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Genetics - MEIOSIS
Background Information
During interphase, DNA is uncondensed
and is called chromatin.
 When cells get ready to divide, the
chromosomes condense into
chromosomes.

Chromosome vs. Chromatin
Structure of a Chromosome

Chromatids – the two identical “sister” parts
of a chromosome

Centromere – where
the two chromatids
attach
You have 2 types of cells!

Somatic Cells – regular body cells (skin
cells, hair cells, heart cells, brain cells,
etc.)


They are considered diploid (2n). They have 2
copies of each chromosome (one from mom
and one from dad).
Gametes – reproductive cells (sperm &
eggs)

They are considered haploid (n). They only
have one copy of each chromosome (mom OR
dad’s).
Mitosis
Somatic cells are
created through
mitosis.
 One cell replicates its
DNA and then divides
to create 2 identical
daughter cells.

HUMAN CHROMOSOMES
•46 total chromosomes
•23 homologous pairs; two sets
•1 set from mom
•1 set from dad
Challenger Questions
What
is our diploid number?
What
is our haploid number?
Challenger Questions
What
is our diploid number?
 46
 23 from dad
 23 from mom
What
 23
is our haploid number?
Meiosis
A type of cell division that produces
gametes with half as many chromosomes
as regular body cells.
The Two Stages of Meiosis
• Meiosis I
• Meiosis II
• Prophase I
• Prophase II
• Metaphase I
• Metaphase II
• Anaphase I
• Anaphase II
• Telophase I
• Telophase II
Interphase
Chromosomes (DNA)
are duplicated in the
S-phase of
interphase (prior to
division).
DNA is uncondensed,
in the form of
chromatin.
Prophase I
•DNA forms
chromosomes.
•Homologous
chromosomes pair up
in a tetrad
•Crossing over
happens
Homologous Chromosome


A pair of replicated chromosomes, one inherited
from each parent, that have corresponding gene
sequences and that pair during meiosis.
They are the same size, have the same genes
and show the same banding pattern in a
karyotype.
Synapsis

The pairing up of homologous
chromosomes.
Tetrad
Homologous chromosomes that are paired
together, ready to divide. (4 chromatids
together)
Crossing Over
Exchange of genetic material between
homologous chromosome tetrads during
Prophase I
Crossing Over: the
result is genetic
recombination: an
exchange of genes.
Produces genetic
variability; any gene can
be found in any gamete
Metaphase I
•Tetrads are arranged in the middle of the
cell.
•Spindle fibers attach
Centriole
Spindle Fibers
Attach to chromatids and pull them apart
during meiosis and mitosis
Metaphase I
Anaphase I
•homologous
chromosomes are
pulled apart
•Sister chromatids
remain together
disjunction

The separation of homologous
chromosomes
Telophase I:
chromatids move to
opposite sides of the
cell.
Cells become haploid
(one set in each cell).
Cytokinesis
•nuclear membrane
forms and 2 new cells
form
Prophase II:
• chromosomes get
ready again
Metaphase II
•Chromosomes line
up down the middle
Anaphase II
•chromosomes move apart
Telophase II & Cytokinesis
•4 haploid sex cells form
•Each is unique
Meiotic Division Results
Initial Cell
Result of
Meiosis I
Result of
Meiosis II
Meiosis reduces
chromosome number
Mitosis vs Meiosis
Diploid Cell
Has the full number of chromosomes;
means “two sets” ; body cells
Haploid
Has half of the normal number of
chromosomes; means “one set”; sex cells
Chromatid
Identical, replicated chromosomes joined by
a centromere
From Parents to Offspring
Parents’ cells (diploid) undergo meiosis to
produce sex cells (haploid)
 Sex & Fertilization occurs
 The fertilized egg is now diploid, with a
complete set of chromosomes
 The fertilized egg has genes from both
maternal and paternal grandparents-the
parents’ genes
 The egg undergoes mitosis to grow

Q: Why is there
meiosis?
A: If gametes (sex
cells) had a full set of
chromosomes, the
number of
chromosomes in the
adult would double
with every generation!!
Meiosis!
Quick “practice” quiz!
1. The gamete that contains genes
contributed only by the mother is
_______________.
a. a sperm
b. an egg
c. a zygote
d. syngamy
e. ovary
2. The paternal gamete is _________.
a. a sperm
b. an egg
c. a zygote
d. syngamy
e. gonad
3. Chromosomes that contain genes
for the same trait in the same order
are called ____________________.
a. haploid
b. gametes
c. diploid
d. homozygous
e. homologous
4. If a fruit fly has 4 chromosomes,
what is its diploid number?
a. 16
b. 8
c. 6
d. 4
e. 2
5. If a fruit fly has 4 chromosomes,
what is its haploid number?
a. 16
b. 8
c. 6
d. 4
e. 2
6. If a fruit fly has 4 chromosomes
and a fruit fly sperm fertilizes a fruit
fly egg, how many chromosomes
does the zygote have?
a. 16
d. 4
b. 8
e. 2
c. 6
7. Why are the daughter cells
formed during meiosis haploid?
For Q’s 8-12, use the key to match
the term with the proper phrase.
a. Meiosis
b. Mitosis
8. Forms cells that are diploid.
For Q’s 8-12, use the key to match
the term with the proper phrase.
a. Meiosis
b. Mitosis
9. Forms cells with half the
amount of genetic material as a
somatic (body) cell.
For Q’s 8-12, use the key to match
the term with the proper phrase.
a. Meiosis
b. Mitosis
10. Crossing over often occurs.
For Q’s 8-12 use the key to match
the term with the proper phrase.
a. Meiosis
b. Mitosis
11. Produces genetic variability.
12. If a gorilla’s sperm cell has 24
chromosomes, what is the gorilla’s
diploid number?
a. 24
b. 12
c. 26
d. 96
e. 48
CLOSURE

DRAW A PICTURE COMPARING MITOSIS
AND MEIOSIS.