Download Meiosis Powerpoint

+
46 CHROMOSOMES +
=
46 CHROMOSOMES
1. What is this process called?
2. Is this scenario reasonable? Why or why not?
=
92 CHROMOSOMES
+
23 CHROMOSOMES
+ 23 CHROMOSOMES
=
=
= 46 CHROMOSOMES
• The purpose of meiosis is to create gametes
(sperm and eggs) with half the original
number of chromosomes.
• Meiosis begins with one diploid cell (2 of
each type of chromosome) and ends with four
haploid cells (one of each chromosome).
• 4 sperm cells in males
• 1 egg cell with 3 polar bodies in females
• Chromosome: A structure that carries
genes.
• Each human cell has 46 chromosomes
(23 pairs)
• Each human gamete has 23
chromosomes (0 pairs)
Meiosis:
• occurs in testes of males
• occurs in ovaries of females
• Involves two divisions with one
DNA replication.
• Before meiosis begins, DNA
must replicate itself so
enough copies will be
available for the four new
cells being created. This
happens in Interphase.
• Chromosomes in the
replicated state are made of
two SISTER CHROMATIDS
that are connected by a
CENTROMERE.
Occurs in Four Phases
•
Prophase I- Homologous chromosomes pair up.
Crossing over may occur in this phase.
Metaphase I
• Homologous
Chromosomes line up
at the spindle equator.
• The random
arrangement of pairs
on either side of the
spindle equator results
in variation in the
genotypes found in the
gametes.
Anaphase I
• Homologous
chromosomes are
pulled to opposite
ends of the cell by the
spindle fibers.
Telophase I
• The cell begins to
pinch together and
form two distinct
haploid cells.
• How many
chromosomes were in
the original cell?
• A short Interphase
occurs but no DNA
replication
Meiosis II
• Prophase II- No pairing of
homologues (they are in
separate cells).
• Metaphase II- Sister
chromatids line up at the
spindle equator.
• Anaphase II- Sister
chromatids are pulled to
opposite ends of the two
cells.
• Telophase II- Cytokinesis
occurs in both cells
resulting in 4 haploid
• The purpose of
meiosis is to create
gametes.
• The production of
male gametes is
called
spermatogenesis
• The production of female gametes is
called oogenesis.
• Definition: the union of two gametes
(fertilization), thereby combining the
genes from two different organisms.
• It is responsible for the abundant
variation that can be seen in
organisms that reproduce in this way.
• The gametes that
are formed by
meiosis may
randomly combine
to form offspring that
contain a variety of
traits.
• Meiosis results in the production of gametes which
contain a variety of combinations of the parental
genes.
• Variation in gametes results from meiosis I when:
• 1) In prophase I when crossing over may occur
between homologues.
• 2) During metaphase as homologues randomly line
up on either side of the spindle equator.
• Fertilization
contributes to
variation when the
winner of the sperm
marathon hits the
target egg and claims
its prize!