ChromosomesChromosomes

Dr. A. A. Abougabal

In eukaryotic cells the genetic material is In eukaryotic cells the genetic material is organized into a complex structure composed of organized into a complex structure composed of DNA and proteins and localized in a specialized DNA and proteins and localized in a specialized compartment, the nucleus. This structure was compartment, the nucleus. This structure was calledcalled chromatin (from the Greek "khroma" chromatin (from the Greek "khroma" meaning coloured and "soma" meaning body). meaning coloured and "soma" meaning body). Close to two meters of DNA in each cell must be Close to two meters of DNA in each cell must be assembled into a small nucleus of someassembled into a small nucleus of some mm in mm in diameter. Despite this enormous degree of diameter. Despite this enormous degree of compaction, DNA must be rapidly accessible to compaction, DNA must be rapidly accessible to permit its interaction with protein machineries permit its interaction with protein machineries that regulate the functions of chromatinthat regulate the functions of chromatin: :

· · replicationreplication, , · · repair andrepair and · · recombinationrecombination. .

Chromosomes are also located in the nucleus. When a Chromosomes are also located in the nucleus. When a cell is "resting" i.e. in the interphase not dividing,cell is "resting" i.e. in the interphase not dividing,

Chromosomes are also located in the nucleus.Chromosomes are also located in the nucleus. When a cell is "resting" i.e. not dividing, the When a cell is "resting" i.e. not dividing, the

chromosomes are organized into long entangled chromosomes are organized into long entangled structures called chromatin and not into individual structures called chromatin and not into individual chromosomes as we typically think of them. chromosomes as we typically think of them.

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. .

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 high water contentcontent, but are clearly seen during , but are clearly seen during cell division.cell division.

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 during prophase so that the chromosomes become so that the chromosomes become progressively thicker and smaller.progressively thicker and smaller.

Therefore, chromosomes become readily Therefore, chromosomes become readily observable under light microscope. At the observable under light microscope. At the end of cell division, on the other hand, end of cell division, on the other hand, the fibers uncoil and extend as fine the fibers uncoil and extend as fine chromatin threads, which are not visible chromatin threads, which are not visible at light microscopeat light microscope

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

Classification of Classification of Chromosomes according Chromosomes according

to to In order to understand In order to understand chromosomes and their function, chromosomes and their function, we need to be able to we need to be able to discriminate among different discriminate among different chromosomes. chromosomes.

Size of the chromosomes Size of the chromosomes Number of chromosomesNumber of chromosomes Morphological of the Morphological of the

chromosomeschromosomes

First, First, Chromosome SizeChromosome Size In contrast to other cell organelles, the size In contrast to other cell organelles, the size

of chromosomes shows a remarkable of chromosomes shows a remarkable variation depending upon the stages of cell variation depending upon the stages of cell division. division.

Interphase:Interphase: chromosome are longest & chromosome are longest & thinnestthinnest

Prophase:Prophase: there is a progressive decrease in there is a progressive decrease in their length accompanied with an increase in their length accompanied with an increase in thicknessthickness

Metaphase: Metaphase: Chromosomes are the most easily Chromosomes are the most easily observed and studied during metaphase when observed and studied during metaphase when they are very thick, quite short and well they are very thick, quite short and well spread in the cell.spread in the cell.

Anaphase: Anaphase: chromosomes are smallest.chromosomes are smallest. Therefore, chromosomes measurements are Therefore, chromosomes measurements are

generally taken during mitotic metaphase. generally taken during mitotic metaphase.

chromosomes differ greatly in size between chromosomes differ greatly in size between organisms, the size difference can be over organisms, the size difference can be over 100-fold, while within a sp, some 100-fold, while within a sp, some chromosomes are often 10 times as large as chromosomes are often 10 times as large as others.others.

The size of the chromosomes in mitotic phase of The size of the chromosomes in mitotic phase of animal and plants sp generally varies between animal and plants sp generally varies between 0.5 µ 0.5 µ and 32 µ and 32 µ in length, and between in length, and between 0.2 µ and 3.0 µ 0.2 µ and 3.0 µ in in diameter. diameter.

