Stem Cells

Some definitions…

Totipotent cells can mature into any type of cell. Found in early embryos and plants.

Pluripotent cells can form all the cell types in the body (embryonic stem cells).

Multipotent cells can form a number of different cell types, e.g. adult stem cells/cord blood stem cells.

Uses of stem cells

Medical researchMedical treatments

e.g. growth of neurones to treat spinal injuries growth of organs for transplants

Reasons For/Against

Your turn!

Debate – Should the UK government fund stem cell research?

In pairs:- Read the statements on the cards and

discuss what that person might think.

iPS = induced pluripotent stem cells (scientists have a method to turn normal adult cells back into stem cells).

What makes a cell change?How do we get from stem cells to fully

differentiated, specialised cells?

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Controlling development

All organisms begin life as a single cell. This cell divides and the new cells produced start to differentiate and specialize.

‘Switching on’ the expression of a gene or keeping it switched off determines the development of features.

Many organisms contain similar genes that control development of body plans. For example groups of genes called the homeobox genes play an important role in the development of many multicellular organisms.

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Homeobox genes

Homeobox genes code for transcriptional factors. These regulate the expression of other genes important in development.

The genome of the fruit fly contains one ‘set’ or cluster of homeobox genes. These control development, including the polarity of the embryo, polarity of each segment and the identity of each segment.

Mutations in homeobox genes can cause changes in the body plan. For example a mutation in the gene controlling leg placement can cause legs to grow where the antennae are normally found.

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Homeobox genes

There is little variation in many regions of the homeobox genes in different organisms. This suggests that these have been highly conserved throughout evolutionary history. They are thought to be especially important to the basic development of organisms.

Homeobox genes are present in the genomes of most organisms. They control development of body parts in similar ways.

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How is transcription initiated?

In eukaryotic cells, before transcription can begin a gene needs to be stimulated by a regulatory protein, called transcriptional factor.

They cannot initiate transcription alone, but form a pre-initiation complex with RNA polymerase.

Each transcriptional factor contains sites that can bind to a specific region of the DNA.

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Function of transcriptional factors

The action of a transcriptional factor can be switched off by an inhibitor molecule. This can bind to the transcriptional factor, preventing it from attaching to DNA. Without the transcriptional factor the gene cannot be transcribed.

Transcriptional factors function in different ways. Some transcriptional factors recognize parts of the promoter sequence at the start of a gene and bind to them. They can either promote or block the functioning of RNA polymerase.

inhibitor molecule

transcriptional factor

DNA binding

site

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Oestrogen

Some hormones, e.g. oestrogen, have an effect on specific cells due to their ability to influence transcriptional factors, and therefore gene expression in the cell.

Oestrogen diffuses across the cell membrane. Once inside the cytoplasm it combines with a site on a transcriptional factor. The hormone changes the shape of the transcriptional factor causing the inhibitor molecule to be released.

oestrogen

inhibitor molecule

transcriptional factor

DNA binding site

transcription activated

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Small interfering RNA (siRNA)

Small interfering RNA

Petunias

White pigment

Purple pigment

Chalcone synthase

Producing a deep purple petunia

White pigment

Purple pigment

Chalcone synthase

Insert gene encoding chalcone synthase

More mRNA synthesised

More enzyme produced and more pigment

formed

Producing a deep purple petunia

White plant Deep purple plant

Instead of deep purple plants, many

of the plants produced were

white

?

Genetically engineered

plant

Making double-stranded RNA

A U C A G U A C C C A G U A U C G

mRNA is single

strandedRNA-

dependent RNA

polymerase U A G U C A U G G G U C A U A G C

Uses mRNA as a template to produce a complementary RNA strand

Two RNA strands held together by hydrogen bonds

Double-stranded RNA

(dsRNA)

What happens to double-stranded RNA?

Small interfering RNA (siRNA)• Usually 21 base pairs long• Two base overhang at each end

Double-stranded RNA is cut by Dicer

enzyme

Stopping protein synthesis

We will start by simplifying the diagram of the

siRNA molecule

Stopping protein synthesis

We will start by simplifying the diagram of the

siRNA molecule

Stopping protein synthesissiRNA forms a

complex (RISC) with protein

One of the siRNA strands is destroyed

The siRNA–protein complex binds to

mRNA

Stopping protein synthesisThe mRNA is cut by the siRNA–protein

complex

The mRNA is then broken down. This

prevents further protein synthesis

White plant Deep purple plant

So why were white plants produced

instead of deep purple plants?

?

Genetically engineered

plant

Use the information about making double-

stranded RNA and small interfering RNA to

explain why.

The genetically engineered petunia plants had a higher concentration of mRNA

This resulted in RNA-dependent RNA polymerase producing double-stranded RNA from this mRNA

More siRNA molecules were formed that would bind to the mRNA coding for chalcone synthase

Less chalcone synthase was produced so flowers were white, not deep purple