Transcript chapter12_Sections 1-3.ppt

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/chemistry/starr
Chapter 12
Meiosis and Sexual Reproduction
(Sections 12.1 - 12.3)
Albia Dugger • Miami Dade College
12.1 Why Sex?
• Sex mixes up the genes of two parents, so offspring of sexual
reproducers have unique combinations of traits
• Diversity offers sexual reproducers as a group a better
chance of surviving environmental change than clones
• sexual reproduction
• Reproductive mode by which offspring arise from two
parents and inherit genes from both
Sex and Adaptation
• An adaptive trait spreads more quickly through a sexually
reproducing population than an asexually reproducing one
• In asexual reproduction, new combinations of traits arise only
by mutation
• Sexual reproduction generates new combinations of traits in
far fewer generations than does mutation alone
Sexual Reproduction in Plants
• In flowering plants,
pollen tubes with male
gametes grow down
into the ovary,
containing female
gametes
12.2 Sexual Reproduction and Meiosis
• Asexual reproduction produces clones
• Sexual reproduction mixes up alleles from two parents
• Meiosis, the basis of sexual reproduction, is a nuclear division
mechanism that occurs in reproductive cells of eukaryotes
Introducing Alleles
• Somatic cells of sexually-reproducing multicelled organisms
contain pairs of chromosomes: one from the mother and one
from the father
• Except for nonidentical sex chromosomes, the two
chromosomes of every pair carry the same set of genes
• somatic
• Relating to the body
Introducing Alleles (cont.)
• The two genes of a pair are often not identical: Maternal and
paternal genes can encode slightly different forms (alleles) of
the same gene’s product
• alleles
• Forms of a gene that encode slightly different versions of
the gene’s product
Introducing Alleles (cont.)
• Most genes have multiple alleles – one reason individuals of a
sexually reproducing species do not look exactly the same
• Offspring of sexual reproducers inherit new combinations of
alleles, which is the basis of new combinations of traits
Homologous Chromosomes
• One chromosome in a homologous pair is inherited from the
mother, one from the father – different forms are alleles
What Meiosis Does
• Sexual reproduction involves fusion of reproductive cells from
two parents
• Meiosis halves the chromosome number in reproductive cells
so offspring have the same number of chromosomes as the
parents
• meiosis
• Nuclear division process that halves the chromosome
number
• Basis of sexual reproduction
Sexual Reproduction
• The process of sexual reproduction begins with meiosis in
germ cells, which produces gametes
• germ cell
• Diploid reproductive cell that gives rise to haploid gametes
by meiosis
• gamete
• Mature, haploid reproductive cell (egg or sperm)
• Gametes usually form inside special male and female
reproductive structures
Gamete Formation in Humans
Gamete
Formation
in Humans
Fig. 12.3a, p. 176
Gamete
Formation
in Humans
Reproductive organs
of a human male
testis (where sperm originate)
Fig. 12.3a, p. 176
Gamete
Formation
in Humans
Fig. 12.3b, p. 176
Gamete
Formation
in Humans
Reproductive organs
of a human female
ovary (where eggs develop)
Fig. 12.3b, p. 176
Gamete Formation in Flowers
Gamete
Formation
in Humans
Reproductive organs
of a flowering plant
anther (where sexual
spores that give rise
sperm cells form)
ovary (where sexual
spores that give rise
to to egg cells form)
Fig. 12.3c, p. 176
Maintaining Chromosome Number
• Gametes have a single set of chromosomes, so they are
haploid (n): Their chromosome number is half of the diploid
(2n) number
• Diploid number is restored at fertilization, when two haploid
gametes fuse to form a zygote, the first cell of a new
individual
Key Terms
• haploid
• Having one of each type of chromosome characteristic of
the species
• fertilization
• Fusion of two gametes to form a zygote
• zygote
• Cell formed by fusion of two gametes
• The first cell of a new individual
Meiosis I and Meiosis II
• Meiosis starts like mitosis, but sorts chromosomes into new
nuclei twice, forming four haploid nuclei
How Meiosis Halves
the Chromosome Number
How Meiosis Halves
the Chromosome Number
B Homologous partners separate.
The still-duplicated chromosomes
are packaged into two new nuclei.
A In meiosis I, each duplicated
chromosome in the nucleus pairs
with its homologous partner.
C Sister chromatids separate in meiosis II. The now
unduplicated chromosomes are packaged into four new nuclei.
Fig. 12.4, p. 177
How Meiosis Halves
the Chromosome Number
B Homologous partners separate.
The still-duplicated chromosomes
are packaged into two new nuclei.
A In meiosis I, each duplicated
chromosome in the nucleus pairs
with its homologous partner.
C Sister chromatids separate in meiosis II. The now
unduplicated chromosomes are packaged into four new nuclei.
