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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?
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

Chromosomes

Index to this page

Composition

In eukaryotes, chromosomes consist of a single molecule of DNA [Link to visual proof] associated with:

Structure

  • For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope.
  • Before a cell gets ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle).
  • As mitosis begins, the duplicated chromosomes condense into short (~ 5 µm) structures which can be stained and easily observed under the light microscope.
  • These duplicated chromosomes are called dyads.
  • When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains 1–10 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays.
  • While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bona fide chromosome with a full complement of genes.
  • The kinetochore is a complex of proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase.
  • The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm.
  • Staining with the trypsin-giemsa method reveals a series of alternating light and dark bands called G bands.
  • G bands are numbered and provide "addresses" for the assignment of gene loci.
    Diagram of p and q arms,
    G bands, and gene loci

Chromosome Numbers

  • All animals have a characteristic number of chromosomes in their body cells called the diploid (or 2n) number.
  • These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined.
  • The gametes contain the haploid number (n) of chromosomes.

(In plants, the haploid stage takes up a larger part of its life cycle - Link)

Diploid numbers of some commonly studied organisms
(as well as a few extreme examples)
Homo sapiens (human)46
Mus musculus (house mouse)40
Drosophila melanogaster (fruit fly)8
Caenorhabditis elegans (microscopic roundworm)12
Saccharomyces cerevisiae (budding yeast)32
Arabidopsis thaliana (plant in the mustard family)10
Xenopus laevis (South African clawed frog)36
Canis familiaris (domestic dog)78
Gallus gallus (chicken)78
Zea mays (corn or maize)20
Muntiacus reevesi (the Chinese muntjac, a deer)23
Muntiacus muntjac (its Indian cousin)6
Myrmecia pilosula (an ant)2
Parascaris equorum var. univalens (parasitic roundworm)2
Cambarus clarkii (a crayfish)200
Equisetum arvense (field horsetail, a plant)216

Karyotypes

The 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: The karyotype of the human male contains:
  • the same 22 pairs of autosomes
  • one X chromosome
  • one Y chromosome

(A gene on the Y chromosome designated SRY is the master switch for making a male.)

Link to a karyotype of a normal human male stained by the trypsin-giemsa method (the image is 84K)

The X and Y chromosomes are called the sex chromosomes.)

Discussion of 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

Translocations

Karyotype analysis can also reveal translocations between chromosomes. A number of these are associated with cancers, for example

  • the Philadelphia chromosome (Ph1) formed by a translocation between chromosomes 9 and 22 and a cause of Chronic Myelogenous Leukemia (CML) [Link]
  • a translocation between chromosomes 8 and 14 that causes Burkitt's lymphoma [Link]
  • a translocation between chromosomes 18 and 14 that causes B-cell leukemia [Link]

FISH

Location of the gene for muscle glycogen phosphorylase on human chromosome 11

This 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 Content

The 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.

See some of the DNA molecule released from a single human chromosome.

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).

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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.

List of organisms by chromosome count
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This page lists the numbers of chromosomes in various plants, animals, protists, and other living organisms, given as the diploid number (2n)

