- Cells are the building blocks of all living things. Multicellular organisms are made up of collections of cells. The cells can become specialised to do a particular job.
- Groups of specialised cells working together are called tissues and groups of tissues working together are called organs.
- Mitosis is the process by which a cell divides to produce two new cells with identical sets of chromosomes to the parent cell. The new cells will also have all the necessary organelles.
- The purpose of mitosis is to produce new cells for growth and repair and to replace old tissues.
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- When an egg is fertilised by a sperm it becomes a zygote.
- The zygote then divides by mitosis to form a cluster of cells called an embryo.
- Up to (and including) the eight cell stage, all the cells are identical and can produce any sort of cell required by the organism including neurons, blood cells, liver cells etc. They are called embryonic stem cells.
- At the 16 cell stage (approximately four days after fertilisation), most of the cells in an embryo begin to specialise and form different types of tissue.
- Although the cells contain the same genetic information, the position of each cell is different relative to the others.
- The distribution of various proteins in the cells will also be different.
- So, although the genes are the same, the cells are already subtly different from one another.
- At the time of specialisation these differences determine what specific functions a cell will have
- Some cells remain unspecialised. These are adult stem cells.
- At a later stage they can become specialised. However, unlike embryonic stem cells, adult stem cells cannot become any type of cell.
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- Plants have cells that are like stem cells in animals. The cells are in areas called meristems.
- Only cells within meristems can divide repeatedly (i.e. are mitotically active).
- Cells in the meristem are unspecialised but they can develop into any type of plant cell.
- Under normal hormonal conditions, this would mean that tissues such as xylem and phloem could be formed as well as organs such as leaves, roots and flowers.
- There are two types of meristem: those that result in increased girth (lateral meristems) and those that result in increased height and longer roots (apical meristems).
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Xylem and Phloem
- Xylem is made from specialised cells to transport water and soluble mineral salts from the plant roots to the stem and leaves, and to replace water lost during transpiration and photosynthesis.
- Phloem is made from specialised cells to transport dissolved food made by photosynthesis throughout the plant for respiration or storage.
- When a stem is deliberately cut, special plant hormones can be added. These can send messages to the meristems to start to produce roots.
- As the cutting already has a stem and leaves, it will then grow into a clone of the parent plant.
- There are a wide range of plant hormones. The main group that is used in horticulture is called auxin. Auxins mainly affect cell division at the tip of a shoot, because that is here the meristems are. Just under the tip, the cells grow in the presence of auxins, causing the stem or root to grow longer.
- The growth and development of plants are affected by the environment. Example-phototropism
- Another example is geotropism, where the roots and shoots grow towards and away from the source of gravity.
- Other tropisms also exist.
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- A tropism is the term given to a response by a plant by a stimulus.
- So, phototropism is a response by the plant to light.
- A plant's survival depends on its ability to photosynthesise.
- Plants therefore need strategies to detect light and to respond to changes in intensity. This is demonstrated by the way in which plants will grow towards a light source.
- The cells furthest away from a light source grow more, due to the presence of an auxin, which is sensitive to light.
- Auxin is produced at the shoot tip and migrates down the shoot.
- If a light source is directly overhead, then the distribution of auxin would be the same on both sides of the plant shoot.
- If a light source shines onto the shoot at an angle, the auxin facing the light moves to the side furthest away. The cells on the side furthest from the light source have proportionately more auxin than the closest.
- As a result, the concentration of auxin on the side furthest away from the light increases, causing the cells there to elongate and the shoot begins to bend towards the light.
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- Charles Darwin carried out some simple experiments that demonstrated the role played by plant hormones produced in the shoot tip.
- As we have seen, light causes the shoot to bend towards the source. If the tip of the shoot is removed or covered in opaque material then the plant will continue to grow upwards - as if the light source was not there.
- If the tip is covered with a transparent cap then it will still grow towards the light source. The same thing will happen if an opaque cylinder is wrapped around the stem leaving the tip exposed.
- This experiment proves it is a substance produced in the tip that causes the cells further down the shoot to grow.
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Mitosis and Growth
- Mitosis leads to the production of two new cells, which are identical to each other and to the parent cell.
- Mitosis can only take place when a cell is ready to divide.
- This means that cells go through the cell cycle.
- The cell cycle consists of a growth stage (G1) where the cell gets bigger and the number of organelles increase, then a synthesis stage (S) where the DNA is copied, followed by another very short growth stage (G2) immediately before mitosis (M phase).
- The cycle is a bit like a clock.
- Both new cells need to have a full complement of organelles and DNA to function properly.
