Chromosomes, Mitosis, And
Meiosis
Chapter 8
Do Now
What is DNA?
Where are your chromosomes located?
How many chromosomes do humans have?
DNA
DNA – deoxyribonucleic acid
Long, thin molecule that stores genetic
information
Made up of nucleotide building blocks
(Adenine, Guanine, Cytosine, Thymine)
DNA in a human cell has approximately 6
billion pairs of nucleotides
Chromosomes – rod shaped
structures made of DNA and protein
Chromosomes
1. Contained in nucleus
2. When cell is not dividing,
genetic info is kept in uncoiled
DNA called chromatin
3. When a cell begins to divide
the single DNA molecule coils
around proteins called histones
to form chromosomes
Chromosome Structure
4. Non-histone proteins control
activity of specific DNA
sequences
5. Each chromosome has two
identical parts called – sister
chromatids
5. The point at which they meet
is called a centromere
6. Division-one chromatid from
each chromosome/new cell
Info about Chromosomes
1. Entire human genome mapped – found
chromosome location of approximately
23,000 genes
2. Above the centromere is the “p” region
(small)
3. Below the centromere is the “q” region
(large)
Chromosome Numbers
1. Every Species has a different number of
chromosomes
2. Humans have 46 (23 pairs)
a. 22 autosomes (do not determine the sex)
b. 1 pair sex chromosomes (determine the
sex)
3. Females have 2 X chromosomes, males
have an X and a Y chromosome
Haploid/Diploid
1. Homologous chromosomes (paired
chromosomes) are the same size and shape
and carry genes for the same traits
2. Cells with both chromosomes of a
homologous pair are considered diploid
(2N) – body cells - somatic cells
3. Cells with one of the two chromosomes is
considered haploid (N) – sex cells
Haploid/Diploid
Karyotype
An arranged photomicrograph of chromosomes that
helps determine problems in chromosome # or
deletion
Karyotypes describe the number of chromosomes,
and what they look like under a microscope
Attention is paid to:
chromosome length
position of the centromeres
banding pattern
differences between the sex chromosomes
Do Now
What are chromosomes made of?
What does haploid mean?
What does diploid mean?
Down’s Syndrome
Trisomy 21– 3 copies of chromosome
21 instead of 2
1 in 900 births
More common with older mothers
Some impairment of cognitive ability and
physical growth
A particular set of facial characteristics
Down’s Syndrome
Turner Syndrome

Female missing one X chromosome
(X0)

1 in every 2500 girls

Infertile – ovaries do not develop

Other female characteristics do not
develop

Decreased height
Kleinfelter’s Syndrome

Male with an extra X chromosome
(XXY)
 Occurs in 1 out of every 1700 men
 Testicles do not form properly –
infertile
 Delayed motor function and muscle
formation
 Increased height, increased tiredness
Kleinfelter’s Syndrome
II. CELL DIVISION
– all cells are derived
from pre-existing cells
*Virchow
*Cell division - the process by
which cells produce offspring cells
*It is very different for prokaryotes
and eukaryotes
Cell Division in Prokaryotes
Binary fission –
division of a prokaryotic
cell into two offspring
Single chromosome
copies itself
Cell grows to 2X its size
Cell wall forms between
the 2 chromosomes
Each new cell is
identical
Mitosis - cell division in
eukaryotes
Division of a cell’s nucleus that produces a
genetically identical cell
It does not change the amount of DNA in a
cell
Mitosis occurs in the reproduction of
unicellular organisms and in the addition of
cells to a tissue or organ of a multicellular
organism
Cell Cycle
Repeating set of events that make up the life
of the cell
Consists of:
 mitosis - cell division
 interphase
 G0 Phase – cell exits the cell cycle and
does not prepare for another division
(nerve cells)
Cell Cycle
Interphase
1. Takes up 80-90% of cells lifespan
2. Three distinct parts:
a. G1 (Growth Phase) – cell doubles in size, organelles
double in number
b. S1 (Synthesis Phase) – chromatin replicates
c. G2 (Growth Phase) - rapid growth phase – prepare for
next division
Do Now
What is cell division in prokaryotes called?
What is cell division in eukaryotes called?
What are the 4 main phases of the cell
cycle?
Mitosis
1. 4 phases:

