Chapter 13 Meiosis
and Sexual Life Cycles
Lecture Outline
Overview: Hereditary
Similarity and Variation
·
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.
·
Farmers
have bred plants and animals for desired traits for thousands of years, but the
mechanisms of heredity and variation eluded biologists until the development of
genetics in the 20th century.
·
Genetics
is the scientific study of heredity and variation.
Concept 13.1 Offspring acquire genes from parents
by inheriting chromosomes
·
Parents
endow their offspring with coded information in the form of genes.
°
Your
genome is comprised of the tens of 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.
·
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 language 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 whose
cumulative action produces 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 and an ovum), genes from both parents are
present in the nucleus of the fertilized egg, or zygote.
·
Almost
all the DNA in a eukaryotic cell is subdivided into chromosomes in the nucleus.
°
Tiny
amounts of DNA are also found in mitochondria and chloroplasts.
·
Every
living species has a characteristic number of chromosomes.
°
Humans
have 46 chromosomes in almost all of their cells.
·
Each
chromosome consists of a single DNA molecule associated with various proteins.
·
Each
chromosome has hundreds or thousands of genes, each at a specific location, its
locus.
Like begets like, more or less: a comparison
of asexual and sexual reproduction.
·
Only
organisms that reproduce asexually can produce offspring that are exact copies
of themselves.
·
In
asexual reproduction, a single
individual is the sole parent to donate genes to its offspring.
°
Single-celled
eukaryotes can reproduce asexually by mitotic cell division to produce two genetically
identical daughter cells.
°
Some
multicellular eukaryotes, like Hydra,
can reproduce by budding, producing a mass of cells by mitosis.
·
An
individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals.
°
Members
of a clone may be genetically different as a result of mutation.
·
In
sexual reproduction, two parents
produce offspring that have unique combinations of genes inherited from the two
parents.
·
Unlike
a clone, offspring produced by sexual reproduction vary genetically from their
siblings and their parents.
Concept 13.2 Fertilization and meiosis alternate in sexual life
cycles
·
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 the organism
produces its own offspring.
Human cells contain sets of chromosomes.
·
In
humans, each somatic cell (all cells
other than sperm or ovum) has 46 chromosomes.
°
Each
chromosome can be distinguished by size, position of the centromere, and
pattern of staining with certain dyes.
·
Images
of the 46 human chromosomes can be arranged in pairs in order of size to
produce a karyotype display.
°
The
two chromosomes comprising a pair have the same length, centromere position,
and staining pattern.
°
These
homologous chromosome pairs carry
genes that control the same inherited characters.
·
Two
distinct sex chromosomes, the X and
the Y, are an exception to the general pattern of homologous chromosomes in
human somatic cells.
·
The
other 22 pairs are called autosomes.
·
The
pattern of inheritance of the sex 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).
·
Only
small parts of the X and Y are homologous.
°
Most
of the genes carried on the X chromosome do not have counterparts on the tiny
Y.
°
The
Y chromosome also has genes not present on the X.
·
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 each somatic cell are two sets of 23, a maternal set (from
your mother) and a paternal set (from your father).
·
The
number of chromosomes in a single set is represented by n.
·
Any
cell with two sets of chromosomes is called a diploid cell and has a diploid number of chromosomes, abbreviated
as 2n.
·
Sperm
cells or ova (gametes) have only one
set of chromosomes—22 autosomes and an X (in an ovum) and 22 autosomes and an X
or a Y (in a sperm cell).
·
A
gamete with a single chromosome set is haploid,
abbreviated as n.
·
Any
sexually reproducing species has a characteristic haploid and diploid number of
chromosomes.
°
For
humans, the haploid number of chromosomes is 23 (n = 23), and the diploid number is 46 (2n = 46).
Let’s discuss the role of meiosis in the human
life cycle.
·
The
human life cycle begins when a haploid sperm cell fuses with a haploid ovum.
·
These
cells fuse (syngamy), resulting in fertilization.
·
The
fertilized egg (zygote) is diploid
because it contains two haploid sets of chromosomes bearing genes from the
maternal and paternal family lines.
·
As
an organism develops from a zygote to a sexually mature adult, mitosis
generates all the somatic cells of the body.
°
Each
somatic cell contains a full diploid set of chromosomes.
·
Gametes,
which develop in the gonads (testes or ovaries), 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.
Organisms display a variety of sexual life
cycles.
·
Fertilization
and meiosis alternate in all sexual life cycles.
·
However,
the timing of meiosis and fertilization does vary among species.
·
These
variations can be grouped into three main types of life cycles.
·
In
most animals, including humans, gametes are the only haploid cells.
