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4.2: Meiosis and Sexual Life Cycles


4.2: Meiosis and Sexual Life Cycles The process of creating sex cells i.e.: Eggs (females) Sperm (males) – PowerPoint PPT presentation

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Title: 4.2: Meiosis and Sexual Life Cycles

4.2 Meiosis and Sexual Life Cycles
  • The process of creating sex cells
  • i.e. Eggs (females)
  • Sperm (males)

Cell Reproduction
  • Asexual (vegetative) reproduction
  • A form of duplication using only mitosis.
  • Example, a new plant grows out of the root or a
    shoot from an existing plant.
  • Produces only genetically identical offspring
    since all divisions are by mitosis.
  • Offspring called clones meaning that each is an
    exact copy of the original organism
  • This method of reproduction is rapid and
    effective allowing the spread of an organism
  • Since the offspring are identical, there is no
    mechanism for introducing diversity.

Sexual reproduction
  • Formation of new individual by a combination of
    two sex cells (gametes).
  • Fertilization- combination of genetic information
    from two separate cells
  • Gametes for fertilization usually come from
    separate parents
  • Female- produces an egg
  • Male produces sperm
  • Both gametes are haploid, with a single set of
  • The new individual is called a zygote, with two
    sets of chromosomes.
  • Meiosis is a process to convert a diploid cell to
    a haploid gamete, and cause a change in the
    genetic information to increase
    diversity/variation in the offspring.

4.2.1 State that meiosis is a reduction division
of a diploid nucleus to form haploid nuclei.
Obj. 1
  • The goal of meiosis is to make gametes.
  • Gametes are sperm cells and egg cells

  • In humans, each somatic cell has 46 chromosomes.
  • Made up of 23 pairs of homologous chromosomes?
    chromosomes that make up a pair that have the
    same length, centromere position and pattern of
    coded genes.
  • The two chromosomes of each pair carry genes
    controlling the same inherited characters. (i.e.
    if a gene for eye colour is situated at a
    particular spot (locus) on a certain chromosome,
    then the homologue of that chromosome will also
    have a gene specifying eye colour at the
    equivalent locus.
  • Except for the two sex chromosomes the
    chromosomes that determine an individuals sex
    (all other chromosomes are called autosomes.

How Do Scientists Read Chromosomes?
4.2.2 Homologous chromosomes
  • Homologous chromosomes two chromosomes that are
    the same size and show the same banding pattern.

4.2.5 State that, in karyotyping, chromosomes are
arranged in pairs according to their size and
  • Make a Karyotype

Karyotype a visual representation of the
organization of the chromosomes in the cell of an
Ploidy Number of sets of chromosomes in a cell
  • Haploid (n)-- one set chromosomes. In humans
  • Diploid (2n)-- two sets chromosomes. In humans
  • Most plant and animal adults are diploid (2n)
  • Eggs and sperm are haploid (n)
  • Each gamete has a single set of the 22 autosomes
    plus a single sex chromosome, either X or Y.
    These are the only cells in the body not produced
    by mitosis.
  • The formation of a gamete is considered a
    reduction division because the number of
    chromosomes is reduced by half.

Chromosomes in a Diploid Cell
  • Summary of chromosome characteristics
  • Diploid set for humans 2n 46 (2 sets of 23)
  • Autosomes homologous chromosomes, one from each
    parent (humans 22 sets of 2)
  • Sex chromosomes (humans have 1 set of 2)
  • Female-sex chromosomes are homologous (XX)
  • Male-sex chromosomes are non-homologous (XY)

What is Meiosis?
Figure Sexual Life cycle
  • Meiosis involves two successive nuclear
    divisions that produce four haploid cells. The
    first division (meiosis I) is the reduction
    division the second division (meiosis II)
    separates the chromatids.
  • Occurs in the ovaries or testes.
  • Fertilization restores the diploid condition.