Prophase, Metaphase, Anaphase, Telophase
Prophase
1. Chromatin coils and forms chromosomes,
nucleolus and nuclear membrane disappear,
centrosomes and spindle fibers appear
Metaphase
1. Chromosomes line up at center, or equator
of cell
Anaphase
Spindle Fibers pull chromosomes towards
poles of cell
Telophase
1. Centrioles and Spindle fibers disappear
2. Chromosomes unwind, and nuclear
membrane and nucleolus forms in each cell
Mitosis in Plant Cells (onion)
Cytokinesis
1. Takes place after mitosis
2.Cytoplasm and organelles split and form
two daughter cells
3. Each cell receives and identical
copy of the original cell’s
chromosomes and ~ half
of the cytoplasm and organelles
Do Now
What are the 4 stages of mitosis?
What are the structures that pull the
chromosomes toward opposite ends of the
cell?
How does cytokenesis differ in animals and
plants?
Meiosis – reduction division
Process of nuclear division that reduces the #
of chromosomes in each new cell to half of the
original cell
This is necessary in reproductive cells called
gametes so that they can fuse and form a
zygote with a normal # of chromosomes
Example – human egg (haploid -1n) and sperm
(haploid -1n) form an embryo (diploid -2n)
Meiosis
A. Two nuclear divisions take place
1. Meiosis I – homologous chromosomes are
separated
2. Meiosis II – chromosomes are separated into
chromatids for each haploid cell
3. 4 haploid cells are produced - gametes
Meiosis I – DNA copies itself just
before beginning
Prophase I - similar to mitosis:
DNA coils into chromosomes
 nuclear membrane and nucleolus disassemble
 spindle fibers appear
THEN
 Every chromosome lines up with its homologue
and forms a tetrad
 Synapsis – pairing of homologous
chromosomes – does not occur in mitosis

Meiosis Helps Genetic Diversity
A. During Prophase I,
homologous
chromosomes line up
forming a tetrad
B. Chromatids can
exchange pieces,
called crossing –
over
C. Genetic
recombination results
Meiosis I
Metaphase I
Tetrads line up randomly along the midline of
the cell
 Spindle fibers attach to centromeres and poles
of cell

Meiosis I
Anaphase I
Each homologous chromosome moves to an
opposite pole of dividing cell
 Independent assortment – the random
separation of homologous chromosomes
 Genetic recombination occurs again due to
random separation of maternal (mom’s) and
paternal (dad’s) chromosomes

Meiosis I
Telophase I
Chromosomes reach the opposite ends of the
cell
 Cytokinesis begins
 New cells are now haploid (1n):

they contain only one homologue from each
homologous pair
 But they have two copies of each chromosome
because the DNA copied itself right before
beginning Meiosis I

Meiosis II
DNA does not copy itself before Meiosis II
Occurs in both cells formed in Meiosis I
Exactly like mitosis
Results in four new cells from each cell that
began Meiosis
Each new cell (haploid – 1n) has half the
number of chromosomes as the original cell
(diploid – 2n)
Gamete Formation
Meiosis produces haploid reproductive cells
called gametes
Meiosis only occurs in cells of the
reproductive organs
Humans:
Oogenesis - female ovaries produce 1 egg and 3
polar bodies (disintegrate)
 Spermatogenesis - male testes produce 4 sperm

Oogenesis
Spermatogenesis
Asexual Reproduction
• Production of offspring from one parent:
•
•
•
no union of gametes
no genetic diversity
offspring genetically identical
• Unicellular organisms -binary
fission or mitosis
• Multicellular organisms - budding
Sexual Reproduction
Production of offspring through meiosis and
the union of egg and sperm
Offspring are genetically different from
parents
Sexual Reproduction
Except in identical twins, offspring are
unique combinations of their parents’ genes
Sexual Reproduction
Gives an evolutionary advantage in an
environment that can change – survival
of the fittest