Transcript Recap: Cell Division and Chromosomes

Chromosomes and Meiosis
DNA and Chromosomes
Some Interesting Facts:
• The DNA molecule in a single human cell is 99 cm
long, so is 10 000 times longer than the cell in which
it resides (<100mm)
• Since an adult human has about 1014 cells, all the
DNA in one human would stretch about 1014 m, which
is a thousand times the distance between the Earth
and the Sun.
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So how does the DNA fit into the cell?
• The DNA is cut into shorter lengths
• Each length is tightly wrapped up
with histone proteins to form a
complex called chromatin.
• Just before cell division the DNA is
replicated.
• Following replication the chromatin
then coils up even tighter to form
short
fat
bundles
called
chromosomes.
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How does the DNA fit into the cell? (cont..)
• Each chromosome is roughly Xshaped because it contains two
replicated copies of the DNA.
• The two arms of the X are
therefore identical. They are
called chromatids, and are joined
at the centromere.
one chromatid
centromere
Chromatin
DNA + histones at any stage of the cell cycle
Chromosome
compact X-shaped form of chromatin formed (and
visible) during mitosis
Chromatid
single arm of an X-shaped chromosome
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Homologous Chromosomes
• Chromosomes come in pairs called homologous
pairs (‘same shaped’)
• The homologous pairs are the maternal (inherited
from the mother) and paternal (inherited from the
father) versions of the same chromosome, so they
have the same sequence of genes.
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Homologous Chromosomes (cont…)
• One pair of chromosomes is different in males and
females.
• These are called the sex chromosomes, and are nonhomologous in one of the sexes.
• In humans the sex chromosomes are homologous in
females (XX) and non-homologous in males (XY). (In
birds it is the other way round!)
• The non-sex chromosomes are sometimes called
autosomes, so humans have 22 pairs of autosomes,
and 1 pair of sex chromosomes.
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Meiosis
• A form cell division that produces gametes
• Two important functions:
– To form haploid cells with half the number of chromosomes
– To re-arrange the chromosomes with a unique combination
of genes
• There are two successive divisions, without DNA
replication in between
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Meiosis
Meiosis in detail
http://www.thelifewire.com/content/ch09/ch09xe02.htm
OR
go to www.thelifewire.com
Click on ‘animated tutorials’
Scroll down to ‘9.2 Meiosis’
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