Chromosome term was given by Waldeyer.
- Chromosomes are seen during the Metaphase stage of cell division.
- Chromatin reticulum seen during the interphase coils to form chromosomes during cell division.
Chromatin shows 2 types of coiling behavior:
- Euchromatin: loosely coiled chromatin. It is transcriptionally active.
- Heterochromatin: densely coiled chromatin. It is transcriptionally inactive and consists of repetitive DNA. It is present in centromere, telomere & satellite.
Packaging of DNA in chromatin:
Chromatin is made of DNA and histone proteins, in the ratio 1:1.
Histone proteins are basic proteins, rich in Lysine and Arginine.
- They are of 5 types H1, H2A, H2B, H3 & H4.
Chromatin consists of repeating units called Nucleosomes.
- Nucleosomes appear like ‘beads on a string’.
- Kornberg proposed the Nucleosome model.
- A nucleosome consists of the core particle and a linker DNA.
- Core particle is an octamer of 4 histone proteins; H2A, H2B, H3 & H4 each present twice. DNA strand of 146 bp surrounds the octamer. It makes 1¾ coil around the octamer.
- Linker DNA; is a DNA strand of about 8 to 114 bp that joins 2 core particles & completes the 2nd coil around the octamer. H1histone protein stabilizes the 2 coils around the octamer.
Diameter of the nucleosome is 11nm.
Nucleosomes are coiled in the form of a solenoid, having 6 nucleosomes per coil. They form the chromatin thread having a diameter of 30nm.
The chromatin thread further coils with the help of Non-histone chromosomal proteins.
Non-histone chromosomal proteins form a central scaffold around which the solenoid forms loops. It further coils to form chromosomes.
- Non-histone chromosomal proteins are rich in Tryptophan & Tyrosine.
A chromosome consists of two chromatids attached to each other at the centromere or the primary constriction.
Lying on the either side of the centromere are disc-like structures, the kinetochore to which spindle fibers attach during cell division.
Specialized ends of chromosomes are Telomeres.
- Telomeres help the chromosomes to maintain their identity.
- Telomeres are tandem repeats of ‘TTAGGG‘.
Satellite chromosomes / SAT-chromosomes:
- In some chromosomes besides the primary constriction, a secondary constriction is also present.
- Also referred as the Nucleolar organizer (NOR). They are associated with the nucleolus and help in the formation of nucleolus at the end of cell division.
- In man chromosome 13, 14, 15, 21, 22 have the NOR.
- Satellite is the region outer to the secondary constriction & is usually spherical.
- SAT represents Sine Acid Thymidine i.e., this region is without Thymidilic Acid.
Chromosomes are divided into different types according to the position of centromere:
- Metacentric: the centromere is present at the center, dividing the chromatid into 2 equal arms.
- Sub-metacentric: the centromere is present slightly away from the center, dividing the chromatid into 2 unequal arms.
- Acrocentric: the centromere is present slightly away from the tip, dividing the chromatid into 2 unequal arms.
- Telocentric: the centromere is at the tip.
Complete set of chromosomes in a species or in an individual organism.
- Karyotypes describe the chromosome number in an organism and how these chromosomes look like under a light microscope.
- Length of a chromosome, the position of the centromere, its banding pattern, differences between the sex chromosomes and autosomes and any other physical characteristics are considered.
Idiogram is a diagrammatic representation of a karyotype of a species.
Genome: is the complete set of chromosomes/genes in an organism.
Autosomes: Chromosomes other than sex chromosomes (X & Y) are autosomes.
- Autosomes appear in pairs.
Homologous chromosomes are chromosomes having the same number, type, and arrangement of genes. But one is paternal and one is maternal.