Notes on Chromosomes

Chromosomes

• These are rod or ribbon-shaped, dark stained bodies within the cell.
• That contains the cell’s genetic material.
• Only seen using basic dye under a light microscope.

• Chromosome term is a combination of two Greek words, Chroma and Soma.
• Chroma – Colour
• Soma – Body
• Because of strongly stained by some dyes.
• In prokaryotes the Chromosomes are circles of DNA.
• While linear and complex structural chromosomes are present in eukaryotes.

Named by

•  Heinrich Wilhelm Gottfried Von Waldeyer – Hartz in 1888.

•  He gave this name to the structure that appears during the process of cell division(Reproduction).

Chromosomes are clearly visible as distinct bodies during the stage of cell division only and their number can be counted with relative ease only during mitotic metaphase.

Chromosome number

• Can describe in two parts as
1. Somatic chromosome number
2. Gemetic chromosome number

1. Somatic chromosome number

• These are the number of chromosomes found in somatic cells. E.g. Tissues and Meristemetic cells.
• These are represented as 2n.
• 44 are autosomal chromosomes.

2. Gemetic chromosome number

• The number of chromosomes found in germ cells. E.g. Ovum and Sperms.
• These are represented by n.
• 2 (X and Y) are sex chromosomes.

Chromosomal Morphology

• Composition of chromosomes

1. Negatively charged DNA

- It’s a coiled DNA that contains genes, regulatory elements and nucleotide sequences.
- Chromosomes are composed of 10,000 to 1,000,000,000 nucleotides.
- The DNA that presents in a chromosome is approximately long as 8.5 centimetres.

2. Positively charged Histone Proteins

• Histone proteins are 80% of the chromosomal proteins.
• Molecular weight range from 10,000 to 30,000 D.
• Types—H1, H2a, H2b, H3, H4
• H1—Lysine rich
• H2a, H2b—slightly Lysine rich
• H3, H4 – Arginine rich
• These all completely devoid to tryptophan.

Structural features of chromosomes

• Chromatids
• Centromere
• Telomere
• Secondary Constrictions
• Satellite Chromosomes
• Chromomere

Chromatid

• Metaphase Chromosomes are to be longitudinally divided into two identical parts.
• These two parts appear like two identical arms and known as two sister chromatids and individually called Chromatids.
• And these sister chromatids are appeared to be joined together at the centromere.
• These separate only at the anaphase stage and move to the opposite poles.

Centromere

• The region where two sister chromatids are appeared to be jointed or held together.
• Divide chromosomes into two equal or unequal sections or arms.
• If divide into two unequal parts then
o Short arms represented by “p”.
o Long arms represented by “q”.
• Also known as primary constriction region.

Primary Constriction region - The narrow region in the chromosome arm.

• In anaphase the centromere and spindle fibres attached together by a protein called KINETOCHORE.
• Location of centromere in chromosome provide specific shape and describe the location of genes.

Telomere

• Ends of chromatids called Telomeres.
• These are highly stable.
• Not easily fused with the telomere of other chromosomes.
• Fusion is very rare if telomere is broken.
• Do not have any structural features under a light microscope.
• These are the highly condensed form of 300 A° Chromatin fibres.

Secondary Constrictions

• These are the additional constrictions to the centromere.
• Generally located near the short arm of the chromosome.
• Also called NOR(Nucleolar Organizing Region).

Satellite Chromosomes

• The region between the secondary constriction and the telomere called SATELLITE.
• The chromosomes having satellite called Satellite Chromosomes.
• These also known as Sat Chromosomes and NOC(Nucleolar Organism Chromosome).

Chromomere

• Appear like small beads like structures during the Pachytene stage of the first prophase of meiosis I.

Types of Chromosomes

• On the basis of the location of Centromere in Chromosomes

1. Submetacentric – Centromere is located at one side of the centre of chromosomes. In this type chromosome is J shaped at the metaphase stage.
2. Metacentric— Centromere is present in the centre of Chromosomes. In this type of chromosome is V-shaped at the metaphase stage.
3. Acrocentric– Centromere is located close to the end of the chromatids. A chromosome is rod-shaped at the metaphase stage.
4. Telocentric – Centromere appeared to located at the end of a chromosome.Chromosomes are rod-shaped at the metaphase stage.

• On the basis of the number of Centromere in Chromosomes

1. Acentric – Without any centromere and it is very rare.
2. Monocentric – Having one centromere and this type is very common in cells.
3. Diacentric – Having two centromeres.

Acentric and dicentric chromosomes are produced due to chromosomal aberrations such as deletion, duplication, translocation or breakage.

• Significance of Chromosomes

o The organ of heredity.
o Form link between two generations.
o Maintain and replicate the genetic information.
o Ideograms show the Bending Patterns and describe the location of genes.
o Karyotyping of chromosomes shows chromosomal aberrations, cellular functions, taxonomic relationship and to gather the information about the evolutionary events.