CHAPTER
13
MEIOSIS AND SEXUAL LIFE CYCLES
·
Living
organisms are distinguished by their ability to reproduce their own kind.
·
Offspring
resemble their parents more than they do less closely related individuals of
the same species.
·
The
transmission of traits from one generation to the next is called heredity or inheritance.
·
However,
offspring differ somewhat from parents and siblings, demonstrating variation.
· Genetics is the study of heredity and variation.
1. Offspring acquire genes from parents by
inheriting chromosomes
·
Parents
endow their offspring with coded information in the form of genes.
·
Your
genome is derived from the thousands of genes that you inherited from your
mother and your father.
·
Genes
program specific traits that emerge as we develop from fertilized eggs into
adults.
·
Your
genome may include a gene for freckles, which you inherited from your mother.
·
Genes
are segments of DNA.
·
Genetic
information is transmitted as specific sequences of the four
deoxyribonucleotides in DNA.
·
This
is analogous to the symbolic information of letters in which words and
sentences are translated into mental images.
·
Cells
translate genetic “sentences” into freckles and other features with no
resemblance to genes.
·
Most
genes program cells to synthesize specific enzymes and other proteins that
produce an organism’s inherited traits.
·
The
transmission of hereditary traits has its molecular basis in the precise
replication of DNA.
·
This
produces copies of genes that can be passed from parents to offspring.
·
In
plants and animals, sperm and ova (unfertilized eggs) transmit genes from one
generation to the next.
·
After
fertilization (fusion) of a sperm cell with an ovum, genes from both parents are
present in the nucleus of the fertilized egg.
·
Almost
all of the DNA in a eukaryotic cell is subdivided into chromosomes in the
nucleus.
·
Tiny
amounts of DNA are found in mitochondria and chloroplasts.
·
Every
living species has a characteristic number of chromosomes.
·
Humans
have 46 in almost all of their cells.
·
Each
chromosome consists of a single DNA molecule in association with various
proteins.
·
Each
chromosome has hundreds or thousands of genes, each at a specific location, its
locus.
2. Like begets like, more or less: a comparison of
asexual and sexual reproduction
·
In
asexual reproduction, a single
individual passes along copies of all its genes to its offspring.
·
Single-celled
eukaryotes reproduce asexually by mitotic cell division to produce two identical
daughter cells.
·
Even
some multicellular eukaryotes, like hydra, can reproduce by budding cells
produced by mitosis.
·
Sexual reproduction results in greater
variation among offspring than does asexual reproduction.
·
Two
parents give rise to offspring that have unique combinations of genes inherited
from the parents.
·
Offspring
of sexual reproduction vary genetically from their siblings and from both
parents.
·
A
life cycle is the
generation-to-generation sequence of stages in the reproductive history of an
organism.
· It starts at the conception of an organism and continues until it produces its own offspring.
1. Fertilization and meiosis alternate in sexual
life cycles
·
In
humans, each somatic cell (all cells
other than sperm or ovum) has 46 chromosomes.
·
Each
chromosome can be distinguished by its size, position of the centromere, and by
pattern of staining with certain dyes.
·
A
karyotype display of the 46
chromosomes shows 23 pairs of chromosomes, each pair with the same length,
centromere position, and staining pattern.
·
These
homologous chromosome pairs carry
genes that control the same inherited characters.
·
Karyotypes,
ordered displays of an individual’s chromosomes, are often prepared with
lymphocytes.
·
An
exception to the rule of homologous chromosomes is found in the sex chromosomes, the X and the Y.
·
The
pattern of inheritance of these chromosomes determines an individual’s sex.
·
Human
females have a homologous pair of X chromosomes (XX).
·
Human
males have an X and a Y chromosome (XY).
·
Because
only small parts of these have the same genes, most of their genes have no
counterpart on the other chromosome.
·
The
other 22 pairs are called autosomes.
·
The
occurrence of homologous pairs of chromosomes is a consequence of sexual
reproduction.