The longest metaphase chromosomes found in The longest metaphase chromosomes found in TrilliumTrillium - - 32 µ32 µ. .

The giant chromosomes found in The giant chromosomes found in diptera diptera and they may and they may be as long as be as long as 300 µ 300 µ and up to and up to 10 µ 10 µ in diameter.in diameter.

In general, In general, plants have longer chromosomes plants have longer chromosomes than than animal and species having lower chromosome animal and species having lower chromosome numbers have long chromosomes than those having numbers have long chromosomes than those having higher chromosome numbershigher chromosome numbers

Among plants, Among plants, dicots dicots in general, have a higher number in general, have a higher number of chromosome than monocots.Chromosomes are of chromosome than monocots.Chromosomes are longer in monocot than dicots. longer in monocot than dicots.

KaryotypeKaryotype&& Idiotype Idiotype In a species In a species KaryotypeKaryotype, a pictorial or , a pictorial or

photographic representation of all the different photographic representation of all the different chromosomes in a cell of an individual, chromosomes in a cell of an individual, chromosomes are usually ordered by size and chromosomes are usually ordered by size and numbered from largest to smallestnumbered from largest to smallest

KaryotypeKaryotype: is the general morphology of the : is the general morphology of the somatic chromosome. Generally, karyotypes somatic chromosome. Generally, karyotypes represent by arranging in the descending order represent by arranging in the descending order of size keeping their centromeres in a straight of size keeping their centromeres in a straight line.line.

Idiotype: the karyotype of a species may be Idiotype: the karyotype of a species may be represented diagrammatically, showing all the represented diagrammatically, showing all the morphological features of the chromosome; morphological features of the chromosome; such a diagram is known as Idiotype. such a diagram is known as Idiotype.

Second, Second, Number of Number of chromosomeschromosomes

Normally, all the individuals of a Normally, all the individuals of a species species have the same number have 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 HaploidHaploid

Somatic cells usually contain two sets of Somatic cells usually contain two sets of chromosome - chromosome - 2n : Diploid2n : Diploid

Organism No. chromosomes

Human 46 Chimpanzee 48 Dog 78 Horse 64 Chicken 78 Goldfish 94 Fruit fly 8 Mosquito 6 Nematode 11(m), 12(f) Horsetail 216 Sequoia 22 Round worm Round worm 22

Organism No. chromosomes

OnionOnion 1616 MoldMold 1616 Carrot Carrot 2020 Tomato Tomato 2424 Tobacco Tobacco 4848 Rice Rice 2424 Maize Maize 2020 Haploppus gracilis Haploppus gracilis 44 Crepis capillaris Crepis capillaris 6

On the extreme, On the extreme, round worm round worm shows shows only only two chromosomestwo chromosomes, while the , while the other extreme is represented by other extreme is represented by Protozoa Protozoa having having 300 or more 300 or more chromosomeschromosomes. .

However, most organisms have However, most organisms have numbers between numbers between 12 to 5012 to 50. .

3-8 3-8 in fungiin fungi From From 8 – 16 8 – 16 in Angiosperms (Most in Angiosperms (Most

common number being common number being 1212).).

ThirdThird , ,Morphology of Morphology of Eukaryotic ChromosomesEukaryotic Chromosomes

A A chromatidchromatid is a condensed DNA subunit of a is a condensed DNA subunit of a chromosome. The two chromatids of a duplicated chromosome. The two chromatids of a duplicated chromosome are held together at a region of DNA chromosome are held together at a region of DNA called the centromere (see figure below). Centromeres called the centromere (see figure below). Centromeres are the attachment points for are the attachment points for microtubulesmicrotubules, which are , which are responsible for the guiding the movement of responsible for the guiding the movement of chromosomes during mitosis and meiosischromosomes during mitosis and meiosis

CHROMOSOMAL CHROMOSOMAL STRUCTURESTRUCTURE

The nuclei of most human The nuclei of most human cells contain 46 cells contain 46 chromosomes. chromosomes.

