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8.2 Gene expression is controlled by a number of features - Coggle Diagram
8.2 Gene expression is controlled by a number of features
8.2.1 lesson 1: differentiation and stem cells
differentiation and stem cells
differentiation
specialisation of cells/formation of specialised cells from stem cells
occurs by switching genes on and off
not normally reversible in adults but reversible in plants
stem cells
an undifferentiated cell that is able to differentiate into different cell types and self renew (divide indefinitely)
found in embryonic tissue and adult tissue (bone marrow) in animals and plant meristems
expressed
if a gene is expressed then it is transcribed and translated - producing a polypeptide
embryonic stem cells
totipotent stem cells
able to differentiate into all cell types
found in zygote (fertilised egg), early embryonic cells, plant meristems
pluripotent stem cells
Able to differentiate into almost any cell type
Found in the inner cell mass of the blastocyst (late embryo) and early foetal cells.
unable to form placental cells - which form from the outer cells of the blastocyst
adult stem cells
multipotent stem cells
Able to form a limited number of cell types.
Found in umbilical cord blood and some adult tissues like bone marrow
Important for growth and tissue repair (cell replacement)
unipotent stem cells
Able to form a single cell type.
Found in small numbers in many adult tissues.
Important for repairing damaged tissues.
what can we use stem cells for
research into the process of differentiation
develop specialised tissues for use of drug testing
stem cell tissue transplant e.g. bone marrow transplant for leukaemia patients
develop specialised tissues for transplantation e.g. nerve tissues to repair spinal cord damage
8.2.1 lesson 2: using stem cells
sources of stem cells
embryonic stem cells mostly from 'spare' embryos from IVF treatmen
adult stem cells from donors
issues with using stem cells in medicine
using embryonic stem cells causes harm to the embryo
there are concerns that using stem cells for tissue transplants could lead to cancer
if stem cells used come from a different individual the tissue transplant could be rejected
stem cell transplants can transfer viruses
evaluate the use of adult (multipotent stem cells) versus embryonic (totipotent/pluritpotent stem cells in medicine)
adult
pros
can give consent for use of stem cells
cons
can only differentiate into a limited number of cells so therefore can only be used to treat a limited number of conditions
embryonic
pros
spare embryos from IVF are common so less likely to have to wait a long time for treatment
can differentiate into all types of cell so can be used to treat many more types of conditions
cons
the embryos are harmed
making induced pluripotent Stem cells (IPS)
adult somatic cells are treated with transcription factor to reverse the process of differentiation, producing induced pluripotent stem cells
tissues used to treat conditions such as paralysis or diabetes
adult somatic cells are collected from patient
IPS cells can be treated to develop into many different types of tissue
evaluate the use of stem cells in medicine
pros
IPS would not be rejected from the body if they were initially taken from the patient
can be used to treat long-term debilitating human disorders e.g. paralysis, diabetes etc
prevents patients and their families from suffering
can be used to discover new drugs for treating illnesses more efficiently
can be used to track the development of a disease from outset
cons
moral and ethical issues surrounding the sanctity of life i.e. the human embryo is considered to be a potential life and should not be interfered with - religious arguments
rejection of stem cells that originate from human embryos - patients need to take immunosuppressants which increase he risk of contracting other diseases
misuse of stem cell terminology e.g. cloning humans from IPS cells
cells could divide uncontrollably leading to cancer/tumour development
8.2.2 lesson 1: transcription factors
gene expression
a gene is expressed when it is transcribed and translated so the polypeptide it codes for is made
if a gene is 'down regulated' or expression is decreased - less polypeptide is made
if a gene is 'up regulated' or expression is increased - more of the polypeptide is made
regulation of gene expression
switching genes on/off pr increasing/decreasing expression if genes
regulation of transcription
transcription factors
epigenetics
regulation of translation
RNA interference (RNAi) by small interfering RNA (siRNA)
transcription factors
proteins found in the cytoplasm
move from the cytoplasm to the nucleus through nuclear pores
bind to a section of DNA upstream/before of the target gene called the promoter
this stimulates (or possibly inhibits) transcription of the target gene by RNA polymerase (either aids or prevents RNA polymerase binding)
oestrogen
oestrogen is a steroid hormone
oestrogen is non-polar and lipid soluble
oestrogen can diffuse through the phospholipid bilayer of the cell surface membrane by simple diffusion
binds to receptors inside the cell (no need for secondary messenger)
how oestrogen affects gene transcription
