Chromosomes
DNA STORAGE STRUCTURE
• Eukaryotic cells have
their DNA in
structures called
chromosomes stored
in the nucleus.
A cell’s chromosomes
Chromosome Structure
• Chromosomes:
– Contains a single
DNA molecule.
– Have the DNA
wrapped around
proteins called
histones.
Chromosome structure
Chromosome condensation
• Chromosomes
condense only for
cell reproduction.
• Condensation:
– Twist
– Become shorter
– Become thicker
– Become visible
• Chromosomes can
be moved about cell
easier.
Chromosome condensation
Chromosome
replication
• Before cells divide
chromosomes replicate
so each daughter cell
has a complete set of
DNA.
• Each copy is called a
chromatid.
• Sister chromatids are
connected by a
centromere
Chromosome pairs
• Chromosomes are
arranged in pairs
called homologous
pairs.
• In humans one pair
determines sex:
sex chromosomes.
• All other
chromosomes are
called autosomes.
Sex Chromosomes
• Either X or Y.
– XX creates
female.
– XY creates
male.
Sex Chromosomes
Haploid Vs Diploid
• Haploid: Having one of each
chromosome.
Sperm/ pollen and egg
(gametes)
• Diploid: having a pair of
chromosomes.
All cells except
gametes have pairs of
chromosomes.
Human sperm at the surface of
an egg
Chromosome Numbers
• Each species has
characteristic
number of
chromosomes.
–
–
–
–
Humans = 46
Fruit flies = 8
Crayfish = 200
Chimpanzee = 48
Fruit fly chromosomes
White board review
1. When DNA
replicates what are
the copies of the
chromosomes
called?
2. What are the 2 types
of chromosome
pairs called?
3. What does haploid
mean?