°
Gametes
do not divide but fuse to form a diploid zygote that divides by mitosis to
produce a multicellular organism.
·
Plants
and some algae have a second type of life cycle called alternation of generations.
°
This
life cycle includes two multicellular stages, one haploid and one diploid.
°
The
multicellular diploid stage is called the sporophyte.
°
Meiosis
in the sporophyte produces haploid spores
that develop by mitosis into the haploid gametophyte
stage.
°
Gametes
produced via mitosis by the gametophyte fuse to form the zygote, which grows
into the sporophyte by mitosis.
·
Most
fungi and some protists have a third type of life cycle.
°
Gametes
fuse to form a zygote, which is the only diploid phase.
°
The
zygote undergoes meiosis to produce haploid cells.
°
These
haploid cells grow by mitosis to form the haploid multicellular adult organism.
°
The
haploid adult produces gametes by mitosis.
·
Note
that either haploid or diploid cells can divide by mitosis, depending on the
type of life cycle. However, only diploid cells can undergo meiosis.
·
Although
the three types of sexual life cycles differ in the timing of meiosis and
fertilization, they share a fundamental feature: each cycle of chromosome
halving and doubling contributes to genetic variation among offspring.
Concept 13.3 Meiosis reduces the number of
chromosome sets from diploid to haploid
·
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,
resulting in four daughter cells.
°
The
first division, meiosis I, separates homologous chromosomes.
°
The
second, meiosis II, separates sister chromatids.
·
The
four daughter cells have only half as many chromosomes as the parent cell.
·
Meiosis
I is preceded by interphase, in
which the chromosomes are replicated to form sister chromatids.
°
These
are genetically identical and joined at the centromere.
°
The
single centrosome is replicated, forming two centrosomes.
·
Division
in meiosis I occurs in four phases: prophase I, metaphase I, anaphase I, and
telophase I.
Prophase
I
·
Prophase
I typically occupies more than 90% of the time required for meiosis.
·
During
prophase I, the chromosomes begin to condense.
·
Homologous
chromosomes loosely pair up along their length, precisely aligned gene for
gene.
°
In
crossing over, DNA molecules in nonsister chromatids break at corresponding
places and then rejoin the other chromatid.
°
In
synapsis, a protein structure called the synaptonemal complex forms between
homologues, holding them tightly together along their length.
°
As
the synaptonemal complex disassembles in late prophase, each chromosome pair
becomes visible as a tetrad, or
group of four chromatids.
°
Each
tetrad has one or more chiasmata,
sites where the chromatids of homologous chromosomes have crossed and segments
of the chromatids have been traded.
°
Spindle
microtubules form from the centrosomes, which have moved to the poles.
°
The
breakdown of the nuclear envelope and nucleoli take place.
°
Kinetochores
of each homologue attach to microtubules from one of the poles.
Metaphase
I
·
At
metaphase I, the tetrads are all arranged at the metaphase plate, with one
chromosome facing each pole.
°
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.
Anaphase
I
·
In
anaphase I, the homologous chromosomes separate. One chromosome moves toward
each pole, guided by the spindle apparatus.
·
Sister
chromatids remain attached at the centromere and move as a single unit toward
the pole.
Telophase
I and cytokinesis
·
In
telophase I, movement of homologous chromosomes continues until there is a
haploid set at each pole.
°
Each
chromosome consists of two sister chromatids.
·
Cytokinesis
usually occurs simultaneously, by the same mechanisms as mitosis.
°
In
animal cells, a cleavage furrow forms. In plant cells, a cell plate forms.
·
No
chromosome replication occurs between the end of meiosis I and the beginning of
meiosis II, as the chromosomes are already replicated.
Meiosis
II
·
Meiosis
II is very similar to mitosis.
°
During
prophase II, a spindle apparatus forms and attaches to kinetochores of each
sister chromatid.
§
Spindle
fibers from one pole attach to the kinetochore of one sister chromatid, and
those of the other pole attach to kinetochore of the other sister chromatid.
·
At
metaphase II, the sister chromatids are arranged at the metaphase plate.
°
Because
of crossing over in meiosis I, the two sister chromatids of each chromosome are
no longer genetically identical.
°
The
kinetochores of sister chromatids attach to microtubules extending from
opposite poles.
·
At
anaphase II, the centomeres of sister chromatids separate and two newly
individual chromosomes travel toward opposite poles.
·
In
telophase II, the chromosomes arrive at opposite poles.
°
Nuclei
form around the chromosomes, which begin expanding, and cytokinesis separates
the cytoplasm.
·
At
the end of meiosis, there are four haploid daughter cells.
