CELL : STRUCTURE AND FUNCTIONS
Chromosomes
- They are rod shaped or threadlike deeply stainable condensed chromatin fibres which are hereditary vehicles as they store and transmit coded hereditary information.
- Chromosomes appear only during karyokinesis.
- They are meant for equitable distribution of genetic material.
- The number is fixed and is the same in all the individuals of a species.
- There is a single set in gametophytic or haploid forms and two sets in sporophytic or diploid forms.
- Size and shape of individual chromosomes are quite distinct.
- The shape is more clearly visible in late prophase and metaphase (as well as anaphase) when primary constric tion or centromere becomes distinct.
- During prophase and metaphase, the chromosomes are replicated.
- There are two chromosome halves or chromatids.
- The two chromatids are attached to each other by a narrow area called centromere or primary constriction.
- Anaphasic chro mosomes do not have chromatids.
- The two parts of a chromosome or chromatid on either side of primary constriction are called arms.
- The two arms are equal in isobrachial chromosomes and un equal in heterobrachial chromosomes.
- The ratio between the two arms of a chromosome is called centromeric ratio.
- Based on the position of centromere, chromosomes are of four types
- Telocentric
- Centromere terminal in the area of telomere
- Acrocentric
- Centromere inner lo telomere ( = subterminal )
- Submetacentric
- Centromere submedian
- Metacentric
- Centromere median.
- Besides primary constriction or centromere, a chromosome may have one or more secondary constrictions.
- A secondary constriction present near the distal part of an arm may develop a small outgrowth or fragment called satellite.
- Satellite is connected to secondary constriction through a chromatin thread.
- A chromosome having satellite is called sat chromosome.
- Sat chromosomes are called marker chro mosomes.
- Other secondary constrictions can also function as markers because they occupy a constant position.
- Under light microscope, cytologists found that a chro mosome contains a coiled filament called chromonema.
- Chromonema was thought to be gene bearing part.
- Some workers thought that a chromosome may have several chromonemata.
- Electron microscope has revealed that a chromosome is actually formed by direct condensation of loops of chromatin fibre attached to a scaffold.
- It is 30 nm in diameter and contains a single DNA duplex .
- Chromosomes contain genes.All the hereditary information is located in the genes.
- Chromosomes control the synthesis of structural proteins and thus help in cell division and cell growth.
- They control cellular differentiation.
- By directing the synthesis of particular enzymes, chromosomes control cell metabolism.
- Chromosomes can replicate themselves or produce their carbon copies for passage to daughter cells and next generation.
- Sat chromosomes produce nucleoli for synthesis of ribosomes.
- Their haploid or diploid number respectively bring about gametophytic and sporo phytic characteristics to the individual.
- Chromosomes form a link between the offspring and the parents.
- Some chromosomes called sex chromosomes (e.g. ,X and Y or X and 0 ) determine the sex of the individual.
- Through the process of crossing over, chromosomes introduce variations.
- Mutations are produced due to change in gene chemistry.
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