Human Chromosomes ,Genetics, genetics problems explain chromosomes and disorder related to chromosomes

1. 09 人类染色体
human chromosome

2. A chromosome is the visible state of genetic
material during a phase of the division of the cell
(metaphase).

3. Humans have 23 pairs of chromosomes, which
makes the diploid number 46. The diploid number is
the number of chromosomes of a normal cell.

4. The haploid number is the number of
chromosomes in a gamete. Usually, the diploid
number is twice the haploid number.

5. The number of chromosomes varies
depending on the species.

6. 1. chromatin
Chromatin is that portion of the cell nucleus which
contains all of the DNA of the nucleus in animal cells.
(A small amount of special DNA is also found in the
mitochondria of the cell cytoplasm outside of the cell
nucleus.)

8. A. heterchromatin and euchromatin
The degree of condensation of the nuclear DNA is variable.
Strongly condensed DNA is called heterochromatin,
Less condensed one euchromatin.
Heterochromatin is stained more intense than euchromatin by certain
certain nuclear dyes.

9. Heterochromatin is the condensed form of
chromatin organization. It is seen as dense patches of
chromatin.

10. Abundant heterochromatin is seen in resting, or
reserve cells such as small lymphocytes (memory cells)
waiting for exposure to a foreign antigen.
Heterochromatin is considered transcriptionally
inactive.

11. There are two types of heterochromatin:
Facultative heterochromatin and Constitutive
heterochromatin.

12. Facultative heterochromatin is condensed
chromatin that unfolds and becomes transcriptionally
active during some portion of the cell cycle (Barr
body).

13. Constitutive heterochromatin refers to the chromatin that remains
transcriptionally inert during the entire cell cycle.

14. The bulk of constitutive heterochromatin is
composed of non-coding DNA sequences that are
repeated hundreds of thousands or even millions of
times.

16. Euchromatin is threadlike, delicate. It is most abundant in active,
transcribing cells. Thus, the presence of euchromatin is significant
because the regions of DNA to be transcribed or duplicated must
uncoil before the genetic code can be read.

17. Condensation and decondensation of the DNA are at
least partially reversible processes. Each DNA segment
that has to be replicated or transcribed has to be
decondensed during this time.

18. The highest degree of condensation can be found
in the chromosomes (mitosis, meiosis) during cell
division.

21. B. the nucleosome
The fundamental structural unit of chromatin is an assemblage,
called the nucleosome, composed of five types of histones (designated
(designated H1, H2A, H2B, H3, and H4) and DNA.

22. A nucleosome consists of approximately 1.8 turns of DNA wound
around a core particle of histone proteins. The core particle is a
roughly heart-shaped octamer of 4 types of histones: two each of the
H2A, H2B, H3, and H4 proteins.

23. The DNA between two nucleosomes is called the
linker segment. This linker segment gives unfolded
chromatin a beads-on-a-string appearance (the
nucleosomes are the beads).

27. DNA is bound to the histones through electrostatic forces
between the negatively charged phosphate groups in the DNA
backbone and positively charged amino acids (e.g., lysine and
arginine) in the histone proteins.

28. At times, histone proteins are modified by the addition of acetyl,
methyl, or phosphate groups, and this alters the strength of the
bonding between the histones and DNA.

29. Modifications such as these are usually associated with the
regulation of biological processes such as DNA replication, gene
expression, chromatin assembly and condensation, and cell division.

30. C. Scaffold-radial loop structure model
Chromatin refers
to proteins that help
organize the long
DNA molecule. The
protein supports and
organizes small loops
of DNA.

32. 2.Chomosome
At some point during the cell division process, the
chromatin will be packaged tightly enough that we
are able to visualize distinct chromosomes under the
light microscope.

33. A. chromosome biology
The chromosomes do not actually become visible in the eukaryotic
cell until after the DNA is replicated. So, a newly-visible chromosome
is a duplicated chromosome, and contains two copies of the DNA
double helix.

34. These chromosomes are composed of two threads of chomatin,
called chromatids, which are attached to one another at the
centromere of the chromosome. The 2 chromatids that make up a
duplicated chromosome are the products of DNA replication, and are
known as sister chromatids.

