What is a Chromosome?
A chromosome is a structure made up of DNA and proteins. In bacteria, the large, circular molecule of DNA that bathes in the cytoplasm is called the bacterial chromosome. Bacteria also often have small circular pieces of DNA in their cytoplasm: the plasmids.
In eukaryotic cells, i.e. cells with a cell nucleus, the chromosomes are located in the cell nucleus. Their number varies depending on the species. Outside of the times when the cell is dividing, the chromosomes cannot be visualized individually. When cell division begins, they condense and become distinctive.
Chromosomes are made up of two identical chromatids joined at a point called a centromere. During cell division (mitosis), the chromosomes divide into two halves: each chromatid becomes a daughter cell. In a phase of DNA synthesis, the DNA in the daughter cell is then duplicated so that each chromosome again consists of two chromatids.
The human chromosomes
A human cell contains 23 pairs of chromosomes (2n = 46 chromosomes), 22 of which are common to both sexes: the autosome pairs. The remaining two chromosomes are the sex chromosomes. In women there are two X chromosomes. In males they are different, one is an X chromosome and the other, much shorter, is called the Y chromosome.
Only 23 chromosomes are present in gametes such as sperm (n). That's because during meiosis, a cell division essential to the formation of gametes, the two chromosomes in a pair separate. Gametes are cells involved in sexual reproduction .
Some genetic diseases are linked to chromosomal aberrations . For example, trisomy 21 is caused by the presence of an extra chromosome 21. Likewise, Klinefelter syndrome affects boys who have three sex chromosomes: XXY. Women with Turner syndrome only have one X chromosome.
Chromosomal Abnormalities
Chromosomal abnormalities cause various diseases. Abnormalities affecting the autosomes (the 22 paired chromosomes that are similar in males and females) are more common than those affecting the sex chromosomes (X and Y).
Chromosomal abnormalities fit into several categories, but broadly they can be viewed as either numerical or structural abnormalities.
Numerical anomalies include
Trisomy (an extra chromosome)
Monosomy (a missing chromosome)
Include structural anomalies
Translocations (abnormalities in which a whole chromosome or parts of chromosomes are wrongly connected to other chromosomes)
Deletions and duplications of different parts of chromosomes
.png)
The human chromosomes
A human cell contains 23 pairs of chromosomes (2n = 46 chromosomes), 22 of which are common to both sexes: the autosome pairs. The remaining two chromosomes are the sex chromosomes. In women there are two X chromosomes. In males they are different, one is an X chromosome and the other, much shorter, is called the Y chromosome.
Only 23 chromosomes are present in gametes such as sperm (n). That's because during meiosis, a cell division essential to the formation of gametes, the two chromosomes in a pair separate. Gametes are cells involved in sexual reproduction .
Some genetic diseases are linked to chromosomal aberrations . For example, trisomy 21 is caused by the presence of an extra chromosome 21. Likewise, Klinefelter syndrome affects boys who have three sex chromosomes: XXY. Women with Turner syndrome only have one X chromosome.
Chromosomal Abnormalities
Chromosomal abnormalities cause various diseases. Abnormalities affecting the autosomes (the 22 paired chromosomes that are similar in males and females) are more common than those affecting the sex chromosomes (X and Y).
Chromosomal abnormalities fit into several categories, but broadly they can be viewed as either numerical or structural abnormalities.
Numerical anomalies include
Trisomy (an extra chromosome)
Monosomy (a missing chromosome)
Include structural anomalies
Translocations (abnormalities in which a whole chromosome or parts of chromosomes are wrongly connected to other chromosomes)
Deletions and duplications of different parts of chromosomes
.png)
.jpeg)
