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Chromosomes
PART 1
Honors
Genetics
Ms. Day
Why is Cell Division Important?
¡  Unicellular
organisms
l  Reproduce by cell division
à increasing the population.
100 µm
(a) Reproduction. An amoeba,
a single-celled eukaryote, is
dividing into two cells. Each
new cell will be an individual
organism (LM).
Why Do Multicellular Organisms
Depend on Cell Division?
1. 
2. 
3. 
Development & Growth
Repair (ex: tissue renewal)
Maintenance
200 µm
(b) Growth and development.
This micrograph shows a
sand dollar embryo shortly
after the fertilized egg divided,
forming two cells (LM).
20 µm
(c) Tissue renewal. These dividing
bone marrow cells (arrow) will
give rise to new blood cells (LM).
Cell Division (Mitosis)
2 genetically identical
daughter cells from 1 parent
cell
¡  Before cells divide
l  They duplicate their genetic
material à ensures that each
daughter cell receives an exact
copy of the genetic material, DNA
¡  Makes
Organization of the Genetic
Material
¡  All
of a cell’s DNA (genetic
information) is called its
genome
l  In
prokaryotes
¡ Genome=single,
long DNA
molecule in a circle
l  In
eukaryotes
¡ Genome
= several DNA
molecules grouped in clumps
(called chromosomes)
Forms of DNA
¡  DNA
(in nucleus of eukaryotes)
can be in 2 forms
l  Chromatin
: DNA is not tightly
packed together (loosely coiled)
¡ Occurs during interphase (most
of cell’s life)
l  Chromosomes : tightly packed
together (TIGHTLY coiled)
¡ Occurs during mitosis (cell
division)
Tightly
coiled?
Not tightly
coiled?
How Does DNA Fit into a
Small Cell?
¡ 
Prokaryotic
organisms
l  No nucleus;
instead have a
nucleiod
region
¡ DNA = 1
chromosome
How Does DNA Fit into a
Small Cell?
¡  Eukaryotic
l 
chromosomes
stored within the cell's nucleus
¡  DNA
is coiled around proteins
known as histones
l Group of histones is called a
nucleosome
¡  helps
form a “supercoil”
DNA + histones è form
nucleosomes
(help to “super coil DNA)
Nucleosomes
•  Like beads
on a string
Histone Tails
+!
-!
Tails have positive charge; DNA
has negative charge
Histone Modification
¡  Adding
acetyl
group to the tail
(acetylation)
1. 
2. 
3. 
Neutralizes
charges
Making DNA
less tightly
coiled
Increasing
transcription
DNA Modification
¡  Adding
a
methyl group
to the DNA
(methylation)
Maintains the
positive charge
l  Makes DNA
more coiled
l  Reduces
transcription
l 
Gene is “active”!
•  Switched on
•  Open chromatin
•  Transcription can
happen
•  Unmethylated
cytosines (white
circles)
•  Acetyl groups ARE
on histones
Gene is “inactive” !
•  No transcription
•  Methylated
cytosines (red
circles)
•  No acetyl groups on
histones
Making Chromosomes
**Occurs right before cell division
(mitosis)
Steps:
1.  DNA copies itself through DNA
Replication
2.  The DNA coils (wraps) around
proteins called histone proteins.
3.  Chromosomes are made.
The DNA molecules in a cell
Are packaged into chromosomes
50 µm
Chromosome Structure
Two shapes of chromosomes
1. Single chromosome (1 copy of DNA);
“V” shaped
2. Double Chromosome (2 copies of
DNA); “X” shaped; a.k.a-duplicated
chromosome
Occurs right
before cell
division
(mitosis)
Chromosomes and Cell
Division
preparation for cell
division (mitosis)
l DNA is replicated
(single à double
chromosome)
¡ In
l V
shape à X shape
has two sister chromatids, which
separate during cell division
0.5 µm
A eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
Figure 12.4
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Centromeres
Sister
chromatids
Sister chromatids
Double Chromosome
Structure
Kinetochore
attaches to
spindle fibers
Sister
Another view…
Draw and Label the Pieces of a
Duplicated Chromosome
Chromosomes
¡  Every
eukaryotic species
has a characteristic, unique
# of chromosomes in EACH
cell nucleus
l  Ex:
Humans have 46
chromosomes
¡  #
of chromosomes does
NOT necessarily equal
complexity
Different Types of Cells
¡ 
There are 2 types of cells in the
human body
1.  Somatic cells: all body cells
2. Gametes: reproductive cells
(sperm and egg)
Different cell types à different
cell divisions
¡  Eukaryotic
cell division consists of
1. Mitosis
l  Occurs in somatic cells
2. meiosis
l  Occurs in GERM LINE CELL (a
special somatic cell in gonads)
l  CREATES gamete cells
Chromosome Number
¡  Gametes
(sperm/egg)
have 1 copy of each chromosome
l  called haploid
l 
¡  Somatic
(body)
have 2 copies of each chromosomes
l  called diploid
l 
Chromosomes come in PAIRS
l  All
pairs of chromosomes differ in
size, shapes, and set of genes.
Haploid vs. Diploid
¡ 
Haploid= when a cell has 1 copy of
each chromosome.
¡  Expressed
¡ 
as n = 23
Diploid= when a cell has 2 copy of
each chromosome. (a.k.a TOTAL # of C’s in
organisms)
¡  Expressed
Haploid egg cell
as 2n = 46
n
n
Haploid sperm cell
Fertilization
2n
1st diploid Zygote cell of an organism
Set of Chromosomes
¡ 
¡ 
¡ 
Each chromosome set contain 2
“homologues” or “twins”
Chromosomes have a “twin” or matching
pair
Homologous Chromosome Pair = 2
chromosomes that are similar in shape,
size, and genes.
¡ 
Each homologue in a pair comes from each
parent!