- rER, vesicles and the Golgi apparatus are involved un protein transport. Proteins are made at the ribosome on the surface of rER. The protein chain is passed through the rER’s cisternae to be folded into their 3D shape – by forming bonds.
- New proteins are then transported to the Golgi apparatus in vesicles, the vesicle fused with the Golgi’s membrane. The protein then undergo further processing i.e. carbohydrate groups are added.
The protein than enters more vesicles to be transported around the cell. Extra cellular enzymes such as digestive enzymes moves to the cell surface and are secreted via exocytosis.
- Similar cells (Xylem tissues or squamous epithelium tissues) / tissue are grouped into tissues (Leaf or Lunges) / organs system (i.e. respiratory or digestive system) that perform a particular function.
The cell cycle and mitosis
- The cell cycle starts when a cell is produced by cell division and ends with the cell dividing to produce two identical cells. The cell cycle is the process that all body cells from Multicellular organisms used to grow and divide.
- The cell cycle consists of a period of cell growth and DNA replication, called interphase, followed by a period of cell division – called mitosis. The interphase is sub-divided into 3 separate growth stages.
G1 – cell grows bigger and new organelles are made (as are proteins)
S – Cell replicates DNA
G2 – Cell continues growing and protein & energy required for cell division are made
- Mitosis is required for the growth of multicellular organism and for repairing damaged tissue.
Some organisms (i.e. plants and fungi) reproduce asexually using mitosis. So new organisms produced are genetically identical to the original parent organism.
- There are 4 main division stages in mitosis and it follows interphase:
- Carries out normal function and prepares to divide
- The cell’s DNA is unravelled and replicated – to double its genetic content.
- Organelles are also replicated so it has spare ones for the new daughter cells.
- Its ATP content increase – providing energy required for cell division.
- The chromosomes condense, getting shorter and fatter.
- The centrioles move to opposite ends of cells
- A network of spindle fibres forms
- The nuclear envelop breaks down and chromosome lies free in the cytoplasm.
- The chromosome line up along the equator of the cell. The chromosome consists of 2 sister chromatids attached by the centromere.
- The spindle fibres become attached to the centromere.
- The centromere divides, separated the two sister chromatids.
- The spindle fibre than contracts, pulling the chromatids to opposites ends of the cells, with its centromere first.
- Chromatids reach the opposite poles of the cells, they then uncoil and become long again, they’re now called chromosome.
- A nuclear envelop forms around each groups of chromosomes -> 2 nuclei
- Cytokinesis then occurs where the cytoplasm divides and 2 new identical daughter cells are produces.
- Stem cells are unspecialized cells that can develop into any type of cells, they divide by mitosis to become new cells, which then become specialize. The process in which stem cell specializes is called differentiation.
- In humans, stem cells are found in the embryo (where they differentiate into all the cells required to form a fetus) and in some adult tissues (where they differentiate into specialized cells that need to be replaced - i.e. Bone marrow can differentiate into red blood cells):
- Totipotency: The ability to produce all cell types, including all the specialized cells in an organism & extraembryonic cells, i.e placenta & umbilical cord
- Pluripotency: The ability of stem cells to produce all the specialized cells in an organism but not extraembryonic cells
Totipotent cells differentiate into pluripotent cells and extraembryonic cells, and then the pluripotent cells will differentiate into the specialized in a fetus.
- Plants have stem cells in the area where growth occurs, i.e. Roots and shoots. All stem cells in plants are totipotent, meaning they can produce all cell types and can grown into a whole new plant. Totipotency can be shown in plants suing tissue culture - growing a new plant from a single cell.
- A single cell is taken from a growing area on a plant.
- The cells is than placed in some growth medium (i.e. Agar) that contains nutrients and growth hormones. This medium is sterile, so microorganisms can’t grown and compete with the plant cells.
- Plant cells will grow and divide into a mass of unspecialized cells. If conditions are suitable (i.e. correct hormones) the unspecialized cells will then differentiate into specialized cells.