These 46 chromosomes These 46 chromosomes consist of 23 pairs of consist of 23 pairs of homologous chromosomeshomologous chromosomes, , or or homologs, meaning each homologs, meaning each of these pairs are alike, but of these pairs are alike, but not necessarily identical.not necessarily identical.

The convention is to describe The convention is to describe the chromosome number in the chromosome number in humans as 2humans as 2nn = 46 because = 46 because the cells are diploid, meaning the cells are diploid, meaning they have two complete sets they have two complete sets of chromosomesof chromosomes. .

CHROMOSOMAL CHROMOSOMAL STRUCTURESTRUCTURE

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

The centromere divides the The centromere divides the chromosome into two arms, chromosome into two arms,

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

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

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

Telocentric Acrocentric

Submetacentric

Metacentric

According to the position of the According to the position of the centromere, the eukaryotic centromere, the eukaryotic chromosomes may be chromosomes may be

- rod-shaped (telocentric and - rod-shaped (telocentric and acrocentric), acrocentric),

- J-shaped (submetacentric) and - J-shaped (submetacentric) and

- V-shaped (metacentric) - V-shaped (metacentric)

All house mouse chromosomes are All house mouse chromosomes are telocentric, while human chromosomes telocentric, while human chromosomes include both metacentric and include both metacentric and acrocentric, but no telocentric. acrocentric, but no telocentric.

Chromosome shape

Chromosome name Centro mire location

Rood shapedRoad like

Telocentric or Monobractial

Chromosome

Terminal

Achro centricAchro centric

J shapedJ shaped Subtelocentric or

Cephalobracial chSub terminal

L shaped Hook shaped

Submetacentric or Heterobracial ch.

Sub medium

V shaped Metacentric or isobracial chromosome

"Median , medium

Autosomal pairAutosomal pair Sex Sex chromosomechromosome

DiploidDiploid No. of No. of No. of No. of X Y X Y

(2n) metacentrics acrocentric or (2n) metacentrics acrocentric or telocentrictelocentric

CatCat 3838 1616 22 M M M M

DogDog 7878 00 3838 M A M A

PigPig 3838 1212 66 M M M M

GoatGoat 6060 00 2929 A A M M

Sheep Sheep 5454 33 2323 A A M M

Cow Cow 6060 00 2929 M M M M

HorseHorse 6464 1313 1818 M A M A

M – Metacentric; A – AcrocentricM – Metacentric; A – Acrocentric

Third , Morphology of Third , Morphology of Eukaryotic ChromosomesEukaryotic Chromosomes

Beside centromere, the Beside centromere, the chromosomes may chromosomes may chromosomes contain an chromosomes contain an additional specialized additional specialized segment, the nucleolus segment, the nucleolus organizer, which is organizer, which is associated with the associated with the nucleolus.nucleolus.

A. Nucleolus organizer A. Nucleolus organizer B. Chromosome B. Chromosome C. Nucleolus C. Nucleolus

When the DNA of a prokaryote, such When the DNA of a prokaryote, such as E.coli, is isolated, fragmented as E.coli, is isolated, fragmented and centrifuged to equilibrium in and centrifuged to equilibrium in a Cesium chloride (CsCl) density a Cesium chloride (CsCl) density gradient, the DNA usually forms a gradient, the DNA usually forms a single band in the gradient.single band in the gradient.

On the other hand, CsCl density-On the other hand, CsCl density-gradient analysis of DNA from gradient analysis of DNA from eukaryotes usually reveals the eukaryotes usually reveals the presence of one large band of DNA presence of one large band of DNA (usually called “(usually called “MainbandMainband” DNA) ” DNA) and one to several small bands. and one to several small bands.

These small bands are referred to as These small bands are referred to as ““Satellite DNAsSatellite DNAs”.”.

For e.g., in Drosophila virilis, For e.g., in Drosophila virilis, contain three distinct satellite contain three distinct satellite DNAs.DNAs.

