2. Homologous chromosomes
• Pair of chromosomes that
one comes from the male
parent and the corresponding Homologous
one from the female parent chromosomes
Homologous ≠ identical Alleles
same genes in same loci
same length
same shape
may not the same alleles
for one gene
3. Diploid & Haploid Cells
• two copies of each • one copy of each
chromosome chromosome
• homologous pairs • no homologous pairs
• body cells • gametes (sex cells)
Sperm
Egg
5. The purpose of meiosis
is to produce gametes.
A (haploid) gamete
(sex cell, sperm or egg
cell) has half the
number of
chromosomes compared
to a (diploid) somatic
cell (body cell). A male
and a female gamete
may then fuse to form a
zygote, which will have
the same number of
chromosomes as a
somatic cell.
6. Human chromosomes
– Diploid cells: 23
pairs of homologous
chromosomes
– 1st-22nd pairs:
autosomes
– 23rd pair: sex
chromosome
9. Karyotype (the type of nucleus)
• The number and appearance of the chromosomes in an
organism
10. Karyotyping
• Arrange the chromosomes in pairs
according to their size and structure
• Collect cells
• Culture cells
• Stop cell division at metaphase
• Staining chromosome
• Pair (Length, Shape, Position of
centromeres, Bands)
• Diagnosis (number, gender,
abnormality)
11. How many chromosoems
8
How many pairs of
homologous
chromosomes ？
4
How many pairs of
autosomes ？
3
How many pairs of
Fruit fly’s somatic cells
sex
chromosomes ？
1 : x,y
12. A B
germ cell
C D
What is the correct gamete
produced by the germ cell
13. Meiosis Overview
• reduction division
• number of chromosomes per cell halved
• separation of homologous chromosomes
• 1 diploid 4 genetically different
haploid
• DNA copied once, cell divide twice
14.
15. Sperms are produced by … meiosis
Diploid parent
cell The
chromosomes in
the nucleus are
copied
Four haploid
daughter cells
16. Diploid Eggs are also produced by
parent cell meiosis
Haploid
17. Phases of meiosis
MEIOSIS I MEIOSIS II
PROPHASE I PROPHASE II
interphase no interphase
(DNA replication METAPHASE I (no DNA replication METAPHASE II
before meiosis I) ANAPHASE I before meiosis II) ANAPHASE II
TELOPHASE I TELOPHASE II
18. Meiosis I
(reduction division)
Interphase I Prophase I Metaphase I Anaphase I Telophase I
Cells undergo a round Each chromosome pairs Spindle fibers attach to The fibers pull the
of DNA replication, with its corresponding the chromosomes. homologous chromosomes
forming duplicate homologous chromosome toward the opposite ends
Chromosomes. to form a tetrad. of the cell.
19.
20. Prophase 1
• Supercoil of
chromosomes
• Spindle form
• Synapsis
(homologous pair to
form bivalents)
• Crossing over
• Nuclear envelope
breaks down
21. Synapsis & Crossing over
• The key to the process of
meiosis is the Synapsis in
Prophase I and the splitting of
the homologues in Anaphase I.
• One event that can take place
during meiosis is crossing
over.
• Crossing over is the exchange
of genetic material between two
homologous chromosomes
which have paired up during
Prophase I.
• The result of crossing over is
genetic recombination.
22. Metaphase 1
1. Spindle network
complete
2. Bivalents line up on
equator
3. Centromere
attached to
microtubules
25. Meiosis II
(similar to mitosis)
There is no
DNA
replication
Prophase II Metaphase II Anaphase II Telophase II
Meiosis I results in two The chromosomes line up The sister chromatids Meiosis II results in four
haploid (N) daughter cells, in a similar way to the separate and move toward haploid (N) daughter cells.
each with half the number metaphase stage of opposite ends of the cell.
of chromosomes as the mitosis.
original.
