Download sex chromosomes

Section B Chromosome
General Review
DNA – Gene – Chromosome - Genome
DNA
•DNA: the chemical inside the nucleus of a
cell that carries the genetic instructions for
making living organisms.
•The material inside the nucleus of cells that
carries genetic information.
• The scientific name
deoxyribonucleic acid.
for
DNA
is
http://www.ncbi.nlm.nih.gov/
Chromosomes
• A chromosome is one of the threadlike "packages" of
genes and other DNA in the nucleus of a cell.
• Different kinds of organisms have different numbers of
chromosomes. 不同有机体有不同的染色体数
• Humans have 23 pairs of chromosomes, 46 in all: 44
autosomes and two sex chromosomes. 人44条常染色体，2
条性染色体。23对
• Each parent contributes one chromosome to each pair,
so children get half of their chromosomes from their
mothers and half from their fathers. 每对染色体一条来自父
方，一条来自母方
Chr.
DNA
http://www.ncbi.nlm.nih.gov
Gene
• A gene is the functional and physical unit of
heredity passed from parent to offspring.
• Genes are pieces of DNA, and most genes
contain the information for making a specific
protein.
http://www.ncbi.nlm.nih.gov
Chromosomes and Genome
• A genome is all the DNA contained in an
organism or a cell, which includes the
chromosomes plus the DNA in mitochondria
(线粒体） (and DNA in the chloroplasts （叶
绿体） of plant cells).
http://www.ncbi.nlm.nih.gov
Chromosomes
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Definition of “Chromosome”
Chromosome structures
Chromosome morphology
Function of chromosome
Molecular structure of chromosome
Functional and nonfunctional chromatin
Alteration to chromosome numbers
Alteration to chromosome structure
Cell division and sex determination
Non-mendel genetic material
1. DEFINITION OF CHROMOSOME
It is a combination of two words, i.e., “Chroma”-means
‘colour’ and “Somes”-means ‘body’.
So the coloured thread like bodies present in the nucleoplasm
of the living cells, which helps in the inheritance (transmission)
of characters in form of Genes from generation to generation
are known as CHROMOSOMES.
Prokaryote and eukaryote chromosomes
 The structures of chromosome in prokaryotes and
eukaryotes are different:
 In prokaryote, consists of a single circular DNA double
helix, relatively few proteins
 In eukaryote, many linear chromosomes located in
nucleus, large amount of specific proteins; much greater
in the amount of DNA per chromosome
2. PHYSICAL STRUCTURE
Size varies from 1 to 30 micron （微米） in length and diameter
from 0.2 to 2 micron.
CENTROMERE （ 着 丝 粒 ） :-The non-stainable part of the
chromosome making a primary constriction.
CHROMATIDS （ 染 色 单 体 ） : Two chromatids join at the
centromere to form a chromosome.
CHROMONEMA（染色丝）： In each chromatid, there are two
longitudinal chromonemata coiled with each other.
CHROMOMERES（染色体）: In each chromonemata, there are
“bead” like chromomeres（染色粒）present through out the coil.
GENES（基因）: Each chromomeres contains genes, the unit
of inheritance of character.
SATELLITE （ 卫 星 ） :- In some chromosomes a round and
elongated satellite is present.
CONSTRICTION（缢痕）:- Presence of centromere shows the
primary constriction. But in some cases there is an additional
Secondary Constriction.
SURFACE VIEW
Specialized chromosomal structure
• Centromere, Telomere, Nucleolar organizer
regions (NORs) and Chromosomal satellite
(secondary constriction)
• 着丝粒，端粒，核仁组织区，染色体卫星
（次缢痕）
• Centromere: spindle attaches centromere via
kinetochore [ki'ni:təkɔ:]; consists of highly repeated
satellite DNA; A chromosome lacking centromere will
be lost.
• 纺锤丝通过动粒连接在着丝粒上，有大量的重复微
卫星DNA。
• Telomeres （端粒）: contain multiple repeats of
simple, short DNA sequences; prevent recombination
between different chromosomal ends; a molecular
marker of the aging process; telomere length is
maintained by the enzyme telomerase （端粒酶）
• NORs: usually at secondary constrictions;
consists of tandemly repeated 5.8S, 18S and
28S rRNA genes; a nucleolus forms around
NOR; chromosomes that have NOR attached
at short arms
• 通常形成次缢痕，含有rRNA的基因，是参
与形成核仁的染色质区，通常在短臂上。
（注意：并非所有染色体都含有NORs）
• Secondary constriction and Chromosomal
satellite: can be seen on human
chromosomes 13,14,15,21 and 22.
