1
,2
,DNA & Chromosomes
All the DNA in one human would stretch about 10 14 m, which is one thousand times the
distance between the Earth and the Sun.
In order to fit into the cell nucleus, the DNA in eukaryotes is tightly wrapped up with
histone proteins to form a complex called chromatin.
Just before cell division the DNA is replicated forming [temporarily] twice the normal
amount of chromatin.
Following replication, the chromatin then coils up even tighter to form short fat bundles
called chromosomes. These are about 100 000 times shorter than fully stretched DNA,
and therefore 100 000 times thicker, so are thick enough to be seen with the light
microscope.
Each chromosome is roughly X-shaped because it contains two replicated copies of the
DNA. The two arms of the X are therefore identical and each arm is called a [sister]
chromatid. Pairs of chromatids are joined at the centromere.
The complex folding of DNA into chromosomes is shown below:
3
, Karyotypes & Homologous Chromosomes
If a dividing cell is stained with a special fluorescent dye and examined under a
microscope during cell division, the individual chromosomes can be distinguished. They
can then be photographed and studied [it helps if the chromosomes are cut out and
arranged in order of size].
4
,2
,DNA & Chromosomes
All the DNA in one human would stretch about 10 14 m, which is one thousand times the
distance between the Earth and the Sun.
In order to fit into the cell nucleus, the DNA in eukaryotes is tightly wrapped up with
histone proteins to form a complex called chromatin.
Just before cell division the DNA is replicated forming [temporarily] twice the normal
amount of chromatin.
Following replication, the chromatin then coils up even tighter to form short fat bundles
called chromosomes. These are about 100 000 times shorter than fully stretched DNA,
and therefore 100 000 times thicker, so are thick enough to be seen with the light
microscope.
Each chromosome is roughly X-shaped because it contains two replicated copies of the
DNA. The two arms of the X are therefore identical and each arm is called a [sister]
chromatid. Pairs of chromatids are joined at the centromere.
The complex folding of DNA into chromosomes is shown below:
3
, Karyotypes & Homologous Chromosomes
If a dividing cell is stained with a special fluorescent dye and examined under a
microscope during cell division, the individual chromosomes can be distinguished. They
can then be photographed and studied [it helps if the chromosomes are cut out and
arranged in order of size].
4
