ChromosomeChromosome

FatchiyahFatchiyah

fatchiya@brawijaya.ac.idfatchiya@brawijaya.ac.id

What Exactly is a What Exactly is a chromosome?chromosome?

Chromosomes are the Chromosomes are the rod-shapedrod-shaped, , filamentous bodies filamentous bodies present in the present in the nucleusnucleus, which become visible , which become visible during during cell divisioncell division. .

They are the They are the carriers of the gene carriers of the gene or unit or unit of heredity.of heredity.

Chromosome are Chromosome are not visible not visible in active in active nucleus due to their nucleus due to their high water contenthigh water content, , but are clearly seen during cell division. but are clearly seen during cell division.

Chromosomes were first described Chromosomes were first described by by Strausberger Strausberger in in 18751875. .

The term “Chromosome”, however The term “Chromosome”, however was first used by was first used by Waldeyer Waldeyer in in 18881888. .

They were given the name They were given the name chromosome (Chromo = colour; chromosome (Chromo = colour; Soma = body) due to their marked Soma = body) due to their marked affinity for basic dyesaffinity for basic dyes. .

Their number can be counted easily Their number can be counted easily only during only during mitotic metaphase.mitotic metaphase.

Chromosomes are composed of Chromosomes are composed of thin thin chromatin chromatin threads called threads called Chromatin Chromatin fibersfibers. . These fibers undergo These fibers undergo foldingfolding, , coiling coiling and and supercoiling supercoiling during prophase so that the during prophase so that the chromosomes become progressively chromosomes become progressively thicker and smaller. thicker and smaller. Therefore, chromosomes become Therefore, chromosomes become readily observable under light readily observable under light microscope. microscope. At the end of cell division, on the other At the end of cell division, on the other hand, the fibers uncoil and extend as hand, the fibers uncoil and extend as fine chromatin threads, which are not fine chromatin threads, which are not visible at light microscopevisible at light microscope

Number of chromosomesNumber of chromosomesNormally, all the individuals of a Normally, all the individuals of a species have species have the same number the same number of chromosomes. of chromosomes. Closely related species usually have similar Closely related species usually have similar chromosome numbers. chromosome numbers. Presence of a whole sets of chromosomes is Presence of a whole sets of chromosomes is called called euploidyeuploidy. . It includes haploids, diploids, triploids, It includes haploids, diploids, triploids, tetraploids etc.tetraploids etc.Gametes normally contain only one set of Gametes normally contain only one set of chromosome – this number is called chromosome – this number is called HaploidHaploidSomatic cells usually contain two sets of Somatic cells usually contain two sets of chromosome - chromosome - 2n : Diploid2n : Diploid

3n – triploid3n – triploid4n – tetraploid4n – tetraploidThe condition in which the chromosomes sets The condition in which the chromosomes sets

are present in a multiples of “n” is are present in a multiples of “n” is PolyploidyPolyploidy

When a change in the chromosome number When a change in the chromosome number does not involve entire sets of does not involve entire sets of chromosomes, but only a few of the chromosomes, but only a few of the chromosomes - is chromosomes - is Aneuploidy.Aneuploidy.Monosomics (2n-1)Monosomics (2n-1)Trisomics (2n+1)Trisomics (2n+1)Nullisomics (2n-2)Nullisomics (2n-2)Tetrasomics (2n+2)Tetrasomics (2n+2)

OrganismOrganism No. No. chromosomeschromosomes

HumanHuman 4646ChimpanzeeChimpanzee 4848DogDog 7878HorseHorse 6464ChickenChicken 7878GoldfishGoldfish 9494Fruit flyFruit fly 88MosquitoMosquito 66NematodeNematode 11(m), 12(f)11(m), 12(f)HorsetailHorsetail 216216SequoiaSequoia 2222Round worm Round worm 22

Characteristics of eukaryotic chromosomeCharacteristics of eukaryotic chromosome

- found in the nucleus of a cell.

- consists of DNA and protein (histone etc.).

- in linear arrangement.

- ready to absorb dyes (it’s named after; in Greek: chroma is colour, soma is body).