3. CHEMICAL STRUCTURE
Chemically the chromosomes are made of
proteins and nucleic acids.
PROTEINS
It is mainly Protamines, Histones and smaller amount
of acidic proteins. 精蛋白，组蛋白，酸性蛋白
NUCLEIC ACIDS
It is de-oxy ribose Nucleic Acids (DNA). Genes
are nothing but the segments of DNA.
4. CHROMOSOME MORPHOLOGY
• Chromosomes are classified on the basis of their
morphology, which is determined by the position of the
centromere (ce).据着丝粒的位置分类
• Four typical morphologies :
– Telocentric （端着丝粒…）at the end of Chr, only one arm (T)
– Acrocentric （近端着丝粒…）: close to the end of Chr (A)
– Submetacentric（亚中着丝粒…) : far away from the midpoint
(S)
– Metacentric（ 中着丝粒） : ce is close to the midpoint of Chr
(M)
TYPES OF CHROMOSOMES :
1. TELOCENTRIC:- The centromere is
CENTROMERE
present at the end of the chromosomes.
SHORT ARM
p
LONG ARM
CENTROMERE
LONG ARM
q
2. ACROCENTRIC:-The centromere
is almost terminal. It has one large
and another very small arm.
TYPES OF CHROMOSOMES :
SHORT ARM
CENTROMERE
3. SUB-METACENTRIC:- Here the centromere
is not at the middle position of the chromosomes.
So the arms are unequal and it is ‘L-Shaped’ in
appearance.
LONG ARM
4. METECENTRIC:- The centromere
is at the middle position. So the arms
are equal and it is ‘V-Shaped’ in
appearance.
TWO EQUAL ARMS
CENTROMERE
M
S
A
T
Chromosome designation based on centromere position
5. FUNCTION OF CHROMOSOMES
[I]- The chromosomes are capable of selfduplication. During duplication process the DNA
strands unwind. As unwinding starts, each template
of DNA forms its complementary strand in doublehelix nature. The conversion of the old DNA
molecule into two new molecules, helps in
duplicating the chromosomes.
self duplication of DNA molecule
(it helps in the duplication of chromosomes )
Single DNA
molecule in
double helical
structure
Mother templates
unwind and new
complementary
strands originate
Unwinding
continues along
with new template
formation
Two separate DNA
molecules formed
having an old and a
new strand
[II]- They help in expression of different characters
in an organism by synthesizing proteins in cells. A
definite protein is accumulated to produce a
definite character.
[III]- As carrier of genes they transmit characters from
generation to generation , i.e. parents to offspring.
[IV]- The chromosomes control the physiological and
biochemical processes in the body of the organism.
6. NUMBERS OF CHROMOSOMES
• Constant for each cell in the body (except sex
cells which only have half sets).
• 每个细胞都有一致的染色体数量
• Constant throughout the life of an individual
(you don’t lose or gain chromosomes)
• 在整个生命活动中数量保持一致
• Constant for all members of a species
• 同一物种个体间的染色体数量一致
© 2007 Paul Billiet ODWS
Mouse
20对
Maize
20条
Organism
Human
Chromosome
numbers
46
Chimpanzee
48
House Mouse
40
Maize
20
© 2007 Paul Billiet ODWS
• Karyotype（核型）: the complete diploid (二
倍体）set of chromosomes.
– In a karyotype, autosomes are numbered in
order of decreasing size, sex chromosomes
referred to as X or Y
– Ideogram: the conventional way to display
karyotype 用符号表示
– 着丝粒、随体、次缢痕等的位置是确定核型的
重要参考指标。
有丝分裂中期染
色体（男性）
(chromosomes at
metaphase of
mitosis , human,
male)
核型
(karyotype)
M
A
S
T
The shape of chromosome during anaphase in mitosis
• Chromosome banding techniques
• G-banding: chromosomes with a series of dark Gbands (A and T rich) and pale interbands (G and C
rich)
– Mild pretreatment of chromosome preparation slides
with protease(蛋白酶） (e.g. trypsin胰蛋白酶) →stain
chromosomes with Giemsa
– Aid chromosome identification, reveal sub-chromosomal
regions so as to be used in gene mapping, medical
genetics et al.
