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Structure of chrmosomes
• A chromosome is an organized structure of DNA and protein found
in cells. It is a single piece of coiled DNA containing
many genes, regulatory elements and othernucleotide sequences.
Chromosomes also contain DNA-bound proteins, which serve to package
the DNA and control its functions. Chromosomal DNA encodes most or all
of an organism's genetic information; some species also
contain plasmids or other extrachromosomal genetic elements.
• Chromosomes vary widely between different organisms. The DNA
molecule may be circular or linear, and can be composed of 100,000 to
over 3,750,000,000[1][2]nucleotides in a long chain.
Typically, eukaryotic cells (cells with nuclei) have large linear
chromosomes and prokaryotic cells (cells without defined nuclei) have
smaller circular chromosomes, although there are many exceptions to this
rule. Also, cells may contain more than one type of chromosome; for
example, mitochondria in mosteukaryotes and chloroplasts in plants have
their own small chromosomes.
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In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure
called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure
of chromosomes and chromatin varies through the cell cycle. Chromosomes are the essential unit
for cellular division and must be replicated, divided, and passed successfully to their daughter cells
so as to ensure the genetic diversity and survival of their progeny. Chromosomes may exist as either
duplicated or unduplicated. Unduplicated chromosomes are single linear strands, whereas
duplicated chromosomes contain two identical copies (called chromatids) joined by a centromere.
Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic fourarm structure (pictured to the right). Chromosomal recombination plays a vital role in genetic
diversity. If these structures are manipulated incorrectly, through processes known as chromosomal
instability and translocation, the cell may undergo mitotic catastrophe and die, or it may
unexpectedly evade apoptosis leading to the progression ofcancer.
In practice "chromosome" is a rather loosely defined term. In prokaryotes and viruses, the
term genophore is more appropriate when no chromatin is present. However, a large body of work
uses the term chromosome regardless of chromatin content. In prokaryotes, DNA is usually
arranged as a loop, which is tightly coiled in on itself, sometimes accompanied by one or more
smaller, circular DNA molecules called plasmids. These small circular genomes are also found in
mitochondria and chloroplasts, reflecting their bacterial origins. The simplest genophores are found
in viruses: these DNA or RNA molecules are short linear or circular genophores that often lack
structural proteins.
Chromosomal
ALterations
1.Deletion
2.Duplication
3.inverstion
• The four types of alterations of
chromosome structure are deletion,
duplication, inversion, and reciprocal
translocation.
• Deletion is the loss of one or more
nucleotides from a gene by mutation; the
loss of a fragment of a chromosome.
• Duplication is repition of a part of a
chromosome resulting from fusion with a
fragment froma homologous chromosome.
Duplication can result from an error in
meiosis or from mutagenesis.
• Inversion is a change in a
chromosomeresulting from reattachment of
a chromosome fragment to the original
chromosome, but in a reverse direction.
Mutagens and errors during mesosis can
cause inversions.
• Translocation is the attachment of a
chromosomal fragment to a
nonhomologous chromosome.
Variations in
chromosomes
number(Ploidy)
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Ploidy is the number of sets of chromosomes in
the nucleus of a biological cell. Normally
a gamete (sperm oregg) carries a full set of
chromosomes that includes a single copy of each gene,
as aneuploidy generally leads to severe genetic disease
in the offspring. The haploid number (n) is the number
of chromosomes in a gamete. Two gametes form
a diploid zygote with twice this number (2n) i.e. two
copies of autosomalchromosomes
Cells are described according to the number of sets
present: haploid (1 set), diploid (2 sets), triploid (3
sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid
(6 sets), heptaploid[4] or septaploid[5] (7 sets), octoploid
(8 sets), nonaploid (9 sets), decaploid (10 sets),
undecaploid (11 sets), dodecaploid (12 sets),
tridecaploid (1 sets), tetradecaploid (14 sets) etc.
Because chromosome number is generally reduced
only by the specialized process of meiosis, the somatic
cells of the body inherit and maintain the diploid
chromosome number of the zygote. However, in many
situations somatic cells double their copy number by
means of endoreduplication as an aspect of cellular
differentiation.
• Euploidy is the state of a cell or organism having an integral
multiple of the monoploid number, possibly excluding the sexdetermining chromosomes. For example, a human cell has 46
chromosomes, which is an integer multiple of the monoploid
number, 23. A human with abnormal, but integral, multiples of this
full set (e.g. 69 chromosomes) would also be considered as
euploid. Aneuploidy is the state of not having euploidy. In humans,
examples include having a single extra chromosome (such as Down
syndrome), or missing a chromosome (such as Turner syndrome).
Aneuploid karyotypes are given names with the suffix -somy (rather
than -ploidy, used for euploid karyotypes), such
as trisomy and monosomy.