Mutations and Gene expression Flashcards
what is a gene mutation
● A change in the base sequence of DNA
● Can arise spontaneously during DNA replication (interphase)
What is a mutagenic agent?
A factor that increases rate of mutation, eg. ultraviolet (UV) light or alpha particles
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 codons on mRNA
- So changes sequence of amino acids in the encoded polypeptide
- So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
- So changes tertiary structure (shape) of protein
- Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form
what is a substitution gene mutation
A base / nucleotide is replaced by a different base / nucleotide in DNA
what is an addition gene mutation
1 or more bases / nucleotides are added to the DNA base sequence
what is a deletion gene mutation
1 or more bases / nucleotides are lost from the DNA base sequence
what is a duplication gene mutation
A sequence of DNA bases / nucleotides is repeated / copied
what is an inversion gene mutation
A sequence of bases / nucleotides detaches from the DNA sequence, then rejoins at the same position in the reverse order
what is a translocation gene mutation
A sequence of DNA bases / nucleotides detaches and is inserted at a different location within the same or a different chromosome
Explain why not all gene mutations affect the order of amino acids
● Some substitutions change only 1 triplet code / codon which could still code for the same amino acid
○ As the genetic code is degenerate (an amino acid can be coded for by more than one triplet)
● Some occur in introns which do not code for amino acids
explain why a change in amino acid sequence is not always harmful
● May not change tertiary structure of protein (if position of ionic / disulphide / H bonds don’t change)
● May positively change the properties of the protein, giving the organism a selective advantage
What are stem cells?
Undifferentiated / unspecialised cells capable of:
1. Dividing (by mitosis) to replace themselves indefinitely
2. Differentiating into other types of (specialised) cells
Explain what is meant by a frameshift
● occurs when gene mutations change the number of nucleotides / bases by any number not divisible by 3
● This shifts the way the genetic code is read, so all the DNA triplets/ mRNA codons downstream from the mutation change
● The sequence of amino acids encoded changes accordingly and the effects on the encoded polypeptide are significant
Describe how stem cells become specialised during development
● Stimuli lead to activation of some genes (due to transcription factors )
● So mRNA is transcribed only from these genes and then translated to form proteins
● These proteins modify cells permanently and determine cell structure / function
Describe totipotent cells
● Occur for a limited time in early mammalian embryos
● Can divide AND differentiate into any type of body cell (including extra-embryonic cells eg. placenta)
Describe pluripotent cells
● Found in mammalian embryos (after first few cell divisions)
● Can divide AND differentiate into most cell types (every cell type in the body but not placental cells)
Describe multipotent cells
● Found in mature mammals
● Can divide AND differentiate into a limited number of cell types
Describe unipotent cells, using an example
● Found in mature mammals
● Can divide AND differentiate into just one cell type
Explain how stem cells can be used in the treatment of human disorders
● Transplanted into patients to divide in unlimited numbers
● Then differentiate into required healthy cells (to replace faulty / damaged cells)
give examples of how stem cells are used in the treatment of human disorders
● Potential treatment of Type 1 diabetes by creating healthy islet cells that produce insulin
● Bone marrow stem cell transplant for SCD / blood cancers
1. Destroy patient’s bone marrow before treatment → so no faulty cells are produced
2. Transplant stem cells from healthy person → divide and differentiate into healthy cells
Explain how induced pluripotent stem (iPS) cells are produced
- Obtain adult somatic (body) cells (non-pluripotent cells or fibroblasts) from patient
- Add specific protein transcription factors associated with pluripotency to cells so they express
genes associated with pluripotency (reprogramming)
○ Transcription factors attach to promoter regions of DNA, stimulating or inhibiting transcription - Culture cells to allow them to divide by mitosis
What are transcription factors?
● Proteins which regulate (stimulate or inhibit) transcription of specific target genes in eukaryotes
● By binding to a specific DNA base sequence on a promoter region
Describe how transcription can be regulated using transcription factors
- Transcription factors move from cytoplasm to nucleus
- Bind to DNA at a specific DNA base sequence on a promoter region (before / upstream of target gene)
- This stimulates or inhibits transcription (production of mRNA) of target gene(s) by helping or
preventing RNA polymerase binding
Explain how oestrogen affects transcription
- Oestrogen is a lipid-soluble steroid hormone so diffuses into cell across the phospholipid bilayer
- In cytoplasm, oestrogen binds to its receptor, an inactive transcription factor, forming an oestrogen-receptor complex
- This changes the shape of the inactive transcription factor, forming an active transcription factor
- The complex diffuses from cytoplasm into the nucleus
- Then binds to a specific DNA base sequence on the promoter region of a target gene
- Stimulating transcription of target genes forming mRNA by helping RNA polymerase to bind
Explain why oestrogen only affects target cells
Other cells do not have oestrogen receptors.
