Nucleus & Chromosomes | Biology for JAMB PDF Download

Structure of Chromosome

(Parts which appears in metaphase chromosome)

1. Pellicle – This is outermost, thin proteinaceous covering or sheath of chromosome.

2. Matrix – This is a liquid nongenetic achromatic ground substance of chromosome, which has different type of enzymes, minerals, water, proteins.

3. Chromonema (singular Chromonemata) →Term by Vejdovsky. This is an important, genetical, highly coiled thread, throughout the length of a chromosome or chromatid. It was called chromonema.

• Chromonema lie embeded in matrix.  Each chromonemata is consist of a single long thread of DNA associated with histone.
• Sometimes bead like structure are seen on chromonema fibres, which are called as chromomeres.

4. Centromere/Kinetochore :- (Primary constriction) 

• Each chromosome (at metaphase) is consist of two half chromosome or two chromatids. Both the chromatids of a chromosome are joined or connected by a structure called Centromere. At this point or centromere two protein discs are present which is called Kinetochore. 
• Kinetochores constitute the actual site of attachement of spindles to chromosomes during cell division. 

Centromeric DNA is called as alphoid DNA.  

• At the region of centromere the chromosome is comparatively narrower than remaining part of chromosome, thus it is termed as Primary constriction. 

5. Chromatid – At metaphase stage each chromosome is consist of two cylindrical structures - called chromatids.

Both sister chromatids or longitudinal half chromosome are joined together by a common centromere. A chromosome, may have single chromatid (in Anaphase or Telophase) or two chromatid. (as in metaphase)

6. Secondary Constriction : Besides primary constrictions one or two, other constriction may also occurs on some chromosome, which are known as secondary constriction.

•  Secondary constriction is also known as NOR (Nucleolar organizer region)(13,14,15,21,22 chromosomes in human)

7. Satellite : part of chromosome remains after the NOR is known as chromosomes satellite/ trabent.

•  Chromosomes with satellite part are called as SAT chromosome (SAT = Sine Acid Thymonucleinico)

8. Telomere : Chromosomes have polarity and polar ends of chromosomes is known as Telomere.

•  Telomere prevents fusion of one chromosomes to other chromosome. Telomere rich in Guanine base. 

•  Enzyme Telomerase presents in telomere part of chromosome, which is a Ribonucleoprotein.

Types of Chromosomes on the basis of Position of Centromere

(i) Telocentric :- When centromere is terminal or located at the tip of chromosome.

ii) Acrocentric :- When the centromere is sub-terminal or located near the tip.

(iii) Metacentric :- When the centromere is located at mid of the chromosome.

(iv) Sub metacentric :- When the centromere located near centre or mid point of chromosome.

• The ratio of length of the long arm to the short arm of a chromosome is called arm ratio. Arm ratio is maximum in acrocenteric chromosome.

Ultra Structure or Fine Structure of the Chromosome

Nucleosome Model

DNA and an Octamer (Core particle) of four types ( H2A, H2B, H3& H4) of histone proteins". Nucleosome is also known as Nu-body or g-particle.

Nucleosome= Binding DNA (146 bp)+Octamer Core (H2A, H2B, H3, H4 × 2)+Linker DNA +H1 Histone 

• 6 Nuclesome units united (or super coiling) to forms Solenoid structure. (by Klug - 1982)
• H1 histone protein (sealing histone) joined the turns of binding DNA in nucleosome.
• Nucleosome unit have 1.75 or (1*3/4) turns of binding DNA. 

Special Types of Chromosomes

1. Salivary Gland Chromosome
This type of  chromosome was discovered by E.G. Balbiani,  in Chironomous larva of Drosophila . Size of this chromosome may upto 2000 micron (2mm) and number of chromatids may be 512 to many thousands. Thus, this type of chromosome also known as Giant chromosome.

• Koller named it as Polytene chromosome, because number of chromatids is very high.
• Swollen areas present at some places in polytene chromosome, which are called as Balbiani rings or puffs. These puffs helps in synthesis of RNA & proteins.
• Salivary gland chromosome concerns with metamorphosis and moulting process of insect larva. (This chromosome is related to a moulting hormone - Ecdyson) 

2. Lampbrush Chromosome
Discovered by Flemming and Ruckert from oocytes of vertebrates (Amphibia) during diplotene stage of cell division. These chromosomes look like lamp - brush, thus called as lamp brush chromosomes.

Size of these chromosomes may upto 5900 micron, and also called as giant chromosome. 

• Axis of lamp-brush chromosome is consist of DNA, while matrix is consist of RNA & proteins.
• Lamp brush chromosome is concerned with "Vitellogenesis" (Yolk formation) 

Old NCERT Syllabus

• 2n = number of chromosome in diploid cell. n = number of chromosome in haploid cell.
• The number of chromosome is definate for each species. For example every normal human being has 46 chromosomes in each body cell.
•  Gametes of all organisms contain only one of each chromosome. The number of chromosomes in a gamete is called "Genome" or haploid chromosome. (Human 23) ‘‘A complete set (n) of chromosomes (all genes) inherited as a unit from one parent is known as genome,,.
• Karyotype: Karyotype is external morphology of all Chromosomes of a cell which is specific for each species of living organisms. Karyotype can be studied in metaphase of mitosis.
• Karyotype includes the number of chromosomes, relative size, position of centromere, length of the arms, secondary constrictions and banding patterns. 

Banding technique is used to study of the specific pattern of bands and interbands on chromosome. This includes the use of fluorochromes (fluorescent dyes):

(i) Q–banding : It is obtained when chromosomes are stained with quinacrine mustard. It stains A–T rich areas (developed by casperson for Y chromosomes).

