Chromosomes are the smallest during anaphase and studied during metaphase. They range in size from 0.5-32 micrometers in length and 0.2-3 micrometers in diameter, with the longest being 32 micrometers in the plant Trillium. Each metaphase chromosome is longitudinally divided into two identical parts called chromatids joined at the centromere. The position of the centromere determines whether the chromosome is metacentric, submetacentric, acrocentric, or telocentric. Telomeres are repetitive units at the ends of chromosomes that maintain structural integrity. Secondary constrictions are associated with nucleoli and are nuclear organizing regions.
2. Chromosome
Described by Strassburger (1875)
Word coined by Walder (1888)
Word Chroma means colour, Soma means body
Somatic chromosome no. 2n
Gametic chromosome no. n
3. Chromosome size
Chromosome are smallest during Anaphase and
studied during Metaphase
Size 0.5 -32µ length and 0.2 -3µ in diameter
Longest metaphase chromosome is in Trillium 32µ
4. Chromosome morphology
Chromatid
They are the structural and functional unit of chromosome .
Each metaphase chromosome is longitudinally divided into two
identical parts and appears to be joined at centromere.
Two chromatids separate during anaphase
Two chromatids replicate during prophase and metaphase from a
same chromatids called as Sister chromatids
Chromatids of homologous chromosome is called Non sister
chromatids.
5.
6. Centromere
Movement of chromosome is due to centromere hence it
is known as Kinetochore .
Position of centromere
Metacentric - centromere is in median
Sub metacentric - centromere is in sub median
Acrocentric - centromere is in sub terminal
Telocentric - centromere is in terminal
No of centromere
Monocentric - single centromere
Polycentric - more than one centromere
7. Telomere
Two ends of chromosome are called telomere which are
highly stable and do not fuse with telomere of other
chromosome .
Telomere has a repetitive unit.
It maintains the structural integrity and individuality .
8. Secondary constriction
Region between secondary constriction and telomere is
called satellite so chromosome having secondary
constriction are called satellite chromosome or Sat
chromosome .
Nucleolus is always associated with secondary constriction
so it is called Nuclear Organising Region (NOR) and
chromosome is Nuclear Organising Chromosome (NOC).
Its position remains fixed and its number is constant
Human have 10 Sat chromosome 13,14,15,21and 22
9. Chromomere
During pachytene small bead like structure called
chromomere.
Distribution is highly specific . Homologous chromosome
has have identical distribution .
They are clearly observed in gaint chromosome of salivary
gland in dipterans .
