Chromosomes and Cell Cycle
All genetic material of a cell is
called the genome
• Genome is composed of DNA
• Long molecules of DNA organized for cell
division are called chromosomes
• Each chromosome contains thousands of
genes
Chromosomes contain genes
Chromosomes stained orange
DNA material in a cell
• Found to be loosely coiled in the cell and is
called chromatin
• For cellular division the DNA is condensed
to form chromosomes
DNA condensation generates
chromosomes
DNA is packaged into
chromosomes
DNA wraps around a histone core forming nucleosomes. Multiple
nucleosomes are organized into solenoids
solenoid
Condensed further to form
chromosomes
What happens to a chromosome if
you remove the scaffolding proteins?
• Histones have been
removed from this
metaphase
chromosome
• Scaffolding proteins
retain shape of the
chromosome
Do all organisms have DNA?
• Yes!
Do all organisms have
• The same amount of DNA?
• The same number of chromosomes?
The number of chromosomes varies
with each organism
Humans have 23 pairs of
chromosomes
• 22 pairs of autosomes
• 1 pair of sex chromosomes
Chromosome analysis Giemsa staining
• Giemsa stain is used
to visualize
chromosomes
• There are 22 pairs of
autosomes and 1 pair
of sex chromosomes
Chromosome analysis Fluorescence in-situ hybridization
(FISH)
A technique to visualize
specific regions of
chromosomes using
fluorescent DNA
probes.
Chromosome analysis Chromosome Paints
• DNA is labeled
with different
fluorescent dyes
• Dyes “paint’
specific regions
Remember that chromosomes
represent duplicated DNA
• These structures are formed to facilitate the
proper partitioning of DNA during cell
division
• The structure represents twice as much
DNA, since DNA replication has occurred
Chromosome Structure
p (short arm)
centromere
q (long arm)
Chromosome split the chromatids
during cell division
All cells need to divide to pass on
their genetic information by cell
division
Cell Cycle reflects the life of a
cell
Cell Cycle
• G1 phase
– First growth phase, cells are making proteins
• S phase
– DNA replication occurs
• G2 phase
– Growth phase, checking of DNA
• M phase
– Cell division
Mitosis
Mitosis
Mitosis
How does cancer relate to the cell
cycle?
• There are checkpoints in the cell cycle that
regulate each stage
– G1
– G2
–M
• Cancer cells have lost the control of cell
division
Checkpoints in the cell cycle
Do all human cells divide by
mitosis?
• Remember that mitosis provides a new cells
with a copy of the EXACT same
chromosomes, 23 pairs
• What about the sex cells? How many
chromosomes are present in them?
Meiosis is a special type of
cellular division
• Sex cells have ONE set of 23 chromosomes
• Sex cells are considered haploid
• When two haploid cells fuse they form a
diploid cell (one that has 23 PAIRS of
chromosomes)
How do we end up with half the
number of chromosomes in
meiosis?
• Need TWO rounds of division
– Meiosis I
– Meiosis II
• Final result is 4 daughter cells with haploid
chromosomes
Meiosis Overview
Meiosis I
Meiosis II
Crossing Over in Meiosis
Summary
Comparison of mitosis and
meiosis
Key points to notice
• Only in meiosis I does synapsis occur
which allows for recombination or
crossing over of chromosomes
• Does this process occur in mitosis?
Nondisjunction occurs when
chromosomes fail to separate
• Can occur either during mitosis or meiosis
• What happens if it occurs in Meiosis I?
• What happens if it occurs in Meiosis II?
Trisomy 21
• Down syndrome
• Most common chromosome number
abnormality
• Occurs at a rate of 1/700 births
Incidence of Down syndrome
increases with age of mother
Human Sex Determination
Textbook Story:
XX = Female
XY = Male
With a few
exceptions….
Aneuploidy can also occur with Sex
Chromosomes
XXY = Klinefelter Syndromephenotypically male with normal
intelligence, sterile
XO = Turner syndromephenotypically female, sterile,
usually normal intelligence
XYY = no general traits (tall)
XXX = healthy, “normal”
The SRY Gene
Down Syndrome may be
inherited
• Robertsonian
translocation
accounts for 5% of
down syndrome
cases
• Occurs between
chromosomes 21
and 14