Download Chapter 10 Meiosis

Chapter 10
Meiosis
Asexual Reproduction
• Asexual Reproduction: one parent passes
a duplicate of its genetic information to its
offspring, which can only be genetically
identical clones of the parent
• Clone of the parent
• Example: Bacteria
Sexual Reproduction
• Each parent contributes one gene for each
trait
– Genes: unit of information for inheritable trait
–passed from parent to offspring
– Genes for each trait come in slightly different
forms called alleles, originally produced by
mutations
– Meiosis shuffles the alleles during gamete
formation, and fertilization produces offspring
with unique combinations of alleles
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• The variation generate by sexual
reproduction is the testing ground for
natural selection and is the basis for
evolutionary change
How Meiosis Halves the
Chromosome Number?
• Meiosis begins with diploid (2n=46 in
humans) germ cells can produce haploid
gametes (n=23)
– In 2n cells there are two chromosomes of
each type called homologous chromosomes
– Homologous chromosome pairing up one
from each parent except sex cells
– Homologous chromosomes line up (even
unequally matched sex chromosome) during
meiosis
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• Autosome: all cells except sex cells
• Sex Chromosomes: determine the sex of
an organisms
– Humans (23rd pair)
– Males XY and Female XX
• Meiosis produces gametes that have one
of each pair of homologous chromosomes
Two Division, Not One
• In some ways, meiosis resembles mitosis
• The chromosomes are duplicated during
interphase to form sister chromatids held
together at the centromere
• Chromosomes are moved by the
microtubules of the spindle fibers
Continue…
• Meiosis has two division (meiosis I and
meiosis II)
– During meiosis I, homologous chromosomes
pair and cytokinesis follows
• Each of the daughter cells receives a haploid
number of chromosomes
• Each chromosome is still duplicated
– In meiosis II does not go through
interphase
– Four haploid cells are produced
Crossing Over in Prophase I
• Homologous chromosomes pair up
(synapsis)
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– Nonsister chromatids exchange segments in
a process called crossing over
– Because alleles for the same trait can vary,
new combinations of genes in each
chromosome can result; this is one source of
genetic variation
– After crossing over, the nonsister chromaids
begin to partially separate but remain
attached by chiasmata
Metaphase I aligns
• During metaphase I, homologous
chromosomes randomly line up at the
spindle fibers equator
• During anaphase I, homologous
chromosomes (still duplicated) separated
into two haploid cells each of which has a
random mix of maternal and paternal
chromosomes
From Gametes to Offspring
Plants
• Germ cells within plant tissues produce
haploid spores by meiosis
– Each spore undergoes meiosis to produce a
haploid spores by meiosis
– Fertilization results in a diploid sporophyte
(example: Pine tree)
Gamete Formation Animals
• The life cycle of mulitcelled animals
proceeds from meiosis to gamete
formation  fertilization  growth by
mitosis
Gamete Formation in Males
• In males, meiosis and gamete formation
are called spermatogenesis
– Germ cells (2n)  primary spermatocyte (2n)
 Meiosis I  two secondary spermatocytes
(n)  meiosis II  four spermatids (n)
– Spermatids change in form; each develops a
tail to become mature sperm.
Female Gamete Formation
• In females, meiosis and gamete formation
are called oogenesis
– Germ cells (2n) primary oocyte (2n) 
Meiosis I  secondary oocyte (n, and large in
size) plus polar body (n and small in size) 
Meiosis II  one large ovum (n), plus three
polar bodies (n, small) (dissolved back in the
body)
– The single ovum is the only cell capable of
being fertilized by a sperm, the polar bodies
wither and die
Gene Shufflings at Fertilization
• The diploid chromosome number is
restored at fertilization when two very
different gamete nuclei fuse to form the
zygote
Variation
• 1- Crossing over occurs during prophase I
• 2- Random alignment at metaphase I lead
to millions of combinations of maternal and
paternal chromosomes in each gamete
• 3- Of all the genetically diverse gametes
produced, chance will determine which
two will determine which two will meet
Meiosis and Mitosis
• Look at drawing