Meiosis - LincolnLions.org
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Transcript Meiosis - LincolnLions.org
The other form of cell division
Chapter 8
Genes are passed from parents to offspring on
chromosomes.
Human body cells contain 46 chromosomes
23 from mother & 23 from father
Homologous- Each chromosome has a
corresponding, or homologous chromosome.
Chromosomes occur in pairs
Diploid: 2 complete sets of each chromosome (2N)
(where N = single set of chromosomes)
Haploid: 1 single set of chromosomes (N)
In humans:
Body cells are diploid, and contain 46 chromosomes
Gametes are haploid, and contain 23 chromosomes
each.
Why?
Sperm (23) + egg (23) = you! (46)
Karyotype: The arrangement of chromosomes by
homologous pairs
homologous
chromosomes
Picture taken during cell division
Stained to show banding
pattern- location of genes
Use size and banding
pattern to match pairs
Meiosis
The cell division that results in half the number of
chromosomes in each cell
Occurs in human gametes (sex cells)
Let’s follow a cell through meiosis where 2N=4
1. Interphase- DNA is replicated and cell prepares to
divide
2. Prophase I – Each replicated chromosome pairs up
with its homologous chromosome forming a tetrad.
*Plus everything else that
we already learned occurs
in prophase.
Crossing over- Chromatids of homologous
chromosomes cross over each other and exchange
some alleles.
3. Metaphase I Paired homologous chromosomes line up across the
center of the cell.
Spindle fibers attach to each tetrad.
tetrad
4. Anaphase I- Spindle fibers pull apart each
homologous chromosome pair toward the poles.
Crossing over that occurred
5. Telophase I- Separated chromosomes cluster on
either side of the cell and nuclear membrane reforms
around each.
Cytokinesis- Cytoplasm divides and cell membrane is
pinched off forming two non-identical cells (due to
crossing over).
The two cells now enter meiosis II
They do not re-copy DNA, however some cells might go through G1 and
G2 checkpoints again.
Meiosis II just like the steps of mitosis
1. Prophase II Nuclear membrane breaks down
Chromosomes each consisting of 2 sister chromatids
condense to become visible (if not so already).
Spindle fibers form and centrioles begin to move to
opposite sides of the cell.
2. Metaphase II- Chromosomes line up in the middle
of the cell, spindle fibers attach to centromeres.
3. Anaphase II- Sister chromatids split and spindle
fibers pull them to either side of the cell.
4. Telophase/Cytokinesis II- Nuclear membrane
reforms and cytoplasm is divided during the pinching
off of the cell membrane.
Four haploid daughter cells (not genetically identical)
N=2
The resulting 4 haploid cells are gametes (in humans)
A video of meiosis