Download CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES

CHAPTER 13
MEIOSIS AND SEXUAL
LIFE CYCLES
13.1 Offspring acquire genes from parents by
inheriting chromosomes
13.2 Fertilization and meiosis alternate in sexual life cycles
13.3 Meiosis reduces chromosome number from diploid to haploid
diploid to haploid
13.4 Genetic variation produced in sexual life p
cycles contributes to evolution
1. Parents give their offspring coded information in the form of genes.
the form of genes.
2. Genes are segments of DNA.
3. Genetic information is transmitted as specific sequences of the four deoxyribonucleotides in DNA.
4 Most genes program cells to synthesize specific 4.
M t
ll t
th i
ifi
enzymes and other proteins that produce an organism’s inherited traits.
5 G
5.
Genes program specific traits that emerge as we ifi t it th t
develop from fertilized eggs into adults.
Fertilization and meiosis alternate ‫ﻳﺘﻌﺎﻗﺒﺎ‬
Fertilization and meiosis alternate in sexual life cycles
life cycles
™ A life cycle
A life cycle of an organism is the generation‐
of an organism is the generation‐to
to‐‐generation sequence of stages in
in its reproductive history.
its reproductive history
p
g
p
p g
™ It starts at the conception of an organism until it produces its own offspring ‫ﻧﺴﻞ‬..
™ In humans, each somatic cell
IIn humans, each h
h somatic cell
i
ll (all cells other than sperm or ovum) has ll ll h h
) h 46 ) has 46 46
chromosomes.
™ These homologous chromosome
These homologous chromosome ‫ اﻟﻜﺮوﻣﻮﺳﻮﻣﺎت اﻟﻤﺘﻤﺎﺳﻠﺔ‬pairs carry genes that control the same inherited characters.
™ A karyotype
A karyotype ‫ اﻟﻄﺮز اﻟﻜﺮوﻣﻮﺳﻮﻣﻰ‬display of the 46 display of the 46 chromosomes shows chromosomes shows 23 23 pairs pairs of chromosomes, each pair with the same length, centromere
p
g centromere p
position, and of chromosomes, each pair with the same length, staining pattern.
٣
The Karyotype
The Karyotype ‫اﻟﻄﺮز اﻟﻜﺮوﻣﻮﺳﻮﻣﻰ‬:
It is a display of an individual’s chromosomes that arranged according to size and shapes))
Fig. 13.3, Page 237
٤
Chromosomes (sex and autosomes)
•
A
An exception to the rule of homologous chromosomes is found in the sex An exception to the rule of homologous chromosomes is found in the ti t th
l fh
l
h
i f
d i th sex chromosomes, the chromosomes
, the X and the Y.
•
The pattern of inheritance of these chromosomes determine an individual’s sex ‫ ﻧﻮع ﺟﻨﺲ اﻟﻔﺮد‬.
– Human females have a homologous pair of X chromosomes (
Human females have a homologous pair of X chromosomes (XX
g
p
(XX).
)).
– Human males have an X and a Y chromosome (
Human males have an X and a Y chromosome (XY
XY).
).
•
The other 22 The other 22 pairs are called pairs are called autosomes p
autosomes ‫اﻟﺬاﺗﻴﺔ‬
‫اﻟﻜﺮوﻣﻮﺳﻮﻣﺎت اﻟﺬاﺗﻴﺔ‬.
‫اﻟﻜﺮوﻣﻮﺳﻮﻣﺎت‬.
• We inherit one chromosome of each homologous pair ‫ اﻷزواج اﻟﻤﺘﻤﺎﺛﻠﺔ‬from each p
parent.
– The 46 The 46 chromosomes in a somatic cell can be viewed as two sets of chromosomes in a somatic cell can be viewed as two sets of 23
23, a , a maternal maternal set ‫ ﻣﺠﻤﻮﻋﺔ اﻷم‬and a paternal set and a paternal set ‫ﻣﺠﻤﻮﻋﺔ اﻷب‬.
‫ﻣﺠﻤﻮﻋﺔ‬.
