gene expression and cancer Flashcards
gene expression can be regulated:
Gene expression can be regulated at 4 steps:
1. Transcriptional regulation: is mRNA being made? 2. Post-transcriptional regulation: is the mRNA getting translated? 3. Translational regulation 4. Post-translational regulation
epigentic regulation
Epigenetic regulation - a form of transcriptional regulation.
This regulation is needed for cells to differentiate —> STEM CELLS
When this regulation goes wrong —> CANCER
promoter
indicates the start of the gene
RNA polymerase binds to this sequence to transcribe the gene
exons
sequence that code for amino acids in polypeptide
introns
sequence that do not code for amino acids in polypeptide
get transcribed by RNA polymerase but removed from mRNA by splicing
regulatory sequences
at the start (and sometimes end) of a gene that are involved in switching the gene on and off
how can transcription be regulated
Either encouraging RNA Pol to bind to the promoter and tc the gene
Or preventing RNA Pol from binding to .... This is done by proteins known as TRANSCRIPTION FACTORS.
transcription factor
is a protein that binds to the regulatory sequence or to the promoter of a gene / DNA and either causes transcription (ACTIVATOR) or prevents transcription (REPRESSOR).
The 3o structure of the TF determines how it interacts with the DNA and with RNA Pol —> acts as an activator or repressor.
oestrogen
• Oestrogen is secreted by the ovary directly into the blood.
• Since it is a lipid, it can diffuse through the membrane of ALL cells and enter them.
• Only the ovaries, the uterus and the mammary / breast tissues respond to oestrogen because they make a specific (tertiary) protein receptor called oestrogen receptor.
• Oestrogen is complementary to the binding site of the receptor.
• Oestrogen binds to the Oe Receptor. This acts as the TF.
• The TF enters the nucleus and binds to the regulatory sequences / promoter of genes involved in cell division.
• It acts as an activator and causes RNA Pol to transcribe these genes.
• Results in cell division.
post transcriptional regulation
Occurs after transcription, ie, the mRNA is already made.
Many post-tc mechanisms exist, but only one in your syllabus:
RNA interference - get rid of the mRNA / prevents it being translated => ie, the gene is not expressed / gene is silenced.
short interfering RNA - siRNA
- The cell naturally produces short dsRNA molecules (using ‘junk’ DNA in the genome as templates) OR we can artificially introduce short dsRNA. One strand is complementary to the target mRNA.
- An enzyme called DICER cuts the dsRNA into short fragments (about 20-22 nt long) - called siRNA.
- There is a complex of proteins / enzymes called RISC (RNA-induced silencing complex) which will bind to one of the two strands of the ds siRNA.
Half the RISC complexes will have one strand, and the other half will have the other strand.
- The RISC complex with the complementary RNA will bind to the target mRNA by complementary base-pairing.
- Argonaute hydrolyses the target mRNA into short fragments which cannot be translated.
micro RNA - miRNA
- The cell naturally produces short ssRNA molecule which folds back / loops back on itself to form a hairpin loop. (Using ‘junk’ DNA in the genome as templates) OR we can artificially introduce it. Part of this strand is complementary to the target mRNA.
- An enzyme called DICER cuts the ssRNA into short fragments (about 16-24 nt long) - called miRNA (and the loop part is not used).
- There is a complex of proteins / enzymes called RISC (RNA-induced silencing complex) which will bind to one of the two strands of the miRNA.
Half the RISC complexes will have one strand, and the other half will have the other strand.
- The RISC complex with the complementary RNA will bind to the target mRNA by complementary base-pairing.
- Argonaute hydrolyses the target mRNA into short fragments which cannot be translated.
types of stem cells
- Totipotent - can form any type of cells in the body plus extra embryonic cells.
- Pluripotent - these cells can form any cell type in the body, however cannot form extra embryonic cells. They are also found in the early stages of an embryo. These are often used in replacing damaged tissues in human disorders.
- Multipotent - can differentiate into other cells types but are more limited e.g. the cells in the bone marrow and umbilical cord.