There are key differences between mitosis and
meiosis.
·
Mitosis
and meiosis have several key differences.
°
The
chromosome number is reduced from diploid to haploid in meiosis but is
conserved in mitosis.
°
Mitosis
produces daughter cells that are genetically identical to the parent and to
each other.
°
Meiosis
produces cells that are genetically distinct from the parent cell and from each
other.
·
Three
events, unique to meiosis, occur during the first division cycle.
1. During prophase I of
meiosis, replicated homologous chromosomes line up and become physically
connected along their lengths by a zipperlike protein complex, the synaptonemal
complex, in a process called synapsis. Genetic rearrangement between nonsister
chromatids called crossing over also occurs. Once the synaptonemal complex is
disassembled, the joined homologous chromosomes are visible as a tetrad.
X-shaped regions called chiasmata are visible as the physical manifestation of
crossing over. Synapsis and crossing over do not occur in mitosis.
2. At metaphase I of meiosis,
homologous pairs of chromosomes align along the metaphase plate. In mitosis,
individual replicated chromosomes line up along the metaphase plate.
3. At anaphase I of meiosis,
it is homologous chromosomes, not sister chromatids, that separate and are
carried to opposite poles of the cell. Sister chromatids of each replicated
chromosome remain attached. In mitosis, sister chromatids separate to become
individual chromosomes.
·
Meiosis
I is called the reductional division
because it halves the number of chromosome sets per cell—a reduction from the
diploid to the haploid state.
·
The
sister chromatids separate during the second meiosis division, meiosis II.
Concept 13.4 Genetic variation produced in sexual life cycles
contributes to evolution
·
What
is the origin of genetic variation?
·
Mutations
are the original source of genetic diversity.
·
Once
different versions of genes arise through mutation, reshuffling during meiosis
and fertilization produce offspring with their own unique set of traits.
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 in each generation.
·
Three
mechanisms contribute to genetic variation:
1. Independent assortment of
chromosomes.
2. Crossing over.
3. Random fertilization.
·
Independent assortment of
chromosomes
contributes to genetic variability due to the random orientation of homologous
pairs of chromosomes at the metaphase plate during meiosis I.
°
There
is a fifty-fifty chance that a particular daughter cell of meiosis I will get
the maternal chromosome of a certain homologous pair and a fifty-fifty chance
that it will receive the paternal chromosome.
·
Each
homologous pair of chromosomes segregates independently of the other homologous
pairs during metaphase I.
·
Therefore,
the first meiotic division results in independent assortment of maternal and
paternal chromosomes into daughter cells.
·
The
number of combinations possible when chromosomes assort independently into
gametes is 2n, where n is the haploid number of the organism.
°
If
n = 3, there are 23 = 8
possible combinations.
°
For
humans with n = 23, there are 223,
or more than 8 million possible combinations of chromosomes.
·
Crossing over produces recombinant chromosomes, which combine
genes inherited from each parent.
·
Crossing
over begins very early in prophase I as homologous chromosomes pair up gene by
gene.
·
In
crossing over, homologous portions of two nonsister chromatids trade places.
°
For
humans, this occurs an average of one to three times per chromosome pair.
·
Recent
research suggests that, in some organisms, crossing over may be essential for
synapsis and the proper assortment of chromosomes in meiosis I.
·
Crossing
over, by combining DNA inherited from two parents into a single chromosome, is
an important source of genetic variation.
·
At
metaphase II, nonidentical sister chromatids sort independently from one
another, increasing by even more the number of genetic types of daughter cells
that are formed by meiosis.
·
The
random nature of fertilization adds
to the genetic variation arising from meiosis.
·
Any
sperm can fuse with any egg.
°
The
ovum is one of more than 8 million possible chromosome combinations.
°
The
successful sperm is one of more than 8 million possibilities.
°
The
resulting zygote could contain any one of more than 70 trillion possible
combinations of chromosomes.
°
Crossing
over adds even more variation to this.
·
Each
zygote has a unique genetic identity.
·
The
three sources of genetic variability in a sexually reproducing organism are:
1. Independent assortment of
homologous chromosomes during meiosis I and of nonidentical sister chromatids
during meiosis II.
2. Crossing over between
homologous chromosomes during prophase I.
3. Random fertilization of an
ovum by a sperm.
·
All
three mechanisms reshuffle the various genes carried by individual members of a
population.
Evolutionary adaptation depends on a
population’s genetic variation.
·
°
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.
·
If
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 mutation continually generate new genetic variability.
·
Although
·
Gregor
Mendel, a contemporary of
°
However,
this work was largely unknown until 1900, after Darwin and Mendel had both been
dead for more than 15 years.