– 显示亚染色体区域，便于基因定位和其它遗传学研究
• C-band：produces a number of dark bands
which are largely confined to areas around
centromeres (constitutive heterochromatin)
• 异染色质通常位于着丝粒周围并常含有高
度重复序列的DNA，染色使着丝粒处的异染
色质着色。因为通常都在着丝粒
(Centromere)处出现，所以称之为C-带。
7. MOLECULAR STRUCTURE OF
CHROMOSOMES
染
色
质
包
装
的
结
构
模
型
染色质纤丝
组蛋白八聚体
• Nucleosome:
– a core of histones: 2 molecules of H2A, H2B, H3
and H4
– DNA: 146bp
– H1 histone: sits on the outside of the
nucleosome complex acting as a seal
– Linker DNA: about 60bp
• Solenoid（螺线管）: nucleosomes coiled
together to form a solenoid (30nm in
diameter)
• Chromosome （染色体）: the binding of
chromatin fibers on to a chromosomal
scaffold (largely topoisomerase Ⅱ); scaffold
attachment regions of DNA link the DNA
molecule to chromosomal scaffold
• Chromosome and chromatin（染色质）:
Chromatin is a mixture of DNA and protein;
proteins: histones and nonhistones ( or acidic
proteins)
• Histones: H2A, H2B, H3 and H4; H1; with
molecular masses of less than 23kDa; have a
basic charge due to Lys and Arg; highly
conserved throughout evolution
8. FUNCTIONAL AND NONFUNCTIONAL
CHROMATIN
• Euchromatin (真染色质）：active
• heterochromatin（异染色质）: inactive
• Heterochromatin types：
– Constitutive heterochromatin（结构性异染色质）:永久
性的呈现异固缩的染色质，含有高度重复的随体DNA，
分布于大多数染色体的着丝粒区、端粒和次缢痕处，呈
现C带染色
– Facultative heterochromatin（兼性异染色质）: 在一定的
细胞类型或一定的发育阶段呈现凝集状态的异染色质。
在一定时期的特种细胞的细胞核内, 原来的常染色质可转
变成兼性异染色质。如 X chromosome in female
mammals; randomly inactivated; reactivated during
gametogenesis
9. ALTERATIONS TO CHROMOSOME
NUMBERS
• Differences in chromosomes are associated with difference
in the way we grow.
• The karyotypes of males and females are not the same
Females have two large X chromosomes
Males have a large X and a small Y chromosome
The X and the Y chromosomes are called sex chromosomes
The sex chromosomes are placed at the end of the
karyotype
• Unusual growth can be associated with chromosome
abnormalities
• e.g. People who develop Down’s syndrome（唐氏综合症）
have trisomy （三体性染色体）21
© 2007 Paul Billiet ODWS
• Caused by errors in meiosis or mitosis.
• Aneuploidy（非整倍体、异倍体）:
– Nullisomy 缺对染色体 : 2n-2
– Monosomy单体性染色体 : 2n-1, Turner’s syndrome (45,
XO)
– Trisomy三体性染色体: 2n+1, Down’s syndrome( 47, +21),
1/750
– Anuploid gametes arise from errors at both first and
second division of meiosis through non-disjunction
Trysomy-21  Down’s syndrome
Trysomy-18  Edward’s syndrome
Images believed to be in the Public Domain
Polyploidy（多倍体）: Organisms with more than
two complete sets of chromosomes, have
undergone polyploidy.
This may occur when a normal gamete fertilizes
another gamete in which there has been
nondisjunction of all its chromosomes.
The resulting zygote would be triploid (3n)（三
倍体）.
Alternatively, if a 2n zygote failed to divide after
replicating its chromosomes, a tetraploid (4n)（四
倍体） embryo would result from subsequent
successful cycles of mitosis.
• Polyploidy is relatively common among plants and much
less common among animals.
• 10% of spontaneously aborted human fetuses.
• The spontaneous origin of polyploid individuals plays an
important role in the evolution of plants.
• Both fishes and amphibians （两栖动物） have polyploid
species.
• Recently, researchers in Chile have
identified a new rodent
• (啮齿类动物)species which
• may be the product of
polyploidy.
• Polyploids are more nearly normal in
phenotype than aneuploids.
• 多倍体在表型上比非整倍体正常得多
• One extra or missing chromosome apparently
upsets the genetic balance during development
more than does an entire extra set of
chromosomes.