Satellite DNAs

1.Secondary constrriction

2.Satellite3.Primary constriction

or centromere

Morphology of Morphology of Eukaryotic ChromosomesEukaryotic Chromosomes

According to the classical According to the classical cytological studies, each cytological studies, each chromosome structurally consists chromosome structurally consists of two very -thin, highly coiled of two very -thin, highly coiled filaments called chromonema or filaments called chromonema or chromonemata. chromonemata.

Each chromonemata is 800AEach chromonemata is 800A00 thick thick and contains 8-microfibriis, each of and contains 8-microfibriis, each of which in its turn contains two which in its turn contains two double helices of DNA. double helices of DNA.

Both chromonematae remain Both chromonematae remain coiled in spiral manner with each coiled in spiral manner with each other and have a series of other and have a series of microscopically visible microscopically visible bead-like bead-like swelling along its length called swelling along its length called chromomeres. chromomeres.

1.Two chromonemata2.Satellite

3.Centromere

The early geneticists have attached The early geneticists have attached great significance to the great significance to the chromomeres and considered chromomeres and considered them as hereditary unit, hereditary them as hereditary unit, hereditary or Mendelian factors or genes; but or Mendelian factors or genes; but modern cytological investigations modern cytological investigations have confirmed that the have confirmed that the chromomeres are not genes but chromomeres are not genes but the regions of super-imposed coils.the regions of super-imposed coils.

Morphology of Morphology of Eukaryotic ChromosomesEukaryotic Chromosomes

A-External Structure1.Secondary constrriction

2.Satellite3.Primary constriction or centromere

B-Internal Structure 1.Two chromonemata

2.Satellite3.Centromere

Structure of a metaphase chromosome

Centromeres and Centromeres and TelomeresTelomeres

CentromeresCentromeres and and telomeres telomeres are two are two essential features of all eukaryotic essential features of all eukaryotic chromosomes.chromosomes.

Each provide a unique function i.e., Each provide a unique function i.e., absolutely necessary for the stability absolutely necessary for the stability of the chromosomeof the chromosome..

CentromeresCentromeres are required for the are required for the segregationsegregation of the centromere of the centromere during meiosis and mitosis.during meiosis and mitosis.

Teleomeres Teleomeres provide provide terminal terminal stability stability to the chromosome and to the chromosome and ensure its survival ensure its survival

Centromere Centromere The region where two The region where two

sister chromatids of a sister chromatids of a chromosome appear chromosome appear to be joined or “to be joined or “held held togethertogether” during ” during mitatic metaphase is mitatic metaphase is called called Centromere Centromere

When chromosomes When chromosomes are stained they are stained they typically show a typically show a dark-dark-stainedstained region that is region that is the centromere. the centromere.

Also termed as Also termed as Primary constrictionPrimary constriction

Centromere Centromere During During mitosismitosis, the centromere that is shared , the centromere that is shared

by the sister chromatids must divide so that the by the sister chromatids must divide so that the chromatids can migrate to opposite poles of the chromatids can migrate to opposite poles of the cell. cell.

On the other hand, during the On the other hand, during the first meiotic first meiotic division the centromere of sister chromatids division the centromere of sister chromatids must remain intact whereas during must remain intact whereas during meiosis II meiosis II they must act as they do during mitosis. they must act as they do during mitosis.

Therefore the centromere is an important Therefore the centromere is an important component of chromosome structure and component of chromosome structure and segregation.segregation.

As a result, centromeres are the first parts of As a result, centromeres are the first parts of chromosomes to be seen moving towards the chromosomes to be seen moving towards the opposite poles during anaphase.opposite poles during anaphase.

The remaining regions of chromosomes lag The remaining regions of chromosomes lag behind and appear as if they were being pulled behind and appear as if they were being pulled by the centromere. by the centromere.

KinetochoreKinetochore

Within the centromere region, most Within the centromere region, most species have several locations species have several locations where spindle fibers attach, and where spindle fibers attach, and these sites consist of DNA as well these sites consist of DNA as well as protein.as protein.

The actual location where the The actual location where the attachment occurs is called the attachment occurs is called the kinetochore kinetochore and is composed of and is composed of both DNA and protein. both DNA and protein.

