Meiosis and Sexual Reproduction
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Meiosis, on the other hand, is used for just one
purpose in the human body: the production
of gametes – sex cells, or sperm and eggs. Its goal is
to make daughter cells with exactly half as many
chromosomes as the starting cell.
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To put that another way, meiosis in humans is a
division process that takes us from a diploid cell – one
with two sets of chromosomes – to haploid cells –
ones with a single set of chromosomes. In humans,
the haploid cells made in meiosis are sperm and eggs.
When a sperm and an egg join in fertilization, the two
haploid sets of chromosomes form a complete
diploid set: a new genome.
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Homologue pairs separate during a first round of cell
division, called meiosis I. Sister chromatids separate
during a second round, called meiosis II.
Meiosis I
Prophase I
- At this stage, chromosomes become compact, dense
structures and are easily visible under the
microscope. The homologous chromosomes pair
together. The two sets of sister chromatids resemble
two X's lined up next to each other. Each set
exchanges bits of DNA with the other and
recombines, thus creating genetic variation. This
process is known as crossing over.
- Even though in humans the male sex chromosomes
(X and Y) are not exact homologs, they can still pair
together and exchange DNA. Crossing over occurs
within only a small region of the two chromosomes.
- By the end of prophase I, the nuclear membrane
breaks down.
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Metaphase I
- The meiotic spindle, a network of protein filaments,
emerges from two structures called the centrioles,
positioned at either end of the cell. The meiotic
spindle latches onto the fused sister chromatids. By
the end of metaphase I, all the fused sister
chromatids are tethered at their centromeres and
line up in the middle of the cell. The homologs still
look like two X's sitting close together.
Anaphase I
- The spindle fibers start to contract, pulling the fused
sister chromatids with them. Each X-shaped complex
moves away from the other, toward opposite ends of
the cell.
Telophase I
- The fused sister chromatids reach either end of the
cell, and the cell body splits into two.
Meiosis I results in two daughter cells, each of which contains
a set of fused sister chromatids. The genetic makeup of each
daughter cell is distinct because of the DNA exchange
between homologs during the crossing-over process.
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Meiosis II
Meiosis II is like mitosis. In other words, by the end of the
process, the chromosome number is unchanged between
the cells that enter meiosis II and the resulting daughter cells.
Prophase II
- The nuclear membrane disintegrates, and meiotic
spindles begin to form once again.
Metaphase II
- The meiotic spindles latch onto the centromere of
the sister chromatids, and they all line up at the
center of the cell.
Anaphase II
- The spindle fibers start to contract and pull the sister
chromatids apart. Each individual chromosome now
begins to move to either end of the cell.
Telophase II
- The chromosomes reach opposite ends of the cell.
The nuclear membrane forms again, and the cell
body splits into two.
Meiosis II results in four haploid daughter cells, each with the
same number of chromosomes. However, each
chromosome is unique and contains a mix of genetic
information from the maternal and paternal chromosomes
in the original parent cell.
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9
Sexual Reproduction in Plants
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