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How do you find out the number of chromosomes of an animal? What are chromosomes? What happens to our chromosomes during reproduction? How does a fertilized egg grow into a baby? How does trisomy arise? What is mosaicism? How does mosaicism occur? How is chromosomal mosaicism diagnosed? How does chromosomal mosaicism affect the health of a developing baby or grown adult? What is chromosome testing (karyotyping)? Cytogenetics – chromosome test (karyotype) * How long does the test take? * What if my prenatal test takes longer than 2 weeks? * What if the test is abnormal? * What if I am worried about how my test is going? * Is the result always correct? * What if the test fails and there is no result? FastDNA™ * How is the test done? * How long does the FastDNA test take? * If my result is not abnormal, does that mean my baby has normal chromosomes? * Is the result always correct? * What if the test is abnormal? * Do I still need to have routine chromosome analysis as well as FastDNA? * What if the test fails? * What is the advantage of the FastDNA test? Biochemistry * When can the first trimester blood test be done? * Where can I have blood taken? * What does it mean if I get a high-risk result? * What does it mean if I get a low-risk result? Molecular aneuploidy screening: miscarriages * How long does the test take? * What if the test is abnormal? * What if I'm worried how the test is going? * Is the result always correct? * What if the test fails and there is no result? Molecular DNA testing * What type of samples are required? * Is the sample collection painful? * Is the DNA test 100% accurate? * How long does the testing take? * Can I take the samples myself at home? * Do you check for paternity with the DNA sample? * Will you destroy my DNA once testing is complete? Microarray CGH testing * How long does the test take? * What type of samples are required? * Is the sample collection painful? * What if I am worried about how my test is going? * What if the test is abnormal? * Is the result always correct? * What if the test fails and there is no result? * Will you destroy my DNA once testing is complete? | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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The contents of the nucleus
The nucleus contains some coiled strand-like bodies known as chromosomes , which are only really visible in dividing cells, when they will absorb stain. These contain the genetic information, encoded in the structure of the DNA (Deoxyribonucleic acid) molecule, which enables the cell to carry out all its functions. Genes - the units of inherited information - are parts of chromosomes, and these are passed out equally amongst the cells of the body and in even shares from one generation to the next. In other words, chromosomes carry genes from one cell to the next. DNA has a very special structure: it can split into two halves, each of which can then be built up to reform the missing section. It is effectively a molecule which is capable of reproduction, the basis of life. It can also pass instructions out to the cytoplasm, to produce proteins. Mitosis - nuclear division Before a cell divides, its nucleus must divide, in order to pass on this information to both daughter cells. In higher plants and animals a process known as mitosis ensures that chromosomes, after dividing, are distributed evenly between the two cells. In other words, nuclear division comes before cell division. As a result of mitosis, each dividing cell produces two cells, each of which contains identical sets of chromosomes. So the main function of mitosis is for growth or replacement of cells. Cell division vertical section through a root tip Growth occurs by an increase in the number of cells and/or by a (limited) increase in cell size. Increase in cell number takes place as a result of division of cells. This is the basis of asexual reproduction. This cell division process frequently occurs during the development of a plant or animal. A plant continues to grow throughout its life, so that cell division is a perpetual process in some regions, such as the tips of roots and shoots. These regions of active cell division in plants are known as meristems. See the diagram alongside In all organisms that reproduce sexually, cell division is an important phase in the life-cycle because it results in the production of many copies of cells by duplication. In developing from a fertilised egg or zygote, animal cells also undergo division to make a ball of cells which eventually become the embryo (and foetus in mammals). Cells from an early embryo cannot be distinguished from one another, but as the process progresses, the cells become specialised to perform different functions (differentiation). Once they have reached their normal size, animals do not continue to grow but cell division still takes place, in order to repair damage and replace worn-out tissues, e.g. there is a continual turnover of red blood cells (produced in the bone marrow) and skin is replaced by division of cells from an underlying layer (the Malpighian layer). Each cell normally divides into two halves , each of which then usually enlarges to the size of the original cell. Usually, one cell is able to divide again. See the diagrams below. cell division in Amoeba cell division in plants At cell division, animal cells pinch into two because they only have a flexible membrane on the outside of the cells, but plant cells form a thin structure (middle lamella) across the centre of the old cell, which then thickens up to form a cell wall. In a complex organism made of many cells, all cells are derived by division from the fertilised egg. Mitosis ensures that each cell receives the same genetic information, but different groups of cells become developed