• 缺失或多一条染色体会打乱 遗传平衡。
10. ALTERATIONS TO CHROMOSOME
STRUCTURE
•
•
•
•
Deletion（缺失）
Duplication（重复）
Inversion（倒位）
Translocation（易位）
• Breakage of a chromosome can lead to four types
of changes in chromosome structure.
• A deletion occurs when a chromosome fragment
lacking a centromere is lost during cell division.
– This chromosome will be missing certain genes.
• A duplication occurs when a fragment becomes
attached as an extra segment to a sister
chromatid.
缺失的表型效应：
染
色
体
结
构
的
改
变
：
缺
失
1.致死或出现异常
2.假显性（pseudo
domiant）
染色体结构的改变：重复（duplication）
重复的类型：顺接、反接、同臂、异臂
重复产生的原因：断裂－融合桥的形成，染色体扭结，
不等交换
重复的表型效应：改变连锁关系或交换率，位置效应，
剂量效应，表型异常
重复的应用：研究位置效应；固定杂种优势
• An inversion occurs when a chromosomal
fragment reattaches to the original
chromosome but in the reverse orientation.
• In translocation, a chromosomal fragment joins
a nonhomologous chromosome.
– Some translocations are reciprocal, others are not.
11. CHROMOSOMES AND CELL
DIVISION
• Multicellular organisms copy their
chromosomes before cell division.
• They must grow to a mature size.
• The nucleus divides, distributing
the chromosomes into two equal
groups (mitosis).
• The cytoplasm then divides
(cytokinesis) each part taking a
nucleus.
© 2007 Paul Billiet ODWS
Interphase
Image believed to be in the Public Domain
The cell cycle
Cytokinesis
Division of the cytoplasm
M
G0
Some cells may stay
in this stage for over a
year
G1
G2
First growth phase.
Varies in length
Second growth
phase
S
G1 + S + G2 = INTERPHASE
© 2007 Paul Billiet ODWS
Replication of
chromosomes
The cell cycles in different cells
Cell type
Bean root tip
Cell cycle /
hours
19.3
Mouse fibroblast
22
Chinese hamster
fibroblast
Mouse small intestine
epithelium
11
Mouse oesophagus
epithelium
© 2007 Paul Billiet ODWS
17
181
Chromosomes and reproduction
• Chromosomes come in pairs
One of the pair is maternal ，the other is
paternal
• When parents make sex cells the number of
chromosomes must be halved（减半）
One of each type of chromosome is taken
© 2007 Paul Billiet ODWS
Meiosis and fertilisation
Mother
Father
23 pairs of
chromosomes
23 pairs of
chromosomes
Meiosis
Sex cells
23 unpaired
chromosomes
23 unpaired
chromosomes
23 unpaired
chromosomes
Fertilisation
23 pairs of
chromosomes
Child
© 2007 Paul Billiet ODWS
Images believed to be in the Public Domain
23 unpaired
chromosomes
Meiosis
•
•
•
•
A special type of cell division
Used to make sex cells
Meiosis halves the numbers of chromosomes
Meiosis picks one chromosome from each pair
at random and places them in a sex cell. This
results in enormous variation amongst the sex
cells.
© 2007 Paul Billiet ODWS
Timing of Meiosis and
Gamete Formation in Males and Females
• Males
–
–
–
–
Spermatogenesis begins during puberty
Millions of sperm are always in production
Spermatogenesis takes about 48 days
Meiotic division produces 4 sperm
• Females
– Primary oocytes produced during embryonic
development remain in meiosis I until ovulation
– Ovulation begins during puberty
– Meiotic division produces 1 oocyte and 2-3 polar
bodies
Spermatogenesis
Oogenesis and ovulation
12. SEX DETERMINATION
----The inheritance of gender
Is it going to be a boy or a girl?
© University of New Mexico
The inheritance of gender
Mother
Father
XX
XY
Meiosis
Sex cells
X
Fertilisation
X
X
X
Y
X
XX
XY
X
XX
XY
Y
Possible
children
Chance of a girl 50%
Chance of a boy 50%
© 2007 Paul Billiet ODWS
Sex chromosomes
• The sex of many animals is determined by
genes but on chromosomes called sex
chromosomes
• The other chromosomes are called autosomes
• One sex is homogametic （同型配子）
• The other sex is heterogametic （异型配子）
© 2007 Paul Billiet ODWS
Sex determination in different animals
HOMOGAME
TIC SEX
HETEROGAM
ETIC SEX
SEX
DETERMINATION
Female XX
Male XY
Presence of Ychromosome =
maleness
(mammals and
fish)
Presence of second
X-chromosome =
femaleness
(Drosophila, the
fruit fly)
Male ZZ
Female ZW
Birds, amphibians,
reptiles,
butterflies, moths.