35. Organisms that reproduce through sexual reproduction have two
copies of each type of chromosome in their somatic (body) cells, one
inherited from each of their parents. Such organisms are said to be
diploid, whereas cells with only one copy of each type of
(such as germ cells or gametes) are said to be haploid.

36. Cytogenetic
analyses are almost
always based on
examination of
chromosomes fixed
during mitotic
metaphase.

37. Metaphase chromosomes differ from one another in size and
shape, and the absolute length of any one chromosome varies
depending on the stage of mitosis in which it was fixed.

38. A model of a chromosome during metaphase

39. However, the relative position of the centromere is constant, which
means that that the ratio of the lengths of the two arms is constant
for each chromosome. This ratio is an important parameter for
chromosome identification, and also, the ratio of lengths of the two
arms allows classification of chromosomes into several basic
morphologic types.

41. Similar chromosomes inherited from different parents are known
as homologous chromosomes, with corresponding homologous
chromatids. These chromosomes are similar in that they have the
same genes at the same locations on the chromosome, but may have
different forms (or alleles) of those genes.

43. B. chromosome staining

45. 人类染色体非显带核型图

46. Chromosome banding
• Q banding
• G banding
• R banding
• T banding
• C banding
• N banding
• High resolution banding

48. G banding (model)

49. Model banding

50. Model banding

51. Chromosome 1

52. • X chromosome
• Contains over 1400
genes
• Contains over 150
million base pairs,
of which
approximately 95%
have been
determined

53. C显带核型图

54. 3. Sex chromosome and sex-determination
Humans usually have 46 chromosomes per diploid cell consisting
of 22 sets of autosomes and a set of sex chromosomes - either XX or
XY. In the usual course of events, individuals with the karyotype 46,
are female and individuals with 46, XY, are male.

55. The question arises as to how the sex of an individual is
determined. Is sex determined by the number of X chromosomes -
with one X you are male or with 2 X's you are female?

56. Or is sex determined by the presence or absence of the Y
chromosome - the presence of a Y makes for a male or the absence of
a Y produces a female?

57. The answer was provided by individuals resulting from non-
disjunction of the sex chromosomes.

58. Some individuals have 45 chromosomes and have only one X
chromosome; other individuals have 47 chromosomes and have two
X chromosomes and a Y.

59. Chromosome
Constitution
Name of
Syndrome
Sex of
Individual
Frequency in
Population
46, XX Normal Female 0.511*
46, XY Normal Male 0.489*
45, XO
Turner's
Syndrome
Female 1/5,000
47, XXY
Klinefelter's
Syndrome
Male 1/700
*Female (46,XX) and Male (46,XY) frequencies taken from the US Census projection

60. Usually, a woman has two X chromosomes (XX)
and a man one X and one Y (XY).

61. However, both male and female characteristics can sometimes be
found in one individual, and it is possible to have XY women and XX
men.

62. Analysis of such individuals has revealed some of the molecules
involved in sex determination, including one called SRY, which is
important for testis formation.

63. SRY (which stands for sex-determining region Y
gene) is found on the Y chromosome. In the cell, it
binds to other DNA and in doing so distorts it
dramatically out of shape.

64. This alters the properties of the DNA and likely alters the
expression of a number of genes, leading to testis formation.

65. Most XX men who lack a Y chromosome do still have a copy of the
SRY region on one of their X chromosomes. This copy accounts for
their maleness. However, because the remainder of the Y
chromosome is missing they frequently do not develop secondary
sexual characteristics in the usual way.

66. • Contains over 200 genes
• Contains over 50 million base pairs, of which approximately 50% have
been determined

68. 4. chromosomal polymorphism
Polymorphism is a general concept in biology,
referring to a more than one version of a trait being
actively present in a population.

69. Chomosomal polymorphism refers to the condition where one
species contains members with varying chromosome counts or
shapes. In most cases, the difference in chromosome counts is the
result of a single gene undergoing fission, where it splits into two
smaller chromosomes, or two undergoing fusion, where two
chromosomes join to form one.

