control of gene expression Flashcards
what is a point mutation
the change of 1 base as it occurs at only 1 point in the sequence
what are the 3 types of substitution mutation
- nonsense
- mis-sense
- silent
what is a nonsense mutation
an amino acid codon mutates to become a stop codon, stopping protein synthesis (translation)
what is a mis-sense mutation
an amino acid sequence changes, possibly changing the structure and function of the protein
what is a silent mutation
the genetic code is degenerate so a mutation doesnt always change the AA sequence
what is an inversion mutation
a group of bases separates from dna sequence and rejoins at the same position but in inverse order. the base sequence is reversed and affects the resulting AA sequence
what is a duplication mutation
can be just 1 base duplicated (has the same effect as an addition). causes a frame shift to the right. if 3 bases are duplicated, a new AA is added
what is a translocation mutation
a base group separates from a dna sequence on 1 chromosome and becomes inserted into the dna sequence of a different chromosome. the mutations often have significant effects on gene expression, leading to an abnormal phenotype
examples of mutagens
- high energy radiation (uv radiation and x-rays)
- carcinogens in cigarettes and car exhaust fumes
- alcohol
effect of mutations in somatic cells (non-reproductive body cells)
it will only affect cells that derive from that cell, so will likely have a small local effect like a birthmark
effect of mutations in germ-line cells (reproductive cells/gametes)
they will affect every single cell of the resulting organism as well as its offspring. these mutations are one source of genetic variation
what are stem cells
undifferentiated cells that can divide by mitosis to form more stem cells (keep dividing and make more cells) OR cells that can differentiate into a specific cell type
why do different cells make different proteins
different genes are expressed/transcribed and then translated/switched on. some genes are switched on in all cells e/g those involved in respiration while some are switched on/off as required
how can gene expression be controlled
- prevention of transcription so prevention of mRNA production
- prevention of translation
key features of stem cells
- potency - have the potential to differentiate into specialised cell types
- immortality - they can divide indefinitely
ethical concerns of stem cell use
can be objected to on religious grounds if a person believes life begins at conception
types of stem cell: totipotent
can differentiate into any cell type. found in early embryos up to the 32 cell stage
types of stem cell: pluripotent
can differentiate into nearly all cell types. found in blastocytes (5-day old embryos containing ~150 cells before implantation)
types of stem cell: multipotent
found in some tissues throughout life so are adult/somatic stem cells. can differentiate into a related family of cells (few cell types). used by the body to replace and repair damaged tissues. have a limited capacity to the types of cell they can change into
types of stem cell: unipotent
can only differentiate into a single type of cell. derived from multipotent cells and are made in adult tissues
sources of animal stem cells
- embryos - unused from IVF (any cell type) (licensed research is only allowed on embryo cells created outside the body)
- adult cells (few cell types)
- umbilical cord (limited cell types)
- placental cells (limited cell types)
what are induced pluripotent stem cells (iPS cells)
a type of pluripotent cell that is produced from a unipotent cell (derived from multipotent) - the unipotent cell can be any body cell
what are iPS cells used for
they are genetically altered in the lab to make them acquired characteristics of embryonic stem cells. this involves inducing genes and transcription factors within the cell i.e turning on genes that were switched off
features of iPS cells
- very similar to embryonic stem cells in form and function but are not exact duplicates
- capable of self-renewal so therefore could replace embryonic stem cells in medical research and treatment, thus overcoming many ethical issues
what is regulation of protein synthesis
regulating the switching on or off of genes and hence determining which genes are expressed
how can transcription be regulated
- for transcription to occur, the gene must be stimulated by specific transcriptional factors
- these move from the cytoplasm into the nucleus via a nuclear pore and bind to a specific region (the promoter region) of dna in the nucleus stimulating it to begin transcription
- these TFs are associated with a receptor and an inhibitor in the cytoplasm
- if the receptor is not stimulator then the inhibitor will prevent the TF moving into the nucleus and therefore prevent transcription occurring
regulation of transcription example: oestrogen
- oestrogen is a lipid soluble molecule meaning it can easily cross the phospholipid bilayer and enter the cytoplasm
- it can then bind to the receptor associated with the TF. this causes a conformational change in the molecule which moves the inhibitor from the TFs binding site
- the TF can now enter the nucleus through a nuclear pore and combine with the promoter region of the dna
- the combination of the TF and the dna stimulates transcription of the gene that makes up that part of dna
overview: siRNA
- small interfering rna
- control of translation
- can act as repressors
- 20-25 nucleotides in length
- double-stranded rna
- breaks down mRNA before translation
how does siRNA break down mRNA before translation
- an enzyme (a dicer) cuts large double-stranded rna (dsRNA) into smaller bits (siRNA)
- 1 of 2 siRNA strands (anti-sense) combines with an enzyme. this enzyme is not actually an enzyme - RISC (RNA induced silencing complex)
- the siRNA molecule guides the RISC to an mRNA molecule by pairing its bases with the complementary bases on the mRNA. the enzyme now cuts the mRNA into smaller sections
- the mRNA is no longer able to be used in translation so no polypeptide is made and therefore the gene is not expressed even though the dna codes for it
what are the uses of siRNA
- identifying the roles of genes in a biological pathway. siRNA that blocks a gene could be added to cells and the effects observed to see the role of the blocked gene
- gene-caused diseases could be prevented by using siRNA to block these genes
- could be used to prevent HIV replication in cultures
- could silence oncogenes (cancer treatment)
what is epigenetics
the idea that parents pass on more than genes to their offspring. environmental influences can alter the inheritance of an organism’s offspring. acquired characteristics can be passed on
what is the epigenome
dna and histones are covered in chemicals (tags). this second layer of tags is called the epigenome. it determines the shape of the dna-histone complex
what is methylation
the addition of a methyl group (CH3) (acts as a tag) (to a nerve cell) causes tighter curling of myelin protein around the nerve cell. CH3 is added to the C bases of dna
how can an environmental signal affect genetics
- an environmental signal stimulates proteins to carry its message inside the cell
- here the message passes to a specific protein which can be attached to a specific sequence of bases on the dna
- once attached, the protein can either change:
1. acetylation of histones leading to the activation or inhibition of a gene
2. methylation of dna by attracting enzymes that can add or remove methyl groups
what is acetylation
the process whereby an acetyl group is transferred to a molecule
what is deacetylation
the removal of an acetyl group from a molecule
what is the effect of decreased acetylation of associated histones
decreased acetylation increases the positive charges of histones and therefore increases their attraction to the phosphate groups of dna. this makes the associated between the dna and histones stronger so the dna is less accessible to TFs. the TFs cant initiate mRNA production (transcription) from the dna i.e the gene is switched off
how can increased methylation of dna inhibit the transcription of genes
- preventing the binding of TFs to the dna by causing the surrounding myelin protein to curl tighter around it, making it inaccessible to TFs
- attracting proteins that condense the dna-histone complex (by inducing deacetylation of the histones) making the dna inaccessible to TFs