(ii) G–banding : Chromosomes are stained with Giemsa. It stains sulphur rich protein parts.

A variety of different bands are obtained by the modification of Q–banding and G–banding like C,  T and N–bands. Q, C, G and R banding used for animal karyotypes while C and N banding used in plants.

(iii) C-banding :- It is used to stain constitutive heterochromatin , usually in centromeric region of the chromosome. The process involves denaturation of chromosome by heat or trisodium citrate and then apply giemsa stain.

(iv) R-banding :- The process involved incubation of the chromosomes in a buffer at high temperature followed by use of Giemsa stain. This brings about the visualization of sulphur deficient region of chromosomes thus named as reverse giemsa.

New Techniques for Idiogram Preparation

Modern techniques used in karyotype preparation are:
ISH, FISH (Fluorescence in Situ Hybridisation), Mc FISH (Multicolour fluorescence in situ Hybridisation) and flow cytometry.

•  Idiogram: Diagrammatic representation of Karyotype. In idiogram chromosomes are arranged in decreasing order of size. Sex chromosomes are placed in last but in idiogram of Drosophila sex chromosomes are placed first. Idiogram is specific for every species.

Use of Karyotyping or Idiogram

(i) It suggests primitive or advanced features of an organism. If karyotype shows a large size difference between the smallest and the largest chromosome of the set and having fewer metacentric chromosomes then it is called asymmetric karyotype, which is a relatively advance feature. Symmetric karyotype is primitive feature.

(ii) The karyotype of different species are compared and similarities in them represent the evolutionary relationships.

(iii) Karyotype is helpful in detection of chromosomal abberrations and polyploidy.

(iv) In research of medical genetics Forensic science cytogenetics and  Anthropogenetics.

1. In Situ Hybridization : Using DNA probe labelled with radioactive molecule to locate the position of DNA sequence on chromosome.

2. Fluorescence in Situ Hybridization (FISH) : DNA may also be labelled with fluorochrome to locate the position of DNA sequence on chromosome.

3. Multicolour Fluorescence in Situ Hybridization (Mc FISH) : More fluorochrome colour to locate the position of DNA sequence on chromosome.

4. Flow cytometry : This is recent technique. In this technique a suspension of many thousands of chromosome is made and the suspended chromosome are stained with a DNA binding flurochrome.

• These chromosome pass through the cytometer the fluorescence is measured for individual chromosome and the result is represented in the form of histogram. 
• Each peak in this histogram represent, chromosome or a group of chromosome of same size.
• This technique allow detection of difference as small as 1.5 to 4.0 Mega base pair.
• This technique allow detection of aneuploidy/duplication or deletion. 

Types of Coiling in Chromonema:

(i) Plectonemic-coiling :- When both the chromonema are inter twined and can not be seperated easily. (in mitotic prophase chromosomes)

(ii) Paranemic coiling :- When both chromonema can be easily seperable. (In meiotic prophase)

Types of Chromosomes on the basis of Number of Centromere

(i) Acentric :- Chromosome without centromere.

(ii) Monocentric :- Chromosome with one centromere.

(iii) Dicentric :- When the number of centromere is two.

(iv) Polycentric chromosome :- When the number of centromere is more than two & diffused in throughout chromosome length.

Some Special Types of Chromosomes:

1. B-Chromosome/Accessory Chromosome/Supernuemerary Chromosome 

• Discovered by Wilson in Metapodian Insect. Name supernuemerary chromosome was given by D. Jones -1975.
• These are heterochromatic & small sized chromosome. Thus, no phenotypic effects are known. (Morphological control)
• B-chromosomes also present in plants cells (Maize)
• They are supposed to be involved in ecological adaptation of organisms. 

2. Mega Chromosomes

• Found in hybrid species of tobacco. 

3. Isochromosomes 

• When the both arms of the chromosomes are identical or genetically similar. Then chromosomes called as isochromosomes. If arm of a telocentric chromosome is splitted upto centromere then a metacentric chromosome with two identical arms is formed. such chromosome is called isochromosome. 

4. Ring Chromosome 

Prokaryotic chromosome are ring chromosome or consists of circular folded DNA without histone.

5. Sex Chromosome 

May be XX or XY

6. HACs, MACs, YACs, BACs, etc.

Human Chromosomes

• The normal diploid (2N) chromosome number in human being is 46. It was given by T.H. Tijo and A. Levan in 1956.
• The chromosome complement of a cell depicting the number, size and form of the chromosome as seen in metaphase of mitosis is called karyotype and diagrammatic representation of karyotype is known as Idiogram or Karyogram.
• The chromosomes are morphologically numbered into 7 groups. (size and position of centromere) by conferance in Denver and Colorado. 

Group (A) : 1–3 chromosomes of largest size and submetacentric or metacentric centromere.

Group (B) : 4–5 chromosomes with less larger size, submetacentric 

Group (C) : 6–12 chromosomes with medium sized and submetacentric centromere.

Group (D) : 13–15 chromosomes, shorter than group ‘C’ with centromere near the end (Acrocentric). They are SAT chromosomes or satellite.

Group (E) : 16–18 chromosomes, short sized, with median (Metacenteric) or submedian centromere (Submetaceteric).

Group (F) : 19–20 chromosomes, short sized with median centromere.

Group (G) : 21–22 chromosomes, smallest in size, acrocentric and are also posess satellites. 

• X chromosome is placed in ‘C’ group due to its larger size and submedian centromere.
• Y–chromosome is placed in group ‘G’ due to its short size but satellite absent and is acrocentric type.