Theme 5: Meiosis
11.3 Meio)c Cell Division
• Produces gametes (I.e. eggs, sperm, spores, etc.)
• Makes sexual reproduc9on possible
• Differences from mito9c cell division:
◦ Meio9c cell division results in 4 daughter cells instead of two
◦ Each of the 4 daughter cells contains half the number of
chromosomes as the parent cell
◦ The 4 daughter cells are each gene9cally unique
11.3.1 Pairing of homologous chromosomes is unique to
meiosis
• Follows 1 round of DNA synthesis BUT, two successive cell divisions (I.e.
Meiosis I/ Reduc)onal Division (as it reduces the number of
chromosomes by half) and Meiosis II/ Equa)onal Division)
• Prophase I: Marks the earliest visible manifesta9on of chromosome
condensa9on
◦ Homologous chromosomes pair with each other in the process of
synapsis (including the X and Y chromosomes pairs which pair only
at the 9p where there DNA sequences are nearly iden9cal)
• Synapsis provides an opportunity for the maternal and
paternal chromosomes to exchange gene9c informa9on
◦ Note: Each homologous chromosome is a pair of sister chroma9ds
aSached to a single centromere; A pair of synapsed chromosomes
creates a bivalent which is the whole four-stranded structure;
Chroma9ds aSached to different centromeres are non-sister
chroma9ds
• Note: Sister chroma9ds are replicas of the same
chromosome and are gene9cally iden9cal; Non-sister
chroma9ds are replicas of different chromosomes and are
gene9cally similar, but not iden9cal
,11.3.2 Crossing over between DNA molecules results in
exchange of gene)c material
• Chiasma: A visible manifesta9on of a crossover which is the physical
breakage and reunion between non-sister chroma9ds; Also hold the
bivalents together while they become properly oriented in the center of
the cell during metaphase
◦
• Posi9ons of exchanges are random such that each
chromosome that emerges from meiosis is unique; I.e.
Increases gene9c diversity
• Note: No nucleo9des are gained/lost during the exchange of
material
• Number of chiasmata that are formed during meiosis varies between
species
◦ Example: Humans
• 50-60 chiasmata per meiosis
• Most bivalents have 1/+ chiasma
• X and Y chromosomes are joined by a chiasma
11.3.3 The first meio)c division brings about the reduc)on
in chromosome number
• End of Prophase: Chromosomes are fully condensed and have formed
chiasmata; Nuclear envelope has begun to disappear; Meio9c spindle is
forming
• Metaphase I: Meio9c spindle is completed and the bivalents move so
that they come to lie on an imaginary plane cu\ng transversely across
the spindle
◦ Each bivalent lines up such that its two centromeres lie on
opposite sides of this plane
• Note: Orienta9on of these bivalents is random and so,
increases gene9c diversity
• Anaphase I: The two homologous chromosomes of each bivalent
separate as they are pulled in opposite direc9ons
, ◦ Note: Centromeres don't split; Two chroma9ds of each
chromosome remain together
• Recall: In mitosis, the centromeres split and each pair of
chroma9ds is separated
◦ Ends with the arrival of the chromosomes at the poles of each
spindle
• Example: In human cells, there are 23 chromosomes at each
pole at the end of meiosis I and each consists of two
chroma9ds aSached to a single centromere
• Telophase I: Chromosomes may uncoil slightly; Nuclear envelope briefly
reappears
◦ Cytoplasm divides to produce 2 separate cells
◦ Chromosomes do not completely decondense; So, telophase I
blends into prophase II
◦ Note: No DNA synthesis between the two meio9c divisions
11.3.4 The second meio)c division resembles mitosis
• Prophase II: Nuclei now have the haploid number of chromosomes (as
in, 23 chromosomes in humans)
◦ Chromosomes recondense to their maximum extent
◦ Near the end, nuclear envelope disappears(in those species in
which it has formed) and spindle begins to set up
• Metaphase II: Chromosomes line up so that their centromeres lie on an
imaginary plane cu\ng across the spindle
• Anaphase II: Centromere of each chromosome splits
◦ Separated chroma9ds now full-fledged chromosomes and pulled
toward opposite poles
• Telophase II: Chromosomes uncoil and become diffuse
◦ Nuclear envelope reforms and cytoplasm divides by cytokinesis
• Each resul9ng nucleus has haploid number of chromosomes
• Similarity of meiosis II and mitosis suggests that meiosis likely evolved
from mitosis
• Because the steps of meiosis are the same in all eukaryotes, meiosis is
thought to have evolved in the common ancestor of all eukaryotes and
has been subsequently lost in some lineages
11.3 Meio)c Cell Division
• Produces gametes (I.e. eggs, sperm, spores, etc.)
