Chromosome: Tightly
coiled DNA
• Form when cells divide
– Chromosomes created
for the new cells
• 2 Parts:
– 1) Chromatids: two
identical parts of a
chromosome
– 2) Centromere: Joins
chromatids together
Identifying chromosomes
Chromosomes can be identified by:
• Their size
• Their shape (the position of the
centromere)
NB Chromosomes are flexible
• Banding patterns produced by
specific stains (Giemsa)
Chromosomes are analysed by
organising them into a KARYOTYPE
© 2007 Paul Billiet ODWS
© Biologyreference.com
• Defined: Picture of an
individuals chromosomes
– Help identify sex &
chromosome defects
• First 22 pairs = autosomes
• Size of final pair identifies
sex
– Same size: XX = female
– Different size: XY = male
• Normal human will have 46
chromosomes
Numbers of chromosomes
• Constant for each cell in the body except sex cells
which only have half sets
• Constant throughout the life of an individual (you
don’t lose or gain chromosomes)
• Constant for all members of a species
– Humans have 46 chromosomes
– Chimpanzees have 48
– Maize (corn) has 20
© 2007 Paul Billiet ODWS
• Diploid Cells = Cells with
the full set of
chromosomes
– Paired chromosomes
• Half of our chromosomes
come from each parent
(23 from each parent)
• Somatic (non-sex) cells
are diploid
• Created by mitosis
• Ex: Skin, Muscle, Nerve,
Blood Cells
• Haploid Cells = Cells with ½ the
total number of chromosomes
• Gametes (sex cells) are the only
haploid cells
– Ex: Sperm, Egg, Pollen
• Created by meiosis
– Chromosome number reduced by
1/2
How do humans get 46
chromosomes?
Haploid sperm cell (23) + Haploid egg cell (23) = Diploid zygote (46)
Chromosomes in eukaryotes and prokaryotes
are different
PROKARYOTES
EUKARYOTES
single chromosome plus plasmids
many chromosomes
circular chromosome
linear chromosomes
made only of DNA
made of chromatin, a nucleoprotein
(DNA coiled around histone proteins)
found in cytoplasm
found in a nucleus
copies its chromosome and divides
immediately afterwards
copies chromosomes, then the cell
grows, then goes through mitosis to
organise chromosomes in two equal
groups
© 2007 Paul Billiet ODWS
Chromosomes in eukaryotes
• Found in the nucleus
• Condensed and visible during cell division
• At the beginning of mitosis they can be seen to consist of two
threads (sister chromatids) joined by a centromere
• The sister chromatids are identical copies
• During mitosis the sister chromatids separate and are placed
into two nuclei
© 2007 Paul Billiet ODWS
Image believed to be in the Public Domain
Chromosomes and cell division
• Multicellular organisms copy their
chromosomes before cell division.
• They must grow to a mature size.
Interphase
• The nucleus divides, distributing the
chromosomes into two equal groups
(mitosis).
• The cytoplasm then divides (cytokinesis)
each part taking a nucleus.
© 2007 Paul Billiet ODWS
Image believed to be in the Public Domain
The cell cycle
Some cells may stay in
this stage for over a year
Cytokinesis
Division of the cytoplasm
M
G0
G1
G2
First growth phase.
Varies in length
Second growth
phase
S
G1 + S + G2 = INTERPHASE
© 2007 Paul Billiet ODWS
Copying of
chromosomes
The cell cycles in different cells
Cell type
Bean root tip
Cell cycle /
hours
19.3
Mouse fibroblast
22
Chinese hamster
fibroblast
11
Mouse small intestine
epithelium
Mouse oesophagus
epithelium
17
© 2007 Paul Billiet ODWS
181
How many chromosomes are in brain cells?
Remember…Brain cells are non-sex cells…
…Somatic cells are non-sex cells…
…Somatic cells are diploid…
…Diploid cells have the full set of chromosomes
Answer = 46
What is the human haploid chromosome number?
Remember…Haploid = half number of chromosomes
Answer = 23 chromosomes
How many chromosomes are in female egg cells?
Remember…Egg cells are sex cells…
…Sex cells are gametes…
…Gamete cells are haploid…
…Haploid cells have ½ the set of chromosomes
Answer = 23
What is the human diploid chromosome number?
Remember…Diploid = total number of chromosomes
Answer = 46
What is the human gamete chromosome number?
Remember…Gamete cells are sex cells…
…Sex cells are haploid…
…Haploid cells have ½ the set of chromosomes
Answer = 23
What is the human somatic chromosome number?
Remember…Somatic cells are non-sex cells…
…Somatic cells are diploid…
…Diploid cells have the full set of chromosomes
Answer = 46