Chapter 13 Meiosis
Heredity or inheritance, the transmission of traits from one generation to the
next.
Variation not genetic similar.
Genetics is the scientific study of heredity and hereditary variation.
Concept 13.1
The genes we inherit from our mothers and fathers are our genetic link to our
parents, and they account for family resemblances such as shared eye color or
freckles. Our genes program the specific traits that emerge as we develop from
fertilized eggs into adults.
The genes DNA the polymer of four different nucleotides.
The transmission of heredity traits has its molecular basis in the replication of
DNA, which produces copies of genes that can be passed from parents to
offspring.
In animals and plants, reproductive cells called gametes, are the vehicles that
transmit genes from one generation to the next.
Except for small amounts of DNA in mitochondria and chloroplasts, the DNA of a
eukaryotic cell is packaged into chromosomes within nucleus. Every species has
a characteristic number of chromosomes. (Human has 46 chromosomes)
Somatic cells all cells of the body except the gametes and their precursors.
A gene’s specific location along the length of a chromosome is called the gene’s
locus.
Asexual reproduction a single individual is the sole parent and passes
copies of all its genes to its offspring without the fusion of gametes.
An individual that reproduces asexually gives rise to a clone, a group of
genetically identical individuals. Genetic differences occasionally arise in
asexually reproducing organisms as a result of changes in the DNA called
mutations.
Sexual reproduction two parents give rise to off-spring that have unique
combinations of genes inherited from two parents.
Concept 13.2
A life cycle the generation-to-generation sequence of stages in reproductive
history of an organism, from conception to production of its own offspring.
Karyotype images of chromosomes are arranged in pairs, starting with the
longest chromosomes.
Homologous chromosomes/homologs the two chromosomes of a pair have
the same length, centromere position, and staining pattern.
Example: woman (XX) , men (XY)
Because they determine an individual’s sex, the X and Y chromosomes are called
sex chromosomes. The other chromosomes are called autosomes.
The number of chromosomes in a single set is represented by n, haploid cells.
Any cell with two chromosomes sets is called a diploid cell and has a diploid
number of chromosomes, abbreviated 2n.
In a cell in which DNA synthesis has occurred, all the chromosomes are
, duplicated, and therefore each consists of two identical sister chromatids,
associated closely at the centromere and along the arms.
The human life cycle begins with a haploid sperm from the father fuses with a
haploid egg from the mother. This union of gametes, culminating in fusion of their
nuclei, is called fertilization. The resulting fertilized egg, or zygote, is diploid
because it contains two haploid sets of chromosomes bearing genes representing
the maternal and paternal family lines.
The only cells of the human body not produced by mitosis are the gametes,
which develop from specialized cells called germ cells in the gonads – ovaries in
females and testes in males.
Meiosis this type of cell division reduces the number of sets of chromosomes
from two to one in the gametes, counterbalancing the doubling that occurs at
fertilization.
Plants and some species of algae exhibit a second type of life cycle called
alternation of generations. This type includes both diploid (sporophyte) and
haploid (gametophyte) stages that are multicellular.
Haploid and diploid can be mitosis but only diploid can be meiosis.
Concept 13.3
Meiosis 1 and meiosis 2 these two divisions result in four daughter cells,
each with half as many chromosomes as the parent cell – one set, rather than
two.
The overview of meiosis in figure 13.7 (page 313) shows, for a single pair of
homologues chromosomes in a diploid cell, that both members of the pair are
duplicated and the copies sorted into four haploid daughter cells. Recall that
sister chromatids are two copies of one chromosome, closely associated all along
their lengths; this association is called sister chromatid cohesion. Together, the
sister chromatids make up one duplicated chromosome. In contrast, the two
chromosomes that were inherited from different parents. Homologs appear alike
in the microscope, but they may have different versions of genes, each called an
allele, at corresponding loci. Homologs are not associated with each other in any
obvious way expect during meiosis. figure 13.8, page 314-315.
After interphase, the chromosomes have been duplicated and the sister
chromatids are held together by proteins called cohesins.
Next, the formation of a zipper-like structure called the synaptonemal complex,
holds one homolog tightly to the other. During this association, called synapsis,
the DNA breaks are closed up so that each broken end is joined to the
corresponding segment of the nonsister chromatid.
Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the
crossover point, and vice versa.
