Stem Cells And Totipotency

Question 1

What is cell differentiation?

Answer 1

In multicellular organisms, cells are specialised to perform specific functions- the process by which each cell develops into a specialised structure suited to the role that it will carry out is known as fell differentiation

Question 2

Why can’t single-celled organisms be totally efficient at all functions?

Answer 2

• single-called organisms perform all essential life functions inside the boundaries of a single cell
• although they perform all functions adequately, they cannot be totally efficient at all of them
• because each function requires a different type of cellular structure and enzymes
• no one cell can provide the best conditions for all functions

Question 3

The cells of multicellular organisms are each adapted in different ways to perform a particular role- in early development, an organism is made up of a tiny bundle of cells (all of which are identical). As it matures,

Answer 3

Each cell takes on its own individual characteristics that adapt it to the function that it will perform when it matures

Question 4

All the cells in an organism, such as a human, are derived by mitotic divisions of the fertilised egg (zygote) and it follows they all contain exactly the same genes so every cell is therefore capable of

Answer 4

Making everything that the body can produce e.g. a cell in the lining of the small intestine has the gene coding for insulin just as beta cells of the pancreas = all cells contain all genes, but only certain genes are expressed (switched on) in any one cell at any one time

Question 5

Some genes are permanently expressed (switched on) in all cells, for example

Answer 5

• the genes that code for essential chemicals such as the enzymes involved in respiration, are expressed in all cells
• other genes permanently switched on in all cells include those coding for enzymes and other proteins involved in essential processes like transcription, translation, membrane synthesis and tRNA synthesis

Question 6

Some genes are permanently switched off for example

Answer 6

The gene for insulin in cells lining the small intestine

Question 7

Should be noted that further genes are switched on and off

Answer 7

As and when they’re needed

Question 8

Differentiated cells differ from each other, often visibly so- this is mainly because

Answer 8

They each produce different proteins, and the proteins that a cell produces are coded for by the genes it possesses or more accurately, by the genes that are switched on

Question 9

An organism develops from a single fertilised egg- a fertilised egg clearly has the ability to give life types of cells. Cells such as fertilised eggs, which can mature into any body cell are called

Answer 9

Totipotent cells

Question 10

The early cells that are derived from the fertilised egg are also totipotent- these later differentiate and become more specialised for a function, for example

Answer 10

• muscle cells become specialised for contraction, and this is because during the process of cell specialisation, only some of the genes are expressed
• = only part of the DNA of a cell is translated into proteins
• = cell therefore only makes those proteins that it requires to carry out its specialised function
• these proteins include those required for essential processes like respiration and membrane synthesis
• although it is still capable of synthesising all the other proteins, these are not needed and would be wasteful to produce them

Question 11

In order to conserve energy and resources, a variety of stimulus (controlling factors) ensure the genes for these other proteins are not expressed- the ways in which genes are prevented from expressing themselves include:

Answer 11

• preventing transcription and so preventing the production of mRNA
• preventing translation

Question 12

Differentiation results from

Answer 12

Differential gene expression

Question 13

If specialised cells still retain all the genes of the organism, can they still develop into any other cell?

Answer 13

• it depends
• xylem vessels and red blood cells for example are so specialised they lose their nuclei once they are mature
• as the nucleus contains the genes,then clearly these cells cannot develop into other cells
• in fact, specialisation is irreversible in most animal cells
• once cells have matured and specialised they can no longer develop into other cells
• in mature mammals, only a few cells retain the ability to differentiate into other cells = STEM CELLS

Question 14

What are stem cells?

Answer 14

Undifferentiated dividing cells that occur in adult animal tissues and need to constantly be replaced- they therefore have the ability to divide to form an identical copy of themselves in a process called self-renewal

Question 15

Stem cells originate from various sources in mammals:

Answer 15

• embryonic stem cells: come from embryo in the early stages of development and can differentiate into any type of cell in the initial stages of development
• umbilical cord blood stem cells: derived from umbilical cord blood and similar to adult stem cells
• placental stem cells: found in placenta and develop into specific types of cells
• adult stem cells: despite their name, are found in the body tissues of the fetus through to the adult- they are specific to a particular tissue or organ within which they produce the cells to maintain and sues through an organism’s life

Question 16

There are a number of different stem cells which are classified according to

Answer 16

Their ability to differentiate

Question 17

What are the different classifications of stem cells?

