• Mitosis divides one diploid cell to form two diploid
• For example: A human cell with 46 chromosomes divides to
form two cells with 46 chromosomes.
• If each parent were to pass on a diploid cell to the
offspring, that offspring would then have 4 copies of
• 46 chromosomes from each parent would yield a 92
• Meiosis allows for two divisions to divide a one
diploid cell into four haploid cells.
• Diploid- contains 2 sets
• (2n = 46)
• Example: human somatic
• Haploid- contains 1 set of
• (n = 23)
• Example: human gametes
ORGANISMS HAVE TENS OF THOUSANDS OF GENES THAT DETERMINE INDIVIDUAL
TRAITS. A CHROMOSOME CAN CONTAIN A THOUSAND OR MORE GENES ALONG ITS
DIPLOID AND HAPLOID CELLS
IN THE BODY CELLS OF ANIMAL AND MOST PLANTS, CHROMOSOMES OCCUR IN PAIRS.
ONE CAME FROM MALE PARENT AND FEMALE PARENT. A CELL WITH TWO OF EACH
KIND OF CHROMOSOME IS CALLED A DIPLOID CELL (2N), NUMBER OF
• Organisms produce gametes that contain one of each kind of chromosome. A cell
containing one of each kind of chromosome is called a haploid cell and is said to
contain a haploid, or n, number of chromosomes.
• Homologous chromosomes
The two chromosomes of each pair in a diploid cell are called homologous
chromosomes. On homologous chromosomes, these genes are arranged in the same
order, but because there are different possible alleles for the same gene, the two
chromosomes in a homologous pair are not always identical to each other.
• When cells divide by mitosis, the new cells have exactly the same number and kind of
chromosomes as the original cells.
• Cell division that allows offspring to have the same number of chromosomes as their
parents. This kind of cell division, which produces gametes containing half the
number of chromosomes as parent’s body cell, is called Meiosis.
• Meiosis occurs in the specialized body cells of each parent that produce gametes.
THE PHASES OF MEIOSIS
• Interphase- The cell replicates its chromosomes. The chromosomes are
replicated during interphase that precedes meiosis I, also.
• Prophase I – behaves in a similar way to one entering prophase of mitosis.
DNA coils, homologous chromosomes line up with each other, gene by gene
along their length, to form a four part structure called a tetrad.
• A tetrad consists of two homologous chromosomes, each made up of two sister
chromatids. The chromatids in a tetrad pair tightly. In fact, they pair so tightly that
non-sister chromatids from homologous chromosomes can actually break and
exchange genetic material in a process known as crossing over. Crossing over
results in a new combinations of alleles on a chromosome.
• Crossing over can occur almost anywhere at random on a chromosome. This
means that an almost endless number of different possible chromosomes can
be produced by crossing over, providing additional variation to the variation
already produced by the random assortment of chromosome.
• This reassortment of chromosomes and the genetic information they carry,
either by crossing over or by independent segregation of homologous
chromosomes, is called a genetic recombination.
• Metaphase I- the centromere of each chromosome becomes attached to spindle
fiber. Homologous chromosomes are lined up side by side as tetrads.
• Anaphase I- The homologous chromosomes move to opposite ends of the cell.
• Telophase I- The reverse order from the events of prophase I. The spindle is broken
down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells.
However another cell division is needed because each chromosome is still doubled.
THE PHASES OF MEISOIS II
• The second division in meiosis is simply a mitotic division of the products of
• Meiosis II consists of prophase II, metaphase II, anaphase II, and telophase II.
Meiosis follows the same steps as the phases of mitosis.
MEIOSIS II : THE EQUATIONAL DIVISION
• Organisms produce gametes (sex cells) that contain one of
each kind of chromosome.
• A cell with only one of each kind of chromosome is called
• Sex cells have one of each kind of chromosome so that
when they combine (as egg and sperm do during
fertilization), the resulting cell is diploid.
• Each species has a specific
number of chromosomes.
• Humans have 23 pairs (46 total)
• Fruit Flies have 4 pairs (8 total)
• Dogs have 39 pairs (78 total)
• Humans have 23 pairs of chromosomes
• 22 pairs of autosomes
• 1 pair of sex chromosomes
• Half of each pair came from one parent
and half came from the other parent
TYPES OF NONDISJUNCTION
• Remember: In normal fertilization, a zygote would get
one copy of a chromosome from each parent
resulting in one pair of each type of chromosome
(humans: 23 pairs)
• Monosomy – when the zygote gets a copy of a
chromosome from only one parent so it is missing
• Most zygotes with monosomy do not survive
• One exception is the case of Turner’s Syndrome
• Females have only one X chromosome instead of two
• These people will still have female sexual characteristics but they
will generally be underdeveloped
• Makes 4 cells genetically different from parent cell & from each other
• Production of gametes = sperm & egg
• Contain half the number of chromosomes = haploid (1N)
• Called the “reduction” division =
• Used in sexual reproduction
MEIOSIS IS A SPECIAL KIND OF CELL DIVISION
THAT HALVES THE NUMBER OF
CHROMOSOMES IN GAMETES. THIS INSURES
THAT THE CORRECT NUMBER OF
CHROMOSOMES ARE PASSED ON.
Copyright Pearson Prentice Hall
• Pair of chromosomes - maternal & paternal - similar in
shape & size.
• Homologous pairs – tetrads - carry genes controlling the
same inherited traits.
• Each locus - position of a gene - is in the same position on
• Meiosis has a large role in maintaining variability in a
• Through sexual reproduction, offspring are not simply
replicas of one organism but a genetic combination of
• Crossing over during Prophase I
insures that a parent organism can
pass on different gametes each time
it reproduces, creating a
variety of offspring.
• Errors can also occur during Meiosis.
• Sometimes the homologous chromosomes do not
separate properly – this is called nondisjunction
• This results in gametes with either an extra copy of a
chromosome or no copy at all.
• Chromosomal mutations can happen when chromosomes break and do not repair
PG. 143 MEIOSIS VOCAB