Download Cell Reproduction Part II

Meisosis
8.12
Chromosomes are matched in
homologous pairs

The somatic (body) cells of each species


Contain a specific number of chromosomes
For example human cells have 46

Making up 23 pairs of homologous chromosomes

The chromosomes of a homologous pair

Carry genes for the same characteristics at the same place,
or locus
Chromosomes
Centromere
Figure 8.12
Sister chromatids
8.13

Cells with two sets of chromosomes


Gametes have a single set of chromosomes
Are said to be diploid
Gametes, eggs and sperm, are haploid

With a single set of chromosomes

Sexual life cycles

Involve the
alternation of
haploid and
diploid stages
Haploid gametes (n = 23)
n
Egg cell
n

Web activity
Sperm cell
Meiosis
Fertilization
Multicellular
diploid adults
(2n = 46)
Diploid
zygote
(2n = 46)
Mitosis and
development
Figure 8.13
2n
1.
2.
3.
4.
5.
Name 2 functions of mitosis(why do cells
divide?).
Two chromosomes composing a pair are
Chromosomes
called?
What is a somatic Cell?
Centromere
What is a gamete?
What are you thankful Sister chromatids
for this Thanksgiving?
8.14
Meiosis reduces the chromosome number
from diploid to haploid

Meiosis, like mitosis


Is preceded by chromosome duplication
But in meiosis


The cell divides twice to form four daughter cells
Produces haploid gametes in diploid organism.

The first division, meiosis I


Starts with synapsis, the pairing of homologous
chromosomes.
XX
XX
Tetrad
Tetrad
In crossing over

Homologous chromosomes exchange corresponding segments

Meiosis I separates each homologous pair


And produce two daughter cells, each with one set of
chromosomes
Meiosis II is essentially the same as mitosis

The sister chromatids of each chromosome separate
The result is a total of four haploid cells

Web activity


The stages of meiosis
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
PROPHASE I
METAPHASE I
Sites of crossing over
Spindle
Chromatin
Figure 8.14 (Part 1)
Sister
chromatids
Tetrad
ANAPHASE I
Microtubules
Metaphase
attached to
plate
kinetochore
Centromere
(with kinetochore)
Sister chromatids
remain attached
Homologous
chromosomes separate
MEIOSIS II: Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister chromatids
separate
Figure 8.14 (Part 2)
Haploid daughter cells
forming
8.15
Review: A comparison of mitosis and meiosis
Mitosis
Meiosis
Parent cell
(before chromosome replication)
Meiosis i
Prophase I
Prophase
Duplicated
chromosome
(two sister chromatids)
Chromosomes
align at the
metaphase plate
Tetrads
align at the
metaphase plate
Anaphase
Telophase
Sister chromatids
separate during
anaphase
Homologous
chromosomes
separate during
anaphase I;
sister
chromatids
remain together
Daughter cells
of mitosis
Figure 8.15
2n = 4
Metaphase
2n
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
replication
Chromosome
replication
2n
No further
chromosomal
replication; sister
chromatids
separate
during
anaphase II
Metaphase I
Anaphase I
Telophase I
Haploid
n=2
Daughter
cells of
meiosis I
Meiosis ii
n
n
n
n
Daughter cells of meiosis II
8.16
Independent orientation of chromosomes in
meiosis and random fertilization lead to varied
offspring

Each chromosome of a homologous pair


Differs at many points from the other member of the pair
The arrangement of homologous pairs at metaphase I of
meiosis affects the resulting gametes.

Random arrangements of chromosome pairs at
metaphase I of meiosis

Lead to many different combinations of chromosomes in
eggs and sperm
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Possibility 2
Metaphase II
Gametes
Combination 1 Combination 2
Figure 8.16
Combination 3 Combination 4

Random fertilization of eggs by sperm

Greatly increases this variation
8.18
Genetic recombination


Which results from
crossing over during
prophase I of meiosis,
increases variation
still further
TEM 2,200

Crossing over further increases genetic variability
Web activity
Tetrad
Chiasma
Centromere
Figure 8.18A

