8A 2 Mutations and Gene Expression- stem cells, regulation and expression

What are totipotent stem cells?

stem cells- unspecialised cells- that are able to mature into any type of body cell. Only found in mammals in the first few cell divisions of an embryo.

What are pluripotent stem cells?

embryonic stem cells that can specialise into any cell in the body but lose the ability to become cells that make up the placenta.

What stem cells can be present in adult mammals?

multipotent stem cells- able to differentiate into a few different types of cell, eg. cells found in bone marrow can become both red and white blood cells

unipotent stem cells- able to differentiate into only one type of cell

How do stem cells become specialised?

Not all of the DNA in stem cells is transcribed and translated to be expressed

mRNA is only transcribed from specific genes, and this is translated into proteins that modify the cell, determining cell structure and processes that cause the cell to become specialised.

the changes caused by the proteins cannot be reversed, so once a cell is specialised, it stays specialised.

Why are cardiomyocytes relevant in stem cell research?

these are heart muscle cells, and it is thought that they cannot divide to replicated themselves.

scientists now think that new cardiomyocytes can be developed from a small supply of unipotent stem cells in the heart.

scientists disagree on how quickly this can happen

How can stem cell therapies be used to treat disorders?

stem cells can divide into a range of specialised cell types and so could be used to replace cells damaged by illness/injury.

this has been used successfully to treat leukaemia, lymphoma and sickle-cell anaemia.

What are the three main potential sources of human stem cells?

• adult stem cells- from body tissues of an adult, eg. bone marrow. Can be obtained in a relatively simple operation with low risk, but do not have as large a range of potential cell types to differentiate into. Multipotent.

• embryonic stem cells- from embryos at an early stage of development. Embryos created using IVF, stem cells removed from them and the rest of the embryo is destroyed. Pluripotent.

• induced pluripotent stem cells (iPS cells)- created by reprogramming specialised adult body cells to become pluripotent in a lab.

How are iPS cells produced?

Induced Pluripotent Stem Cells

Adult cells are made to express a series of transcription factors that are usually found in pluripotent cells, causing the adult cells to express genes associated with pluripotency

This can be done by infecting the cells with a modified virus that has genes coding for the transcription factors within its own DNA so that when infected, the genes are passed into the adult cell DNA and the adult cell is able to produce the transcription factors.

What are transcription factors?

proteins that control the transcription of target genes.

How do transcription factors work in eukaryotes?

transcription factors move from the cytoplasm into the nucleus.

they bind to specific DNA sites at the start of their target genes

they control expression by controlling the rate of transcription.

eg. activators stimulate or increase rate of transcription

repressors inhibit or decrease the rate of transcription.`

How does oestrogen initiate transcription of target genes?

• oestrogen is a steroid hormone that binds to a transcription factor called an oestrogen receptor, forming an oestrogen-oestrogen receptor complex.

• the complex moves from the cytoplasm to the nucleus where it binds to specific DNA sites near the start of the target gene.

• the complex can act as an activator, helping RNA polymerase to bind to the start of the target gene.

How does RNA interference (RNAi) inhibit translation of mRNA?

• RNAi is where small, double-stranded RNA molecules stop mRNA from target genes from being translated into proteins. Two types of molecules are involved: siRNA and miRNA. This occurs in eukaryotes; a similar process occurs in prokaryotes.

How does siRNA lead to RNAi in animals?

once mRNA is transcribed, it moves to the cytoplasm.

Here, double-stranded small interfering RNA (siRNA) associates with several proteins and unwinds.

A single strand of siRNA binds to the target mRNA through complementary base pairing with sections of the target mRNA.

The proteins associated with the siRNA cut the mRNA into fragments so it cannot be translated. The fragments move to a processing body which can degrade them.

A similar process happens with microRNA (miRNA) in plants.

How does miRNA lead to RNAi in mammals?

once mRNA is transcribed, it moves into the cytoplasm.

In the cytoplasm, double-stranded microRNA (miRNA) associates with several proteins and unwinds.

miRNA is not usually fully complementary to the target mRNA and so is less specific and can target more than one mRNA molecule.

the miRNA-protein complex physically blocks the translation of the target mRNA, and the mRNA is moved to a processing body where it will be stored or degraded. If stored, it can be returned and translated at another time.

How does epigenetic control work in eukaryotes?

epigenetic control determines whether a gene is expressed by the attachment or removal of chemical groups to or from DNA or histone proteins. They alter how easy it is for enzymes and other proteins needed for transcription to interact with and transcribe DNA.

epigenetic changes to gene expression play a role in normal cellular processes or can occur in response to changes in environment, eg. pollution

How can epigenetic changes be inherited?

organisms inherit DNA base sequences from parents

most epigenetic marks are removed between generations, but some escape removal and are passed on to offspring

therefore, expression of some genes can be affected by environmental changes that impacted parents and grandparents.

how does increased methylation affect a gene epigenetically?

methylation of DNA:

a methyl group is attached to the DNA coding for a gene.

the group always attaches at a CpG site, where a cytosine and guanine base are next to each other and joined by phosphodiester bond.

increased methylation changes the DNA structure so that transcription machinery can’t interact with the gene, so the gene is not expressed.

increased methylation of DNA switches genes off.

How does decreased acetylation of histones affect a gene epigenetically?

DNA wraps around histones to form chromatin, which makes up chromosomes and can be highly condensed or less condensed. How condensed affects the accessibility of the DNA and whether it can be transcribed.

histones can be epigenetically modified by addition or removal of acetyl groups.

When histones are acetylated, chromatin is less condensed, so transcription machinery can access the DNA and the gene can be transcribed.

When acetyl groups are removed from histones, chromatin becomes highly condensed and genes in the DNA can’t be transcribed because transcription machinery cannot access them.

Decreased acetylation reduces transcription and therefore expression of a gene.

What enzymes are responsible for removing acetyl groups from histones?

histone deacetylases

How can epigenetics lead to development of disease?

• cancer- interaction between methyl groups and tumour suppressor/proto-oncogenes

• mutations can lead to more CpG sites in DNA, increasing methylation which leads to a gene being switched off.

How can diseases caused by epigenetic diseases be treated by drugs?

epigenetic changes are reversible and so are good targets for drugs

eg. drugs can stop DNA methylation or acetylation of histones