BOTANY Higher Secondary Second Year - Textbooks Online

chromosome has one centromere and the polycentric chromosome has
many centromeres.
The centromere contains a complex system of fibres called
kinetochore. Each centromere has two kinetochores lined with
chromosomal arms. The kinetochore is made up of protein fibres and
microtubules which assist in the formation of spindles during mitosis
and meiosis.
All constrictions other than primary are called secondary
constrictions. In a given set of chromosomes only one or two
chromosomes have secondary constrictions. The nucleoli develop from
secondary constrictions and such secondary constrictions are called
nucleolar organisers.
A satellite is a short chromosomal segment and separated from
the main chromosome by a relatively elongated secondary constriction.
A chromosome with a satellite is called SAT-chromosome.
Chromatin is a viscous gelatinous substance that contains DNA,
RNA, histone and non–histone proteins. H1, H2A, H2B, H3 and H4
are the five types of histones found in the chromatin. The chromatin is
formed by a series of repeated units called nucleosomes. Each
nucleosome has a core of eight histone subunits.
Telomere is the terminal part of chromosome. It offers stability
to the chromosome. DNA of the telomere has specific sequence of
nucleotides. Eukaryotic chromosome has DNA, RNA, histones, non–
histone proteins and metallic ions like Ca +2 , Mg +2 , etc.,
Types of chromosomes
The chromosomes are classified into different types based on shape
and position of the centromere. According to the position of centromere,
Centromere
Arm
Telocentric Acrocentric Sub-metacentric Metacentric
Fig. 3.2 Four morphogenic types of chromosomes
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