Organism Scientific name Diploid number of chromosomes Notes
African Wild Dog Lycaon pictus 78[1]
Alfalfa Medicago sativa 32[2] Cultivated alfalfa is tetraploid, with 2n=4x=32. Wild relatives have 2n=16.[2]
American Badger 32
American Marten 38
American Mink 30
Aquatic Rat Anotomys leander 92[3] Tied for highest number in mammals with Ichthyomys pittieri.
Arabidopsis thaliana 10
Barley Hordeum vulgare 14[2]
Bat-eared Fox Otocyon megalotis 72[1]
Bean Phaseolus sp. 22[2] All species in the genus have the same chromosome number, including P. vulgaris, P. coccineus, P. acutifolis, and P. lunatus[2].
Beaver (American) Castor canadensis 40
Beaver (Eurasian) Castor fiber 48
Beech Marten 38
Bengal Fox Vulpes bengalensis 60
Moonworts Botrychium 90
nagaho-no-nastu-no-hana-warabi Botrypus strictus 88 B. strictus and B. virginianus have been shown to be paraphyletic in the genus Botrypus
Rattlesnake fern Botrypus virginianus 184
Cabbage Brassica oleracea 18[2] Broccoli, cabbage, kale, kohlrabi, brussels sprouts, and cauliflower are all the same species and have the same chromosome number[2].
Carp 104
Capuchin Monkey 54[4]
Cat Felis catus 38
Chicken Gallus gallus domesticus 78
Chimpanzee Pan troglodytes 48 [5]
Chinchilla Chinchilla lanigera 64 [6]
Coatimundi 38
Cotton Gossypium hirsutum 52[2] 2n=4x; Cultivated upland cotton is derived from an allotetraploid
Cow Bos primigenius 60
Coyote Canis latrans 78[1]
Deer Mouse 48
Dhole 78
Dingo Canis lupus dingo 78[1]
Dog Canis lupus familiaris 78[7] 76 autosomal and 2 sexual.[8]
Dolphin Delphinidae Delphis 44
Donkey 62
Dove 78[9] Based on African collared dove
Fruit fly Drosophila melanogaster 8[10] 6 autosomal, and 2 sexual
Duck-billed Platypus 52
Earthworm Lumbricus terrestris 36
Echidna 63/64 63 (XXY, male) and 64 (XXXX, female)
Elephant 56
Elk (Wapiti) Cervus canadensis 68
Eurasian Badger 44
European honey bee Apis mellifera 32 32 for females, males are haploid and thus have 16.
European Mink 38
European Polecat 40
Fennec Fox Vulpes zerda 64[1]
Ferret 40
Field Horsetail 216
Fisher (animal) 38 a type of marten
Fossa 42
Giraffe Giraffa camelopardalis 62
Goat 60
Golden Jackal Canis aureus 78[1]
Gorilla 48
Gray Fox Urocyon cinereoargenteus 66[1]
Gypsy moth 62
Hawkweed 8
Hare[11][12] 48
Hedgehog Genus Atelerix (African hedgehogs) 90
Hedgehog Genus Erinaceus (Woodland hedgehogs) 88
fern-like plant Helminthostachys zeylanica 94
Horse Equus ferus caballus 64
Human Homo sapiens 46[13] 44 autosomal and 2 sex
Hyena 40
Crab-eating rat (semiaquatic rodent) Ichthyomys pittieri 92[3] Tied for highest number in mammals with Anotomys leander.
Jack jumper ant Myrmecia pilosula 2[14] 2 for females, males are haploid and thus have 1; smallest number possible. Other ant species have more chromosomes.[14]
Kangaroo 12
Kit Fox 50
Lion Panthera leo 38
Long-nosed Cusimanse (a type of mongoose) 36
Maize Zea mays 20[2]
Maned Wolf Chrysocyon brachyurus 76
Mango Mangifera indica 40[2]
Meerkat 36
Mosquito Aedes aegypti 6[15] The 2n=6 chromosome number is conserved in the entire family Culicidae, except in Chagasia bathana which has 2n=8.[15]
Mouse Mus musculus 40
Mule 63 semi-infertile
Oats Avena sativa 42[2] This is a hexaploid with 2n=6x=42. Diploid and tetraploid cultivated species also exist.[2]
Adders-tongue Ophioglossum reticulatum 1200 or 1260 This fern has the highest known chromosome number.
Orangutan 48
Oriental Small-clawed Otter 38
Pea Pisum sativum 14[2]
Pig 38
Pigeon 80
Pine Marten 38
Pineapple Ananas comosus 50[2]
Potato Solanum tuberosum 48[2] This is a tetraploid; wild relatives mostly have 2n=24.[2]
Porcupine Erethizon dorsatum 34 [6]
Rabbit 44
Raccoon (Procyon lotor) 38[16]
Raccoon Dog Nyctereutes viverrinus 42 some sources say sub-species differ with 38, 54, and even 56 chromosomes
Raccoon Dog Nyctereutes procyonoides 56
Radish Raphanus sativus 18[2]
Rat 42
Red Deer Cervus elaphus 68
Red Fox Vulpes vulpes 34[1] Plus 3-5 microsomes.
Red Panda 36
Reeves's Muntjac Muntiacus reevesi 46
Rice Oryza sativa 24[2]
Rhesus Monkey 48
Rye Secale cereale 14[2]
Sable 38
Sable Antelope 46
Grape ferns Sceptridum 90
Sea Otter 38
Sheep 54
Shrimp Penaeus semisulcatus 86-92 [17]
Slime Mold Dictyostelium discoideum 12 [18]
Snail 24
Spotted Skunk 64
Starfish 36
Striped skunk 50
Swamp Wallaby Wallabia bicolor 10/11 10 for male, 11 for female
Tanuki/Raccoon Dog Nyctereutes procyonoides albus 38
Tiger Panthera tigris 38
Tibetan fox 36
Tobacco Nicotiana tabacum 48[2] Cultivated species is a tetraploid[2].
Turkey 82
Virginia Opossum Didelphis virginiana 22[19]
Wheat Triticum aestivum 42[2] This is a hexaploid with 2n=6x=42. Durum wheat is Triticum turgidum var. durum, and is a tetraploid with 2n=4x=28[2].
White-tailed deer Odocoileus virginianus 70
Wolf 78
Woolly Mammoth 58 extinct; tissue from a frozen carcass
Wolverine 42
Yellow Mongoose 36
Yeast 32
Bittersweet nightshade Solanum dulcamara 24[20] [21]
Husk Tomato Physalis pubescens 24[22]


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