Meiosis 15 October, 2004 Text Chapter 13

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Meiosis15 October, 2004Text Chapter 13
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Asexual Reproduction
In asexual reproduction, individuals give rise to
genetically identical offspring (clones). All
cell division in this process is mitotic.
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Important Terms (review)
genome all of a cells DNA
chromosome individual DNA molecule
chromatin chromosomal DNA and its protein
scaffold
sister chromatids the two identical copies of
a duplicated chromosome
centromere the point of attachment for two
sister chromatids
somatic cells (most cells) have two copies of
each chromosome (diploid)
gametes (reproductive cells sperm and eggs)
have one copy of each chromosome (haploid)
locus the location of a gene on a chromosome
(sometimes gene)
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More Important Terms
homologous chromosomes the two similar but
nonidentical chromosomes that make up a pair.
sex chromosomes in humans, X and Y chromosomes.
Not truly homologous
autosomes all other chromosomes
fertilization the uniting of two gametes to
form a zygote (n n goes to 2n)
meiosis the process of gametogenesis (2n goes
to n n n n)
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Describing Chromosomes
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Timing of Meiosis and Fertilization
Some organisms have free-living multicellular
haploid phases.
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Chromosomes and Meiosis
Meiosis reduces the chromosome number from
diploid to haploid.
First, the DNA in a diploid cell is duplicated in
S phase.
Cells resulting from the first meiotic division
are haploid duplicated.
The products of meiosis are haploid gametes.
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Meiosis I with Crossing Over
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Meiosis II -
The products of meiosis are genetically
different from each other and from the parent
cell.
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Mitosis and Meiosis
Meiosis differs from mitosis in the number of
divisions, the process of synapsis, and the
number and genetic composition of daughter cells
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Independent Assortment
Independent assortment of chromosomes during the
first meiotic division contributes to genetic
variation.
Here, there are 22 4 possible gametes
In humans, there are 223 about 8 million
possible gametes
Random fertilization allows for (8 million)2 64
trillion possible offspring from any two parents.
Crossing over occurs two to three times per
chromosome, and increases the number of possible
offspring even further.
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