The nucleus. The chromosomes

The nucleus. It is a constant component of all living cells. There are two different nuclear states. One is mitotic, another is interphase. Such division was made because of different nucleus activity and appearance during these periods of cell cycle. Previously, it was believed that interphase nucleus was inactive. But now it has been proven an adverse statement, it is very active during interphase. All plastic processes occur during interphase. Different cells have different nucleuses. But commonly nucleus has a sphere or ellipsoid shape.

The shape of nucleus depends on cell shape containing it and it may vary in wide range. Nucleus sizes vary no only between different cell types but also within one cell type. Cells of internal organs may have a polymorphism in sizes or volumes. The functional cell state may have an influence to nucleus size. It is stated that functional nucleus enlargement may be considered as a criteria of increased cell activity. The ratio between nucleus and cytoplasm volume is called as nucleus/cytoplasm ratio. It may serve as an indicator of cell activity and may be a factor of cell division. The nucleus consists of karyolemm (nuclear envelope), nucleoplasm, nucleolus and chromatin.

The karyolemm - is a good visible in a light microscope. But structure that is more definite may be revealed only by electronic microscope. The karyolemm is made of two biological membranes, each having 0.006-0.009 mcm. of width. The space between them is called perinuclear space. It has a width 0.01-0.02 mcm. The external membrane extends to membranes of endoplasmic reticulum.

The nuclear envelope is semipermeable. In some regions, membranes of karyolemm fuses together to make a pore in a nuclear envelope. These pores have a diameter 0.08-0.09 mcm. Pores aren’t just holes in envelope. They contain substance with moderate electronic density. Pores contain a protein structure, which is called pore complex. It regulates a substance flow through nuclear pore.

Nuclear pores extend to protein layer underlying nuclear envelope (lamina densa). It shows a complex mechanism of regulation of nuclear/cytoplasmic relations. It is possible that close connection of lamina densa and karyolemm helps to bring an order in interphase chromosome localization. The function of nuclear envelope is separation of eukaiyotic cell hereditary information from cytoplasm and regulation of nuclear/cytoplasmic relations.

The nucleoplasm. It makes an internal environment of a nucleus. It has proteins as a main part of it. It plays an important role in providing normal functioning of genetic apparatus. Also it has fibrillar proteins and may give a support to nucleus structures.

The nucleolus. It is a structural component of interphase nucleus. It is dissolved in prophase and it is newly formatted in telophase. It is formed from special thread like structures of proteins and giant molecules of RNA precursors. Them mature RNA is made from such precursors. Genes, which are responsible for RNA synthesis, are in different regions of different chromosomes. They are called nucleolus organizers. Merging into one structure, these regions forms nucleolus. In mitotic chromosomes these regions are seen as secondary strips.

The chromatin is the interphase form of hereditary information being. It’s organization was described above in chapter 2.2.

The chromosomes - are components of cell nucleus, which are good visible during mitosis. They have a complex structure, ability to replicate itself and transmit hereditary information to offspring. The chromosomes usually look like straight or curved stabs (pic 3.7). Each chromosome contains two chromatids. Chromosome shape may be defined by primary and secondary strip position. In place of primary stripe, there is a chromosome region without DNA.

Pic. 3.7. The chromosomes’ shapes: I - telocentric; II- acrocentric; III - submetacentric; IV - metacentric (1 - centromere; 2 - satellite, 3 - short arm, 4 - long arm), (by Yarygm, 1997)

Inside of it, there is a special structure - centromere (kynetochore). The spindle is attached to this structure. The centromere divide chromosome on two arms. Accordinäry centromere position and arm length it may distinguished following chromosome types: metacentric (with equal arms), submetacentric (arms slightly different), acrocentric (arms significantly different) and telocentric (without one arm). Chromosome arms are appointed the Latin letters, “q” for long arm and “p” for short arm. The percentage ratio of small arm length to total chromosome length is considered as centromere index. If centromere index is about 50%, it is a metacentric chromosome. If centromere index is less then 50% it is submetacentric chromosome.

If centromere index is around zero, it is acrocentric chromosome. Some chromosomes have a secondary strip, which divides a chromosome satellite from main chromosome part. Chromosome satellite is pointed by letter “S”. Chromosomes strictly follow such rules as: rule of constant chromosome number, i.d. somatic cells of every species have their own chromosome number (drosophila has 8, human - 46), rule of chromosome pairs (chromosome which make a pair are homologues chromosome), Ascaris lumbricoideus has only 1 pair, human has 23, rule of individuality - non homologues chromosomes differ from each other, rule of continuity - an ability of chromosomes to autoreproduction.

All features of somatic cell chromosomes structure taken together make a karyotype. This term was firstly introduced by Russian scientist G.A. Levitansky (1878 - 1542) in 1924. Normal human karyotype includes 46 chromosomes (23 pair in a diploid set). 44 from them are somatic chromosomes and 2 are sex chromosomes (pic 3.8 and 3.9). List of chromosomes placed accordinary size decreasing is called idiogramm.

Pic. 3.8. Male karyotype. Upper part is metaphase plate; lower part - idiogramm (in Biology Science an inquiry into life, 1980)

Pic. 3.9. Female karyotype

The term “idiogramm” and listing principle was suggested by cytologist S.G. Navashin (1857-1930) in 1921. Chromosomes are divided by size and by centromere position accordinaiy Denver’s classification (1960). In the same year, K. Pattaw suggested to divide chromosomes to 7 groups, pointing each group by Latin alphabet letter (table 3.1).

Table 3.1. The human karyotype chromosomes characteristics

Later the classification was updated on a base of new findings achieved by selected metaphase chromosome regions staining and chromosome mapping. The localization of specifically stained region is unique for each non-homologues chromosome. It allows making a “chemical chromosome maps”. Using selective chromosome staining in 1971 the human linear chromosome maps was developed in Paris (pic 3.10).