animal meiosis 2
DESCRIPTION
Discusses MeiosisTRANSCRIPT
MEIOSIS
MEIOSIS
• Occurs in plants and
animals reproducing
sexually.
• In the course of
gametogenesis or
maturation of gametes.
• Reduction of chromosomes
to haploid conditions.
• Two successive divisions
but chromosomes divide
only once.
Meiosis
• a type of cell division that occurs in specialized cells of the ovaries and testes
• at sexual maturity
– The ovaries and testes produce haploid gametes by meiosis
Figure 13.5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
MEIOSIS
• Consists of two successive nuclear divisions:
a. First meiotic division (Meiosis I)
- reduction division
- involved reduction in the number of chromosomes
- diploid to haploid
- 46 chromosomes – 23 chromosomes
b. Second meiotic division(MeiosisII)
- equational division
- no reduction in the number of chromosomes
Each of the two divisions has the same four stages:
prophase, metaphase, anaphase and telophase
• In animals
– meiosis occurs during gamete formation
– gametes are the only haploid cells
Gametes
Figure 13.6 A
Diploid
multicellular
organism
Key
MEIOSIS FERTILIZATION
n
n
n
2n 2n Zygote
Haploid
Diploid
Mitosis
(a) Animals
Homologous Chromosomes
- carry genes for the same traits
- same length
- similar pattern
Figure 13.4 Describing chromosomes
in a diploid cell at metaphase - in a
eukaryote
Key
Maternal set of
chromosomes (n = 3)
Paternal set of
chromosomes (n = 3)
2n = 6
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Centromere
Meiosis
sexual reproduction
haploid, diploid, gamete
homologous chromosome
two stages of meiosis
independent assortment
crossing-over
Synapsis
- a process where the
homologous chromosomes line
up side by side
- process of linking of
homologous chromosomes
Tetrad
- also called bivalent
- a thick (4) structure being
composed of (2) homologous
chromosomes
Crossing over
- nonsister chromatid of (2)
homologous chromosomes in a
tetrad exchange DNA segments
Chiasmata
- point at which 2 non-sister
chromatids intertwine
Figure 13.8 The Meiotic Division
of an Animal Cell
Centrosomes
(with centriole pairs)
Sister
chromatids
Chiasmata
Spindle
Tetrad
Nuclear
envelope
Chromatin
Centromere
(with kinetochore)
Microtubule
attached to
kinetochoreTetrads line up
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Pairs of homologous
chromosomes split upChromosomes duplicate Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
INTERPHASE MEIOSIS I: Separates homologous chromosomes
PROPHASE I METAPHASE I ANAPHASE I
Meiosis
two stages of meiosis
Prophase I
• Nuclear membrane disintegrates or breaks up
• Synapsis
• Crossing over
Metaphase I
• Nuclear membrane disappeared
• Spindle apparatus is formed
• Homologous chromosomes line up at the center of the metaphase plate
Anaphase I
• Homologoues separate and begin to move toward each pole
Telophase I
• similar with telophase in mitosis
• daughter cells have one chromosome from each homologous pair
• transition to the second nuclear division is called interkinesis
TELOPHASE I AND
CYTOKINESISPROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND
CYTOKINESIS
MEIOSIS II: Separates sister chromatids
Cleavage
furrow Sister chromatids
separate
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
Two haploid cells
form; chromosomes
are still double
Meiosis
two stages of meiosisFigure 13.8 The Meiotic Division
of an Animal Cell
Meiosis II
• resemble an ordinary mitotic division except that the number of
chromosomes has been reduced by half
• Prophase II, Metaphase II, Anaphase II,Telophase II and
Cytokinesis
• 4 new cells with haploid chromosomes
Comparison between Mitosis and Meiosis
Mitosis Meiosis
- Formation of two daughter - formation of four daughter cells
cells (2n) (n)
- Cells divide only once for - two nuclear divisions resulting
every cell division cells with haploid number of
chromosomes
- Chromosomes duplicate - chromosomes form pairs
but do not pair which are segregated into (2)
daughter cells of the first
division
- in the second division
homologous chromosomes
split and form 4 haploid cells
- occurs in somatic/body cells - occurs in gametes
Meiosis in relation to Gametogenesis
• Maturing gametes divide
by meiosis and undergo
series of development and
transformation
through“gametogenesis”
two types:
1. spermatogenesis
- male gametogenesis
- testis
2. oogenesis
- female
- ovary
Cross-section of frog’s
testis Seminiferous tubules
showing spermatogenesis
Spermatogenesis in frog’s testis
Oogenesis
Oogonia
oogonium
Primary
oogonium/oocyte
Secondary oogonia
Spermatogenesis
spermatogonia
Primary spermatocytes
Secondary spermatocytes
Spermatozoa