Meiosis
15 October, 2004
Text Chapter 13
Asexual
Reproduction
In asexual reproduction,
individuals give rise to genetically
identical offspring (clones). All
cell division in this process is
mitotic.
Important Terms (review)
genome: all of a cell’s 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)
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)
Describing Chromosomes
Timing of Meiosis
and Fertilization
Some organisms
have free-living
multicellular
haploid phases.
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.
Meiosis I with Crossing Over
Meiosis II - The products of meiosis are genetically different from
each other and from the parent cell.
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
Independent assortment of
chromosomes during the first
meiotic division contributes to
genetic variation.
Independent Assortment
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.