Lecture 7: Meiosis
Meiosis and sexual life cycles:
 In the sexual reproduction ‫ التكاثر الجنسي‬process, two gametes fuse
during fertilization to produce a zygote.
 Meiosis is a specialized type of cell division which reduces the
chromosome number by half (reduces chromosome number from
diploid to haploid).
 This process occurs in animals and plants.
 Meiosis occurs only in sex gonads ‫ الغدد التناسلية الجنسية‬to form
gametes.
 Fertilization restores ‫ يُعيد‬the diploid condition by combining
two haploid sets of chromosomes.
 Gametes, produced by meiosis, are the only haploid cells.
 Gametes undergo no divisions
themselves, but fuse ‫ تندمج‬to form
a diploid zygote that divides by
mitosis
to
produce
a
multicellular organism.
•
Meiosis and mitosis are preceded by the
replication of chromosomes.
• However, in meiosis, chromosomes
replicate once followed by two
consecutive ‫ متعاقب‬cell divisions, meiosis
I and meiosis II, which results in four
daughter cells.
• Meiosis reduces chromosome number
by copying the chromosomes once, but
dividing twice.
1) The first division is known as meiosis I
(a reductional division ‫)االنقسام المنصف‬:
separates homologous chromosomes.
2) The second is known as meiosis II (an
equational division ‫(االنقسام المتساوي‬:
separates sister chromatids.
3
The phases of meiotic division:
Occurs in two steps
1) Meiosis I
Prophase I
2) Meiosis II
Anaphase I
Metaphase I
Telophase I
Prophase II
Anaphase II
Metaphase II
Telophase II
The phases of meiosis I
•
Prior to meiosis, chromosome duplication occurs during the S of the
interphase stage in order to double the sister chromatid pairs.
1. During prophase I:
 Homologous chromosomes (tetrad) ‫ الكروموسومات المتماثلة‬pair up
by the synaptonemal complex in a process called synapsis
 Each pair of homologous chromosomes is known as a bivalent
(tetrad).
 The pairs of chromosomes consist of four chromatids, with
one chromosome coming from each parent.
 Crossing over between homologous chromosomes occurs and
DNA is exchanged between the bivalents in a process called
homologous recombination.
Crossing over
6
2. At metaphase I:
 homologous pairs of chromosomes, not individual
chromosomes are aligned along the metaphase plate.
 In humans, you would see 23 tetrads.
3. At anaphase I:
 Homologous chromosomes, not sister chromatids are
separated from each other, but the sister chromatids
remain attached to each other at the centromeres until
anaphase II.
 The processes during the second meiotic division (meiosis II)
are identical to those in mitosis.
7
8
Comparison between mitosis and meiosis
9
Definitions
 Genes: The units that specify an organism’s inherited characteristic.
 Chromatin: A DNA-protein complex which is organized into a long
thin fiber.
 Chromosome: The package that formed from a condensed, coiled
and folded chromatin.
 Chromatids: Two sister chromatids formed from each duplicated
chromosome. They contain identical copies of the chromosome’s DNA.
 Centromere: The narrow region which the two sister chromatids
are connected together.
 Mitosis: A type of cell division that occurs in somatic cells and generates
two identical daughter cells
 Meiosis: A type of cell division that occurs in the gonads, and yields
four non-identical daughter cells, each with half the chromosomes of
the parent.