Why does chromatin condense during mitosis

3. Chromosomes and chromatin

The structure of DNA: chromosomes and chromatin
The chromosomes are tightly packed DNA packages that are formed before each cell division. This compact form (heterochromatin, condensed DNA) is necessary so that the DNA strands do not tangle when the nucleus divides. When no cell division is taking place (= interphase of the cell cycle), the DNA is in the form of open chromatin threads (euchromatin).
1. DNA double helix
2nd chromatin strand (DNA with histones)
3. Chromatin during interphase with centromere
4. chromatin; after replication; before cell division (now consisting of two chromatids because the DNA has doubled in the S phase (during the interphase) of the cell cycle)
5. Cell division chromosome (consisting of two chromatids = 2 chromatid chromosome)
The DNA lies in the form of a Double helix before that around so-calledHistones is wrapped. It then continues to turn into one Chromatin fiber together. It is normally found in this form in the cell nucleus. It is only during cell division that the chromatin fibers are compressed into bundles, the Chromosomes to be named. Humans have \ (23 \) pairs of chromosomes.
In every organism, the DNA is broken down into a number of pieces of DNA. Humans have \ (23 \) pieces of DNA in duplicate (one each from father and mother). Before and during cell division there are \ (46 \) chromosomes in the nucleus.
Chromosome structure with drawn chromatids and direction of division
Chromosomes before and after nuclear division of mitosis
A chromosome consists of one or two chromatids, a middle section (centromere), end sections (telomere), and short and long chromosome arms.
Chromosome theory of inheritance
The chromosomes are the molecular basis for the inheritance of Features. Gregor Mendel did not have the knowledge of the molecular basis of inheritance when he formulated the inheritance rules. However, knowing about chromosome inheritance helps to better understand Mendel's rules of inheritance (and their exceptions).
  • The chromosomes are called independent units passed on to the next generation.
  • Every chromosome occurs in pairs in every cell in the body. One chromosome each comes from the father and one from the mother (= homologous chromosome pairs). One speaks of one as a whole double set of chromosomes (diploid) - in humans these are \ (23 \) pairs of chromosomes \ (= \) \ (46 \) chromosomes.
  • A chromosome consists of a cell division (mitosis or meiosis) two identical chromatids (= \ (1 \) - chromatid chromosomes). One chromatid each is passed on to the two daughter cells.
Pair of chromatids on the left before cell division and on the right after cell division
  • In a meiosis (reduction division) the Set of chromosomes halved and the chromatid pairs on the paternal and maternal side are distributed "randomly" or according to the laws of probability to the germ cells (= gametes: sperm cells & egg cells). The set of chromosomes is only simple (haploid).
Meiosis with a pair of chromosomes (chromatids double /
Chromatid pairs become coincidentally split / chromatid pair is halved)
The entirety of the DNA of an organism is made into one Karyogram shown. These are always metaphase chromosomes (= \ (2 \) - chromatid chromosomes, already duplicated chromatin), i.e. the chromosomes are just before the cell division and are therefore X-shaped. The chromosomes are called homologous chromosome pairs arranged. The karyogram for the human genome looks like this:
Human karyogram
Every person has \ (23 \) chromosomes from father and mother and thus has \ (46 \) chromosomes in the form of \ (23 \) homologous chromosome pairs - in diploid body cells. The human germ cells are haploid and contain random combinations of \ (23 \) these paternal and maternal chromosomes.
Inheritance of gender
Of the \ (23 \) pairs of chromosomes in humans are \ (22 \) autosome pairs (independent of gender) and \ (1 \) heterosome pairs ("sex chromosomes"). The two chromosomes in this pair can look different - one shape looks like an X, the other looks like a Y.

X & Y chromosome
Males have XY as the 23rd pair of chromosomes.
At this point women have XX.
It follows that the sex of a child is determined by the sperm cell. The father inherits either an X or a Y chromosome (the probability is (50:50)). The mother can only inherit one X chromosome. This is called the configuration of the heterosome pair genetic sex. The heterosome pair is therefore also called that Sex chromosome pair (= also Pair of gonosomes).

Supplement: In the animal kingdom, in addition to the XX / XY system for gender development, there is also the X / XX system (e.g. in grasshoppers) and the haploid / diploid system (e.g. in bees), in which the males are haploid and the females are diploid.
In addition to genetic sex, one also differentiates between that biological sex (depending on the development of the primary genital organs) and that psychological gender (to which the individual feels they belong).
Supplement: Homosexuality occurs frequently in nature and is likely to play an important role in the development of social systems, but is fundamentally independent of biological and psychological gender.
As a result of mutations, there may be deviations in the sex chromosome pair:
  • XXY: Klinefelter Syndrome (male, small testicles, often tall, female body shapes, sterile)
  • XXX: These women are physically no different from other women
  • X0: (only one X): Gymnastics Syndrome (female, no secondary sexual characteristics, sterile, short stature)
The Y chromosome causes the testes to develop, while it prevents the ovaries from developing.
The heterosome pair determines the genetic sex in humans. XX = female / XY is male.
Ruso, Bernhart. 2011. BIOLOGY. Script. Vienna: Dr. Roland GmbH, 2011. 3rd edition
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