How crossing
over leads to
genetic variation
Coat-color
genes
C
Eye-color
genes
E
e
c
1
Breakage of homologous chromatids
C
E
c
e
2
Tetrad (homologous
pair of chromosomes
in synapsis)
Joining of homologous chromatids
E
C
Chiasma
e
c
3
C
E
C
e
c
E
c
4
Figure 8.18B
Separation of homologous
chromosomes at anaphase I
e
Separation of chromatids at
anaphase II and completion
of meiosis
C
E
C
e
c
E
c
e
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
8.21
Accidents during meiosis can alter
chromosome number

Abnormal chromosome count is a result of
nondisjunction


The failure of homologous pairs to separate during
meiosis I
The failure of sister chromatids to separate during
meiosis II
Nondisjunction
in meiosis I
Normal
meiosis I
Normal
meiosis II
Nondisjunctio
n in meiosis II
Gametes
Gametes
n+1
n+1
n+1
n 1
Number of chromosomes
Figure 8.21A
n 1
n 1
n
Number of chromosomes
Figure 8.21B
n

Fertilization after nondisjunction in the mother
Egg cell
n+1
Zygote
2n + 1
Sperm cell
n (normal)
Figure 8.21C
8.22
Abnormal numbers of sex chromosomes
do not usually affect survival

Nondisjunction can also produce gametes with
extra or missing sex chromosomes
 Leading to varying degrees of malfunction in
humans but not usually affecting survival
Poor beard
growth
Breast
Development
Under-developed
testes
Characteristic facial
features
Web of
skin
Constriction
of aorta
Poor breast
development
Under developed
ovaries
Figure 8.22A
Figure 8.22B

Human sex chromosome abnormalities
8.23
Alterations of chromosome structure can
cause birth defects and cancer

Chromosome breakage can lead to
rearrangements

That can produce genetic disorders or, if the changes
occur in somatic cells, cancer
Chromosomal Abnormalities
Deletions, duplications, inversions, and translocations
Reciprocal
translocation
Deletion
Nonhomologous
chromosomes
Figure 8.23B
Chromosome 9
Duplication
Homologous
chromosomes
Chromosome 22
Reciprocal
translocation
Inversion
“Philadelphia chromosome”
Figure 8.23A
Activated cancer-causing gene
Figure 8.23C
8.17
Homologous chromosomes carry different
versions of genes

The differences between homologous chromosomes

Are based on the fact that they can bear different
versions of a gene at corresponding loci
Brown coat (C); black eyes (E)
Coat-color
genes
Eye-color
genes
Brown
Black
C
E
C
E
C
E
c
e
c
e
Meiosis
c
White
e
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Figure 8.17A
Chromosomes of
the four gametes
Figure 8.17B
White coat (C); pink eyes (e)
8.19
A karyotype is a photographic inventory of an
individual’s chromosomes

A karyotype

Is an ordered arrangement of a cell’s chromosomes

Preparation of a karyotype from a blood sample
Packed red and
white blood cells
Hypotonic
solution
Fixative
Stain
Blood
culture
White
blood
cells
Centrifuge
1 A blood
Fluid
culture is
centrifuged to separate the
blood cells from the culture fluid.
2 The fluid is discarded, and a hypotonic
3 Another centrifugation step separates the swollen white
blood cells. The fluid containing the remnants of the red
solution is mixed with the cells. This makes
blood cells is poured off. A fixative (preservative) is mixed
the red blood cells burst. The white blood
with the white blood cells. A drop of the cell suspension
cells swell but do not burst, and their
is spread on a microscope slide, dried, and stained.
chromosomes spread out.
Centromere
Sister
chromosomes
2,600X
Pair of homologous
chromosomes
Figure 8.19
4 The slide is viewed with a microscope equipped with a digital
camera. A photograph of the chromosomes is entered into a
computer, which electronically arranges them by size and shape.
5 The resulting display is the karyotype. The 46 chromosomes here include
22 pair of autosomes and 2 sex chromosomes, X and Y. Although difficult to
discern in the karyotype, each of the chromosomes consists of two sister
chromatids lying very close together (see diagram).
8.20
An extra copy of chromosome 21 causes
Down syndrome

A person may have an abnormal number of
chromosomes

Which causes problems

Down syndrome is caused by trisomy 21
An extra copy of chromosome 21
5,000

Figure 8.20A
Figure 8.20B

The chance of having a Down syndrome child

Goes up with maternal age
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
Figure 8.20C
25
30
35
40
Age of mother
45
50