4.2: Sexual Reproduction
pg. 169
Asexual Reproduction is reproduction that requires only one
parent and produces genetically identical offspring.
Sexual Reproduction is reproduction that requires two parents
and produces genetically distinct offspring.
Figure 4.12 When gametes combine in fertilization, the resulting cell is diploid.
Haploid ad Diploid Cells in Sexual Reproduction
Gametes are the male or female reproductive cells (haploid).
Zygote is a cell formed by the fusion of two gametes.
Fertilization occurs in humans, the joining of male and female
haploid gametes.
Haploid is a cell that contains half the number of chromosomes as
the parent cell.
Diploid is a cell that contains pairs of homologous chromosomes.
During sexual reproduction the male sex gamete (sperm) fuses
with the female sex gamete (ovum) to create a zygote. The process
of the male and female sex cells fusing is called fertilization.
Each of the sex gametes are haploid (23 chromosomes) cells, and
when the zygote is formed the cell is now diploid (23 pairs of
chromosomes).
Sperm
Ovum
Human
= n = 23 chromosomes
= n = 23 chromosomes
= 2n = 23 pairs of chromosomes (46)
Meiosis – Producing Haploid Gametes
The process that produces gametes with a haploid number of
chromosomes is called meiosis.
Two key outcomes of Meiosis;
1. Genetic Reduction: Meiosis is a form of cell division that
produces daughter cells with half the number of
chromosomes of the parent cell.
2. Genetic Recombination: The products of meiosis have
different combinations of alleles. Genetic recombination
gives rise to offspring that are genetically different from
one another and their parents. This greatly increases the
genetic variation in a population.
Interphase
During this stage the cell grows, functions and replicates its DNA.
The content of genetic information is now 46 sister pairs of
chromatids or 92 individual chromosomes.
Phases of Meiosis
Meiosis is divided into two stages, Meiosis I and Meiosis II, each
having four phases. Each phase is identified by its name and than
follow by a roman numeral to identify the stage it occurs in.
Stage:
Meiosis I: responsible for dividing the genetic material
in half (haploid), but each cell will still contain 46
chromosomes.
Prophase I: The nuclear membrane disappears, spindle
fibres appear and centrosomes migrate to the poles.
Homologous align themselves across from each other
near the equator. Here a synapsis can occur, where
genetic material maybe exchanged, this called Crossing
Over. This leads to genetic diversity.
Metaphase I: Here the homologous pairs are line up a
long the equator. Spindle fibres appear to be attached
to the centrosomes of the chromatids.
Anaphase I: At this point the spindle fibres pull
homologous to opposite poles from the equator. The
sister chromatids do not separate at this point. The
chromosome number is reduced from 2n (diploid) to 1n
(haploid).
Telophase I: Homologous chromosomes begin to
uncoil and the spindle fibres disappear. Cytokinesis
starts and the two (haploid) cells are created,
containing 46 chromosomes, and the genetic material is
enclosed in a nuclear membrane.
Stage:
Meiosis II: is responsible for the reduction of the
number of chromosomes from 46 chromosomes to 23
chromosomes.
Prophase II: The nuclear membrane disappears,
spindle fibres appear and centrosomes migrate to the
poles. Each chromatid pair aligns themselves along the
equator.
Metaphase II: Here the chromatid pairs are lined up a
long the equator. Spindle fibres appear to be attached
to the centrosomes of each chromatid pairs.
Anaphase II: At this point the spindle fibres pull
chromatid pairs apart and chromosomes travel to
opposite poles from the equator. The chromosome
number is reduced from 46 to 23 chromosomes.
Telophase II: the 23 chromosomes begin to uncoil and
the spindle fibres disappear. Cytokinesis starts and the
two (haploid) cells are created, and the genetic
material is enclosed in a nuclear membrane.
Figure 4.13 Meiosis involves two complete cycles of four phases. Notice that each cell
contains some chromosomes from the mother (yellow), some chromosomes from the
father (blue), and some chromosomes with segments that have been exchanged (yellow
and blue).
A comparison of Mitosis and Meiosis
Mitosis
Meiosis
One stage/one division
Produces two diploid cells
Identical to the parent cell
Two stages/two divisions
Produces four haploid cells
Non-identical cells
Figure 4.14 Comparing mitosis and meiosis can help to understand key differences
between the two processes.
Learning Check, questions 7 – 12, page 172
Gamete Formation in Animals
Spermatogenesis is the process of producing male gametes (sperm)
in mammals.
Spermatogenesis takes place in the testes of the male organism.
Spermatogonia cells found in the testes are responsible for sperm
production. The process of meiosis creates four haploid cells
expressing genetic diversity. These immature sperms cells move to
the epididymis of the testes where they mature into fully developed
cells. The head region contains the genetic information; the
midsection contains a large number of mitochondria to supply
energy, and then the tail section, flagellum for locomotion.
