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1. How might mitochondrial genes become transferred to the nucleus, if the SET theory is correct?
Answer
This is a difficult topic and no really dominant
theories exist. Some ideas focus on the idea that there is direct
transfer of the DNA from the organelle, whereas others suggest the
transfer of complementary DNA (cDNA) made from the messenger RNA (mRNA) produced by the organelle, especially in self-fertilizing plants. In Arabidopsis, the entire mitochondrial DNA is present in the nucleus and appears to have been transferred as a whole. In some plants, genes have transferred, whereas in other species the same genes have not. The majority of gene transfer from organelles to the nucleus probably occurred during the early stages of the integration of these organelles into the cytosol of their host; but these events probably still continue to occur to the present day.
2. Can viruses be considered a life form?
Answer
Generally viruses are not thought to be a life form as they are unable to replicate themselves without the help of other cells and organisms; they lack any process of metabolism or respiration and they are acellular. They thus lack basic characteristics generally used to define a living organism. However, they mutate and consist of self-replicating molecules so there are some fundamental components of life and evolution.
3. What determines how a cell becomes one specific type?
Answer
Many factors contribute to the eventual differentiation of a cell into a specific cell type. This is called terminal differentiation and is set in motion in many multicellular organisms by the formation of early germ layers in the embryonic organism. Some cells are totipotent (able to form any other cell type in an organism) and in some these cells are only represented by the initial zygote; however, in other organisms totipotency can be retained or retrieved by some cells. Pluripotent cells can turn into a range of cell types – in animals these are considered to be embryonic stem cells, whereas in higher plants these derive from meristematic cells. Cellular differentiation then proceeds through to multipotent cells before terminal differentiation. Cellular differentiation is caused by the gradual honing down to specific gene expression patterns that differ between different types of differentiated cell. The pattern of expression is controlled by transcription factors and signalling molecules that help to specify why one set of genes is expressed and another not, even though all cells (except gametes) contain a complete identical copy of the organism’s genes.
4. Where does endoplasmic reticulum (ER) in a cell originally arise if it is needed for its own synthesis?
Answer
The endoplasmic reticulum of a cell comes from the cytoplasm of the female egg cell. The male sperm is too small to carry much more than the nuclear material and it sheds its tail, which has some mitochondria in it, on fertilization. Hence, all ER is derived from the female egg cell going way back in time to the beginning of complex cells.
5. Why should proteins have different isoforms?
Answer
Different isoforms enable the same basic protein to be used for slightly different jobs. They can be modified to have slightly different properties. For example, in the hearing organ (cochlea) there are gradients in various anatomical and physiological properties. Changes in an ion channel that are needed for encoding different frequencies could be produced by combining different amounts of two different isoforms in a systematic way. Another example is actin, which has different isoforms in muscle from those found in other cells in which it does a different job.