mutations, gene expression and cancer Flashcards
what is meant if the codon is universal
the code is for all organisms, with same triplet coding for the same amino acids
what is meant if the code is non overlapping
the ribosome reads each base only once in the codons, first three read first followed by the second three, etc
what is meant if the code is degenerate
more than one triplet codes for an amino acid
what is a gene mutation
a change in the sequence of bases in a gene
what happens when a mutation takes places in an exon
change in sequence of amino acids, affecting the positions of different bonds, changing the tertiary structure, results in faulty protein
what happens if a mutation occurs in an intron
an intron is a non coding region so there will be no affect as there is no change in the mrna being translated into amino acids
OR
the mutation in the intron could prevent mrna splicing in the nucleus so faulty mrna is produced and a different amino acid sequence is translated
what are addition / deletion mutations
complete loss / gain of a base in a region of DNA coding for a gene
results in alteration in the base sequence , causing a frame shift - all base triplets downstream of the mutation are altered
what are substitution mutations
substitution of one base for another
however as the genetic code is degenerate it will not always change the encoded amino acid - this is known as a silent mutation
could cause a change in sequence and therefore change in tertiary structure etc
could cause a stop codon to be produced causing the polypeptide chain to terminate prematurely
what is an inversion mutation
when a segment of bases is reversed end to end
what is a duplication mutation
a doubling of sequence of base pairs so resultant cells have more than 2 copies of a gene
what is a translocation mutation
when groups of base pairs relocate from one area of the genome to another, usually between non homologous chromosomes
what are the causes of mutations
can occur spontaneously during DNA replication
mutation rates increase with the presence of mutagenic agents e.g. x rays and gamma rays which damage DNA molecules and chemicals that alter the DNA structure or interfere with DNA replication
what is a totipotent cell
cells that can divide into any type of body cell or whole organism
what is a pluripotent cell
stem cells that can divide in unlimited numbers and can be used in treating human disorders
can be isolated from spare embryos produced in fertility clinics - the embryo can be grown and pluripotent cells can be isolated.
how is a totipotent cell formed
occur for a limited time in mammalian embryos and can develop into any type of cell
most cells loose their totipotency as they differentiate to carry out specific functions - they become limited in the parts of their DNA they translate
this is because during cell specialisation only the genes required for essential processes within the cell and those needed to produce proteins required for specialised function of the cell are expressed
what are multipotent cells
a small number of cells within the adult mammal retain the ability to differentiate into a limited number of different cells
what are unipotent cells
only capable of giving rise to only one cell type
certain parts of DNA are inactive, genes are switched on that are needed and others switched off e.g. cardiomyocytes
what are induced pluripotent cells
laboratory grown pluripotent cells that scientists have found can be produced from unipotent cells using certain protein translation factors
what can stem cells be used for
1) producing tissues for skin graphs
2) research into producing organs for transplant
3) research into how cells become specilaised
4) research into cancer
5) research into serious disease and the use of stem cells to cure such diseases as parkinsons disease
what are ethical concerns about the use of stem cells
- not right to use stem cells from embryos as they have human status from the moment of contraception
- however some believe that embryos have no moral rights
- use of stem cells also questioned as animals must be used in experiments with stem cells before the stem cells can be trialled on humans
- concerns with IPS technology also include the patient donor giving consent for their cells uses
what does it mean if a gene is “ switched on “
greater transcription of a gene, so more mrna is produced, more translation takes place
how can transcription of target genes be stimulated or inhibited
when specific transcriptional factors move from the cytoplasm into the nucleus
when is RNA plymerase activated
to transcribe a gene only when specific transcription factors bind to the genes promotor
what is the promotor region
one or more base sequences found upstream of a gene that controls the expression of that gene
what are transcription factors
protiens which when activated bind to the promotor region of a gene stimulating RNA polymerase to transcribe the target gene
describe the control of transcription
1) the transcription factors bind to promotor region upstream of the gene
2) this stimulates RNA polymerase to bind to the gene
3) to make mRNA / transcription
describe the oestrogen control of transcription
1) oestrogen is lipid soluble and passes through the cell membrane and nuclear membrane into the nucleus of the cell
2) oestrogen binds to a complementary receptor ( ER alpha ) - inactive transcription factor
3) ER alpha oestrogen receptor changes shape and is released from the protien complex which its inhibited its action
4) oestrogen binds to promotor region of target gene and stimulates RNA polymerase to transcribe the target gene
what are the two types of RNA interference that can inhibit translation of mRNA
- microRNA ( miRNA )
- small interfering RNA ( siRNA )
how does RNA interference work
a single strand of miRNA and siRNA can bind to a protien in the cytoplams to form a complex, called an RNA induced silenceing complex ( RISC ). The miRNA / siRNA has a complementary base sequence to part of a specific mRNA molecule.
the RISC blocks gene expression by binding to the mRNA with the following outcomes :
- siRNA binds to the target mRNA and the mRNA is hydrolysed into fragments by an enzyme - RNA hydrolyse
- miRNA binds to the target mRNA and prevents ribosomes from attatching to the mRNA, blocking translation
translation does not take place, polypeptide is not produced and the expression of the gene has been blocked
what is epigenetics
inheritable changes in gene function without changes to the DNA base sequences
these inheritable changes are caused by changes in the genes environment that inhibit transcription by increased methylation of DNA or decreased acyetylation of histones
what is methylation of DNA
a methyl group is added to carbon atom 5 of a ctyosine residue
happens most commonly where there is a high proportion of cytosine followed by guanine residues ( CpG islands )
methyltransferase catalyses the reaction
repeated CpG sequences are common at the 5 end of many genes e.g near the promotor region
during development methylation of these islands silences the affected genes by preventing activation of RNA polymerase, as RNA polymerase cant bind, the gene does not get transcribed
what is actylation of histones
the chromosomal DNA of eukaryotes is wrapped around molecules of the protien histone, forming bead-like nucleosomes. The winding may be tight or loose.
Histones molecules have side branches or “ tails “
these “ tails “ contain the amino acid leucine. Leucine can be acytlated e.g. an aceytl group is transferred to from acetyl - coA
when acetylated, the histones become more loosely packed and the promotor and target genes are accessible by transcription factors and RNA polymerase and the target genes can be transcribed.
what is the difference between methylation and acetylation
M switches off genes and prevents transcription by inhibiting RNA polymerase from binding, whereas A switches genes on and allows transcroiption
M adds a methyl group to cytosine residues of DNA whereas A causes DNA to be looselt packed around histones, target genes and become accessible to transcription factors and RNA polymerase
what is the difference between benign tumours and maliganant
begnin are capsulated by connective tissue, slower growing and do not metastaisse wherease malignant tumours are fast grwoing, non capsulated and they do not metastasise.
what are proto - oncogenes
stimulates cell division
what are tumour suppressor genes
code for protiens which inhibit cell division
how do tumours develop
if a DNA mutation occurs in a proto - oncogene, it alters to become an oncogene. This results in an over stimulation of a cell division, so that cell division is permanently switched on, resulting in a tumour.
If DNA mutation occurs in a tumour supressor gene, the gene becomes inactived, so the protiens coded for stop inhibiting cell division, so rate of cell division increases.
how do epigenetics play an important part in development of tumours
if the tumour supressor gene is over methylated, the gene becomes inactivated, so the protiens coded for stop inhibiting cell division, so the rate of cell division increases
if an oncogene is under methylated the gene becomes more activated, so that cell division is switched on, so the rate of cell division increases.