Figure 13.10 summarizes the key differences between meiosis and
mitosis in diploid cells. (page 316)
Heredity or inheritance, the transmission of traits from one generation to the
next.
Variation not genetic similar.
Genetics is the scientific study of heredity and hereditary variation.
Concept 13.1
The genes we inherit from our mothers and fathers are our genetic link to our
parents, and they account for family resemblances such as shared eye color or
freckles. Our genes program the specific traits that emerge as we develop from
fertilized eggs into adults.
The genes DNA the polymer of four different nucleotides.
The transmission of heredity traits has its molecular basis in the replication of
DNA, which produces copies of genes that can be passed from parents to
offspring.
In animals and plants, reproductive cells called gametes, are the vehicles that
transmit genes from one generation to the next.
Except for small amounts of DNA in mitochondria and chloroplasts, the DNA of a
eukaryotic cell is packaged into chromosomes within nucleus. Every species has
a characteristic number of chromosomes. (Human has 46 chromosomes)
Somatic cells all cells of the body except the gametes and their precursors.
A gene’s specific location along the length of a chromosome is called the gene’s
locus.
Asexual reproduction a single individual is the sole parent and passes
copies of all its genes to its offspring without the fusion of gametes.
An individual that reproduces asexually gives rise to a clone, a group of
genetically identical individuals. Genetic differences occasionally arise in
asexually reproducing organisms as a result of changes in the DNA called
mutations.
Sexual reproduction two parents give rise to off-spring that have unique
combinations of genes inherited from two parents.
Concept 13.2
A life cycle the generation-to-generation sequence of stages in reproductive
history of an organism, from conception to production of its own offspring.
Karyotype images of chromosomes are arranged in pairs, starting with the
longest chromosomes.
Homologous chromosomes/homologs the two chromosomes of a pair have
the same length, centromere position, and staining pattern.
Example: woman (XX) , men (XY)
Because they determine an individual’s sex, the X and Y chromosomes are called
sex chromosomes. The other chromosomes are called autosomes.
The number of chromosomes in a single set is represented by n, haploid cells.
Any cell with two chromosomes sets is called a diploid cell and has a diploid
number of chromosomes, abbreviated 2n.
In a cell in which DNA synthesis has occurred, all the chromosomes are
, duplicated, and therefore each consists of two identical sister chromatids,
associated closely at the centromere and along the arms.
The human life cycle begins with a haploid sperm from the father fuses with a
haploid egg from the mother. This union of gametes, culminating in fusion of their
nuclei, is called fertilization. The resulting fertilized egg, or zygote, is diploid
because it contains two haploid sets of chromosomes bearing genes representing
the maternal and paternal family lines.
The only cells of the human body not produced by mitosis are the gametes,
which develop from specialized cells called germ cells in the gonads – ovaries in
females and testes in males.
Meiosis this type of cell division reduces the number of sets of chromosomes
from two to one in the gametes, counterbalancing the doubling that occurs at
fertilization.
Plants and some species of algae exhibit a second type of life cycle called
alternation of generations. This type includes both diploid (sporophyte) and
haploid (gametophyte) stages that are multicellular.
Haploid and diploid can be mitosis but only diploid can be meiosis.
Concept 13.3
Meiosis 1 and meiosis 2 these two divisions result in four daughter cells,
each with half as many chromosomes as the parent cell – one set, rather than
two.
The overview of meiosis in figure 13.7 (page 313) shows, for a single pair of
homologues chromosomes in a diploid cell, that both members of the pair are
duplicated and the copies sorted into four haploid daughter cells. Recall that
sister chromatids are two copies of one chromosome, closely associated all along
their lengths; this association is called sister chromatid cohesion. Together, the
sister chromatids make up one duplicated chromosome. In contrast, the two
chromosomes that were inherited from different parents. Homologs appear alike
in the microscope, but they may have different versions of genes, each called an
allele, at corresponding loci. Homologs are not associated with each other in any
obvious way expect during meiosis. figure 13.8, page 314-315.
After interphase, the chromosomes have been duplicated and the sister
chromatids are held together by proteins called cohesins.
Next, the formation of a zipper-like structure called the synaptonemal complex,
holds one homolog tightly to the other. During this association, called synapsis,
the DNA breaks are closed up so that each broken end is joined to the
corresponding segment of the nonsister chromatid.
Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the
crossover point, and vice versa.
Figure 13.10 summarizes the key differences between meiosis and
mitosis in diploid cells. (page 316)
