Download Meiosis And Sexual Reproduction

Chapter 9
Meiosis And Sexual
Reproduction
Genes and Alleles
Heredity
• The transmission of traits from one generation to
next is called inheritance , or heredity
Inheritance of Genes
• Genes are segments of DNA that encode heritable
information about traits
Alleles
– Slightly different forms of the same gene
– Each specifies a different version of gene product
Sexual and Asexual Reproduction
• Asexual reproduction (1 parent)
– Offspring inherit parent’s genes
– Clones (identical copies of parent)
• Sexual reproduction (2 parents)
– Offspring differ from parents and each another
– Different combinations of alleles
– Different details of shared traits
Sexual Reproduction
• Meiosis, gamete formation, and fertilization
occur in sexual reproduction
• Meiosis and fertilization shuffle parental alleles
– Offspring inherit new combinations of alleles
Sexual Reproduction
Meiosis
• Nuclear Division process that halves the chromosome
number in half
• Basis of sexual reproduction
Fertilization
• Fusion of a sperm nucleus and an egg nucleus result in
the formation a single - celled zygote
Gamete
• Reproductive cells
• Vehicles that transmit genes from one generation to next
• Male and female gametes (Sperm and egg)
Where Gametes Form
Key Concepts:
SEXUAL VS. ASEXUAL REPRODUCTION
• By asexual reproduction, one parent alone
transmits its genetic information to offspring
• By sexual reproduction, offspring typically inherit
information from two parents that differ in their
alleles
• Alleles are different forms of the same gene; they
specify different versions of a trait
What Meiosis Does
• Meiosis
– Nuclear division mechanism that precedes gamete
formation in eukaryotic cells
– Halves parental chromosome number
• Fertilization
– Fusion of two gamete nuclei
– Restores parental chromosome number
– Forms zygote (first cell of new individual)
2n
germ cell
germ cell
each chromosome
duplicated during
interphase
n
MEIOSIS I
separation of
homologues
MEIOSIS II
separation of
sister chromatids
gametes
gametes
2n
diploid number
restored at
fertilization
zygote
Fig. 9.12, p.150
Homologues
• Sexual reproducers inherit pairs of
chromosomes
– 1 from maternal parent, 1 from paternal parent
• The pairs are homologous (“the same”)
– Except non-identical sex chromosomes (X and Y)
– Same length, shape, genes
• All pairs interact at meiosis
– One chromosome of each type sorts into gametes
Homologous Chromosomes
p.141b
Meiosis I
• The first nuclear division
• Each duplicated chromosome lines up with its
homologous partner
• The two homologous chromosomes move
apart, toward opposite spindle poles
p.141c
Prophase I
• Chromosomes condense and align tightly with
their homologues
• Each homologous pair undergoes crossing over
• Microtubules form the bipolar spindle
• One pair of centrioles moves to the other side of
the nucleus
Prophase I (cont.)
• Nuclear envelope breaks up
– Microtubules growing from each spindle pole
penetrate the nuclear region
• Microtubules tether one or the other
chromosome of each homologous pair
Metaphase I
• Microtubules from both poles position all
pairs of homologous chromosomes at the
spindle equator
Anaphase I
• Microtubules separate each chromosome from
its homologue, moving to opposite spindle poles
• Other microtubules overlap midway between
spindle poles, slide past each other to push poles
farther apart
• As anaphase I ends, one set of duplicated
chromosomes nears each spindle pole
Telophase I
• Two nuclei form
– Typically, the cytoplasm divides
• All chromosomes are still duplicated
– Each still consists of two sister chromatids
Meiosis I
one pair of homologous
chromosomes
newly forming
microtubules of
the spindle
plasma
membrane
breakup
of nuclear
envelope
Prophase I
spindle equator (midway
between the two poles)
centrosome with
a pair of centrioles,
moving to opposite
sides of nucleus
Metaphase I
Chromosomes were Prior to metaphase I, one
duplicated earlier, in set of microtubules had
interphase.
tethered one chromosome
of each type to one spindle
pole and another set tethered
its homologue to the other spindle
pole.
Anaphase I
One of each duplicated
chromosome, maternal
or paternal, moves to a
spindle pole; its homologue
moves to the opposite pole.
Telophase I
One of each type
of chromosome has
arrived at a spindle
pole. In most species,
the cytoplasm divides
at this time.
Fig. 9.5a, p.142
Meiosis II
there is
no DNA
replication
between the
two divisions
Prophase II
In each cell, one of two
centrioles moves to the
opposite side of the
cell, and a new bipolar
spindle forms.
Metaphase II
By now, microtubules
from
both spindle poles
have finished a tug-ofwar.
Anaphase II
The sister chromatids of
each chromosome move
apart and are now
individual, unduplicated
Telophase II
A new nuclear envelope
encloses each parcel of
chromosomes, so there
are now four nuclei.
Fig. 9.5b, p.142
Haploid Daughter Cells
• When cytoplasm divides, four haploid cells
result
• One or all may serve as gametes or, in plants,
as spores that lead to gamete-producing
bodies
Key Concepts:
STAGES OF MEIOSIS
• Diploid cells have a pair of each type of
chromosome, one maternal and one paternal
• Meiosis, a nuclear division mechanism, reduces
the chromosome number
• Meiosis occurs only in cells set aside for sexual
reproduction
Key Concepts:
STAGES OF MEIOSIS (cont.)
