Unit 8: control of gene expression Flashcards
what is a gene mutation
a change in the base sequence of DNA on chromosomes that can arise spontaneously during DNA replication in the interphase
explain how a gene mutation can lead to the production of a non-functional protein or enzyme
- changes sequence of base triplets in DNA so changes sequence of condons on mRNA
- so changes sequence of amino acids in the encoded polypeptide
- so changes position of hydrogen/ionic/disulphide bonds between amino acids
- changes tertiary structure of protein
- enzymes active site changes shape, so substrate cant bind, ESC cannot form
what are the different types of mutations
substitution
addition
deletion
duplication
inversion
translocation
what are insertion/deletion mutations and what are the effects of this mutation
where one or more nucleotide pairs are inserted or deleted from the sequence. this type of mutation alters the sequence of nucleotides after the insertion/deletion point, known as frameshift
3 effects of substitution
-formation of stop codon, which will stop production of the polypeptide prematurely, protein produced wont be functional
-formation of a codon for a different amino acid, polypeptide may differ in shape and be dysfunctional
-different codons produces the same amino acid because code is degenerate so the polypeptide produced is the same
what is a duplication and what is the effect of this mutation
a sequence of DNA bases are repeated and therefore produces a frameshift
what is inversion and what effect does this mutation have
a sequence of bases detach from the DNA sequence and then rejoin at the same position but in reverse order, this therefore affects the amino acid that is produced
what is translocation and what effect does this mutation have
a sequence of DNA bases detaches and is inserted at a different location within the same or a different chromosome
this can often lead to significant effects on the phenotype
what is the effect of translocation on bases
leads to an abnormal phenotype
e.g. development of some cancers or reduced fertility
what is ‘frameshift’ mutation
occurs when mutations change the number of nucleotides/bases by a number not divisible by 3
-this shifts the way that the genetic code is read, so all the DNA triplets/ mRNA codons downstream from the mutation change
explain why not all gene mutations affect the order of amino acids
some substitutions change only one triplet code/codon which could still code for the same amino acid as the genetic code is degenerate
-some also occur in introns which do not code for amino acids as they are removed during splicing
explain why a change in amino acid sequence is not always harmful
may not change the tertiary structure of the protein and instead may positively change the properties of the protein, giving the organism a selective advantage
what are mutagenic agents and examples
factors that increase the rate of mutations occurring
e.g. radiation (UV), chemicals (benzene) and some viruses (HPV)
how can mutagenic agents increase the rate of mutations
deleting bases or changing their chemical structure so that they pair with bases that they wouldn’t normally do so.
-they can also change the structure of the dna itself, which can cause problems during dna replication
what are acquired mutations
mutations which occur after fertilisation
-if these occur in the genes that control mitosis, can cause uncontrolled cell division and hence may result in a tumor
describe how tumours and cancers form
mutations in DNA/genes controlling mitosis can lead to uncontrolled cell division
tumour formed if this results in a mass of abnormal cells
benign tumors
non invasive
-usually grow slowly
-cells have normal/regular nuclei
-cells are well differentiated
malignant tumors
invasive
-grow rapidly
-invade and destroy surrounding tissue
-cells can break off and travel around in the blood or lymphatic system
function of tumour suppressor genes
code for proteins that:
-inhibit/slow cell cycle
-or cause self destruction of potential tumour cells
what role do tumor suppressor genes have in the development of tumors
-mutation in DNA base sequence leads to the production of a non-functional polypeptide due to a change in the amino acid sequence which changes the proteins tertiary structure.
-decreased histone acetylation or increased DNA methylation prevents the production of protein as it prevents the binding of RNA polymerase to the promoter region, inhibiting transcription
-both lead to uncontrolled cell division
function of (proto-)oncogenes
code for proteins that stimulate cell division
what role do oncogenes have in the development of tumors
-when a mutation occurs in the DNA base sequence of a proto-oncogene, the gene can become overactive causing an overproduction of protein produced or a permanently activated protein
-decreased DNA methylation or increased histone acetylation increases the production of protein by stimulating the binding of RNA polymerase to the promoter region, stimulating transcription
-both lead to uncontrolled cell division
(an oncogene is a mutated form of the corresponding proto-oncogene)
how can increased oestrogen concentration lead to tumors forming
- some breast cancers cells have oestrogen receptors, which are inactive transcription factors
- if oestrogen concentration is increased, more oestrogen binds to oestrogen receptors,
forming more oestrogen-receptor complexes which are active transcription factors - these bind to promoter regions of genes that code for proteins stimulating cell division
- this increases transcription / expression of these genes, increasing the rate of cell division
why can it be difficult to interpret data on the risk factors of cancer
-some cancers are polygenic- triggered by more than one gene
-some cancers triggered by many environmental factors- difficult to know which environmental factors are having the greatest effect
-can have a control group of lab animals but not people