gene expression Flashcards
universal
same 3 bases on triplet/codon code for the same amino acids in all organisms
non overlapping
each base is read as part of only one triplet/codon
degenerate
more than one triplet codes for an amino acid
mutations
- substitution (silent)
- addition (frame shift
- chromosomal nondisjunction (chromosomes fail to split during metaphase 1. down syndrome caused by chromosome 21, 3 instead of 2 chromosomes)
- inversion
- duplication
- translocation (bw non homologous chromosomes)
mutagenic agents
high energy ionising radiation (gamma rays, beta particles) interferes with dna replication
- dna reactive chemicals: benzene, hydrogen peroxide
- biological agents: viruses/ bacteria
totipotent stem cells
can give rise to complete human/all cell types
- translate only part of their DNA, producing specific proteins leading to cell specialisation
- divide by mitosis
- occurs during zygote, up to 8 cell stage
pluripotent
- divide by mitosis
- only translate part of their DNA
- can only give rise to some cell types (ie: cells that make up tissue in the mammalian feotus)
- embryonic and fetal stem cells
multipotent
- exist in tissues
- retain the ability to differentiate into a limited number of cell types (eg: bone marrow only RBCs and WBCs)
- used for vital growth and repair of damaged tissues
- adult stem cells
unipotent
give rise to only one type of cell
(eg: heart stem cells differentiate into cardiomyocytes)
- used for vital growth and repair of damaged tissues
key points
- only stem cells can divide by mitosis
- differentiation of stem cells determined by gene expression (only some parts of dna expressed, others inactive)
explain how cells produced from stem cells ahve the same genes yet be of different types?
not all genes are switched on/active
- gene expression involves transcription factors (proteins that bind a genes promoter region and either promote or inhibit the transcription of a gene)
describe the mechanism by which a signal protein causes the synthesis of mRNA
- signal protein binds to the receptor on surface membrane
- messenger molecule moves from cytoplasm and enters nucleas
- activate transcription factor
- binds to promoter region
- RNA polymerase transcribes target gene
applications for stem cells
- produces tissues for skin grafts
- organ transplant research
- research into how cells become specialised
- cancer research
stem cell concerns
- may divide rapidly out of control, forming tumours
- embryos have human status from mone tof conception
- not true human being, ebnefits outweigh risks
- no moral rights
iPSC
induced pluripotent stem cells
- lab grown pluripotent cells prpduced from somatic cells using transcription factors
-capable of self renewal and limitless supply
promoter region
one or more base sequences found upstream of a gene
control the expression of that gene
transcription factors
proteins, when activated, bind to the promoter region of the gene
stimulates RNA polymerase to begin transcription of the target gene
describe how oestrogen enables RNA polymerase to begin transcription of its target gene
- oestrogen diffuses via phospolipid cell membrane and diffuses through the nuclear envelope (is lipid soluble)
- binds to the oestrogen receptor
- changes the 3 structure of the receptor
- releases transcription factor which binds to the dna
- at the promoter region
- stimulates RNA polymerase to transcribe the gene
rna interference
miRNA = microRNA
small interfering RNA = siRNA
- single strand of miRNA and siRNA binds to a protein in cytoplasm to form a RISC (RNA-induced silencing) complex
- complementary base sequence to specific mRNA molecule
RISC
RISC inhibits gene expression by binding to complementary mRNA; mRNA hydrolysed by enzyme into fragments OR initiation of ribosomal translation inhibited.
- translation doesnt take place, polype[tide not produced
- expression of gene silenced
why might a protein still be present even after the introduction of RNAi?
not all mRNA has been destroyed
so some translation still occurs
epigenetics
inheritable changes in gene expression
without changes in DNA base sequence
inhibits translation via methylation of DNA / acylation of histones
compare the structure of dsRNA and DNA
dsRNA = double stranded RNA
- polymer of nuclepotides
- A,C and G
pentose sugar
double stranded
- dsRNA contains U, DNA contains T
- dsRNA ribose DNA deoxyribose
dsRNA shorter than DNA
explain how methylation of tumour suppressor genes can lead to cancer
-methylation prevents transcription of a gene
-protein that prevents cell division not produced
-no control of mitosis
process of methylation
CH3 added to c5 of cytosine base
methyltransferase enzyme catalyses this reaction
cpg islands silence genes by preventing TF binding to promoter
process of acetylation of histones
DNA in chromosomes wrapped around histones creating nuclosomes
- become loosely packed so dna less condensed
- promoter regions are exposed TF bind
- RNA polynerase to bind an dtranscribe target gene
describe how alterations of tumour suppressor gene can lead to development of tumours
- increased methylation of TSG
- mutation in TSG
- TSG not transcribed
- uncontrolled cell division
DESCRIBE what is meant by a MALIGNANT tumour
mass of undifferentiated cells
- uncontroled cell division
- metastasis forms new tumours
- spread to other parts of body
- fast growing
- non capsulated
describe benign tumours
surrounded by a capsuke
dont metastasise
slow growing
describe how altered DNA can lead to cancer
- dna altered by mutation
- mutation changes base sequence
- of gene controlling cell growth (oncogene)
-of TSG
changes protein structure
TSG produces proteins that inhibit cell division - mitosis: uncontrolled cell division forming malignant tumours
define epigenetics
heritable changes not involving changes in DNA base sequence
proto-oncogenes
stimulate cell division
codes for proteins that increase rate of CD
Tumour suppresor genes
slow cell division
code for proteins decreasing rate of CD
- code for proteins repair mistakes in DNA
- code for proteins instructing cells to die
eg: BRCA2, TP53
mutations in genes
proto oncogene mutation causes oncogene
- overstimulation of cd so persmanently switched on
tsg mutation = inactivated
- stops inhibiting cd, rate of cd increases
epigenetic changes to TSG
increased methylation of tsg increases rate of cd
increased acetylation of tsg decreases rate of cd
epigenetic changes to oncogene
decreased methylation increases rate of cd
decreased acetylation of histones decreases rate of cd
