biology unit 5 (biol5) gene expression

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Unit 5.7 Gene expression is controlled at DNA level during transcription Know totipotent cells are body cells that can mature into any body cell. Understand during development some genes are switched off/only some of them are translated resulting in cell specialisation. Appreciate once this has happened it cannot be reversed. Understand how in mature plants many cells remain totipotent and be able to interpret data relating to tissue culture of plants from totipotent cells. Understand transcription only occurs when certain substances move from the cytoplasm into the nucleus. Appreciate the effect oestrogen has on transcription. Small interfering RNA (SiRNA) is short double stranded RNA that interferes with the expression of a specific gene. Understand in an animal a few cells remain totipotent (stem cells) and these can be used to treat genetic diseases, as can embryonic stem cells. Evaluate the use of stem cells in treating human disorders Interpret data relating to oncogenes/tumour suppressor genes in prevention, treatment and cure of cancer. Evaluate the effect on diagnosis and treatment of disorders caused by hereditary mutations and acquired mutations Key words: Totipotent, in vitro, somatic cell, germ cell, tissue culture, promoter region, transcription factor, siRNA. Totipotency and cell specialisation Totipotent cells can specialise into anything. But during specialisation/differentiation only some genes are expressed this means that only part of the DNA of a cell is translated into proteins. The cell therefore only makes proteins that it requires to carry out its specialised function. Although it is still capable of making all the other proteins, but they are not needed so this would be wasteful. Therefore the genes for these other proteins are not expressed. The ways in which genes are prevented from expressing themselves include: o Preventing transcription and hence preventing the production of mRNA o Breaking down mRNA before its genetic code can be translated.

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Page 1: Biology unit 5 (BIOL5) Gene expression

Unit 5.7

Gene expression is controlled at DNA level during transcription

Know totipotent cells are body cells that can mature into any body cell.

Understand during development some genes are switched off/only some of them are translated resulting in cell specialisation.

Appreciate once this has happened it cannot be reversed.

Understand how in mature plants many cells remain totipotent and be able to interpret data relating to tissue culture of plants from totipotent cells.

Understand transcription only occurs when certain substances move from the cytoplasm into the nucleus.

Appreciate the effect oestrogen has on transcription. Small interfering RNA (SiRNA) is short double stranded RNA that interferes with the expression of a specific gene.

Understand in an animal a few cells remain totipotent (stem cells) and these can be used to treat genetic diseases, as can embryonic stem cells.

Evaluate the use of stem cells in treating human disorders

Interpret data relating to oncogenes/tumour suppressor genes in prevention, treatment and cure of cancer.

Evaluate the effect on diagnosis and treatment of disorders caused by hereditary mutations and acquired mutations

Key words:

Totipotent, in vitro, somatic cell, germ cell, tissue culture, promoter region, transcription

factor, siRNA.

Totipotency and cell specialisation

Totipotent cells can specialise into anything.

But during specialisation/differentiation only some genes are expressed – this means

that only part of the DNA of a cell is translated into proteins. The cell therefore only

makes proteins that it requires to carry out its specialised function. Although it is still

capable of making all the other proteins, but they are not needed so this would be

wasteful.

Therefore the genes for these other proteins are not expressed. The ways in which

genes are prevented from expressing themselves include:

o Preventing transcription and hence preventing the production of mRNA

o Breaking down mRNA before its genetic code can be translated.

Page 2: Biology unit 5 (BIOL5) Gene expression

Regulation of transcription and translation

The effect of oestrogen on gene transcription

o For transcription to begin the gene needs to be stimulated by specific

molecules that move from the cytoplasm into the nucleus. These molecules

are called transcriptional factors.

o Each transcriptional factor has a site that binds to a specific region of the

DNA in the nucleus.

o When it binds, it stimulates this region of DNA to begin the process of

transcription.

o Messenger RNA (mRNA) is produced and the genetic code it carries is then

translated into a polypeptide.

o When a gene is not being expressed, the site on the transcriptional factor that

binds to DNA is blocked by an inhibitor molecule.

o This inhibitor molecule prevents the transcriptional factor binding to DNA and

so prevents transcription and polypeptide synthesis.

Hormones like oestrogen can switch on a gene and thus start transcription by

combining with a receptor on the

transcriptional factor. This releases

the inhibitor molecule. The process is

illustrated in the diagram and

operates as follows;

Oestrogen is a lipid-soluble

molecule and therefore diffuses

easily through the phospholipid

portion of cell-surface membranes

Once inside the cytoplasm of

a cell, oestrogen changes the shape

of the receptor molecule of the

transcriptional factor. The shape of

this site and the shape of the

oestrogen molecule complement one

another.

By combining with the site,

the oestrogen changes the shape of

the receptor molecule. This change

of shape releases the inhibitor

molecule from the DNA binding site

on the transcriptional factor

The transcriptional factor can

now enter the nucleus through a

nuclear pore and combine with DNA.

The combination of the

transcriptional factor with DNA

stimulates transcription of the gene

that makes up the portion of DNA.

Page 3: Biology unit 5 (BIOL5) Gene expression

The effect of siRNA on gene expression

Gene expression can be prevented by breaking down messenger RNA before its

genetic code can be translated into a polypeptide. Essential to this process are small

double-stranded sections of RNA called small interfering RNA (siRNA). The process

operates as follows;

o An enzyme cuts large double-stranded molecules of RNA into smaller

sections called small interfering RNA (siRNA);

o One of the two siRNA strands combines with an enzyme;

o The siRNA molecule guides the enzyme to a messenger RNA molecule by

pairing up its bases with complementary ones on a section of the mRNA

molecule;

o Once in position the enzyme cuts the mRNA into smaller sections;

o The mRNA is no longer capable of being translated into a polypeptide

o This means that the gene has not been expressed, that is, it has been

blocked;

The siRNA has a number of potential scientific and medical uses

o It could be used to identify the role of genes in biological pathway. Some

siRNA that blocks a particular gene could be added to cells. By observing the

effects (or lack of them) we could determine what the role of the blocked gene

is.

o As some diseases are caused by genes, it may be possible to use siRNA to

block these genes and so prevent the disease.