#ORGANISMS AND #NUMBER OF CHROMOSOMES IN BODY CELLS OR SOMATIC CELLS:
¤The chromosomes are arranged in pairs.for example,in humans,there are 23pairs of chromosomes.
¤There are 22different pairs of chromosomes called #Autosomes and a pair of #sex chromosomes/ #Gonosomes.
¤The autosomes are numbered as chromosomes number 1,2,3 up to chromosomes number 22;and there are two of each,the two chromosomes making up each pair are identical to each other.
¤In female,the two sex chromosomes are also identical and are referred to as XX chromosomes.
¤In males,one of these is an X chromosome and the other is a Y chromosome.
¤Therefore,instead of saying that the somatic cells of humans have 46 chromosomes,it is more informative to say that they have 44+XX chromosomes (in female) and 44+XY chromosomes (in males).
¤Since there are really two sets of 23 different kinds of chromosomes in each somatic cells, we say that the cells have a diploid(2n) number of chromosomes,and in humans the diploid or 2n number is 46.
¤Sex cells or gametes of humans have just one set of 23 different chromosomes,we say that the sex cell or gametes have a haploid (n) number of chromosomes and in humans the haploid number is 23.
¤Notice that the two chromosomes making up each pair of autosomes are identical,one member of each pair is maternal origin (comes from our mothers) and the other is of the paternal origin (comes from our fathers).
¤A pair of identical chromosomes,one of maternal origin and the other of paternal origin,is called a #Homologous pair of chromosomes.
¤Just prior to cell division (after DNA replication),the chromosomes appear double,each being made up of two #Chromatids being held together by a #Centromere.
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