Diploid multicellular organism
Advantages of Meiosis
  • Mitotic cell division produces new cells
    genetically identical to the parent cell.
  • VS
  • Meiosis increases genetic variation in the
    population. exchange of information can occur
    during meiosis I while mutations can also occur

Unique Features of Meiosis Comparison of Meiosis
and Mitosis
4.2.3 Outline the process of Meiosis
  • Interphase is an important stage preceding
    meiosis. Without this stage meiosis would not
  • During this stage, each individual chromatid
    replicates, similar to mitosis..
  • At this stage, the chromosomes are long and
    stringy and are not visible.
  • Remember All somatic cells are diploid in
    number (2n), therefore for each chromatid there
    also exists its homologue, which also replicates
    during interphase.

Prophase I
  • Prophase I is one of the most important stages of
    meiosis. The chromotid threads begin to twist
    and condense, creating chromosomal structures
    which are visible to the microscope.
  • In a process called synapsis, homologous
    chromosomes, each made up of two sister
    chromatids, come together as pairs.
  • After the homologous chromosomes pair, the
    structure is referred to as a tetrad (four
    chromatids). The point at which two non-sister
    chromatids intertwine is known as a chiasmata
    (sing. chiasma).
  • Sometimes a process known as crossing over occurs
    at this point. This is where two non-sister
    chromatids exchange genetic material. This
    exchange does not become evident, however, until
    the two homologous pairs separate. (10.1.1
    Describe the behaviour of the chromosomes in the
    phases of meiosis. Obj. 2)

10.1.2 Outline the process of the formation of
chiasmata during crossing over
Meanwhile, centrosomes move away from each other
and spindle fibres form between them. The nuclear
membrane disappears.
Metaphase I
  • At metaphase, each chromosome has reached its
    maximum density.
  • The homologous pairs and their sister chromatids
    also prepare for separation.
  • They interact with spindle fibers which form from
    either side of the nuclear envelope of the cell.
  • There is a centriole at opposite ends of the
    cell, which is referred to as poles.
  • During metaphase, the chromosomes are lined by
    the spindle fibers at what is known as the
    metaphase plate.

Anaphase I
  • Spindle fibres pull apart the tetrad, separating
    each homologous chromosome. Sister chromatids
    still remain attached to each other and move as a
    single unit toward the same pole.
  • It is by random chance that a certain chromosome
    is pulled to a certain pole.

Telophase I
  • Telophase I varies from species to species.
  • Sometimes Telophase I is skipped and meiosis
    starts its second division immediately.
  • In general, however, two nuclear envelopes begin
    to surround the separate chromosomes and
    cytokinesis (splitting of the cytoplasm into two
    separate entities) will sometimes occur.
  • Each pole now has a haploid chromosome set, but
    each chromosome still has two sister chromatids.
  • Then a phase called interkinesis will follow,
    which essentially is a resting period from
    Telophase I to Prophase II.
  • This differs from mitosis because DNA replication
    does not occur again.

Prophase II
  • During Prophase II, each dyad (1/2 a tetrad) is
    composed of a pair of sister chromatids and they
    are connected by a centromere.
  • The centrioles (replicated during Telophase I)
    which produce the spindle fibers also start to
    move toward the poles of the cell.

Metaphase II
  • Metaphase II is similar to Metaphase I in that
    the dyads are lined up at a metaphase plate by
    the spindle fibers.

Anaphase II
  • The centromeres of sister chromatids finally
    separate, and the sister chromatids of each pair,
    now individual chromosomes, move toward opposite
    poles of the cell.
  • Each sister chromatid ends up on one side of the

Telophase II
  • At the end of Telophase II, the nuclear envelopes
    forms around each set of DNA at opposite poles of
    the cell and the cytoplasm divides once again
  • As a result, four haploid daughter cells have
    formed from one diploid cell.
  • The chromosomal content of a haploid cell in
    one-half the chromosomal content of a diploid
    cell (n as opposed to 2n)

Cell Photograph of Telophase II the chromosomes
are ½ the number in a somatic cell
Stages of Meiosis
  • End result of meiosis ? gametogenesis
    production of gametes Spermatogenesis process
    of male gamete production, one diploid cell gives
    rise to 4 sperm cells Oogenesis process of
    female gamete production, one diploid cell gives
    rise to 1 viable egg cell and three polar bodies,
    occurs in the ovaries once a month starting at