The DNA sequence within these The DNA sequence within these regions is called regions is called CENCEN DNA. DNA.

Typically CEN DNA is about Typically CEN DNA is about 120 base 120 base pairspairs long and consists of several sub- long and consists of several sub-domains, domains, CDE-I, CDE-II CDE-I, CDE-II andand CDE-III. CDE-III.

Mutations in the first two sub-Mutations in the first two sub-domains have no effect upon domains have no effect upon segregation,segregation,

but a point mutation in the CDE-III but a point mutation in the CDE-III sub-domain completely sub-domain completely eliminates the eliminates the ability of the centromere to function ability of the centromere to function during chromosome segregation. during chromosome segregation.

Therefore Therefore CDE-III must be actively CDE-III must be actively involved in the binding of the spindle involved in the binding of the spindle fibers to the centromere. fibers to the centromere.

Protein complex of the Protein complex of the kinetochore kinetochore

The protein component of the The protein component of the kinetochore is only now being kinetochore is only now being characterized. characterized.

A complex of three proteins A complex of three proteins called called Cbf-III Cbf-III binds to normal binds to normal CDE-III regions but can not CDE-III regions but can not bind to a bind to a CDE-III region with CDE-III region with a point mutationa point mutation that that prevents mitotic segregation. prevents mitotic segregation.

TelomereTelomere The two ends of a chromosomeThe two ends of a chromosome are are

known as telomeres.known as telomeres. It required for the It required for the replication and replication and

stabilitystability of the chromosome. of the chromosome. When telomeres are damaged or When telomeres are damaged or

removed due to chromosome removed due to chromosome breakage, the damaged chromosome breakage, the damaged chromosome ends can readily fuse or unite with ends can readily fuse or unite with broken ends of other chromosome. broken ends of other chromosome.

Thus it is generally accepted that Thus it is generally accepted that structural integrity and individuality structural integrity and individuality of chromosomes is maintained due to of chromosomes is maintained due to telomeres.telomeres.

McClintock noticed that McClintock noticed that chromosomes have unique chromosomes have unique

features.features. McClintock noticed that if two chromosomes McClintock noticed that if two chromosomes

were broken in a cell, the end of one could were broken in a cell, the end of one could attach to the other and vice versa. attach to the other and vice versa.

What she never observed was the What she never observed was the attachment of the broken end to the end of attachment of the broken end to the end of an unbroken chromosome. an unbroken chromosome.

Thus the ends of broken chromosomes are Thus the ends of broken chromosomes are sticky, whereas the normal end is not sticky, sticky, whereas the normal end is not sticky, suggesting the ends of chromosomes have suggesting the ends of chromosomes have unique features. unique features.

Telomere Repeat Telomere Repeat SequencesSequences

Species Repeat Sequence

Arabidopsis TTTAGGGHuman TTAGGGOxytricha TTTTGGGG

Slime Mold TAGGGTetrahymena TTGGGGTrypanosome TAGGG

Until recently, little was known about molecular structure of telomeres. However, during the last few years, telomeres have been isolated and characterized from several sp.

The telomeres of this The telomeres of this organism end in the organism end in the sequence 5'-TTGGGG-sequence 5'-TTGGGG-3'. 3'.

The The telomerasetelomerase adds a adds a series of 5'-TTGGGG-3' series of 5'-TTGGGG-3' repeats to the ends of repeats to the ends of the lagging strand. the lagging strand.

A hairpin occurs when A hairpin occurs when unusual base pairs unusual base pairs between guanine between guanine residues in the repeat residues in the repeat form. form.

Finally, the hairpin is Finally, the hairpin is removed at the 5'-removed at the 5'-TTGGGG-3' repeat. TTGGGG-3' repeat.

Thus the end of the Thus the end of the chromosome is chromosome is faithfully replicated. faithfully replicated.

TetrahymenaTetrahymena - - protozoa organism.protozoa organism.

RNA Primer  -  Short stretches of ribonucleotides (RNA substrates) found on the lagging strand during DNA replication. Helps initiate lagging strand replication

The end The end