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Chromosomes, Mitosis, Meiosis
I. The Chromosome
A. How all these terms fit together:
Nucleotide – Basic unit of DNA
Gene – Sequence of DNA coding for a protein
Chromosome – Many genes
Genome – all the chromosomes in one cell
46 chromosomes in the human genome
B. Structure of a chromosome
1. DNA wrapped around specialized proteins called histones
a. this is a “single chromosome” (meaning DNA has
not been replicated).
b. when we see a chromosome under the microscope,
the DNA has been replicated, so what we see
are “doubled chromosomes”. We can’t see them
C. Terms to help you understand Mitosis and Meiosis
1. Mitosis – division of the nucleus
2. Meiosis – making of gametes (egg and sperm)
3. Single chromosome – DNA has not been replicated
4. Doubled chromosome – DNA has been replicated
5. Homologous chromosomes
a. all chromosomes occur in pairs (you get one from your
mom {egg} and one from your dad {sperm}) so you
can think of one in the pair being the “female
chromosome” and the other in a pair being the “male
b. they are the same size and shape
c. they are carrying the same genes on them (ie…each
gene you have occurs as a pair)
6. Diploid cell (abbreviated 2n) – a cell that contains both
chromosomes of a homologous pair (in a human, this
would be a cell with 46 chromosomes in it)
7. Haploid cell (abbreviated n) – a cell that contains only
one chromosome of a homologous pair
a. egg and sperm cell
8. Gametes – fancy term for reproductive cells
a. haploid (egg and sperm)
b. when gametes fuse, you get a zygote – one diploid
cell that divides many times to make a whole
A. Interphase
1. Most of the cell’s life is
spent in this phase
2. This phase is divided
into 3 parts
a. G1 = Gap 1
The cell makes proteins
The cell grows
The cell makes more organelles
b. S = Synthesis
When DNA replication occurs (single chromosomes become doubled)
c. G2 = Gap 2
More proteins are produced to prepare for cell division
B. Mitosis
1. This is where the nucleus divides into two nuclei
C. Cytokinesis
1. This is where the cytoplasm is divided and two cells are produced
Mitosis: What is it?
One nucleus dividing into two.
The two nuclei are identical
Occurs in all cells except egg and
sperm. Occurs in “My--Toes”.
Phases of Mitosis
 Prophase
 Metaphase
 Anaphase
 Telophase
Chromosomes coil
Nuclear envelope breaks down
Spindle forms
Early Prophase
Late Prophase
 Chromosomes line
up in the Middle
of the cell
 Doubled chromosomes move
Apart to opposite sides of the cell
Early Anaphase
Late Anaphase
Chromosomes uncoil
Nucleus reappears
Spindle breaks down
Cytokinesis occurs – cytoplasm splits resulting
in 2 cells.
Purpose: To separate the homologous pairs.
Interphase 1
DNA replicates
Interphase 1
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Chromosomes coil up
Spindle forms
Nucleus disappears
Homologous pairs form tetrads
Crossing over may occur
Tetrads line up in the middle
Tetrads (homologous pairs) separate
Chromosomes uncoil
Spindle disappears
Nucleus reappears
Result: 2 haploid
cells with doubled
Purpose is to separate doubled chromosome
Interphase 2
Prophase 2
Metaphase 2
Cell rests, nothing happens
Chromosomes coil
Spindle forms
Nucleus disappears
Chromosomes line up in the
Chromosomes separate
Anaphase 2
Telophase 2
Chromsomes uncoil
Nucleus reappears
Spindle disappears
Result: 4 cells that are haploid
Normal Female
46 XX
Chromosomal Mutations
A. Deletion
1. The loss of a chromosome region.
a. Causes include – radiation, viral attach, environmental
b. Results in loss of at least one gene…probably more
c. Example: “cri-du-chat”
High-pitched cry sounds like a cat
slow growth
Small head
Wide-set eyes
Downward slant to the eyes
Small jaw
Low-set ears (may be malformed)
Skin tags just in front of the ear
Partial webbing or fusing of fingers or toes
Single line in the palm of the hand
Mental retardation
Slow or incomplete development of motor skills
B. Duplication/Insertion
1. Repeated gene sequence on a chromosome.
2. Example: Fragile X syndrome
a. twice as many males as females have this.
b. there is a region on the X chromosome that has many
replications which elongates the X chromosome.
Family history of Fragile X syndrome,
especially a male relative
Mental retardation
Large testicles after puberty
Large body size
Tendency to avoid eye contact
Hyperactive behavior
Large forehead or ears with a prominent
C. Inversion
1. A segment that separated from a chromosome and then
was inserted at the same place but in reverse. This
reversal alters the position and order of the
chromosome’s genes.
a. may or may not have a major effect.
b. may cause mental retardation and/or birth defects
D. Translocation
1. A part of one chromosome that has exchanged places
with a corresponding part of another, nonhomologous
chromosome. (example—chromosome 14 and 21
exchange information)
a. may cause a miscarriage
b. may not effect carrier, but their offspring may be at
risk of having Down syndrome or never be viable.
D. Nondisjunction – When chromosomes fail to separate
during meiosis.
1. can cause abnormalities in autosome number.
a. Down Syndrome (have an extra chromosome 21)
2. can cause abnormalities in sex chromosome number
a. Turner Syndrome (XO)
Female that has only one X chromosome
(the egg did not have an X chromosome)
The ovaries never become functional
Breasts do not develop
Stocky build
A webbed neck
Difficulty in spatial patterns
Mental retardation
b. Triple X (XXX)
An egg with two X’s is fertilized by a sperm with an X
Tendency toward learning disabilities.
Usually no physical abnormalities
c. Klinefelter Syndrome (XXY)
An egg with two X’s is fertilized by a sperm with a Y
Male in appearance, but testes are underdeveloped,
and breasts may be enlarged.
Arms and legs tend to be longer than average
Poor muscular development
Sparse body hair
Some learning disabilities
d. Jacob Syndrome (XYY)
Taller than average
Persistent acne
Slightly below normal intelligence.
At one time people used to think humans with
Jacob syndrome were likely to be criminally
aggressive, but this has been found not to be
Down Syndrome
Klinefelter syndrome
Turner Syndrome
Triple X