Unit 2- Genetics
Meiosis
Fertilization
Egg
____ chromosomes
X
Sperm
_____ chromosomes
Zygote
46 chromosomes
Which is diploid ?
Which is haploid ?
Sex cells
Egg
23 chromosomes
23 chromosomes
How can a diploid cell become haploid cells
Answer: by the process of meiosis
Meiosis: process of cell division in which a
diploid cell produces four haploid daughter cells;
each with half the number of chromosomes
found in the parent cell
Recall where
DNA is within
the cell
Interphase
• DNA
replicates/duplicates (S
phase)
• In the form of chromatin
• Not easily visible
• Similar to interphase in
Mitosis
Before any cell division, DNA must be duplicated
• Were going to use a simpler cell
in which 2n=4
• This cell have 4 unduplicated
chromosomes right before
meiosis occurs
Thus, this cell has 2 pairs of
homologous chromosomes
• We are now
ready for meiosis
with Playdoh
What you need
• 3 different colored playdoh
– One color for paternal chromosome
– One color for maternal chromosome
– One color for centromere
• Paper or surface of whiteboard
• Pen/marker to draw other details such as cell
membrane, spindle fibers, centrioles etc.
• Handout
• Group of 3
2 parts of meiosis
• Meiosis 1: reduction division
• Meiosis 2: Gametes are produced
• Unlike mitosis, meiosis does not occur in
somatic cells of our body but rather in
reproductive organs.
Prophase 1
Tetrad: the
homologous
pair made of 4
chromatids.
• Chiasmata:
region where
crossing-over
occurs.
• Tetrad: the
homologous
pair made of 4
chromatids.
• Chiasmata:
region where
crossing-over
occurs.
Prophase 1
• Chromosomes condense and
become visible; each with 2
identical sister chromatids.
• Nuclear membrane dissolves;
centrioles move to opposite cell
poles; spindle fibers form
• Homologous chromosomes pair
up and form a tetrad
• Crossing-over occuring between
homologous chromosomes
produce exchange genetic
information
Your cell should look like this now
Metaphase 1
Homologous chromosomes line up in PAIRS in the middle of the
cell
Again: Homologous chromosomes do not line up in a single file;
they line up in pairs in metaphase 1
Anaphase 1
• Homologous chromosomes (NOT individual sister
chromatids) separate and move to opposite end of
cell.
Telophase 1
• Two haploid cells
(half of the original
amount in each cell
that is, 2 each)
• Original cell (2n= 4
chromosomes)
• Homologues reach opposite sides of the cell.
• A nuclear membrane reforms forming 2 new
nuclei
• Chromosomes relax
Meiosis 2: similar to mitosis
Prophase 2
•Chromatin coil again
•Spindle fibers attach to chromosomes
•THERE IS NO DUPLICATION OF CHROMOSOME AT
THIS STEP
•No Crossing over either
Metaphase 2
• Chromosomes line up in single file
RANDOMLY in the middle of each cell
Anaphase 2
• Centromeres split pulling sister chromatids
to opposite end of each cell
Telophase 2
• Four nuclei form around chromosomes
• Spindle fibers dissolve
• Cells divide
Final result of meiosis
Initial parent cell
2n=4
4 haploid daughter cells
n=2
RECAP MEIOSIS https://www.youtube.com/watch?v=vA8aMpHwYh0
Gametogenesis= the formation of
gametes
involves
Spermatogenesis in male
and oogenesis in female
No DNA duplication
occur here
Chromosome Crossing over
• Only one chiasma is illustrated but many
occur per pair
(Plural: chiasmata)
22 = 4 possible
combinations to
forms a gamete
Some new words
• Gamete: a sex cell; includes sperm cells
and egg cells
• Zygote: a cell produced by the fusion of
two gametes
• Fertilization: the formation of a zygote by
fusing two gametes
• Haploid: a cell containing half the usual
number of chromosomes (n)
• Diploid: a cell containing two copies of
each chromosome (2n)