• Sperm cells or ova (gametes
Sperm cells or ova (gametes) have only one set of chromosomes ) have only one set of chromosomes ‐‐ 22 autosomes and an autosomes and an X
X or a Y
or a Y.
• A cell with a single chromosome set is haploid A cell with a single chromosome set is haploid ‫ﻓﺮدى‬.
– For humans, the haploid number of chromosomes is 23 For humans, the haploid number of chromosomes is 23 (n = (n = 23
23). ). ٥
•
A haploid sperm reaches and fuses A haploid sperm reaches and fuses ‫ﻳﻨﺪﻣﺞ‬
‫ ﺞ‬with a haploid ovum.
•
These cells fuse (syngamy
These cells fuse (syngamy ‫)اﻹزدواج‬ resulting in fertilization.
) resulting in fertilization.
•
The fertilized egg (zygote) now has a diploid ‫زوﺟﻰ‬
The fertilized egg (zygote) now has a diploid set of chromosomes from the maternal and paternal family lines.
•
The zygote and all cells with two sets of chromosomes are diploid cells ‫زوﺟﻰ‬
The zygote and all cells with two sets of chromosomes are diploid cells 46 (2n = 46
n = 46). ). • As an organism develops from a zygote to a sexually mature adult, the zygote’s genes are passes on to all somatic cells by mitosis.
• Gametes, which develop in the gonads, are not
Gametes, which develop in the gonads, are not produced by mitosis.
• Instead, gametes undergo the process of meiosis in which the chromosome Instead, gametes undergo the process of meiosis in which the chromosome number is halved ‫ﻟﻠﻨﺼﻒ‬
number is halved ‫ﻳُﺨﺘﺰل‬.
– Human
Human sperm or ova have a haploid set of 23 Human sperm or ova have a haploid set of sperm or ova have a haploid set of 23 different chromosomes, one 3 different
different chromosomes, one chromosomes, one
from each homologous pair.
٦
•
•
Fertilization restores Fertilization restores ‫ ﻳُﻌﻴﺪ‬the the diploid
diploid ‫ زوﺟﻰ‬condition by combining two condition by combining two haploid
haploid ‫ ﻓﺮدى‬sets of chromosomes.
Fertilization and meiosis alternate in sexual life cycles. Gametes, produced by meiosis, are the only h l id ll
haploid cells.
Gametes undergo no divisions themselves, ‫ ﺞ‬to form a diploid p
but fuse ‫ﺗﻨﺪﻣﺞ‬
zygote that divides by mitosis to produce a multicellular organism
Figure 13.6 A
Fig. 13.
13.4, Page 238
٧
• In Man and Animals
I M
dA i l
– Meiosis occurs during gamete formation
– Gametes are the only haploid cells
Key
Haploid
Diploid
p
n
n
Gametes
n
MEIOSIS
2n
Figure 13.6 A
Diploid
multicellular
organism
FERTILIZATION
Zygote
2n
Mitosis
(a) Animals
Meiosis (Reduction Division) ‫اﻹﻧﻘﺴﺎم اﻹﺧﺘﺰاﻟﻰ‬
Reduces chromosome number from diploid to haploid
educes chromosome number from diploid to haploid :
™ Many steps of meiosis resemble steps in mitosis.
™ Both are preceded by the replication of chromosomes.
™ However, in meiosis, chromosomes replicate once followed by two consecutive ‫ﻣﺘﻌﺎﻗﺐ‬
followed by two consecutive ‫ ﻣﺘﻌﺎﻗﺐ‬cell divisions, meiosis II and meiosis and meiosis meiosis II
II, which results in , which results in four four daughter cells. . daughter cells
™ Each final daughter cell has only half chromosomes number (haploid number (
haploid ‫)ﻓﺮدى‬.
™ Meiosis reduces chromosome number by Meiosis reduces chromosome number by copying y copying py g
the chromosomes once, but dividing twice
the chromosomes once, but dividing twice..
ƒ The first division (meiosis The first division (meiosis II) separates homologous chromosomes.
