A-level Biology - 3.4.9 Regulation of Transcription and Translation
Epigenetics is the study of how gene expression is regulated without altering the DNA sequence itself. It involves chemical modifications, like DNA methylation or histone modification, which can turn genes on or off in response to environmental factors.
What is epigenetics?
Study of changes to gene expression when there’s no change in the gene itself
Key Terms
What is epigenetics?
Study of changes to gene expression when there’s no change in the gene itself
In eukaryotes, what determines whether a gene is switched on or off? (i.e. whether gene is expressed (transcribed and translated) or not)
Epigenetic Control
How does epigenetic control work?
Works through attachment or removal of chemical groups (aka epigenetic marks) to or from DNA or histone proteins
Epigenetic marks don’t alter...
Epigenetic changes can also occur in response to changes in the ________
environment
e.g. pollution and availability of food
Most epigenetic marks on DNA are ______ between generations
removed
Epigenetic changes can be inherited. What is meant by this?
Means expression of some genes in offspring can be affected by environment changes that affected their parents or grandparents
e.g. epigeneti...
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| Term | Definition |
|---|---|
What is epigenetics? | Study of changes to gene expression when there’s no change in the gene itself |
In eukaryotes, what determines whether a gene is switched on or off? (i.e. whether gene is expressed (transcribed and translated) or not) | Epigenetic Control |
How does epigenetic control work? | Works through attachment or removal of chemical groups (aka epigenetic marks) to or from DNA or histone proteins Epigenetic marks don’t alter base sequence of DNA Alter how easy it is for enzymes and other proteins needed for transcription to interact with and transcribe the DNA |
Epigenetic changes can also occur in response to changes in the ________ | environment e.g. pollution and availability of food |
Most epigenetic marks on DNA are ______ between generations | removed |
Epigenetic changes can be inherited. What is meant by this? | Means expression of some genes in offspring can be affected by environment changes that affected their parents or grandparents e.g. epigenetic changes in some plants in response to drought have been passed on to later generations |
Name 2 methods of epigenetic control | Increased methylation of DNA Decreased acetylation of histones |
What effect does increased methylation of DNA have on a gene? | Switches a gene off |
What effect does decreased acetylation of histones have on a gene? | Switch genes off |
What is a promoter? | Sequence of bases before gene where a transcription factor binds |
Where does methyl groups (example of epigenetic mark) always attach to? | CpG site |
What is a CpG site? | Where cytosine and guanine base are next to each other in DNA |
Explain how increased methylation results in a gene not being expressed | Increases methylation changes DNA structure so transcriptional machinery (enzymes, proteins etc.) can’t interact with gene e.g. If promotor methylated transcription factors (protein) cannot bind and recruit RNA polymerase |
What are histones? | Proteins that DNA wraps around to form chromatin which makes up chromosomes |
Chromatin can be highly _____ or less _____ | Chromatin can be highly condensed or less condensed |
What does “how condensed chromatin is” affect? | The accessibility of DNA and whether or not it can transcribed |
How can histones be epigenetically modified? | By the addition or removal of acetyl groups (example of epigenetic mark) |
Explain how the gene is affected when histones are acetylated | When histones are acetylated, chromatin is less condensed Means transcriptional machinery can access DNA = allows genes to be transcribed |
Explain how the gene is affected acetyl groups are removed from histones | Chromatin becomes highly condensed and genes in DNA can’t be transcribed ∵ transcriptional machinery can’t physically access them |
What are histone deacetylase (HDAC)? | Enzymes responsible for removing acetyl groups |
Give an example of epigenetics that can lead to the development of disease | Abnormal methylation of tumour suppressor gene and oncogenes can cause cancer |
Epigenetic changes are ______ | reversible Makes them good targets for new drugs to combat diseases they cause |
Epigenetics Treatment Describe how the drugs work | Drugs designed to counteract epigenetic changes that cause diseases |
Epigenetics Treatment Describe how a drug would counteract increased methylation | Drugs stop DNA methylation = treat diseases e.g. drug azacitidine is used in chemotherapy of types of cancer caused by increased methylation of tumour suppressor genes |
Epigenetics Treatment Describe how a drug would counteract decreased acetylation of histones (genes switched off) | Drugs (HDAC inhibitor drugs) work by inhibiting activity of histone deacetylase (HDAC) enzymes Results in genes remaining acetylated and proteins they code for being transcribed |
State the problem with developing drugs to counteract epigenetic changes | Is that these change take place normally in lot of cells ∴ have to make drugs as specific as possible e.g. drugs used in cancer therapies can be designed to target dividing cells to avoid damaging normal body cells |
What are transcription factors? | Protein molecules that control the transcription of genes |
In eukaryotes, transcription factors move from the to the ______ | In eukaryotes, transcription factors move from the cytoplasm to the nucleus |
What do transcription factors do once they're in the nucleus? | They bind to specific DNA sites near start of their target genes (genes they control the expression of) They control expression by controlling the rate of transcription |
Give some examples of what transcription factors do | Activators: stimulate or increase the rate of transcription e.g. help RNA polymerase bind to start of target gene and activate transcription Repressors: inhibit or decrease rate of transcription e.g. bind to start of target gene, preventing RNA polymerase from binding, stopping transcription |
Give an example of a molecule (other than transcription factors) that affects the expression of genes | oestrogen |
Describe how oestrogen can initiate the transcription of target genes | Oestrogen binds to oestrogen receptor (transcription factor) = forming oestrogen-oestrogen receptor complex Complex moves from cytoplasm into nucleus & binds to promoter which stimulates RNA polymerase Complex acts as activator of transcription - increases it e.g. helping RNA polymerase bind to start of target gene |
