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Summary Meiosis
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MEIOSISREVISION
― Chromosomes occur in the nucleus of every living cell.
― Chromosome consists of a DNA molecule coiled around proteins known as
histones.
― There are various DNA segments in each chromosome strand known as genes.
Each gene controls a particular inherited characteristic.
― When the cell is not dividing, the chromosomes appear as a thread-like network,
the chromatin network.
― When the cell divides, the chromatin network condenses, the threads shortens
and thicken and become visible as single chromosomes.
― Before a cell divides, all the genetic material in the nucleus i.e. the DNA must be
duplicated. This process, where DNA makes an exact copy of itself, is known as
replication.
― Replication is necessary to ensure that both daughter cells formed during mitosis
have identical genetic info to the original mother cell.
― After replication occurs, the single-stranded chromosome [DNA and histones]
consists of two identical units known as the chromatids.
― The two chromatids of the new double-stranded chromosome are joined by a
centromere.
CHROMOSOMES
― Each somatic cell in a particular organism of a particular species contains the
same number of chromosomes.
― In organisms that reproduce sexually, the number of chromosomes is always an
even number because it consists of two sets of chromosomes.
― This double set of chromosomes is known as the diploid chromosome number
[2n].
― Each chromosome of one set normally has a partner in
the other set. These partner chromosomes are similar in
size, shape and genetic composition.
― One chromosome of the pair comes from the mother of
the organism [maternal] and the other from the father
[paternal].
― These pairs of chromosomes are known as homologous
chromosome pairs.
― This ensures that the organism has characteristics of
both parents.
― Gametes [sex cells], i.e. egg and sperm, have only one
set of chromosomes, which is known as the haploid chromosome number [n]. [In
human gametes n = 23].
― The haploid chromosome number [n] is half of the diploid chromosome number
[2n].
― During fertilisation the egg and sperm, each w. one set of chromosomes [n], fuse
to form a zygote [2n].
― Therefore, the zygote contains two sets of chromosomes, i.e. the diploid
chromosome number [2n].
― The zygote divides by mitosis and forms an embryo consisting of many cells, each
w. the diploid chromosome number.
― Each organism has a unique number of chromosomes in each somatic cell.
, KARYOTYPE
― The complete diploid set of chromosomes, arranged according to their size, shape
and number in homologous pairs, w. in a somatic cell of an organism, is known as
a karyotype.
― The human karyotype consists of 22 pairs of autosomes [ordinary chromosomes
that have nothing to do w. sex determination] and one pair of gonosomes [sex
chromosomes], known as the X and Y chromosomes.
― The human diploid chromosome number is 46 [there are 23 homologous
chromosome pairs in each cell].
― A female has two X-gonosomes, whereas a male has one X and one Y gonosomes.
o The chromosome composition in a female is 44 + XX.
o The chromosome composition in a male is 44 + XY.
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