ƒ The second (meiosis The second (meiosis II
II) separates sister ) separates sister chromatids
chromatids..
Fig. 13.7
٩
2‐ Meiosis Division (Reduction Division)
Reduction Division)
Occurs in two steps
A)‐‐ Meiosis I
A)
Reduces the number of
chromosomes from diploid to
haploid
haploid.
B)‐‐ Meiosis II
B)
‐ No further replication of
chromosomes.
‐Occurs in the newly resulting
Occurs in the newly resulting
cells from Meiosis cells from Meiosis I.
I.
Producing (4 Producing (
4 haploid cells n
haploid cells n))
It occurs mainly in sex gonads to form Gametes
Gametes (sperms and ova)
Each of the resulting cells has half number of chromosomes of the original cell ((23 in human). Thus, it called Reduction Division
)
,
Reduction Division
١٠
Meiosis and mitosis can be distinguished
from mitosis
;By three events in Meiosis l:
1-Synapsis and crossing over
H
Homologous
l
chromosomes
h
physically
h i ll connectt and
d exchange
h
genetic information.
2-Tetrads on the metaphase plate
At metaphase I of meiosis, paired homologous
chromosomes ((tetrads)) are p
positioned on the metaphase
p
plates
3- Separation of homologues
3
At anaphase I of meiosis, homologous pairs move toward
opposite poles of the cell
In anaphase II of meiosis,
meiosis the sister chromatids separate
Meiosis
A)‐‐ Meiosis A)
Meiosis II:
is very similar to mitosis.
1)‐ interphase the chromosomes are the chromosomes are
replicated to form sister chromatids.
2) Prophase I,
2)‐
P h
I the chromosomes condense and homologous th h
d
dh
l
chromosomes pair up ‫ ﺗﺰدوج‬to form tetrads ‫ﻣﺠﻤﻮﻋﺎت رﺑﺎﻋﻴﺔ‬.
• Homologous chromosomes are attached together in a process called synapsis ‫اﻟﺘﺸﺎﺑﻚ‬.
– Non –sister Chromatids of homologous chromosomes are crossed over at points called chiasmata and segments of the crossed over at points called chiasmata
and segments of the
chromosomes are exchanged.This is called Crossing Over
Crossing Over.
Figs. 13.8
١٢
3)‐ Metaphase Metaphase II, the tetrads are all arranged at the metaphase plate.
– Microtubules from one pole are attached to the kinetochore of one chromosome of p
each tetrad, while those from the other pole are attached to the other.
4)‐ Anaphase Anaphase II,
the homologous chromosomes separate and are pulled toward opposite poles. Figs-13.8
١٣
5)‐ Telophase I, movement of homologous chromosomes continues until there is a haploid set at each pole
set at each pole.
– Each chromosome consists of linked sister chromatids..
chromatids
• Cytokinesis takes
takes place by the same place by the same
•
mechanisms as in mitosis and usually occurs simultaneously.
In some species, nuclei may reform, but there is no further replication or duplication p
p
of chromosomes before Meiosis II. Figs-13.8
١٤
B) MEIOSIS II:Each
B) MEIOSIS II:Each of the two daughter cells formed after meiosis I undergo Meiosis meiosis I undergo Meiosis II
g
II
1)‐ Prophase Prophase II
II a spindle apparatus forms, attaches to kinetochores
a spindle apparatus forms, attaches to kinetochores of each sister chromatids, and moves them around.
sister chromatids
, and moves them around.
2)‐ Metaphase Metaphase II, the sister Metaphase II
the sister chromatids are
the sister chromatids
are arranged at the metaphase plate.
arranged at the metaphase plate
3)‐ Anaphase Anaphase II
II, the , the centromeres
centromeres of sister of sister chromatids
chromatids separate and the separate sisters travel toward opposite poles. Fig. 13.8
١٥
4)‐ Telophase Telophase II
II, separated sister chromatids arrive at opposite poles.
– Nuclei form around the chromatids.
l if
d h h
id
• Cytokinesis separates the cytoplasm.
separates the cytoplasm
•
At the end of meiosis, there are four haploid daughter cells.
