more on gene expression

transcription factors move from the cytoplasm to the nucleus in eukaryotes They then bind to specific DNA sites called PROTOMERS, which are found at the start of their target gene - which are the genes they control the expression of

they stimulate or increase the rate of transcription e.g. they help RNA polymerase bind to the start of the target gene and activate transcription

these inhibit or decrease the rate of transcription e.g. they bind to the start of the target gene, preventing RNA polymerase from binding, stopping transcription

the expression of genes can also be affected by other molecules in the cell e.g. oestrogen is a steroid hormone that can affect transcription by binding to a transcription factor called an oestrogen receptor

oestrogen binds to the transcription factor called an oestrogen receptor this forms an oestrogen - oestrogen receptor complex the complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene The complex can act as an activator e.g. helping RNA polymerase bind to the start of the target gene

in eukaryotes, gene expression is also affected by RNA interference (RNAi) RNAi is where small, double-stranded RNA molecules stop mRNA from target genes being translated into proteins A similar process to RNAi also occurs in prokaryotes.

they are called siRNA (small interfering RNA) and miRNA (microRNA

once mRNA has been transcribed, it leaves the nucleus for the cytoplasm in the cytoplasm, double-stranded siRNA associates with several proteins and unwinds One of the resulting single strands of siRNA is selected and the other strands are degraded the single strand of siRNA then binds to the target mRNA. The base sequence of the siRNA is complementary to the base sequence to base sequence in sections of the target mRNA The proteins associated with the siRNA cut the mRNA into fragments - so it can no longer be translated. The fragments then move into a processing body which contains “tools” to degrade them this process is similar to miRNA in plants, however. its production is similar to that of mammalian miRNA

the miRNA is not usually fully complementary to the target mRNA therefore us is less specific than siRNA and so it may target more than one mRNA molecules

it exists as a long folded strand It is processed into a double strand, and then into two single strands by enzymes in the cytoplasm Like siRNA, one strand associates with proteins and binds to target mRNA in the cytoplasm Instead of the proteins associated with miRNA cutting mRNA into fragments, the miRNA - protein complex physically blocks the translation of the target mRNA The mRNA is then moved into a processing body where it can either be stored or degraded When it is stored it can be returned at translated at another time

stem cells become specialised because during their development they only transcribe and translate part of their DNA stem cells all contain the same genes but during development not all of them are transcribed and translated (expressed) under one set of conditions, certain genes are expressed and others are switched off

the genes that are expressed get transcribed into mRNA which is then translated into proteins These proteins modify the cell - they determine the cell structure and control cell processes (including the expression of more genes, which produced more proteins) it is these changes to the cell by these proteins that causes the cell to become specialised The change is difficult to be reversed so once a cell is specialised it stays specialised

Genes are expressed 🠮 mRNA is transcribed and translated into proteins 🠮 proteins modify the cell 🠮 cell becomes specialised for a particular function Genes are switched off 🠮 mRNA not transcribed or translated

cardiomyocytes are heart muscle cells that make up a lot of the tissue in our hearts i

it was thought that in mature mammals, they could not divide to replicate themselves therefore everyone thought that we were not able to regenerate our own heart cells at all This is a problem if the heart becomes damaged

recent research has suggested that our hearts do not have some regenerative capability Some scientists now think that old or damaged cardiomyocytes are replaced by new cardiomyocytes derived from a small supply of unipotent stem cells in the heart some say this process is constantly occurring some say it is a slow process and it is possible that some cardiomyocytes are never replaced throughout a person’s entire lifetime

they are found in the body tissue of an adult e.g. found in adult bone marrow

they are not as flexible as embryonic stem cells as they can only specialise in a limited range of cells - so they are multipotent although the operation is simple with very low risk, it can lead to a lot of discomfort in the patient

these are obtained from embryos in the early stages of development these embryos are created in a lab using in vitro fertilisation (IVF) - the egg cell is fertilised by sperm outside of the womb Once the stem cells are approximately 4-5 days old, stem cells are removed from them and the rest of the embryo is destroyed Embryonic stem cells can divide an unlimited number of times and develop into all types of body cells (they’re pluripotent)

iPS cells are created by scientists in a lab The process involved “reprogramming” specialised stem cells to express a series of transcription factors that are normally associated with pluripotent stem cells The transcription factors cause the adult body cell to express genes that are associated with pluripotency

once of the ways that these transcription factors can be introduced to the adult cells is by infecting them with a specially - modified virus The virus has the genes coding for the transcription factors within its DNA When the virus infects the adult cell, these genes are passed into the adult cell’s DNA meaning that the cell is able to produce the transcription factor

• obtaining stem cells from embryos raises ethical issues because the procedure results in the destruction of an embryo that could become a fetus of placed in a womb Some people believe at the moment of fertilisation an individual is formed that has the right to live - so they believe that it is wrong to destroy embryos

• they could save many lives - e.g. many people are waiting for organ transplants could die before one is available • it might be possible to make stem cells genetically identical to the patients’ own cells so reduce the chance of rejection • they could improve the quality of life of many people