that activates DNA repair protein. It can also arrest growth by halting the cell cycle at G1 to allow for DNA repair proteins to work. It binds DNA and activates genes, including p21 proteins. It also initiates apoptosis if the DNA has irreparable damage. It is also a dominant negative suppressor gene.