1) oestrogen diffuses into cell through the phospholipid bilayer
2) oestrogen binds to oestrogen receptor/inactive transcription factor in the cytoplasm (forming a hormone receptor complex). this changes the tertiary structure of the oestrogen receptor so it becomes an active transcription factor
3) transcription factor moves into nucleus through nuclear pore
4) transcription factor binds to the promoter
5) this stimulates transcription of the gene by RNA polymerase
8.2.2 lesson 2: epigenetics
DNA coiling
DNA is coiled around histone proteins
together DNA and associated proteins makes up a chromosome
the DNA can be tightly coiled (e.g. when the chromosomes condense during mitosis) or less tightly coiled
DNA has to be less tightly coiled for transcription factors and RNA polymerase to bind and transcription to take place
epigenetic definition
a heritable change in gene expression that doesnt involve changes in DNA base sequence
this involves attaching methyl groups to the DNA and acetyl groups to the histone proteins
epigenome
chemical tags attached to DNA/histones (methyl/acetyl groups)
heritable (most are removed but some may be passed on)
reversible (can be removed)
affects transcription (and therefore expression) of genes
epigenetic mechanisms
methylation
methyl groups attached to DNA
DNA coils more tightly
harder for transcription factors and RNA polymerase to bind to DNA
inhibits transcription by RNA polymerase
acetylation
acetyl groups attached to histone proteins
DNA coils less tightly
easier for transcription factors and RNA polymerase to bind to DNA
stimulates transcription by RNA polymerase
what affects your epigenetics
environmental chemicals
diet
stress
aging
drugs/pharmaceuticals
hormones
development (in utero, childhood)
8.2.3 lesson 1: oncogenes and tumour suppressor genes
proto-oncogenes and oncogenes
proto-oncogenes
genes that stimulate a cell to divide when expressed (usually triggered by hormones)
oncogenes
mutation can cause a proton-oncogene to become permanently expressed - the gene is now called an oncogene
this can be caused by:
mutation in the gene promotor of the proton-oncogene
mutation of another gene that effects the expression of the photo-oncogene
this causes uncontrolled cell growth and division - a tumour forms
mutation in one allele of a proto-oncogene can cause a tumour
tumour supressor genes (and mutation)
tumour supressor genes are a number of different genes (e.g. BRCA1. BRCA2)
they:
slow down cell division
repair mistakes in DNA
cause a cell to die (apoptosis)
help prevent tumours forming
mutation/switching off of tumour suppressor genes
mutation can cause tumour suppressor genes to produce a non-functional protein or the gene to not be expressed
mutation in the gene - non function protein produced
Mutation in promoter of this gene or regulatory gene e.g. transcription factor) - protein not produced
Hyper methylation could cause the tumour suppressor gene to not be expressed, this leads to cells growing and dividing uncontrollably - forming a tumour
Mutation in both alleles of a tumour suppressor gene necessary to cause a tumour
Mutations of tumour suppressor genes can be inherited, most are acquired
8.2.2 lesson 3: RNA interference
RNA interference causes the destruction of mRNA from a certain gene (prevents translation) which prevents the gene from being expressed
process
1) gene codes for short section of RNA (e.g. miRNA) that is complementary to target mRNA
2) miRNA attaches to hydrolytic enzyme forming an RNA-protein complex
3) miRNA base pairs bind to complementary base pairs on target mRNA (hydrogen bonds formed)
4) target mRNA is hydrolysed and translation is silenced/silent
prevents translation of proteins but can also prevent something from being made even if not a protein as may prevent translation/production of enzymes which are required to catalyse reactions for the production of things
8.2.3 lesson 2: oestrogen and breast cancer
oestrogen, proto-oncogenes and breast cancer
1) raised levels of oestrogen production by breast tissue (in postmenopausal women)
2) more oestrogen diffuses into cells in the breast tissue so more oestrogen binds to receptors/transcription factors which alters the tertiary structure of the receptors/transcription and activating the transcription factors
3) transcription factor binds to promote region for proton-oncogene which stimulates the transcription of the proton-oncogene by RNA polymerase - becomes oncogene
4) oncogene/proto-oncogene is expressed more/translated more which stimulates cells to divide kore - leads to uncontrolled cell division and tumour formation
how could abnormal methylation of oncogenes lead to cancer
less methylation
gene expressed more
uncontrolled cell division leads to tumour formation
cancer treatment
some cancer drugs disrupt some aspect of DNA replication or mitosis
monoclonal antibodies can treat cancer by binding to receptors and preventing binding of growth factors
cancer treatments involving tumour suppressor genes or proto-oncogenes could include
preventing gene expression of proto-oncogenes or increasing expression of tumour suppressor genes
inhibition or activation of enzymes produced from these genes
gene therapy - inserting functional gene e.g. functional tumour suppressor gene