Describe what is meant by epigenetics
● Heritable changes in gene function / expression without changes to the base sequence of DNA
● Caused by changes in the environment (eg. diet, stress, toxins)
Describe what is meant by epigenome
All chemical modification of DNA and histone proteins - methyl groups on DNA and acetyl groups on histones.
explain how methylation can inhibit transcription
- Increased methylation of DNA - methyl groups added to cytosine bases in DNA
- So nucleosomes (DNA wrapped around histone) pack more tightly together
- Preventing transcription factors and RNA polymerase binding to promoter
explain how acetylation can inhibit transcription
- Decreased acetylation of histones increases positive charge of histones
- So histones bind DNA (negatively charged) more tightly
- Preventing transcription factors and RNA polymerase binding to promoter
what factors can lead to epigenetic changes
Environmental factors (eg. diet, stress, toxins)
What is RNA interference (RNAi)?
● Inhibition of translation of mRNA produced from target genes, by RNA molecules eg. siRNA, miRNA
● This inhibits expression of (silencing) a target gene
Describe the regulation of translation by RNA interference
- Small interfering RNA (siRNA) or micro-RNA (miRNA) binds to a protein, forming an RNA-induced silencing complex (RISC)
○ siRNA synthesised as double-stranded RNA → 1 strand incorporated
○ miRNA synthesised as a double-stranded hairpin bend of RNA → both strands incorporated - Single-stranded miRNA / siRNA within RISC binds to target mRNA with a complementary base sequence
- This leads to hydrolysis of mRNA into fragments which are then degraded OR prevents ribosomes binding
- Reducing / preventing translation of target mRNA into protein
Describe how tumours and cancers form
● Mutations in DNA / genes controlling mitosis can lead to
uncontrolled cell division
● Tumour formed if this results in mass of abnormal cells
○ Malignant tumour = cancerous, can spread by metastasis
○ Benign tumour = non-cancerous
main characteristics of benign tumours
-Usually grow slowly (cells divide less often)
-Cells are well differentiated / specialised
-Cells have normal, regular nuclei
-Well defined borders and often surrounded by a capsule so do not invade surrounding tissue
-Do not spread by metastasis (as cell adhesion molecules stick cells together
-Can normally be removed by surgery and they rarely return
main characteristics of malignant tumours
-Usually grow faster (cells divide more often)
-Cells become poorly differentiated / unspecialised
-Cells have irregular, larger / darker nuclei
-Poorly defined borders and not encapsulated so can invade surrounding tissues
-Spread by metastasis - cells break off and spread to other parts of the body, forming secondary tumours (due to lack of adhesion molecules)
-Can normally be removed by surgery combined with radiotherapy / chemotherapy but they often return
Describe the function of tumour suppressor genes
Code for proteins that:
● Inhibit / slow cell cycle (eg. if DNA damage detected)
● OR cause self-destruction (apoptosis) of potential tumour cells (eg. if damaged DNA can’t be repaired)
Explain the role of tumour suppressor genes in the development of tumours
● Mutation in DNA base sequence → production of non functional protein
○ By leading to change in amino acid sequence which changes protein tertiary structure
● Decreased histone acetylation OR increased DNA methylation → prevents production of protein
○ By preventing binding of RNA polymerase to promoter region, inhibiting transcription
● Both lead to uncontrolled cell division (cell division cannot be slowed)
Describe the function of proto-oncogenes
Code for proteins that stimulate cell division
Explain the role of oncogenes in the development of tumours
● Mutation in DNA base sequence → overproduction of protein OR permanently activated protein
○ By leading to change in amino acid sequence which changes protein tertiary structure
● Decreased DNA methylation OR increased histone acetylation → increases production of protein
○ By stimulating binding of RNA polymerase to promoter region, stimulating transcription
● Both lead to uncontrolled cell division (cell division is permanently stimulated)
Suggest why tumours require mutations in both alleles of a tumour suppressor gene but only one allele of an oncogene
● One functional allele of a tumour suppressor gene can produce enough protein to slow the cell cycle
● One mutated oncogene allele can produce enough protein to lead to rapid / uncontrolled cell division
Explain the relevance of epigenetics in cancer treatment
Drugs could reverse epigenetic changes that caused cancer, preventing uncontrolled cell division. For example:
● Increasing DNA methylation OR decreasing histone acetylation of oncogene
○ To inhibit transcription / expression
● Decreasing DNA methylation OR increasing histone acetylation of tumour suppressor gene
○ To stimulate transcription / expression
Explain the role of increased oestrogen concentrations in the development of some (oestrogen receptor-positive) breast cancers
- 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
Suggest how drugs that have a similar structure to oestrogen help treat oestrogen receptor-positive breast cancers
● Drugs bind to oestrogen receptors (inactive transcription factors), preventing binding of oestrogen
● So no / fewer transcription factors bind to promoter regions of genes that stimulate the cell cycle
