Transcript Meiosis - Esperanza High School

MEIOSIS
Meiosis
• The form of cell division by which
gametes, with half the number of
chromosomes, are produced.
• Diploid (2n)

haploid (n)
• Meiosis is sexual reproduction.
• Two divisions (meiosis I and meiosis
II).
Meiosis
• Sex cells divide to produce gametes
(sperm or egg).
• Gametes have half the # of
chromosomes.
• Occurs only in gonads (testes or
ovaries).
Three sexual life cycles differing in the
timing of meiosis and fertilization
Moss with Sporophytes
Gametophytes
Meiosis
sex cell
n=2
sperm
n=2
n=2
2n=4
haploid (n)
n=2
diploid (2n)
n=2
n=2
meiosis I
meiosis II
Spermatogenesis
Spermatocyte
n=23
n=23
n=23
sperm
2n=46
n=23
diploid (2n)
n=23
n=23
meiosis I
meiosis II
haploid(n)
Oogenesis
n=23
Oocyte
ovum
n=23
2n=46
haploid(n)
diploid (2n)
polar
bodies
n=23
meiosis I
meiosis II
Interphase
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome
consists of two identical sister
chromatids attached at their
centromeres.
• Centriole pairs also replicate.
Interphase
• Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Meiosis I (four phases)
• Cell division that reduces the
chromosome number by one-half.
Four Phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase I
• Longest and most complex
phase (90%).
• Chromosomes condense.
• Synapsis occurs:
homologous chromosomes
come together to form a tetrad.
Homologous Chromosomes
• Pair of chromosomes (maternal and
paternal) that are similar in shape and
size.
• Homologous pairs (tetrads) carry
genes controlling the same inherited
traits.
• Each locus (position of a gene) is in the
same position on homologues.
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Prophase I
spindle fiber
aster
fibers
centrioles
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Crossing Over
• Crossing over (variation) may
occur between nonsister
chromatids.
• Crossing over: segments of
nonsister chromatids break and
reattach to the other chromatid.
Crossing Over - variation
nonsister chromatids
site of
crossing over
Tetrad
variation
Metaphase I
OR
equator
equator
Metaphase I
• Shortest phase
• Tetrads align on the equator
• INDEPENDENT ASSORTMENT
OCCURS: genes for different traits
sort independently of one another
into gametes
Anaphase I
Anaphase I
• Homologous chromosomes
separate and move towards the
poles.
• Sister chromatids remain attached
at their centromeres.
Telophase I
Telophase I
• Each pole now has haploid
set of chromosomes.
• Cytokinesis occurs and two
haploid daughter cells are
formed.
Meiosis II
• No interphase
(or very short - no more DNA
replication)
• Remember: Meiosis II is similar
to mitosis but the cells are
different from each other
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis
equator
equator
Anaphase II
• same as anaphase in mitosis
• sister chromatids separate
Telophase II
Telophase II
• Same as telophase in mitosis.
• Nuclei form.
Cytokinesis occurs.
• Remember:
four haploid daughter
cells produced; 3 polar bodies and an
egg in females, 4 sperm in males
gametes = sperm or egg
Fertilization
• The fusion of a sperm and egg to
form a zygote - a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Variation
• Important to population as the
raw material for natural
selection.
• Question:
What are the three sexual
sources of genetic variation?
Answer:
1. crossing over (prophase I)
2. independent assortment
(metaphase I)
3. random fertilization
Remember: variation is good !
The Results of Alternative Arrangements of Two
Homologous Chromosome Pairs on the Metaphase
Plate in Meiosis I
A Comparison of Mitosis and Meiosis
Question:
• A cell containing 20 chromosomes
(diploid) at the beginning of
meiosis would, at its completion,
produce cells containing how
many chromosomes?
Answer:
• 10 chromosomes (haploid)
Question:
• A cell containing 40 chromatids
at the beginning of meiosis
would, at its completion,
produce cells containing how
many chromosomes?
Answer:
• 10 chromosomes
Question:
• In terms of Independent
Assortment -how many different
combinations of sperm could a
human male produce?
Answer
• Formula: 2n
• Human chromosomes:
2n = 46
n = 23
• 223 = ~8 million combinations
1) Three major differences
between meiosis and the mitosis
are:
2) Compare the number of
chromatids present in human cells
before and after DNA replication
occurs in interphase.
3) Compare and contrast oogenesis
with spermatogenesis.
Determine the number of chromosomes in a cell whose diploid (2n)
number is 10 after:
a. meiosis I
b. meiosis II
c. mitosis
What is the primary difference
between sexual life cycles
in animals when compared to fungi
and plants?
Failure of chromosomes to
separate properly during meiosis
may result in a/an ____________
Disorder, such as Down Syndrome.
Explain the evolutionary reason that
Sickle Cell Anemia is more common
among African Americans.
If you understand the process of
meiosis then you know that:
1) It occurs only to make sex cells/gametes.
2) Four different cells are produced.
3) Crossing over (exchange among DNA of
tetrads) occurs in Prophase I to make the
cells genetically different.
4) The amount of DNA is reduced by
half in Meiosis I.