Cdck- cell division activity
  • 10.1.3 Explain how meiosis results in an
    effectively infinite genetic variety in gametes
    through crossing over in prophase I and random
    orientation in metaphase I. Obj. 3
  • 10.1.4 State Mendels law of independent
    assortment. Obj. 1

Genetic Variation
Independent Assortment and Gamete
Diversity Random Orientation of Chromosomes
During Meiosis
Independent Assortment of Alleles
  1. Independent Assortment of Chromosomes
    Arrangements of chromosomes are sorted out/moved
    to opposite poles by chance into gametes
    maternal and paternal (Metaphase I)
  2. Crossing Over Combining DNA inherited from two
    parents into a single chromosome. (Prophase I)
  3. Random Fertilization An egg cell has 1/8 million
    possible chromosomes combinations, is fertilized
    by a single sperm cell, which represents 1/8
    million different possibilities. (Potentially
    producing a zygote with any of 64 trillion
    diploid combinations).

10.1.5 Explain the relationship between the law
of independent assortment and meiosis
  • Independent assortment occurs during metaphase I
    of meiosis, when homologous chromosomes line up
    along the equatorial plane.
  • As chromosomes sort randomly, they create
    opportunities for new recombinants during

4.2.4 Explain that non-disjunction can lead to
changes in chromosomes number
  • Mutations can be inherited if an error occurs in
    the formation of one gamete and that gamete
    fertilizes another to form a zygote ? all the
    cells in the zygote will carry the mutation.
  • Nondisjunction members of a pair of homologous
    chromosomes do not move apart properly during
    meiosis I, or in which sister chromatids fail to
    separate during meiosis II.
  • One gamete receives 2 of the same type of
    chromosome and another gamete receives no copy
  • Anueploidy abnormal chromosome number
  • Trisomy if the chromosome is present in
    triplicate in the fertilized egg (2n1
    chromosomes) e.g. double in egg cell, normal in
    sperm cell e.g.Trisomy 21 Down Syndrome
  • Monosomic if a chromosome is missing (2n-1)
  • Polyploidy when organisms have more than 2
    complete chromosome sets (triploidy 3n,
    tetraploidy 4n) occurs if the cell fails to
    divide after replicating its chromosomes OR
    nondisjunction of all its chromosomes.

Down Syndrome Karyotype
Using Karyotypes to Predict Genetic Disorders
3 of chromosome 21
Other Errors
Syndrome Defect Problems
Petau Extra chromosome 13 Causes numerous physical and mental abnormalities, owing mostly to heart defects Lifespan is measured in days
Edward Trisomy 18 extra chrom 18 Usually naturally aborted or stillborn
Turner Females are missing X chromosome X The most common characteristics of Turner syndrome include short stature and lack of ovarian development. A number of other physical features, such as webbed neck, arms that turn out slightly at the elbow, and a low hairline in the back of the head are sometimes seen in Turner syndrome patients. Individuals with Turner syndrome are also prone to cardiovascular problems, kidney and thyroid problems, skeletal disorders such as scoliosis (curvature of the spine) or dislocated hips, and hearing and ear disturbances.
More errors
Syndrome Defect Problem
Triplo-X Females have an extra X chromosome XXX The symptoms of this syndrome vary widely.  The one consistent feature is tall stature.  Some females exhibit no or very few symptoms, while others have more severe features of developmental delay and/or behavioral abnormalities.
Klinefelter Males have an extra X chromosome XXY XXY males usually have difficulty with expressive language the ability to put thoughts, ideas, and emotions into words. In contrast, their faculty for receptive language-understanding what is said-is close to normal.
Jacobs Males have an extra Y XYY XYY males are fertile, they have testes of normal size, and they have a normal sexual libido and potency. In spite of a somewhat decreased sperm quality with many so-called immature sperm cells, the fertility seems to be normal. In the same way as for triple-X women, who as a rule do not get children with an extra X chromosome, males with XYY most probably only very rarely get sons with two Y chromosomes.
4.2.6 State how karyotyping is performed
  • Karyotyping is performed using cells collected by
    chorionic vilus sampling or amniocentesis, for
    pre-natal diagnosis of chromosome abnormalities.

  • 4.2.7 Analyse a human karyotype to determine
    gender and whether non-disjunction has occurred.

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