Transcript Chapter 5 Gases

Chapter 12
Meiosis and Sexual
Reproduction
Albia Dugger • Miami Dade College
12.1 Why Sex?
• In sexual reproduction, offspring arise from two parents and
inherit genes from both
• Asexual reproduction produces genetically identical copies
of a parent (clones)
Advantages of Sexual Reproduction
Sexual Reproduction in Animals and Plants
Introducing Alleles
• Genes are regions in DNA that traits (ice cream)
• Alleles are different forms of the same gene (vanilla, chocolate)
• Offspring of sexual reproducers inherit new combinations of
alleles, the basis of traits
Life cycle of humans
46
46
X
mitosis
23
23
46
46
92
Some
day I am
going to
be a
sperm,
or 4
even!
Dad’s cell
But, you have to get the correct 23 – what do I want to donate for my
baby?
In order to do this,
our cells use the
machinery set up
for mitosis, that is,
the chromosomes
are copied first and
compacted, then
they can get
moved around
So, let’s talk about replicated chromosomes.
There are 23 pairs of homologues.
Double set/
Diploid #
2N = 46
Single set/
Haploid #
N = 23
One pair
Where is meiosis occurring?
2N
2N
2N
2N
2N
2N
2N
N
2N
Spermatogenesis
In females:
2N
2N
N
2N
2N
2N
2N
2N
2N
Oogenesis
Occurs 3
months in
utero
Homologous
chromosomes
Gene for hair
color
brown
black
Gene for lip
size
G C
A T
T A
thin
fat
T A
G C
C G
Alleles
Gene for hair
color
A
B
brown
a
black
b
T A
G C
C G
C
D
C
Gene for lip
size
d
E
G C
A T
T A
thin
fat
E
Replication of homologues
One chromatid
centromere
Summary of Steps in Meiosis
Meiosis
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
Meiosis
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
Meiosis I – Prophase I
 Synapsis is actually occurring
here, with crossing over
Synapsis, crossing over
pairing of homologues
Sperm Formation in Animals
2
3
4
1
male germ cell
sperm
Stepped Art
Egg Formation in Animals
5
6
7
egg
female germ cell
Stepped Art
Fertilization
• Chance combinations of maternal and paternal chromosomes
through fertilization produce a unique combination of genetic
information
• Fertilization
• The fusion of two haploid gametes (sperm and egg)
resulting in a diploid zygote
Fertilization
Comparing Mitosis and Meiosis
Meiosis I One diploid nucleus to two haploid nuclei
Prophase I
Anaphase I
• Chromosomes condense.
Telophase I
• Homologous
• Homologous chromosomes pair. Metaphase I
• Chromosome clusters arrive
chromosomes
• Crossovers occur (not shown). • Chromosomes
align midway
separate and move opposite spindle poles.
• Spindle forms and attaches
between spindle toward at spindle • New nuclear envelopes form.
chromosomes to spindle poles.
poles.
• Chromosomes decondense.
poles.
• Nuclear envelope breaks up.
Stepped Art
Comparing Mitosis and Meiosis
Mitosis One diploid nucleus to two diploid nuclei
Prophase
• Chromosomes condense.
• Spindle forms and attaches
chromosomes to spindle poles.
• Nuclear envelope breaks up.
Metaphase
• Chromosomes
align midway
between spindle
poles.
Anaphase
Telophase
• Sister chromatids • Chromosome clusters arrive
separate and move opposite spindle poles.
toward at spindle • New nuclear envelopes form.
poles.
• Chromosomes decondense
Stepped Art
Comparing Mitosis and Meiosis
Meiosis II Two haploid nuclei to four haploid nuclei
Prophase II
Metaphase II
Anaphase II
Telophase II
• Chromosomes condense.
• Chromosomes • Sister chromatids • Chromosome clusters arrive
• Spindle forms and attaches
align midway
separate and move at spindle poles.
chromosomes to spindle poles. between spindle toward opposite
• New nuclear envelopes form.
• Nuclear envelope breaks up.
poles.
spindle poles.
• Chromosomes decondense.
Stepped Art
Mouse Cell Nuclei During Meiosis
Practice