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Chromosomes

Topic 3
...
2

Presence of plasmids
Pairs

Biology SL

Sometimes

Never

No

Yes

Diploid and haploid cells
➢ A diploid cell has a nucleus that has chromosomes organized in pairs of
homologous chromosomes
○

2n

○

Most of the cells in the body

➢ Cells containing only one chromosome from each pair are called haploid
○

n

○

The sex cells, gametes

➢ Where n = the haploid number, number of sets of chromosomes a nucleus has
Karyograms and karyotypes
➢ A karyogram is a representation of chromosomes found in a cell arranged according
to a standard format
○

According to their size and shape
■

The shape depends on the position of the centromere

➢ A karyotype is the specific number and appearance of the chromosomes in an
individual’s cells
○

The number of chromosomes is a characteristic feature of members of the
species
■

Although some exceptions might occur

➢ Process:
1
...
Photomicrographs are obtained of the chromosomes during the mitotic
metaphase
3
...
The images of each pair are placed in order by size and position of their
centromeres => decreasing length
Sex determination
➢ The 23rd pair is called the sex chromosomes, because they determine the sex of the
individual
○

XX = female, always gives X
2

Chromosomes
○

Topic 3
...
Cairns, 1962, used this technique to demonstrate that a bacterium’s chromosome
is circular and that it is replicated by being unzipped
○

He called the structures theta structures

3



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