70. 5. An International System for Human Cytogenetic
Nomenclature
Symbols and abbreviated terms used in the
description of chromosomes and chromosomal
abnormalities are listed below. For a detailed
discussion of these terms, consult ISCN (1995): An
International System for Human Cytogenetic
Nomenclature, Mitelman, F (ed); S. Karger, Basel,

71. • add additional material of unknown origin
• arrow (->) from - to, in detailed system
• brackets, square ([]) surround the number of cells
• cen centromere
• colon, single (:)break, in detailed system
• colon, double (::)break and reunion, in detailed system
• comma (,)separates chromosome numbers, sex
chromosomes, and chromosome abnormalities
• decimal point (.) denotes sub-bands

72. • del deletion
• de novo designates a chromosome abnormality which
has not
been inherited
• der derivative chromosome
• dic dicentric
• dup duplication
• fra fragile site
• h heterochromatin, constitutive
• hsr homogeneously staining region
• i isochromosome

73. • ins insertion
• inv inversion
• mar marker chromosome
• mat maternal origin
• minus sign (-)loss
• p short arm of chromosome
• Parenthese ssurround structurally altered
chromosome and breakpoints
• pat paternal origin

74. • plus sign (+) gain
• q long arm of chromosome
• question mark (?) questionable identification of a chromosome or
chromosome structure
• r ring chromosome
• rec recombinant chromosome
• s satellite
• sce sister chromatid exchange

75. • semicolon (;) separates altered chromosomes and breakpoints in
structural rearrangements involving more than one chromosome
• slant line (/) separates clones
• t translocation
• ter terminal (end of chromsome)
• upd uniparental disomy

76. In Situ Hybridization: Symbols and Abbreviations
• minus sign (-) absent from a specific chromosome
• plus sign (+) present on a specific chromosome
• multiplication sign (x) precedes the number of signals seen
• period (.) separates cytogenetic observations from results of in situ
hybridization
• semicolon (;) separates probes on different derivative chromosomes
chromosomes
• fish fluorescence in situ hybridization
• ish in situ hybridization; when used without a prefix applies to
chromsomes (usually metaphase or prometaphase) of dividing cells
• wcpwhole chromosome paint

77. 10 人类染色体畸变
chromosome aberration

78. Any type of change in the chromosome structure or number
(deficiencies, duplications, translocations, inversions, etc.).

79. Although it can be a mechanism for enhancing
genetic diversity, such alterations are usually fatal or
ill-adaptive, especially in animals.

80. 1. factors inducing chromosome aberration
A. Ionizing radiation
Ionizing radiation produces rearrangements of the genome.

82. When irradiation occurs during the G0/G1 phase of the cell cycle,
large-scale rearrangements appear as exchange-type chromosome
aberrations at the next mitosis. Such aberrations can alter cellular
phenotypes, and are important in various areas of biology:

83. Medical and public-health applications include
perinatal diagnostics characterization of specific cancer
types, carcinogenesis risk estimation, radiation
biodosimetry and radiotherapeutic treatment planning.

84. Analyzing chromosome aberrations helps characterize
repair/misrepair pathways involved in the processing of DNA damage.
Ionizing radiation has some unique features as a probe of such
pathways. Compared to other genotoxic agents, it produces copious
quantities of DNA double strand breaks (DSBs), and its timing can be
controlled more accurately.

85. Moreover, initial radiation damage has a discrete, stochastic
character that can be modulated by using different kinds of radiation
(e.g. a -particles versus x-rays) having different ionization densities.

86. The spectrum of different radiation-induced chromosome
aberrations is informative about the geometry of chromosomes
during interphase, and vice versa.

87. Chromosomal instability, in which new aberrations (though often
not of the type characteristic of G0/G1 damage) continue to arise
many generations after irradiation, represents a form of genomic
instability, and genomic instability is prominent during neoplastic
progression.

88. B. chemical mutagen
A kind of substances, such as drugs or toxins ,that causes
chromosomal aberrations of living organisms, or increase the rate of
mutation.