to perform different functions - differentiation, e.g. liver cells, blood cells, muscle cells, nerve cells. Such cells do not usually divide again. There is great current interest in stem cells (present in small numbers in some tissues) which remain undifferentiated and can therefore develop into various cell types, either in normal development or the repair of damage. It is not fully understood how, but in differentiation only a part of the genetic information is decoded to control the development of the cell and the biochemical activity specific to it, such as production of proteins like haemoglobin in red blood cells, and digestive enzymes in cells of the various glands of the gut. Can you think of any other examples of different cell types, and their own (protein) products? > digestive gland cells, > digestive enzymes e.g. salivary e.g. amylase Meiosis Certain organs (sex organs) produce gametes (sex cells). In animals, testes produce the male gametes: sperms, and ovaries produce female gametes: eggs (ova). In flowering plants, anthers produce pollen which contains male gametes, and the embryo sac within the ovary contains a female gamete. Here, a different version of nuclear division takes place: meiosis (also called reduction division). Chromosomes pair up with their partners and then divide, and are distributed into cells after 2 nuclear divisions so that 4 cells are formed, which then develop into gametes. The result is that each gamete contains half the normal number of chromosomes possessed by any ordinary body cell - the haploid number - n. For instance, in Man sperms and eggs each contain 23 chromosomes. Fertilisation At fertilisation, two gametes join together. Nuclear fusion takes place, restoring the normal double number of chromosomes in the (single) cell, the zygote. This (diploid - 2n) number is then retained during cell division by mitosis to provide all the cells of the developing embryo with 2n chromosomes. A similar process occurs in higher plants, resulting in the formation of seeds. NOTE: Asexual reproduction (vegetative propagation) relies on mitosis, whereas sexual reproduction relies on meiosis. Give 2 examples each of types of tissues (not organs) in both higher plants and higher animals, in which mitosis regularly occurs. (animals) > bone marrow > skin (Malpighian layer) making red blood cells (plants) >root tips > shoot tips Towards the end of the cell division process, dividing animal and plant cells do something different. What is the difference? > Animal cells pinch in, whereas plant cells form middle lamella across middle of cell. Explain why this difference occurs. > Animal cell outer layer (cell membrane) is flexible, whereas plant cell outer layer (cell wall) is inflexible. > Asexual reproduction following cell division and mitosis results in identical cells and eventually identical plants (clone) so cuttings etc have the same characteristics. CELL DIVISION and CANCER There are many forms of cancer, but their main feature is that cells of the body divide in a way that is out of control. Biologists believe that DNA in their nuclei has been damaged by something in the environment of the cells. This damage may be caused by a variety of factors, which may be placed under 3 headings: Complete the table below physical e.g. radiation from >rocks,> medical X-rays, > nuclear reactors, waste dumps chemical > chemicals from tobacco, industrial chemicals, food additives, alcohol biological > sexual intercourse, dietary fat and salt, etc If the cells divide to produce a lump of cells it is called a "growth" or tumour; if it stops growing or grows only slowly, and does not spread then it is said to be benign; if it shows no sign of slowing down, breaks up and spreads to other parts of the body it is called malignant. Some types of cancer may be treated by direct means, such as surgery, or by laser treatment. Give some examples of cancers which may be treated in this way. >skin cancer > cervical cancer >breast cancer Chemotherapy and radiotherapy (treatment with chemicals and penetrating radiation) are sometimes used against cancer which is difficult to reach directly. These are often accompanied by unpleasant side-effects such as loss of hair. Try to explain these side-effects. > Hair growth also depends on cell division and this would be prevented by the treatments aimed at stopping cancer cell division. TISSUE CULTURE and MEDICINE Some tumour cells can be grown outside the body, even after the human (or animal) donor has died. These cells may produce chemicals such as hormones, required by the body to regulate its normal growth and development, or antibodies to fight disease. In some cases these cells can accept DNA containing genes from a completely different source, and there are a wide number of possible benefits in medicine as well as industrial manufacturing. Stem cells - which divide in the body and may develop into many different cell types - may conceivably be used in cloning. Other recent lines of research have centred on attempts to understand the mechanism of cell differentiation so as to allow the production of replacement tissues. This is a form of biotechnology - genetic engineering - which may have much promise in the future. Chromosomes
CompositionIn eukaryotes, chromosomes consist of a single molecule of DNA [Link to visual proof] associated with:
Structure
Chromosome Numbers
(In plants, the haploid stage takes up a larger part of its life cycle - Link)
KaryotypesThe complete set of chromosomes in the cells of an organism is its karyotype. It is most often studied when the cell is at metaphase of mitosis and all the chromosomes are present as dyads. The karyotype of the human female contains 23 pairs of homologous chromosomes:
(A gene on the Y chromosome designated SRY is the master switch for making a male.)