- Unipotent - these cells can only differentiate into one type of cell.
what happens to totipotent cells during embryonic development
certain parts of the DNA are selectively translated so that only some genes are switched on in order to differentiate the cell into a specific type and form the tissues that make up the foetus
what types of stem cells are found in embryos
totipotent and pluripotent
multipotent and unipotent cells are only found in mature mammals
how are induced pluripotent stem cells produced
from mature, fully specialised (somatic cells)
the cell regains capacity to differentiate through the use of proteins, in particular transcription factors
what is meant by epigenetics
heritable changes in gene function without genes to the base sequence
DNA-histone complex - tight = gene hidden to transcription hard to take place as transcription factors and DNA polymerase cannot reach
epigenome - chemical tags - attach to DNA and histones - determines shape - added or removed by environmental factors
acetylation - increase - stimulation
methylation - descrease - stimulation
how does increased methylation of DNA affect gene transcription
involves addition of CH3 group to cytosine bases which are next to guanine
prevents transcription factors from binding
therefore gene transcription is suppressed
• DNA is methylated AND / OR Histones are deacetylated. • Causes nucleosomes to be tightly packed / tightly arranged. • Therefore, the gene is not exposed for TF, RNA Pol to bind to. • No / less transcription.
demethylation
• DNA is demethylated AND / OR Histones are acetylated.
• Causes nucleosomes to be loosely packed / loosely arranged.
• Therefore, the gene is exposed for TF, RNA Pol to bind to.
• Transcription occurs / increases.
epigenetics definitino
The DNA base sequence (and or histone protein) is modified by adding or removing methyl groups (or acetyl groups). The DNA base sequence is not changed.
This modification is inherited when the cell replicates its DNA and passes it on to its offspring.
DNA cytosin
DNA cytosine can be
- methylated by methylase enzymes
- demethylated by demethylase enzymes
Histones
Histones can be
- acetylated by acetylase enzymes
- deacetylated by deacetylase enzymes
treating disease using epigenetics
target enzymes as both acetylation and methylation involve enzymes
responsible for adding or removing chemical tag
eg inhibit the enzyme responsible for adding methyl groups
must ONLY target disease cell
cancer
= uncontrolled mitsos
tumour vs cancer
Tumour vs Cancer
Tumour is benign whereas cancer is malignant.
Tumour is enclosed within a membrane whereas cancer is not.
Tumour cannot metastasise whereas cancer cells can metastasise (ie, cells break off from the original cancer and invade another tissue and cause cancer there).
cancer could happen due to genetic mutation or epigenetics
Cancer could happen due to genetic (mutation) or epigenetic causes:
- Mutations in Proto-oncogenes could permanently switch on the gene and convert it to an ONCOGENE, ie, it is always active (transcribed and translated) and cannot be switched off.
- Mutations in Tumour-suppressor genes cause the protein to have eg. the wrong 3o structure, therefore cannot switch off the proto-oncogene.
- Less methylation, more acetylation of the proto-oncogene causes the nucleosomes to be loosely packed, so RNA Pol binds, so more transcription of the proto-oncogene.
- More methylation, less acetylation of the tumour-suppressor genes causes the nucleosomes to be tightly packed, so RNA Pol cannot bind, so less transcription of tumour-suppressor gene, less tu-su protein available to switch off the proto-oncogene.
how does decreased acetylation of DNA affect gene transcription
positively-charged histones bind to negatively-charged DNA
decreasing acetylation increases positive charge of histones
binding becomes too tight and prevents transcription factors from accessing the DNA
therefore gene transcription is suppressed
RNAi
RNA interference - translation inhibition by RNA
microRNA - miRNA - binds to protein complex in cytoplasm - RNA-induced silencing complex - then binds to complementary base pairs on mRNA - prevents translation by preventing ribosome from attaching or enzyme destroys mRNA
SiRNA - cell naturally produces short double stranded RNA - one strand complimentary to mRNA - enzyme called DICER cuts into shorter fragments - siRNA - same as miRNA
describe the role of a tumour-suppressor gene
code for proteins that control cell division
in particular, stopping the cell cycle when damage is detected
they also involved in programming self destruction of the cell
explain how tumour-suppressor genes can be involved in developing cancer
a mutation in the gene could code for a nonfunctional protein
increased methylation or decreased acetylation could prevent transcription
cells will divide uncontrollably resulting in a tumour
proto-oncogenes
control cell division - code for proteins that stimulate cell division
mutation in the gene could turn it into a permanently activated oncogene
this results in uncontrolled cell division and formation of a tumour