Chromosome SizeChromosome SizeIn contrast to other cell organelles, the size of In contrast to other cell organelles, the size of chromosomes shows a remarkable variation chromosomes shows a remarkable variation depending upon the stages of cell division. depending upon the stages of cell division. Interphase:Interphase: chromosome are longest & thinnestchromosome are longest & thinnestProphase:Prophase: there is a progressive decrease in their there is a progressive decrease in their length accompanied with an increase in thicknesslength accompanied with an increase in thicknessAnaphase: Anaphase: chromosomes are smallest.chromosomes are smallest.Metaphase: Metaphase: Chromosomes are the most easily Chromosomes are the most easily observed and studied during metaphase when they observed and studied during metaphase when they are very thick, quite short and well spread in the are very thick, quite short and well spread in the cell.cell.TTherefore, chromosomes measurements are herefore, chromosomes measurements are generally taken during mitotic metaphase. generally taken during mitotic metaphase.

KaryotypeKaryotype: is the general morphology of : is the general morphology of the somatic chromosome. Generally, the somatic chromosome. Generally, karyotypes represent by arranging in the karyotypes represent by arranging in the descending order of size keeping their descending order of size keeping their centromeres in a straight line.centromeres in a straight line.IdiotypeIdiotype: the karyotype of a species may : the karyotype of a species may be represented diagrammatically, showing be represented diagrammatically, showing all the morphological features of the all the morphological features of the chromosome; such a diagram is known as chromosome; such a diagram is known as Idiotype. Idiotype.

Chromosomes may differ in the position of Chromosomes may differ in the position of the the CentromereCentromere, the place on the , the place on the chromosome where spindle fibers are chromosome where spindle fibers are attached during cell division. attached during cell division.

In general, if the centromere is near the In general, if the centromere is near the middle, the chromosome is middle, the chromosome is metacentricmetacentric

If the centromere is toward one end, the If the centromere is toward one end, the chromosome is chromosome is acrocentric acrocentric or or submetacentricsubmetacentric

If the centromere is very near the end, the If the centromere is very near the end, the chromosome is chromosome is telocentrictelocentric. .

Electron micrograph of one of the human chromosomes

Centromere

Structure of cStructure of chromosohromosomeme

Telomere

Chromosome arm

Secondary constriction or NOR (nucleolar organizing region)

Primary constriction or centromere

Telomere

Can distinguish chromosomes by “Can distinguish chromosomes by “paintingpainting” – ” – using DNA hybridization + fluorescent probes – using DNA hybridization + fluorescent probes –

duringduring mitosismitosis

Euchromatin and HeterochromatinEuchromatin and Heterochromatin

Chromosomes may be identified by regions that stain in a Chromosomes may be identified by regions that stain in a particular manner when treated with various chemicals. particular manner when treated with various chemicals.

Several different chemical techniques are used to identify Several different chemical techniques are used to identify certain chromosomal regions by staining then so that they certain chromosomal regions by staining then so that they form form chromosomal bands.chromosomal bands.

For example, darker bands are generally found near the For example, darker bands are generally found near the centromeres or on the ends (telomeres) of the centromeres or on the ends (telomeres) of the chromosome, while other regions do not stain as strongly.chromosome, while other regions do not stain as strongly.

The position of the dark-staining areThe position of the dark-staining are heterochromatic heterochromatic region region oror heterochromatin heterochromatin..

Light staining are Light staining are euchromatic region euchromatic region or or euchromatineuchromatin. .

Staining and Banding chromosome Staining and Banding chromosome Staining procedures have been developed in the past two Staining procedures have been developed in the past two

decades and these techniques help to study the decades and these techniques help to study the karyotype in plants and animals. karyotype in plants and animals.

1.1. Feulgen StainingFeulgen Staining: : Cells are subjected to a mild hydrolysis in 1N Cells are subjected to a mild hydrolysis in 1N

HCl at 60HCl at 6000C for 10 minutes. C for 10 minutes. This treatment produces a free aldehyde group This treatment produces a free aldehyde group

in deoxyribose molecules.in deoxyribose molecules. When When Schiff’s reagent Schiff’s reagent (basic fuschin bleached (basic fuschin bleached

with sulfurous acid) to give a deep pink colour. with sulfurous acid) to give a deep pink colour. Ribose of RNA will not form an aldehyde under Ribose of RNA will not form an aldehyde under

these conditions, and these conditions, and the reaction is thus specific for the reaction is thus specific for DNADNA

22. Q banding:. Q banding:

The Q bands are the The Q bands are the fluorescent bands fluorescent bands observed after observed after quinacrine mustard staining quinacrine mustard staining and and observation with UV light. observation with UV light.

