CELL DIVISION
I. MITOSIS
DEFINITION:
Division is an ensemble of genetic events, which ensures, by means of chromosomes,
transmission of genetic information to daughter cells or to new organisms.
 as a rule, transmission of
genetic information takes place
exactly and with fidelity;
 errors during cell division cause
the chromosome mutations
(numerical and structural),
which lead to abnormalities in
cell and organism function.
MITOSIS is the process of
forming of identical daughter cells;
during
mitosis
replicated
chromosomes are segregated so
that each daughter cell receives the
same number of chromosomes (if
you start with 46 in the parent cell,
you should end up with 46
chromosomes in each daughter
cell).
1. MAIN CHARACTERISTICS:
 during this type of cell division are multiplied most of the cells in human organism (all the
somatic cells, excepting the red blood cells and neurons, skeletal muscle);
 it is a equational division - at the beginning of the division each somatic cell consists of 2n
bichromatidian chromosomes which are separated during mitosis equally between daughter
cells, so each of them contains 2n monochromatidian chromosomes
2n monochromatidian chromosomes
2n bichromatidian chromosomes
2n monochromatidian chromosomes
2. THE IMPORTANCE OF MITOSIS
Mitosis has an important significance because divisions continue during all life
beginning with the first division of zygote.
Mitotic cell division provides the following advantages for organisms:
*
since mitosis produces a pair of genetically identical cells, it ensures stability of
the characteristics of organisms;
*
in human organism, it provides embryogenesis (hysto- and organogenesis) before
birth, and growth and development, in postnatal period;
*
it enables the damaged structures of organisms to be reformed;
*
it ensures the substitution of dead cells.
Cell division. Mitosis. Errors during mitosis
3. STAGES OF MITOSIS
a. PROPHASE:
*
the chromatin fibres condense by coiling and folding to form chromosomes;
*
each prophase chromosome is composed of two chromatids;
*
centrioles divide and migrate to opposite poles of the cell, between them appear the
microtubules of the division spindle;
*
the nucleoli become less visible and the nuclear membrane is fragmented.
b. PROMETAPHASE - is an intermediate stage between prophase and metaphase:
*
cell membrane and nucleoli disappear as structures;
*
chromosomes become attached by their kinetochore to fibres of the division spindle;
c. METAPHASE:
*
condensation and shortening of chromosomes continue;
*
the chromosomes (which consist of 2 chromatids held together by a centromere) migrate
to the equator of the cell;
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Cell division. Mitosis. Errors during mitosis
*
the chromosomes are clearly visible (using the light microscope).
d. ANAPHASE. Is most important phase of mitosis because during this stage the total
and equal distribution of the genetic material takes place by three following events:
*
longitudinal cleavage of centromere;
*
disjunction of chromatids (separation of chromatids), determining the transformation
of bichromatidiam chromosomes into monochromatidian and the appearance of a double number
of chromosomes (92);
*
simultaneous migration of the daughter-chromosomes toward the opposite poles.
e. TELOPHASE - begins with two groups of chromosomes arrival at the opposite
poles:
*
chromosome begin to uncoil, becoming less condensed, and transform into the
chromatin fibres;
*
the nuclear cover reforms and it separates chromatin from cytoplasm;
*
in the equatorial region of the cell appears a strangulation that progresses to form two
daughter cells (cytokinesis).
4. ERRORS PRODUCED DURING THE MITOTIC ANAPHASE
The normal proceeding of events in anaphase may be perturbed because of some
“anaphase accidents”, which cause the errors in distribution of genetic material.
4a. TYPES OF ERRORS:
- transversal cleavage of centromere
 this error determines the formation of isochromosomes, which consist of
only the two short arms (i p) or two long arms (i q).
 each isochromosome presents an one absent arm (deletion) and an extra
another arm (duplication);
 it determines appearance of the chromosome mosaics - 46
46 i p
46 i q
- chromatidian non-disjunction

 the reason of non-disjunction is that a chromatid pair does not separate
during anaphase, both of the chromatids (monochromatidian chromosomes)
migrate together to the same pole of the cell;
 it determines appearance of the cell chromosome mosaics (presence in the
same organism of two or more cell lines (cell clones)), which differ by
chromosome number, for example: 45
46
47 or 45
47.
 if non-disjunction occurs during zygote division, two types of chromosome
mosaic are formed - 45
47;
 non-disjunction can also appear during blastomeres division; in this case three types (one normal, and two - mutant) are formed - 45
46
47.
- anaphase lag
 it appears if one chromosome from the pair migrates to cell pole more slowly
or stops its movement;
 it determines appearance of the next chromosome mosaic - 45
46;
 the lagged chromosome remains in cytoplasm and forms a micronucleus.
4.b. EVOLUTION OF ABNORMAL CELL CLONES AND PHENOTYPIC
CONSEQUENCES
- Evolution depends on viability of cells, which have the chromosome abnormalities
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Cell division. Mitosis. Errors during mitosis
 many of them (severe anomalies) lead to the death of affected cells by selfelimination;
 abnormalities which are less severe can induce the appearance of abnormal cell
clones.
- Chromosome abnormalities can appear at different stages;
 If they appear during embryo development, congenital malformation are produced
 If they appear in postnatal period, perturbation of normal organ structure and function occurs
(e.g. leading to neoplasm – cancer).
Give the definition of the following notions:
Mitosis, Prophase, Metaphase, Anaphase, Telophase, Isochromosome, Monosomy, Trisomy,
Mosaics, Equational Division, Chromatid, Non-disjunction, Anaphase Lag.
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