·
We
inherit one chromosome of each homologous pair from each parent.
·
The
46 chromosomes in a somatic cell can be viewed as two sets of 23, a maternal
set and a paternal set.
·
Sperm
cells or ova (gametes) have only one
set of chromosomes — 22 autosomes and an X or a Y.
·
A
cell with a single chromosome set is haploid.
·
For
humans, the haploid number of chromosomes is 23 (n = 23).
·
By
means of sexual intercourse, a haploid sperm reaches and fuses with a haploid
ovum.
·
These
cells fuse (syngamy) resulting in fertilization.
·
The
fertilized egg (zygote) now has two
haploid sets of chromosomes bearing genes from the maternal and paternal family
lines.
·
The
zygote and all cells with two sets of chromosomes are diploid cells.
·
For
humans, the diploid number of chromosomes is 46 (2n = 46).
·
As
an organism develops from a zygote to a sexually mature adult, the zygote’s
genes are passed on to all somatic cells by mitosis.
·
Gametes,
which develop in the gonads, are not
produced by mitosis.
·
If
gametes were produced by mitosis, the fusion of gametes would produce offspring
with four sets of chromosomes after one generation, eight after a second and so
on.
·
Instead,
gametes undergo the process of meiosis
in which the chromosome number is halved.
·
Human
sperm or ova have a haploid set of 23 different chromosomes, one from each
homologous pair.
·
Fertilization
restores the diploid condition by combining two haploid sets of chromosomes.
·
Fertilization
and meiosis alternate in sexual life cycles.
·
The
timing of meiosis and fertilization does vary among species.
·
The
life cycle of humans and other animals is typical of one major type.
·
Gametes,
produced by meiosis, are the only haploid cells.
·
Gametes
undergo no divisions themselves, but fuse to form a diploid zygote that divides
by mitosis to produce a multicellular organism.
·
Most
fungi and some protists have a second type of life cycle.
·
The
zygote is the only diploid phase.
·
After
fusion of two gametes to form a zygote, the zygote undergoes meiosis to produce
haploid cells.
·
These
haploid cells undergo mitosis to develop into a haploid multicellular adult
organism.
·
Some
haploid cells develop into gametes by mitosis.
·
Plants
and some algae have a third type of life cycle, alternation of generations.
·
This
life cycle includes both haploid (gametophyte)
and diploid (sporophyte)
multicellular stages.
·
Meiosis
by the sporophyte produces haploid spores
that develop by mitosis into the gametophyte.
·
Gametes
produced via mitosis by the gametophyte fuse to form the zygote which produces
the sporophyte by mitosis.
3. Meiosis
reduces chromosome number from diploid to haploid: a closer look
·
Many
steps of meiosis resemble steps in mitosis.
·
Both
are preceded by the replication of chromosomes.
·
However,
in meiosis, there are two consecutive cell divisions, meiosis I and meiosis II,
that result in four daughter cells.
·
Each
final daughter cell has only half as many chromosomes as the parent cell.
·
Meiosis
reduces chromosome number by copying the chromosomes once, but dividing twice.
·
The
first division, meiosis I, separates homologous chromosomes.
·
The
second, meiosis II, separates sister chromatids.
·
Division
in meiosis I occurs in four phases: prophase, metaphase, anaphase, and
telophase.
·
During
the preceding interphase the chromosomes are replicated to form sister
chromatids.
·
These
are genetically identical and joined at the centromere.
·
Also,
the single centrosome is replicated.
·
In
prophase I, the chromosomes condense and homologous chromosomes pair up to form
tetrads.
·
In
a process called synapsis, special proteins attach homologous chromosomes
tightly together.
·
At
several sites the chromatids of homologous chromosomes are crossed (chiasmata)
and segments of the chromosomes are traded.
·
A
spindle forms from each centrosome and spindle fibers attached to kinetochores
on the chromosomes begin to move the tetrads around.