The distal ends of each chromatid are not stained The distal ends of each chromatid are not stained by this technique. by this technique.

The Y chromosome become brightly fluorescent The Y chromosome become brightly fluorescent both in the interphase and in metaphase.both in the interphase and in metaphase.

3. R banding: 3. R banding:

The R bands (from The R bands (from reversereverse) are those ) are those located in the zones that do not fluoresce with the located in the zones that do not fluoresce with the quinacrine mustard, that is they are between the Q quinacrine mustard, that is they are between the Q bands and can be visualized as green. bands and can be visualized as green.

4. G banding:4. G banding: The G bands (from The G bands (from Giemsa) Giemsa) have the same have the same

location as Q bands and do not require fluorescent location as Q bands and do not require fluorescent microscopy. microscopy.

Many techniques are available, each involving Many techniques are available, each involving some pretreatment of the chromosomes. some pretreatment of the chromosomes.

In In ASG (Acid-Saline-GiemsaASG (Acid-Saline-Giemsa) cells are incubated ) cells are incubated in citric acid and NaCl for one hour at 60in citric acid and NaCl for one hour at 6000C and are then C and are then treated with the Giemsa stain. treated with the Giemsa stain.

5. C banding:5. C banding:The C bands correspond to The C bands correspond to constitutive constitutive

heterochromatinheterochromatin. . The heterochromatin regions in a chromosome The heterochromatin regions in a chromosome

distinctly differ in their stainability from euchromatic distinctly differ in their stainability from euchromatic region. region.

Dosage CompensationDosage Compensation

Sex Chromosomes: Sex Chromosomes: females XXfemales XX, males XY, males XY

Females have two copies of every X-linked gene; males Females have two copies of every X-linked gene; males have only one. have only one.

How is this difference in gene dosage compensated for? How is this difference in gene dosage compensated for? OROR

How to create equal amount of X chromosome gene How to create equal amount of X chromosome gene products in males and females?products in males and females?

In cells with more than two X chromosomes, only one X In cells with more than two X chromosomes, only one X remains genetically active and all the others become remains genetically active and all the others become inactivated.inactivated.

Barr BodiesBarr Bodies1940’s two Canadian scientists noticed 1940’s two Canadian scientists noticed a dark staining mass in the nuclei of cat a dark staining mass in the nuclei of cat brain cellsbrain cells

Found these dark staining spots in Found these dark staining spots in female but not malesfemale but not males

This held for cats and humansThis held for cats and humans

They thought the spot was a tightly They thought the spot was a tightly condensed X chromosome condensed X chromosome

Barr bodies represent the inactive X chromosome and are normally found only in female somatic cells.

Barr Bodies

A woman with the chromosome constitution 47, XXX should have 2 Barr bodies in each cell.XXY individuals are male, but have a Barr body. XO individuals are female but have no Barr bodies.

Which chromosome is inactive is a matter of Which chromosome is inactive is a matter of chance, but once an X has become chance, but once an X has become inactivated , all cells arising from that cell inactivated , all cells arising from that cell will keep the same inactive X chromosome. will keep the same inactive X chromosome.

In the mouse, the inactivation apparently In the mouse, the inactivation apparently occurs in early in development occurs in early in development

In human embryos, sex chromatin bodies In human embryos, sex chromatin bodies have been observed by the 16have been observed by the 16thth day of day of gestation. gestation.

Mechanism of X-chromosome Mechanism of X-chromosome InactivationInactivation

A region of the p arm of the X chromosome A region of the p arm of the X chromosome near the centromere called the near the centromere called the X-inactivation X-inactivation centercenter (XIC) is the control unit. (XIC) is the control unit.

This region contains the gene for This region contains the gene for X-inactive X-inactive specific transcriptspecific transcript ( (XISTXIST). This ). This RNARNA presumably coats the X chromosome that presumably coats the X chromosome that expresses it and then expresses it and then DNA methylation locks DNA methylation locks the chromosome in the inactive state.the chromosome in the inactive state.

This occurs about 16 days after fertilization This occurs about 16 days after fertilization in a female embryo.in a female embryo.

The process is independent from cell to cell. The process is independent from cell to cell.

A maternal or paternal X is randomly chosen A maternal or paternal X is randomly chosen to be inactivated.to be inactivated.