١٦
Fig. 13.8
Crossing-over and Recombination During
Meiosis
Chromosomal crossover Chromosomal
crossover
(or crossing over) is the process by which two
process by which two chromosomes pair up and exchange sections of
and exchange sections of their DNA. This often occurs during prophase
occurs during prophase
1 of meiosis in a process called synapsis.
called synapsis.
Sexual life cycles produce genetic variation among offspring
•
Three mechanisms contribute to genetic variation ‫اﻹﺧﺘﻼﻓﺎت اﻟﻮراﺛﻴﺔ‬:
‫اﻹﺧﺘﻼﻓﺎت‬:
1)
2)
3)
independent assortment ‫ﻟﻠﻜﺮوﻣﻮﺳﻮﻣﺎت‬
independent assortment ‫اﻹﻧﺘﻘﺎل اﻟﺤﺮ ﻟﻠﻜﺮوﻣﻮﺳﻮﻣﺎت‬
crossing over ‫اﻟﻌﺒﻮر‬
crossing over ‫اﻟﻌﺒﻮر‬
random fertilization random fertilization ‫اﻟﻌﺸﻮاﺋﻰ‬
‫اﻟﺘﻠﻘﻴﺢ اﻟﻌﺸﻮاﺋﻰ‬
1)‐ Independent assortment:
d
d
of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate.
– There is a fifty
There is a fifty‐
y‐fiftyy chance that a particular daughter cell of meiosis I will get the will get the maternal
maternal chromosome of a certain homologous g
pair and a fifty
pair and a fifty‐‐fifty
chance that it will receive the paternal
receive the paternal
chromosome. Fig. 13.10
١٨
•
Independent assortment alone would find each individual chromosome in a gamete that would be exclusively maternal or paternal in origin.
3)‐ Crossing over:
Homologous portions Homologous portions ‫ﻣﺘﻤﺎﺛﻠﺔ‬
‫ أﺟﺰاء ﻣﺘﻤﺎﺛﻠﺔ‬of two non‐‐sister chromatids exchange places, non
g p
,
producing recombinant chromosomes which combine genes inherited from each parent.
2‐ The random fertilization:
h
d
f l
it adds to the •
genetic variation arising from meiosis.
Any sperm can fuse with any egg.
١٩
Fig. 13.11
Comparison between Mitosis and meiosis • The
The chromosome number is reduced by half in meiosis, but chromosome number is reduced by half in meiosis but
not in mitosis.
– Mitosis produces daughter cells that are genetically Mitosis produces daughter cells that are genetically
identical to the parent and to each other.
– Meiosis produces cells that differ from the parent and Meiosis produces cells that differ from the parent and
each other.
• Three events, unique to meiosis, occur during the first division cycle.
,
q
,
g
y
1. During prophase I
. During prophase I, homologous chromosomes pair up in a process called synapsis.
synapsis
i.
– Later in prophase I, the joined homologous chromosomes are visible as a tetrad.
– At X
At X‐‐shaped regions called chiasmata
shaped regions called chiasmata, sections of , sections of nonsister
nonsister chromatids are exchanged.
– Chiasmata
Chi
iis the physical manifestation of crossing over, a form of genetic h h i l
if
i
f
i
f
f
i
rearrangement.
٢٠
2. At . At metaphase metaphase II homologous pairs of chromosomes, not individual
individual chromosomes are aligned along the metaphase plate.
chromosomes are aligned along the metaphase plate
• In humans, you would see In humans, you would see 23 23 tetrads.
tetrads.
3. At . At anaphase I
. At anaphase anaphase I,, it is homologous chromosomes, not sister it is homologous chromosomes, not sister
chromatids, that separate and are carried to opposite poles of the chromatids
, that separate and are carried to opposite poles of the cell.
– Sister Sister chromatids
chromatids remain attached at the centromere
remain attached at the centromere until anaphase II.
• The processes during the second meiotic division are virtually identical to those of mitosis.
٢١
Fig. 13.8