Female XX
Male Xo
Grasshoppers蝗虫
© 2007 Paul Billiet ODWS
Factors in Sexual Differentiation
• The formation of male and female
reproductive structures depends on:
– Gene action
– Interactions within the embryo
– Interactions with other embryos in the uterus
– Interactions with the maternal environment
Three Levels of Sexual Determination
• The chromosomal sex of an individual (XX or
XY) can differ from the phenotypic sex
• Sex of an individual is defined at three levels
– Chromosomal sex （染色体）
– Gonadal sex （性腺）
– Phenotypic sex （表型）
Chromosomal basis of sex
determination
Male
or
TDF
TDF: TestisDetermining factor
female ?
No TDF
Y Chromosome and Testis Development
• SRY gene
– Sex-determining region of the Y chromosome
– Located near the end of the short arm of the Y
chromosome
– Plays a major role in causing the undifferentiated
gonad(性腺） to develop into a testis（睾丸)
The phenotypic effects of some mammalian genes
depend on whether they were inherited from the mother
or the father (GENOMIC IMPRINTING基因组印记).
• For most genes it is a reasonable assumption
that a specific allele will have the same effect
regardless of whether it was inherited from the
mother or father.
• However, for some traits in mammals, it does
depend on which parent passed along the alleles
for those traits.
– The genes involved may or may not lie on the X
chromosome.
– Involves “essential” silencing of one allele during
gamete formation
基因组印记是指基因根据亲代的不同而有不
同的表达。印记基因的存在能导致细胞中两
个等位基因的一个表达而另一个不表达。
Extra-nuclear genes exhibit a NON-MENDELIAN pattern of
inheritance. 染色体之外的遗传信息
• Not all of a eukaryote cell’s genes are located in
the nucleus.
• Extra-nuclear genes are found on small circles of
DNA in mitochondria（线粒体） and
chloroplasts（叶绿体）.
• These organelles reproduce themselves.
• Their cytoplasmic genes do not display
Mendelian inheritance.
• Karl Correns in 1909 first observed cytoplasmic
genes in plants.
• He determined that the coloration of the
offspring was determined only by the maternal
parent.
• These coloration patterns are due to genes in
the plastids（质体） which are inherited only
via the ovum（卵）,
not the pollen.
• Because a zygote typically inherits all its
mitochondria/chloroplasts only from the ovum,
all such genes in demonstrate maternal
inheritance.
• Several rare human disorders are produced by
mutations to mitochondrial DNA.
– These primarily impact ATP supply by producing
defects in the electron transport chain or ATP
synthase.
– Tissues that require high energy supplies (for
example, the nervous system and muscles) may suffer
energy deprivation from these defects.
– Other mitochondrial mutations may contribute to
diabetes, heart disease, and other diseases of aging.
Key points
• Structural differences in prokaryote and
eukaryote chromosomes
• Explain: centromere, telomere, NOR,
heterochromatin, euchromatin, karotype,
Aneuploidy（trisomy, monosomy),
polyploidy (triploidy, tetraploidy) , genomic
imprinting
• Sex determination
• Non-mendel genetic material
染色质和染色体的多级结构
• 染色质结构的基本单位—核小体:
由核心颗粒和连接区构成；
核心颗粒包括由8个组蛋白分子（H2A，H2B，H3，H4各两个）构成的组蛋白
核心和包绕在核心表面的DNA分子；
包绕在组蛋白核心表面的DNA长140bp，环绕1¾圈；
连接区由DNA分子和H1组蛋白分子构成，长度不定；
• 由连接区将许多核心颗粒相连接构成了染色体的一级结
构；
• 每六个核小体盘绕成直径30nm的螺线管，成为染色体的
二级结构；
• 螺线管进一步螺旋化形成0.4μm的圆筒状结构，称为超
螺线管，是染色体的三级结构；
• 超螺线管进一步缠绕折叠构成染色单体，两条染色单体
通过着丝粒相连成为染色体。