• Makes sexual reproduc9on possible
• Differences from mito9c cell division:
◦ Meio9c cell division results in 4 daughter cells instead of two
◦ Each of the 4 daughter cells contains half the number of
chromosomes as the parent cell
◦ The 4 daughter cells are each gene9cally unique
11.3.1 Pairing of homologous chromosomes is unique to
meiosis
• Follows 1 round of DNA synthesis BUT, two successive cell divisions (I.e.
Meiosis I/ Reduc)onal Division (as it reduces the number of
chromosomes by half) and Meiosis II/ Equa)onal Division)
• Prophase I: Marks the earliest visible manifesta9on of chromosome
condensa9on
◦ Homologous chromosomes pair with each other in the process of
synapsis (including the X and Y chromosomes pairs which pair only
at the 9p where there DNA sequences are nearly iden9cal)
• Synapsis provides an opportunity for the maternal and
paternal chromosomes to exchange gene9c informa9on
◦ Note: Each homologous chromosome is a pair of sister chroma9ds
aSached to a single centromere; A pair of synapsed chromosomes
creates a bivalent which is the whole four-stranded structure;
Chroma9ds aSached to different centromeres are non-sister
chroma9ds
• Note: Sister chroma9ds are replicas of the same
chromosome and are gene9cally iden9cal; Non-sister
chroma9ds are replicas of different chromosomes and are
gene9cally similar, but not iden9cal
,11.3.2 Crossing over between DNA molecules results in
exchange of gene)c material
• Chiasma: A visible manifesta9on of a crossover which is the physical
breakage and reunion between non-sister chroma9ds; Also hold the
bivalents together while they become properly oriented in the center of
the cell during metaphase
◦
• Posi9ons of exchanges are random such that each
chromosome that emerges from meiosis is unique; I.e.
Increases gene9c diversity
• Note: No nucleo9des are gained/lost during the exchange of
material
• Number of chiasmata that are formed during meiosis varies between
species
◦ Example: Humans
• 50-60 chiasmata per meiosis
• Most bivalents have 1/+ chiasma
• X and Y chromosomes are joined by a chiasma
11.3.3 The first meio)c division brings about the reduc)on
in chromosome number
• End of Prophase: Chromosomes are fully condensed and have formed
chiasmata; Nuclear envelope has begun to disappear; Meio9c spindle is
forming
• Metaphase I: Meio9c spindle is completed and the bivalents move so
that they come to lie on an imaginary plane cu\ng transversely across
the spindle
◦ Each bivalent lines up such that its two centromeres lie on
opposite sides of this plane
• Note: Orienta9on of these bivalents is random and so,
increases gene9c diversity
• Anaphase I: The two homologous chromosomes of each bivalent
separate as they are pulled in opposite direc9ons
, ◦ Note: Centromeres don't split; Two chroma9ds of each
chromosome remain together
• Recall: In mitosis, the centromeres split and each pair of
chroma9ds is separated
◦ Ends with the arrival of the chromosomes at the poles of each
spindle
• Example: In human cells, there are 23 chromosomes at each
pole at the end of meiosis I and each consists of two
chroma9ds aSached to a single centromere
• Telophase I: Chromosomes may uncoil slightly; Nuclear envelope briefly
reappears
◦ Cytoplasm divides to produce 2 separate cells
◦ Chromosomes do not completely decondense; So, telophase I
blends into prophase II
◦ Note: No DNA synthesis between the two meio9c divisions
11.3.4 The second meio)c division resembles mitosis
• Prophase II: Nuclei now have the haploid number of chromosomes (as
in, 23 chromosomes in humans)
◦ Chromosomes recondense to their maximum extent
◦ Near the end, nuclear envelope disappears(in those species in
which it has formed) and spindle begins to set up
• Metaphase II: Chromosomes line up so that their centromeres lie on an
imaginary plane cu\ng across the spindle
• Anaphase II: Centromere of each chromosome splits
◦ Separated chroma9ds now full-fledged chromosomes and pulled
toward opposite poles
• Telophase II: Chromosomes uncoil and become diffuse
◦ Nuclear envelope reforms and cytoplasm divides by cytokinesis
• Each resul9ng nucleus has haploid number of chromosomes
• Similarity of meiosis II and mitosis suggests that meiosis likely evolved
from mitosis
• Because the steps of meiosis are the same in all eukaryotes, meiosis is
thought to have evolved in the common ancestor of all eukaryotes and
has been subsequently lost in some lineages
