Heterochromatin. Nucleolus is a small, dense, spherical structure either present singly or in multiples inside the nucleus and it’s not membrane bound. Nucleoli possess genes for rRNA and tRNA. Functions of the nucleus • Controlling all cellular activities • Storing the genetic or hereditary information.
• Coding the information from DNA for the production of enzymes and proteins. • DNA duplication and transcription takes place in the nucleus. • In nucleolus ribosomal biogenesis takes place. .
. Chromosomes Strasburger ( ) first reported its present in eukaryotic cell and the term ‘chromosome’ was introduced by Waldeyer in . Bridges ( ) first proved that chromosomes are the physical carriers of genes. It is made up of DNA and associated proteins.
Structure of chromosome The chromosomes are composed of thread like strands called chromatin which is made up of DNA, protein and RNA. Each chromosome consists of two symmetrical structures called chromatids . During cell division the chromatids forms a well organized chromosomes with definite size and shape. They are identical and are called sister chromatids .
A typical chromosome has narrow zones called constrictions . There are two types of constrictions, namely primary constriction and secondary constriction. The primary constriction is made up of centromere Figure . : Structure of a Chromosome cells, so studies of telomeres and telomerase have the promise of providing new insights into conditions such as ageing and cancer.
Telomeres prevent the fusion of chromosomal ends with one another. Chromonema fibre : A chromatin fibre, – nm in diameter, an element of higher order packing within the chromosome. During prophase the chromosomal material becomes visible as very thin filament called chromonemata, which is called as chromatids in early stages of condensation. Chromatid and chromonema are the two names for the same structure a single linear DNA molecule with its associated proteins Chromomeres : Chromomeres are bead like accumulations of chromatin material which are visible along interphase chromosomes.
They can be seen in polytene chromosomes. At metaphase they are not visible. Types of Chromosomes Metacentric Sub-Metacentric Centromere Telocentric Acrocentric Satellite Arm Figure . :