Types of chromosomes and special forms of chromosomes

genetics

1. TYPES OF CHROMOSOMES R.Dhanalakshmi , MSFBI1504, GENETICS.

2. CHROMOSOMES: In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

3. TYPES OF CHROMOSOMES : AUTOSOMES ALLOSOMES

4. AUTOSOMES: Autosomes contain nonallosomal chromosomes of the genetic hereditary information. It is homologous chromosomes ,they are morphologically similar. In humans 44 autosomes are present.

5. Autosomal Abnormalities: The most well known and most common autosomal abnormality is  Down syndrome.  People with Down syndrome have an irregularity with autosome pair 21.  In most cases, there is an extra chromosome (i.e., trisomy  21).  More rarely (3-5%), there is a structural modification in this chromosome.  Specifically, there is a translocation of all or part of chromosome 21 to chromosome 14 or 15. 

6. The actual genes on chromosome 21 that are responsible for Down syndrome are now being identified.  It is thought that there are at least 350 genes involved.  About 2-4% of the people with Down syndrome are genetically mosaic .  That is, some of their cells have chromosome 21 trisomy while others do not, resulting in generally milder symptoms.  The translocational type of Down syndrome also usually has less severe symptoms.

7. It was first described in 1866 by the English physician John Langdon Down. These include epilepsy , hypothyroidism , crossed eyes, near-sightedness or far-sightedness, cataracts , hearing impairment that makes it difficult to process auditory information, heart defects, intestinal malformations, hernias , and a marked susceptibility to respiratory infections, such as pneumonia.

8. ALLOSOMES: Certain genetic traits are linked to a person sex and are passed on through the sex chromosomes. In humans there are 2 allosomes . Its is heterologous xx and xy Male chromosomes are xy Female chromosomes are xx.

9. Female Sex Chromosome Abnormalities: Turner syndrome:   occurs when females inherit only one X chromosome their genotype.  If they survive to birth, these girls have abnormal growth patterns.  They are short in stature, averaging 4 foot 7 inches as adults, and often have distinctive webbed necks (i.e., extra folds of skin), small jaws, and high arched palates.  They generally lack prominent female secondary sexual characteristics. 

10. Women with Turner syndrome have a higher than average incidence of thyroid disease, vision and hearing problems, heart defects, diabetes, and other autoimmune disorders.  In a few individuals, there is slight mental retardation.  Turner syndrome is rare. 

11. Current estimates of its frequency range from 1 in 2,000 to 1 in 5,000 female infants.  If diagnosed in early childhood, regular injections of human growth hormones can increase their stature by a few inches.  Beginning around the normal age of puberty, estrogen replacement therapy can result in some breast development and menstruation.  These treatments allow Turner syndrome women to appear relatively normal.

12. Triple-X syndrome  occurs in women who inherit three X chromosomes--their genotype is XXX or more rarely XXXX or XXXXX.   As adults, these "super-females" or " metafemales " , as they are sometimes known, generally are an inch or so taller than average with unusually long legs and slender torsos but otherwise appear normal.  They usually have normal development of sexual characteristics and are fertile but tend to have some ovary abnormalities that can lead to premature ovarian failure. 

13. They may have slight learning difficulties, especially in speech and language skills, and are usually in the low range of normal intelligence . Triple-X syndrome is less rare than Turner syndrome, but little is known about it.  The frequency is approximately 1 in 1,000 female infants and it occurs more commonly when the mother is older. 

14. Male Sex Chromosome Abnormalities Klinefelter syndrome   : males inherit one or more extra X chromosomes their genotype is XXY or more rarely XXXY or XY/XXY mosaic.   In severe cases, they have relatively high-pitched voices, asexual to feminine body contours as well as breast enlargement, and comparatively little facial and body hair.  They are sterile or nearly so, and their testes and  prostate gland are small. 

15. they produce relatively small amounts of testosterone. They also are likely to be overweight.  They usually have learning difficulties as children, especially with language and short-term memory. they unable to produce sufficient amounts of sperm for conception. 

16. This may be connected to low testosterone production.  Subsequently, regular testosterone therapy is often prescribed.   The frequency of Klinefelter syndrome has been reported to be between 1 in 500 and 1 in 1000 male births.  This makes it one of the most common chromosomal abnormalities.    Males with Down syndrome sometimes also have Klinefelter syndrome.  Both syndromes are more likely to occur in babies of older mothers.

17. As a result, they produce relatively small amounts of testosterone.  The feminizing effects of this hormonal imbalance can be significantly diminished if Klinefelter syndrome boys are regularly given testosterone from the age of puberty on.

18. XYY SYNDROME: if  males inherit an extra Y chromosome their genotype is XYY.  As adults, these "super-males" are usually tall (above 6 feet) and generally appear and act normal.  However, they produce high levels of testosterone.  During adolescence, they often are slender, have severe facial acne, and are poorly coordinated.  They are usually fertile and lead ordinary lives as adults.  It may be as common as 1 in 900 male births to as rare as 1 in 1500 or even 1 in 2,000.  XYY syndrome is also referred to as Jacobs syndrome. 

19.   However, some researchers suggest that the high testosterone levels of XYY men can make them somewhat more prone to violence and that this may cause higher rates of wife beating.  

20. TYPES OF CHROMOSOMES: Four types of chromosomes: Telocentric Acrocentric Submetacentric Metacentric

21. Metacentric  – centromere is in middle, meaning  p  and  q  arms are of comparable length (e.g. chromosomes 1, 3, 16, 19, 20) Submetacentric  – centromere off-centre, leading to shorter  p  arm relative to  q  arm (e.g. chromosomes 2, 4 - 12, 17, 18, X)

22. Acrocentric   – centromere severely off-set from centre, leading to much shorter  p  arm (e.g. chromosomes 13 - 15, 21, 22, Y) Telocentric   – centromere found at end of chromosome, meaning no  p  arm exists (chromosome not found in humans)

23. SPECIAL TYPES OF CHROMOSOMES: In Eukaryotic organisms certain chromosomes are found only in certain special tissues and are not seen in other tissues. These chromosomes are larger in size and are called giant chromosomes. In certain plants, they are found in the suspensors of the embryo. There are two types of giant chromosomes polytene chromosome and lamp brush chromosome.

24. Polytene chromosomes: Polytene chromosomes were observed by C.G. Balbiani in 1881 in the salivary glands of Drosophila. The characteristic feature of polytenechromosome is that along the length of the chromosome there is a series of dark bands alternate with clear zones called inter bands.

25. The polytene chromosome has extremely large puff called Balbiani ring. It is also known as chromosomal puff. As this chromosome occurs in the salivary gland it is known as salivary gland chromosomes.

26. Uses of polytene chromosomes: Polytene chromosomes are usually found in the larvae, where it is believed these many replicated chromosomes allow for much faster larval growth than if the cells remained diploid. The  Drosophila  Genome Project uses polytene chromosomes as a framework for the map.

27. LAMPBRUSH CHROMOSOMES: Lamp brush chromosomes were first observed by Flemming in 1882. It looked like brushes. They occur at the diplotene stage of meiotic prophase in oocytes of an animal Salamandor and in giant nucleus of the unicellular alga Acetabularia . The highly condensed chromosome forms the chromosomal axis, from which lateral loops of DNA extend as a result of intense RNA synthesis.

28. Uses of lamphbrush chromosomes: It is formed due to the active synthesis of mRNA molecules for future use by the egg cells, when no synthesis of mRNA molecule is possible during the mitotic cell division