·
At
metaphase I, the tetrads are all arranged at the metaphase plate.
·
Microtubules
from one pole are attached to the kinetochore of one chromosome of each tetrad,
while those from the other pole are attached to the other.
·
In
anaphase I, the homologous chromosomes separate and are pulled toward opposite
poles.
·
In
telophase I, movement of homologous chromosomes continues until there is a
haploid set at each pole.
·
Each
chromosome consists of linked sister chromatids.
·
Cytokinesis
by the same mechanisms as mitosis usually occurs simultaneously.
·
In
some species, nuclei may reform, but there is no further replication of
chromosomes.
·
Meiosis
II is very similar to mitosis.
·
During
prophase II a spindle apparatus forms, attaches to kinetochores of each sister
chromatid, and moves them around.
·
Spindle
fibers from one pole attach to the kinetochore of one sister chromatid and those
of the other pole to the other sister chromatid.
·
At
metaphase II, the sister chromatids are arranged at the metaphase plate.
·
The
kinetochores of sister chromatids face opposite poles.
·
At
anaphase II, the centomeres of sister chromatids separate and the now separate
sisters travel toward opposite poles.
·
In
telophase II, separated sister chromatids arrive at opposite poles.
·
Nuclei
form around the chromatids.
·
Cytokinesis
separates the cytoplasm.
·
At
the end of meiosis, there are four haploid daughter cells.
·
Mitosis
and meiosis have several key differences.
·
The
chromosome number is reduced by half in meiosis, but not in mitosis.
·
Mitosis
produces daughter cells that are genetically identical to the parent and to
each other.
·
Meiosis
produces cells that differ from the parent and each other.
·
Three
events, unique to meiosis, occur during the first division cycle.
·
1) During prophase I, homologous chromosomes
pair up in a process called synapsis.
·
A
protein zipper, the synaptonemal complex,
holds homologous chromosomes together tightly.
·
Later
in prophase I, the joined homologous chromosomes are visible as a tetrad.
·
At
X-shaped regions called chiasmata,
sections of nonsister chromatids are exchanged.
·
Chiasmata
is the physical manifestation of crossing over, a form of genetic
rearrangement.
• 2) At metaphase I homologous pairs of chromosomes, not individual
chromosomes are aligned along the metaphase plate.
·
In
humans, you would see 23 tetrads.
• 3) At anaphase I, it is homologous chromosomes, not sister chromatids,
that separate and are carried to opposite poles of the cell.
·
Sister
chromatids remain attached at the centromere until anaphase II.
·
The
processes during the second meiotic division are virtually identical to those
of mitosis.
·
Mitosis
produces two identical daughter cells, but meiosis produces 4 very different
cells.
1. Sexual life cycles produce genetic variation among offspring
·
The
behavior of chromosomes during meiosis and fertilization is responsible for
most of the variation that arises each generation during sexual reproduction.
·
Three
mechanisms contribute to genetic variation:
• Independent assortment.
• Crossing over.
• Random fertilization.
·
All
three mechanisms reshuffle the various genes carried by individual members of a
population.
·
Mutations
are what ultimately create a population’s diversity of genes.
2. Evolutionary adaptation depends on a population’s
genetic variation
·
Darwin
recognized the importance of genetic variation in evolution via natural
selection.
·
A
population evolves through the differential reproductive success of its variant
members.
·
Those
individuals best suited to the local environment leave the most offspring,
transmitting their genes in the process.
·
This
natural selection results in adaptation, the accumulation of favorable genetic
variations.
·
As
the environment changes or a population moves to a new environment, new genetic
combinations that work best in the new conditions will produce more offspring
and these genes will increase.
·
The
formerly favored genes will decrease.
·
Sex
and mutations are two sources of the continual generation of new genetic
variability.
·
Gregor
Mendel, a contemporary of Darwin, published a theory of inheritance that helps
explain genetic variation.
·
However,
this work was largely unknown for over 40 years until 1900.