- Therefore, the number of organelles needs to increase and the DNA has to be copied.
- The chromosomes are copied when the two strands of each DNA molecule separate and new strands form alongside them.
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- Meiosis only takes place in the testes and ovaries. It is a special type of cell division that produces gametes (sex cells, i.e. eggs and sperm) for sexual reproduction.
- Gametes contain half the number of chromosomes of the parent cell.
- This is important becuase it means that when the male and female gametes fuse, the number of chromosomes will increase back to the full number.
- The resulting zygote has a set of chromosomes from each parent.
- Step 1 - Cell with two pairs of chromosomes.
- Step 2 - Each chromosome replicates itself.
- Step 3 - Chromosomes part company and move to opposite sides with their 'copies'.
- Step 4 - Cell divides for the first time.
- Step 5 - Copies now separate and the second cell division now takes place.
- Step 6 - Four gametes, each with half the number of chromosomes of the parental cell.
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- DNA is a nucleic acid, found in the nucleus, and is in the form of a double helix.
- The structure of DNA was worked out by Watson and Crick in 1952, earning them a Nobel prize for their discovery.
- The DNA molecule has a series of bases connected together, like rungs on a ladder. The bases are always in pairs:
- Adenine (A) pairs with thymine (T). Guanine (G) pairs with cytosine (C).
- The DNA is always stored within the nucleus; it does not leave.
- The DNA has sequences of genes, which code for proteins.
- However, the proteins themselves are manufactured in the cytoplasm of the cell.
- Therefore, there is a mechanism for transferring the information stored in the genes into the cytoplasm.
- Imagine the nucleus is a reference library, from which you are not allowed to remove the books. You would need to copy down any information you needed so that you could take it away with you.
- The DNA molecule is too large to leave the cell. So, the relevant section of DNA is unzipped and the instructions copied onto smaller molecules, which can pass through the nuclear membrane of the nucleus into the cytoplasm.
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- The smaller molecules are called messenger RNA (mRNA). These leave the nucleus and carry the instructions to the ribosomes, which follow the instructions to make the specific protein.
- It is the order of bases that determines what protein is made.
- The bases in a gene are read in threes or triplets, called a codon.
- Each triplet means a specific amino acid is attached.
- As the amino acids join together a new protein is formed.
- The mRNA is the opposite of the DNA code, with the exception that mRNA has the base uracil (U) instead of T - so, if the DNA base was A, the mRNA base would be U.
- The triplet codes determine the amino acid produced.
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- It is a fundamental premise in biology that a gene codes for one specific protein.
- All body cells, including stem cells, contain exactly the same genes.
- All cells have to produce proteins for growth and respiration. This means that certain genes (e.g. the gene for making the cell membrane and the gene for making ribosomes) will be switched on in all cells.
- The genes that are not needed will be switched off.
- When a stem cell becomes specialised, the genes for proteins specific to the new cell type will be switched on.
- Any gene in an embryonic stem cell could potentially be switched on, to make the specialised cell type needed.
- In human DNA, there are approximately 20,000 - 25,000 genes, meaning a similar number of different proteins could be made. Each cell type will have a small fraction of these genes switched on.
- Stem cells have the potential to produce cells needed to replace damaged tissues.
- For example, stem cells can be used to replace brain tissue in a patient with Parkinson's disease, or to grow new skin tissue following a burn.
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- To produce the large number of stem cells needed for treatments, it is necessary to clone cells from five-day-old embryos. The stem cells are collected when the embryo is made up of approximately 150 cells. The rest of the embryo is destroyed. At the moment, unused embryos from IVF treatments are used for stem cell reasearch.
- There is an ethical issue as to whether it is right to use embryos to extract stem cells in this way. The debate revolves around whether or not the embryos should be classed as people.
- One view is that if an embryo was left over from IVF (and would never grow into a human being), it would be acceptable for stem cell research to be carried out on it as long as parents gave consent. Another view is destroying an embryo amounts to destroying a life.
- The Government regulates and makes laws on such matters.
- Whole animals have been cloned - Dolly the Sheep was the first to be cloned from adult skin cells. It is, however, illegal to clone a human being in this way.
- Scientists can now take a mature, specialised cell and reactivate (switch on) inactive genes, effectively making it a new specialised cell type. This gives the potential to grow new tissue that is genetically the same as the patient. The benefit is that the tissue will not be rejected by the immune system of the patient. It also means that the patient does not have to take an expensive cocktail of drugs in order to prevent rejection. Such drugs can affect a patient's immune system and can stop it from fighting disease.
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