Unit 3- Genetics Meiosis. Fertilization Zygote 46 chromosomes Egg ____ chromosomes Sperm _____ chromosomes X Which is diploid ? Which is haploid ?
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Fertilization
Zygote46 chromosomes
Egg____ chromosomes
Sperm_____ chromosomes
X
Which is diploid ?Which is haploid ?
Sex cells
Egg23 chromosomes
23 chromosomes
How can a diploid cell become haploid cells?Answer: by the process of meiosis
-Through meiosis, one diploid parent cell produces four haploid daughter cells;
- Each daughter cells has half the number of chromosomes found in the parent 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 duplicatedduplicated
• We’re going to use a simpler cell in which 2n=8 (or n= 4)
• This cell have 4 unduplicated chromosomes right before meiosis occurs
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 in reproductive organs.
Prophase 1Tetrad: the
homologous pair made of 4 chromatids.
• Chiasmata: regions 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 occurring between homologous chromosomes allows exchange of genetic information
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 but rather in pairs in metaphase 1
Draw this cell. This is NOT metaphase 1
Anaphase 1
• Homologous chromosomes (NOT individual sister chromatids) separate and move to opposite end of cell.
Telophase 1
• Homologues reach opposite sides of the cell. • A nuclear member reforms forming 2 new nuclei• Chromosomes relax
Meiosis 2: similar to mitosisProphase 2
•Chromatin coil again•Spindle fibers attach to chromosomes•THERE IS NO DUPLICATION OF CHROMOSOME AT THIS STEP•No Crossing over either
Anaphase 2
• Centromeres split pulling sister chromatids (NOT homologous pairs anymore) to opposite end of each cell
Task: Create a Meiosis Flipbook
• Use Playdoh to recreate the detailed events of meiosis.
• Take picture of each phase of meiosis and compile them into a flip book
• Sample at: https://www.youtube.com/watch?v=uYgbWm4H_gU
• Use the Meiosis flipbook template if you need to
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
Chromosome Crossing over
• Only one chiasma is illustrated but many occur per pair
(Plural: chiasmata)
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)
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