• Meiosis sorts out a reproductive cell’s
chromosomes into four haploid nuclei
• Haploid nuclei are distributed to daughter
cells by way of cytoplasmic division
Introducing Variation in Offspring
• Three events cause new combinations of
alleles in offspring:
– Crossing over during prophase I (meiosis)
– Random alignment of maternal and paternal
chromosomes at metaphase I (meiosis)
– Chance meeting of gametes at fertilization
• All three contribute to variation in traits
How Meiosis Introduces Variation
in Traits
Prophase I: Crossing Over
• Non sister chromatids of homologous
chromosomes undergo crossing over
– They exchange segments at the same place
along their length
• Each ends up with new combinations of
alleles not present in either parental
chromosome
Fig. 9.6c, p.144
Fig. 9.6d, p.144
Fig. 9.6d, p.144
a A maternal chromosome (purple) and
paternal chromosome (blue) were
duplicated earlier, during
interphase. They become visible in
microscopes early in prophase I, when
hey star to condense to
threadlike form.
mom’s
allele
A
mom’s
allele
B
c Here is a simple way to
think about crossing over.
(Chromosomes are still c
ondensed and threadlike,
and each is tightly aligned
with its homologous
partner.)
b Each chromosome
and its homologous partner
zipper
together, so all four
chromatids
are tightly aligned.
dad’s
allele
a
dad’s
allele
b
d Their intimate contact
promotes crossing over
at different places along the
length of nonsister
chromatids.
e At the crossover site,
paternal and maternal
chromatids exchange
corresponding segments.
mom’s
allele
A
mom’s
allele
A
mom’s
allele
B
dad’s
allele
b
f Crossing over mixes
up maternal and paternal
alleles on homologous
chromosomes.
Fig. 9.6, p.144
Fig. 9.6f, p.144
Metaphase I: Chromosome Shuffling
• Homologous chromosomes align randomly
during metaphase I
• Microtubules can harness either a maternal or
paternal chromosome of each homologous pair
to either spindle pole
• Either chromosome may end up in any new
nucleus (gamete)
Chromosome Shuffling:
Random Alignment
CHROMOSOME
RECOMBINATION AND SHUFFLING
Key Concepts:
• During meiosis, each pair of maternal and
paternal chromosomes swaps segments and
exchanges alleles
• Pairs get randomly shuffled, so forthcoming
gametes end up with different mixes of
maternal and paternal chromosomes
From Gametes to Offspring
• Multicelled diploid and haploid bodies are
typical in life cycles of plants and animals
• Gamete Formation in Plants
– Sporophyte: A multicelled plant body (diploid)
that makes haploid spores
– Spores give rise to gametophytes (multicelled
plant bodies in which haploid gametes form)
meiosis
multicelled
sporophyte
(2n)
zygote
(2n)
DIPLOID
fertilization
meiosis
HAPLOID
spores
(n)
gametes
(n)
multicelled
gametophyte
meiosis
Plant life cycle
(n)
meiosis
From Gametes to Offspring
Gamete Formation in Animals
– Germ cells in the reproductive organs give rise to
sperm or eggs
– Fusion of a sperm and egg at fertilization results in
a zygote
meiosis
zygote
(2n)
fertilization
multicelled
body
(2n)
DIPLOID
HAPLOID
gametes
(n)
Animal life cycle
meiosis
Gamete Formation in Animals
• In male animals
• Germ cell develops into a primary sporocyte
• After meiosis, 4 haploid cells are formed and
develop into spermatids
• Then it becomes sperm, a type of mature
male gamete
Sperm Formation in Animals
Gamete Formation in Animals
• In Female Animals
• Germ cell becomes primary oocyte
• Meiosis I followed by unequal cytoplasmic
division produces a small polar body and a
large secondary oocyte. Both are haploid
• Meiosis II and unequal cytoplasmic division
produces one large secondary oozyte and
three small polar bodies
• Secondary oozyte is also called ovum
Egg Formation in Animals
Comparing Mitosis and Meiosis
• Both mitosis and meiosis require bipolar
spindle to move and sort duplicated
chromosomes
• Some mechanisms of meiosis resemble those
of mitosis, and may have evolved from them
– Example: DNA repair enzymes function in both
Differences in Mitosis and Meiosis
• Mitosis maintains parental chromosome number
– Duplicates genetic information
– Occurs in body cells
• Meiosis halves chromosome number
– Introduces new combinations of alleles in offspring
– Occurs only in cells for sexual reproduction
Comparing Mitosis and Meiosis
Comparing Mitosis and Meiosis
no interphase
and no DNA
replication
between the
two nuclear
divisions
Prophase II
All chromosomes still
duplicated. New spindle
forms in each nucleus,
tethers chromosomes
to spindle poles.
Metaphase II
All chromosomes
aligned at the
spindle equator.
Anaphase II
Sister chromatids of
each chromosome
moved to opposite
spindle poles.
Telophase II
Four haploid (n) nuclei
form. After cytoplasmic
division, haploid cells
function as gametes
or spores.