Oogenesis is the process of producing female gametes (eggs) in
mammals.
Oogenesis occurs in the ovaries of the female organism. Oogonium
cells are diploid, before birth they start meiosis, but only get as far
as prophase I. Once the female reaches puberty, one cell once a
month will start from prophase I to produce one mature ovum,
which is haploid. This is an uneven division between four cells.
The one cell that receives the most cytoplasm and nutrients will
develop as the mature ovum. The other three cells form the polar
body. Meiosis II will not finish unless the ovum is fertilized.
Figure 4.15: In spermatogenesis, four haploid sperm cells form from one diploid cell.
Multiple Births
Multiple births can occur when more then one is release at time
and these eggs are fertilized by different sperm. This creates
fraternal twins.
Identical twining occurs when only one egg is release and
fertilized by one sperm, but divides into two separate zygotes.
Identical twins are genetically identical to each other.
The Importance of Meiosis for Genetic Variation
Meiosis is responsible for developing haploid cells that are
genetically diverse from each other. Genetic variation is ensured in
two ways:
- By the creation of gametes that carry different combinations
of maternal and paternal chromosomes. This is done by
independent assortment.
- By the exchange of genetic material between maternal and
paternal chromosomes. This is done by crossing over.
Independent Assortment
Figure 4.18: The diploid cell for this organism has three chromosome pairs. The
potential combinations of chromosomes produce eight genetically different gametes
(23 = 8).
In metaphase I the homologous chromosomes are align along the
equator. At this point depending one the homologous have aligned
different combinations are drawn to opposite poles. The number of
possible genetically distinct gametes can be tabulated by using 2n,
where n represents the number of chromosomes pairs in a diploid
cell. A human has 23chromosome in its diploid cell.
223 = 8 388 608 combinations for a human gamete.
Crossing Over
Crossing over occurs when homologous pairs align beside each
other during prophase I. When non-sister chromatid cross over and
switches genetic information with its homologue (maternal vs.
paternal), new genetic variations are created.
Figure 4.19: During synapsis in prophase I, non-sister chromatids cross over and
exchange segments of DNA to produce a new combination of genes on a chromosome.
Creating Genetic Variation
1. Synapsis (Crossing Over)
2. Independent Assortment
3. Fertilization
Errors during Meiosis
The two processes that lead to genetic variation during meiosis can
also lead to errors. Most gametes do not survive, but in some
situations the gamete does survive. If these situation if fertilization
occurs, a zygote is produced with a genetic error. There are two
situations that can occur that leads to errors;
a) change in chromosome structure, and b) change in
chromosome number.
Errors Caused by Changes in Chromosomes Structure
1) Deletion: a piece of chromosome is deleted.
2) Duplication: a section of a chromosome appears two or
three times in a row.
3) Inversion: a section of a chromosome is inverted.
4) Translocation: a segment of one chromosome becomes
attached to a different chromosome.
Table 4.1: Chromosome Structural Errors
Errors Caused by Changes in Chromosome Number
This type of error occurs because of non-disjunction, where the
homologous pairs fail to separate during prophase I or prophase II.
Figure 4.20: Non-disjunction results in gametes with too many or too few chromosomes.
Non-disjunction may take place during anaphase I (A) or anaphase II (B).
Genetic Disorders Associated with Chromosome Number
The have been a number of genetic disorders identified, which
have caused by an incorrect number of chromosomes present.
Down syndrome is an example of having 3 copies of chromosome
number 23. The individual has 47 chromosomes instead of 46.
This is a Trisomic situation caused by non-disjunction of
chromosome 21 in one of the gametes. This can occur in 1 in 1490
births, at a young age (20 – 24), to 1 in 106 births at age 40, and 1
in 11 births for woman around 49.
Figure 4.21: Many cases of Down syndrome are due to the individual having an extra
chromosome 21.
Trisomies and Monosomies (see Table 4.2 Chromosomal Abnormalities)
Monosomy is the loss of a chromosome as a result of nondisjunction.
Turner syndrome involves a missing X chromosome.
Trisomy is the gain of an extra chromosome as a result of nondisjunction.
This is common for chromosomes 13, 18, and 21, and sex
chromosomes.
There are not therapies or cures for these disorders.
Prenatal Genetic Testing
Prenatal testing is testing performed on the fetus, looking for
genetic abnormalities. These tests are only performed on
pregnant women in high-risk situations, such as; nondisjunction disorders in women over the age of 35, or women
with a family history of genetic disorders.
Decisions that are made after the test results are received are
very difficult and complicated. This results decisions, such as;
pregnancy termination and potential discrimination against
persons with disabilities.
Prenatal Testing Procedures
Figure 4.22: In amniocentesis and chorionic villus sampling, chromosome abnormalities,
genetic disorders, and certain malformations of the spine and brain are monitored.
Section 4.2 Review, questions 1 – 16, page 181