In eukaryotes, gene expression is also affected by ___ ________ | RNA interference (RNAi) |
What is RNAi? | When small, double-stranded RNA molecules stop mRNA from target genes being translated into proteins Similar process to RNAi can also occurs in prokaryotes |
Name 2 molecules involved in RNAi | siRNA Small interfering RNA miRNA microRNA |
What are RNAi molecules? | Small lengths of non-coding RNA (don't code for proteins) |
Describe how siRNA (and miRNA in plants) works | Once mRNA has been transcribed, it leaves nucleus for cytoplasm In cytoplasm, double-standard siRNA associates with several proteins and unwinds A single strand binds to target mRNA Base sequence of siRNA is complementary to base sequence in sections of target mRNA Proteins associated with siRNA cut mRNA into fragments - so no longer translated Fragments move into processing body, which contains ’tools’ to degrade them |
Describe how miRNA in mammals differentiates to siRNA | In mammals, miRNA isn’t fully complementary to target mRNA Less specific than siRNA & so it may target more than one mRNA molecule |
Describe how miRNA in mammals works | miRNA associates with proteins and binds to target mRNA in cytoplasm by specific base pairing miRNA-protein complex physically blocks the translation of the target mRNA by preventing mRNA being read by ribosomes mRNA is then moved in processing body, where it can either be stored or degraded When it’s stored, it can be returned and translated at another time |
What are stem cells? | Unspecialised cells that can develop into any type of cell |
Name 2 places where stem cells are found | Found in embryo Where they become all the specialised cells needed to form a foetus Found in some adult tissues Where they become specialised cells that need to be replaced |
Name 4 types of stem cells | Totipotent Pluripotent Multipotent Unipotent |
What is meant by totipotent? | Stem cells that can mature into any type of body cell in an organism, including placenta |
Where are totipotent stem cells found? | Only present in mammals in first cell divisions of an embryo. After this point become pluripotent. |
What is meant by pluripotent? | Can specialise into any cell in body but lose ability to become cells that make up placenta |
Name the stem cells present in adult mammals | Multipotent Unipotent |
What is meant by multipotent? | Stem cells that are able to differentiate into few different types of cells e.g. red and white blood cells can be formed from multipotent stem cells found in bone marrow |
What is meant by unipotent? | Stem cells that are able differentiate into 1 type of cell e.g. type of unipotent stem cells can only divide to produce epidermal skin cells |
Stem cells contain all the ___ genes | same But during development, not all of them are transcribed and translated |
Describe how stem cells become specialised | Some genes are expressed and others are switched off mRNA is only transcribed from specific genes mRNA from these genes is then translated into proteins These proteins modify the cell Determine cell structure and control cell processes Changes to cell produced by proteins cause cell to become specialised |
What are cardiomyocytes? | Heart muscles that make up lots of tissue in hearts |
Stem cell therapies already exist for some diseases affecting the blood and immune system. Give an example of one. | Bone marrow contains stem cells that can specialise into any type of blood cell Bone marrow transplants used to replace faulty bone marrow in patients that produce abnormal blood cells Stem cells then divide and specialise to produce healthy blood cells |
Name 5 things we could potentially treat using stem cells | Spinal cord injuries Stem cells replace damaged nerve tissue Heart disease and damage caused by heart attacks Stem cells used to replace damaged heart tissue Respiratory diseases Donated windpipes can be stripped down to their simple collagen structure & then covered with tissue generated by stem cells Then transplanted Bladder conditions Stem cells used to grow whole bladders and then implanted Organ transplants Organs grown from stem cells to provide new organs for people on donor waiting lists |
Name and describe 2 benefits of stem cells | Save many lives Many people waiting for organ transplants die before donor organ becomes available Stem cells could be used to grow organs Improve quality of life Stem cells could be used to replace damaged cells in eyes of people who are blind |
Name 3 types of human stem cells | Adult Stem Cells Embryonic Stem Cells Induced Pluripotent Stem Cells (iPS Cells) |
Where are adult stem cells obtained from? | Bone marrow |
Name some advantages of using adult stem cells | Simple operation Little risk |
Name some disadvantages of using adult stem cells | Adult stem cells aren't as flexible as embryonic stem cells = multipotent |
Where are embryonic stem cells obtained from? | Obtained from embryos at early stage of development |
Describe how embryonic stem cells obtained from embryos | Embryos created in lab using in vitro fertilisation Once embryos are 4-5 days old, stem cells are removed from them & rest of embryo is destroyed |
What type of stem cells are embryonic stem cells? | Pluripotent |
Describe how induced pluripotent stem cells (iPS Cells) are made briefly | iPS cells are created by scientists in lab Process involves 'reprogramming' specialised adult body cells so they become pluripotent |
Describe how adult body cells are reprogramed so they become pluripotent | Adult cells made to express a series of transcription factors that are associated with pluripotent stem cells Transcription factors cause adult body cells to express genes associated with pluripotency |
Describe one way of introducing transcription factors to adult cells | Infect them with specifically-modified virus Virus has genes coding for transcription factors within its DNA When virus infects adult cell, genes are passed into adult cell's DNA = cell can produce transcription factors |
State the ethical issue of using embryos | Some believe movement of fertilisation an individual is formed that has right to life Wrong to destroy embryos Destruction of embryo that could become fetus if placed in womb |
State the ethical pros of using stem cells | Use stem cells obtained from egg cells that haven't been fertilised by sperm but has been artificially activated to start dividing ∵ cells couldn't survive past a few days and wouldn't produce fetus if placed in womb iPS cells = potential to be as flexible as embryonic stem cells but obtained from adult tissue Use adult stem ∵ doesn't involve destruction of embryo |
State an advantage of using iPS cells | Made from patient's own cells Would be genetically identical to patient's cells Patient's body wouldn't reject it |