- The cells will grow and differentiate into an entire plant eventually. Tissue culture shows totipotency as a single stem cells produces all the specialized cells to make a whole new plant.
- Stems cells become specialized because different genes in their DNA become active, so they express different genes, which would make different proteins.
- Stem cells all contain the same genes, but not all of them are expressed as not all of them are active. Under the correct conditions. Some genes are activated and others are inactivated.
- mRNA is only transcribed from the active genes, which is then translated into appropriate proteins
- These proteins modify the cell - they determine the cell structure + function & control cell processes. Ore genes are also activated, producing more proteins.
- Changes to the cell produced by these protein cause the cell to become specialized (differentiate). These processes are difficult to reverse, so once a stem cell has differentiated, the cell would stay permanently modified.
- Red blood cells are produced from a type of stem cells from the bone marrow. They contain lots of haemoglobin and have no nucleus (so more room for haemoglobin)
So the stem cell produces a new cell in which the genes for haemoglobin production are activated. Other genes, such as removal of the nucleus, are activated as well. Many other genes are activated/inactivated. The differentiation process results in a specialized red blood cell.
Stem cells in Medicine
- Stem cells can develop into any unspecialised cell types - they could be sued to replace damaged tissue in a range of diseases.
For example, the treatment for leukaemia kills all the stem cells in the bone marrow. So bone marrow transplants can be given to patients to replace them.
- Stem cells can also be used to repair damaged nerve tissues in spinal cord injuries. Replacing damaged heart tissues would also be possible with stem cells.
- Potential benefits of stem cells therapies are considered before decisions are made:
- Stem cell therapy can be used to save many lives, for example, many people needing an organ transplant die before a door organ becomes available. Stem cells can be sued to grow organs for those awaiting transplants.
- Quality of life of many people can be improved, i.e. Stem cells can be used to replace damages tissues in the eyes of people who are blind.
- The are two potential source of human stem cells:
- Adult stem cells can be obtained from body tissues of an adult (i.e. Bone marrow). The donor is anaesthetised, a needle is then inserted into the centre of a bone (i.e. Hipbone) and a small quantity of bone marrow is removed.
Adult stem cells are not as flexible as embryonic stem cells as they can only develop into a limited range of cells.
- Embryonic stem cells come from early embryo; they’re created in a lab using IVF. Once the embryo is approx. 4-5 days old, stem cells are removed from them and the rest of the embryo is destroyed. Embryonic stem cells can develop into all types of specialized cells.
- Ethical issues surrounding the obtainment of embryonic stem cells form embryo:
- The procedure results in the destruction of an embryo that is viable.
- Some believe that a genetically unique individual is created at fertilization, and the embryo has the right to life - so it’d be wrong to destroy embryos.
- Stem cells from an unfertilized embryo are more accepted as they’re not viable.
- Some think only adult stem cells should be used as their production doesn’t destroy any embryos.
- Decisions about important scientific work like stem cell research must be made upon taking into account everyone’s view on the topic.
- Regulatory authorities have been established To consider both the benefits and ethical issue surrounding embryonic stem cell research:
- Looking at proposal of research to decide whether or not it should be allowed ensuring the researches involving embryos is carried out for a good reason. This also makes sure the research isn’t repeated by different groups.
- Licensing and monitoring centres involved in embryonic stem cell research ensures only fully trained staffs carry out the research. They will understand the implication of the research and won’t waste any precious resources - this also helps to avoid unregulated research.
- Producing guidelines and codes of practice making sure all scientists work in a similar method, otherwise their results would be incomparable. This also ensures methods of extraction are controlled.
- Monitoring development in scientific research makes sure any changes in the field are regulated appropriately and that all guidelines are up to date.
- Provide info and advice to government and professionals helps to promote the science involved in embryo research, helping society to understand what’s involved and why embryonic stem cell research is important.