Answer 17

• totipotent stem cells
• pluripotent “
• multipotent
• unipotent

Question 18

What are totipotent stem cells?

Answer 18

Found in the early embryo and can differentiate into any type of cell, but only occur for a limited amount of time in early mammalian embryos (since all body cells are formed from a zygote, it follows that the zygote is totipotent)- as the zygote divides and matures, its cells develop into slightly more specialised cells called pluripotent stem cells

Question 19

What are pluripotent stem cells?

Answer 19

Found in embryos and can differentiate into almost any kind of cell e.g embryonic stem cells

Question 20

What are multipotent stem cells?

Answer 20

Found in adults and can differentiate into a limited number of specialised cells e.g. umbilical cord blood and some adult tissues like bone marrow

Question 21

What are unipotent stem cells?

Answer 21

Derived from multipotent stem cells and made in adult tissue, they can only differentiate into a single type of cell e.g. cardiomyocytes

Question 22

What are induced pluripotent cells (IPS cells)?

Answer 22

• type of pluripotent cell produced from unipotent stem cells (may be almost any body cell e.g. skin cell)
• these body cells are then genetically altered in a laboratory to make them acquire the characteristics of embryonic stem cells

Question 23

Regarding IPS cells, to make the unipotent cell acquire the new characters involves

Answer 23

inducing genes and transcriptional factors within the cell to express themselves I.e. to switch on genes that were otherwise switched off

Question 24

Why could iPS cells potentially replace embryonic stem cells in medical research?

Answer 24

• iPS cells are very similar to embryonic stem cells in form and function
• however they are not exact duplicates of them
• one feature of particular interest is that they are capable of self-renewal = potentially divide indefinitely to provide a limitless supply
• = could replace embryonic stem cells in medical research and treatment and overcome ethical issues associated with the use of embryos in stem cell research

Question 25

Explain the use of pluripotent stem cells in treating human disorders

Answer 25

• many possible uses of pluripotent cells
• cells can be used to regrow tissues that have been damaged in some way, either by accident e.g. skin grafts for serious burn damage
• or as a result of disease e.g. neuro-degenerative diseases, such as Parkinson’s disease e.g. nerve cells

Question 26

During development, totipotent cells translate only part of their DNA resulting in

Answer 26

cell specialisation

Question 27

Currently, embryonic stem cell research is only allowed in uk under licensed and specified conditions- embryos used in this type of research are obtained from IVF, and the process raised a number of ethical issues:

Answer 27

• by taking embryonic stem cells out of the embryo we prevent it from developing in its normal way; we prevent it from becoming what it was ‘programmed’ to be, a human being
• development from a fertilised egg into a baby is a continuous process and any attempt to ‘pinpoint’ when personhood begins is arbitrary- a human embryo is a human being in the embryonic state, just as an infant is a human being in infancy. Although an embryo does not currently have characteristics of a person, it will become a person and therefore should be given the dignity and respect of any other person
• however embryos cannot develop into a child without transfer to a woman’s uterus (needs external help to develop); even then, the probability that embryos used for IVF will develop into full-term successful births is low. So something that could potentially become a person should not be treated as if it were actually a patient
• in contrast, human embryos are not the only source cells- they can be obtained from the bone marrow of adult humans on the condition the patient gives informed consent, and this source of stem cells raises no real ethical issues. But, currently these cells have far more restricted medical application and so cost-benefit ratio needs to be assessed as the ethical costs are decreased, but so are the medical benefits

Question 28

Give two characteristic features of stem cells:

Answer 28

• undifferentiated cells I.e can differentiate into other cells
• possess ability of long-term self renewal through mitosis

Question 29

All cells posses the same genes, yet a skin cell can produce the protein keratin but not the protein myosin, while vice versa for a muscle cell- why is this?

Answer 29

• in skin cells, the gene that codes for keratin is expressed, but not the gene for myosin
• the genetic code for keratin is translated into the protein keratin, which the cell therefore produces
• but genetic code for myosin not translated
• in muscle cells, the gene for myosin is expressed, but not the gene for keratin
• in the same way, the genetic code for myosin rather than keratin is translated and so only myosin is produced

Question 30

Suggest a reason why skin cells retain an ability to divide by being unipotent when the cells of some other organs do not

Answer 30

• skin cells, being on the outside of the body, are subjected to much wear and tear and so need replacing frequently
• many organs are less prone to damage and need little cell replacement