Stepped Art
Fig. 12.4, p. 177
Key Concepts
• Sexual Versus Asexual Reproduction
• In asexual reproduction, one parent transmits its genes to
offspring
• In sexual reproduction, offspring inherit genes from two
parents who usually differ in some number of alleles
• Differences in alleles are the basis of differences in traits
ANIMATION: Reproductive Organs
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ANIMATION: Meiosis I and II
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12.3 The Process of Meiosis
• DNA replication occurs prior to meiosis – the nucleus is
diploid (2n) with two sets of chromosomes, one from each
parent
•
During meiosis, chromosomes of a diploid nucleus become
distributed into four haploid nuclei
Meiosis I: Prophase I
• Homologous
chromosomes condense,
pair up, and swap
segments
• Spindle microtubules
attach to chromosomes
as the nuclear envelope
breaks up
Meiosis I: Metaphase I
• The homologous
chromosome pairs are
aligned midway
between spindle poles
Meiosis I: Anaphase I
• The homologous
chromosomes separate
and begin heading
toward the spindle poles
Meiosis I: Telophase I
• Two clusters of
chromosomes reach the
spindle poles
• A new nuclear envelope
forms around each
cluster, so two haploid
(n) nuclei form
Meiosis II: Prophase II
• The chromosomes
condense
• Spindle microtubules
attach to each sister
chromatid as the
nuclear envelope
breaks up
Meiosis II: Metaphase II
• The (still duplicated)
chromosomes are
aligned midway
between poles of the
spindle
Meiosis II: Anaphase II
• All sister chromatids
separate
• The now unduplicated
chromosomes head to
the spindle poles
Meiosis II: Telophase II
• A cluster of
chromosomes reaches
each spindle pole
• A new nuclear envelope
encloses each cluster,
four haploid (n) nuclei
form
Fig. 12.5a, p. 178
Meiosis I One diploid nucleus to two haploid nuclei
1 Prophase I. Homologous
chromosomes condense,
pair up, and swap segments.
Spindle micro- tubules attach
to them as the nuclear
envelope breaks up.
plasma
membrane
nuclear envelope
breaking up
2 Metaphase I. The
homologous
chromosome pairs
are aligned midway
between spindle
poles.
3 Anaphase I. The
homologous
chromosomes
separate and begin
heading toward the
spindle poles.
4 Telophase I. Two clusters
of chromosomes reach the
spindle poles. A new nuclear
envelope forms around each
cluster, so two haploid (n)
nuclei form.
spindle
centrosome
one pair of
homologous chromosomes
Fig. 12.5a, p. 178
Meiosis I One diploid nucleus to two haploid nuclei
1 Prophase I. Homologous
chromosomes condense,
pair up, and swap segments.
Spindle micro- tubules attach
to them as the nuclear
envelope breaks up.
plasma
membrane
nuclear envelope
breaking up
2 Metaphase I. The
homologous
chromosome pairs
are aligned midway
between spindle
poles.
3 Anaphase I. The
homologous
chromosomes
separate and begin
heading toward the
spindle poles.
4 Telophase I. Two clusters
of chromosomes reach the
spindle poles. A new nuclear
envelope forms around each
cluster, so two haploid (n)
nuclei form.
spindle
centrosome
one pair of
homologous chromosomes
Stepped Art
Fig. 12.5a, p. 178
Fig. 12.5b, p. 178
Meiosis II Two haploid nuclei to four haploid nuclei
5
Prophase II. The
chromosomes condense.
Spindle microtubules attach
to each sister chromatid as
the nuclear envelope breaks
up.
6 Metaphase II. The
(still duplicated)
chromosomes are
aligned midway
between poles of
the spindle.
7 Anaphase II. All
sister chromatids
separate. The now
unduplicated chromo somes head to the
spindle poles.
8 Telophase II. A cluster
of chromosomes reaches
each spindle pole. A new
nuclear envelope encloses
each cluster, so four
haploid (n) nuclei form.
No DNA
replication
Fig. 12.5b, p. 178
Meiosis II Two haploid nuclei to four haploid nuclei
5
Prophase II. The
chromosomes condense.
Spindle microtubules attach
to each sister chromatid as
the nuclear envelope breaks
up.
6
Metaphase II. The
(still duplicated)
chromosomes are
aligned midway
between poles of
the spindle.
7
Anaphase II. All
sister chromatids
separate. The now
unduplicated chromo somes head to the
spindle poles.
8
Telophase II. A cluster
of chromosomes reaches
each spindle pole. A new
nuclear envelope encloses
each cluster, so four
haploid (n) nuclei form.
No DNA
replication
Stepped Art
Fig. 12.5b, p. 178
Animation: Meiosis
3D ANIMATION: Meiosis