section b chromosome general review dna – gene – chromosome - genome

Section B Chromosome

General Review

DNA – Gene – Chromosome - GenomeDNA – Gene – Chromosome - Genome

DNADNA

•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 for DNA is deoxyribonucleic acid.

http://www.ncbi.nlm.nih.gov/


ChromosomesChromosomes

• 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

http://www.ncbi.nlm.nih.gov

DNA

Chr.


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.

Chromosomes and GenomeChromosomes 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).

Chromosomes• 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 STRUCTURESize 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)

1. TELOCENTRIC:- The centromere is

present at the end of the chromosomes.

2. ACROCENTRIC:-The centromere is almost terminal. It has one large and another very small arm.

LONG ARM

CENTROMERECENTROMERE

TYPES OF CHROMOSOMES :

LONG ARM

SHORT ARM

CENTROMERE

p

q

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.

4. METECENTRIC:- The centromere is at the middle position. So the arms are equal and it is ‘V-Shaped’ in appearance.

LONG ARM

CENTROMERE

SHORT ARM

CENTROMERE

TWO EQUAL ARMS

TYPES OF CHROMOSOMES :

Chromosome designation based on centromere position

M S A T

[I]- The chromosomes are capable of self-duplication. During duplication process the DNA strands unwind. As unwinding starts, each template of DNA forms its complementary strand in double-helix nature. The conversion of the old DNA molecule into two new molecules, helps in duplicating the chromosomes.

5. FUNCTION OF 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 Chromosome numbers

Human 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)

The shape of chromosome during anaphase in mitosis

M

A

S

T

• Chromosome banding techniques• G-banding: chromosomes with a series of dark G-

bands (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 ); scaffold Ⅱattachment 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

• Differences in chromosomes are associated with difference in the way we grow.

• The karyotypes of males and females are not the sameFemales have two large X chromosomesMales have a large X and a small Y chromosomeThe X and the Y chromosomes are called sex chromosomesThe 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


9. ALTERATIONS TO CHROMOSOME NUMBERS

• 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.

染色体结构的改变：缺失

缺失的表型效应：

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.

Interphase

© 2007 Paul Billiet ODWSImage believed to be in the Public Domain

The cell cycle

First growth phase.Varies in length

Replication of chromosomes

Some cells may stay in this stage for over a year

Second growth phase

M

G1

S

G2

G0

Cytokinesis Division of the cytoplasm

G1 + S + G2 = INTERPHASE


The cell cycles in different cells

Cell type Cell cycle / hours

Bean root tip 19.3

Mouse fibroblast 22

Chinese hamster fibroblast

11

Mouse small intestine epithelium

17

Mouse oesophagus epithelium

181


Chromosomes and reproduction

• Chromosomes come in pairsOne 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


23 unpaired chromosomes




Fertilisation

Child

Father

23 pairs of chromosomes

Sex cellsMeiosis

Mother



Meiosis and fertilisation

© 2007 Paul Billiet ODWS Images believed to be in the Public Domain

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.


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

YXX X

Fertilisation Possible children

Father

Sex cells

Meiosis

Mother

XX XY

Chance of a girl 50% Chance of a boy 50%

X Y

X XX XY

X XX XY

The inheritance of gender


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）


Sex determination in different animals

HOMOGAMETIC SEX

HETEROGAMETIC SEX

SEX DETERMINATION

Female XX Male XY Presence of Y-chromosome = 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


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 female ?

TDF No TDF

TDF: TestisDetermining factor

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 )

• 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

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 ).

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。

• 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.

Extra-nuclear genes exhibit a NON-MENDELIAN pattern of inheritance. 染色体之外的遗传信息

• 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。

section b chromosome general review dna – gene – chromosome - genome

Documents

chromosome numbersalteration

chromosomesa chromosome

sex chromosomes

pieces of dna

linear chromosomes

pairs of chromosomes

repeated satellite dna

form of genes