Structure of Chromosome
Chromosome structure could be explained with the help of some terminologies as given below.
- Chromonema or Chromonemata
- Primary constriction or Centromere and Kinetochore
- Secondary Constriction
- Nucleolar Organizer
Chromosome has two symmetrical structures at mitotic metaphase, these are called as chromatids
Chromatids are either sister chromatids or non-sister chromatids. Out of these sister chromatids originate from single (only one) chromosome while non-sister chromatids are derived from homologous chromosomes.
Chromonema or Chromonemata
Chromosomes appear as very thin filaments during mitotic prophase, these are called as chromonema or chromonemata, these are chromatids in the early stage of condensation. These are present in coiled and twisted form in a chromatid during all stages of mitosis. Genes are located on this structure. Chromatid may contain one or more chromonema.
These are serially aligned, small bead like accumulations of chromatin material that are visible along length of chromosome esp. during leptotene and zygotene stages of meiosis.
These are regions of tightly folded DNA, visible when chromosomes are relatively uncoiled. In many organisms, chromomeres situated on corresponding positions on homologous chromosomes align with each other to form homologous pairs.
Primary constriction or Centromere and Kinetochore
Centromere is a region of DNA / chromosome (which contains highly repetitive DNA), appear mostly near the middle of a chromosome. It lies within primary constriction. Fibers of mitotic spindle attaches to this during mitosis. Kinetochore is plate or cup like structure (0.20 to 0.25nm) situated upon centromere.
Types of chromosome are listed on the basis of number of centromeres and position of centromere. Visit link to get details.
At the ends of chromosome long stretches of non-coding DNA are present, these are telomers.
Telomers or chromosome ends has polarity, because of this chromosome segments do not fuse with others. If chromosome breaks, the broken ends do not contain telomers, so they can stick with each other. Telomers also assist in the pairing of homologous chromosomes and crossing over.
In addition to primary constriction there is secondary constriction. Present at any point on chromosome. The difference between these two constriction can be noticed during anaphase, as chromosome can bend only at the site of primary constriction. Secondary constrictions are constant in their position and extent, hence prove useful to identify a particular chromosome in a set. Secondary constriction may arise because the rRNA genes are transcribed very actively and thus interfering with chromosomal condensation.
These are some parts of secondary constrictions which contain the genes coding for 5.8s, 18s and 28s rRNA and that induce the formation of nucleolus. Thus named as “Nucleolar Organizing Region”.
These are appendages of chromosome either round, elongated or knob like. Satellite is produced if secondary constriction is present in distal region of chromosome arm. It keeps connection with rest of the chromosome by a thin chromatin filament. Shape and size of satellite remains constant.
Satellite chromosome is a chromosome with satellite. Point to note is satellite DNA is not the same it is a different concept.
For the material of chromosome visit the page.