The X and Y chromosomes are called the sex chromosomes.)
Above is a human karyotype (of which sex?). It differs from a normal human karyotype in having an extra #21 dyad. As a result, this individual suffered from a developmental disorder called Down Syndrome. The inheritance of an extra chromosome, is called trisomy, in this case trisomy 21. It is an example of aneuploidy TranslocationsKaryotype analysis can also reveal translocations between chromosomes. A number of these are associated with cancers, for example
FISHLocation of the gene for muscle glycogen phosphorylase on human chromosome 11This image (courtesy of David C. Ward) provides dramatic evidence of the truth of the story of chromosomes. A piece of single-stranded DNA was prepared that was complementary to the DNA of the human gene encoding the enzyme muscle glycogen phosphorylase. A fluorescent molecule was attached to this DNA. The dyads in a human cell were treated to denature their DNA; that is, to make the DNA single-stranded. When this preparation was treated with the fluorescent DNA, the complementary sequences found and bound each other. This produced a fluorescent spot close to the centromere of each sister chromatid of two homologous dyads (of chromosome 11, upper right). This analytical procedure, which here revealed the gene locus for the muscle glycogen phosphorylase gene, is called fluorescence in situ hybridization or FISH.DNA ContentThe molecule of DNA in a single human chromosome ranges in size from 50 x 106 nucleotide pairs in the smallest chromosome (stretched full-length this molecule would extend 1.7 cm) up to 250 x 106nucleotide pairs in the largest (which would extend 8.5 cm). Stretched end-to-end, the DNA in a single human diploid cell would extend over 2 meters.
In the intact chromosome, however, this molecule is packed into a much more compact structure. [Link]. The packing reaches its extreme during mitosis when a typical chromosome is condensed into a structure about 5 µm long (a 10,000-fold reduction in length).
24 February 2011 Historical note Investigation into the human karyotype took many years to settle the most basic question. How many chromosomes does a normal diploid human cell contain? In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism.[44] Painter in 1922 was not certain whether the diploid number of man is 46 or 48, at first favouring 46.[45] He revised his opinion later from 46 to 48, and he correctly insisted on humans having an XX/XY system.[46] New techniques were needed to definitively solve the problem: 1. Using cells in culture 2. Pretreating cells in a hypotonic solution, which swells them and spreads the chromosomes 3. Arresting mitosis in metaphase by a solution of colchicine 4. Squashing the preparation on the slide forcing the chromosomes into a single plane 5. Cutting up a photomicrograph and arranging the result into an indisputable karyogram. It took until the mid-1950s for it to become generally accepted that the human karyotype include only 46 chromosomes. Considering the techniques of Winiwarter and Painter, their results were quite remarkable.[47][48] Chimpanzees (the closest living relatives to modern humans) have 48 chromosomes. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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List of organisms by chromosome count
From Wikipedia, the free encyclopedia
(Redirected from List of number of chromosomes of various organisms)
This page lists the numbers of chromosomes in various plants, animals, protists, and other living organisms, given as the diploid number (2n)
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chromosomes.html http://www.biotopics.co.uk/life/celdiv.html http://en.wikipedia.org/wiki/Chromosome |
