1. Chapter 18 – Part II
Meiosis

2. Meiosis – The Making of Eggs and Sperm
In meiosis there are two divisions.
During the S phase of interphase, a germ cell duplicates its DNA; each duplicated chromosome consists of two sister chromatids attached to one another. The chromosome number in humans is 46 (diploid).
Interphase is followed by two consecutive divisions of the chromosomes in meiosis; there is no interphase between the two divisions.
After each division, daughter cells form, giving rise to four haploid cells. Each sperm and egg has 23 chromosomes.
The two divisions are called Meiosis I and Meiosis II.

3. MEIOSIS I
MEIOSIS II
interphase
( DNA
replication
before
meiosis I)
no interphase
( no DNA
replication
before
meiosis II)
PROPHASE I
METAPHASE I
ANAPHASE I
TELOPHASE I
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II

4. Meiosis I
During Prophase I, the homologous chromosomes pair up and there is a crossing over (recombination) event.
The paired homologous chromosomes line up on the equator during Metaphase I.
The separation of homologues occurs in Anaphase I.
During Telophase I, a plasma membrane is constructed in the equatorial region and this forms a cleavage furrow which eventually pinches into two haploid cells.

5. Meiosis I

6. Meiosis II
During Prophase II, the chromosomes once again become visible as centrioles migrate to opposite sides.
The chromosomes line up on the equator during Metaphase II.
The separation of sister chromatids occurs in Anaphase II.
During Telophase II, a plasma membrane is constructed in the equatorial region and this forms a cleavage furrow which eventually pinches into four haploid cells.

7. Meiosis II

8. Spermatogenesis Meiosis is the first step in the formation of gametes.
In males, meiosis and gamete formation are called spermatogenesis.
A diploid (2N) germ cell (spermatogonium) undergoes MEIOSIS I at puberty to form two primary (1º) spermatocytes.
The 1º spermatocytes enter MEIOSIS II to become four haploid (N) secondary (2º ) spermatocytes.
These 2º spermatocytes grow tails (flagellum) and turn into spermatids.
The discharge of spermatids from the seminiferous tubules is called spermiation. The spermatids will complete their maturation in the epididymis and the vas deferens before leaving the body during ejaculation.

9. Figure 19.10 Animated! General diagram of spermatogenesis.
cell differentiation, sperm formation
(mature, haploid male gametes)
secondary spermatocytes (haploid)
spermato- gonium
(diploid male germ cell )
primary spermatocyte
Figure Animated! General diagram of spermatogenesis.
spermatids (haploid)
a. Growth
b. Meiosis I and cytokinesis
c. Meiosis II and cytokinesis

10. Oogenesis
In females, meiosis and gamete formation are called oogenesis.
A diploid (2N) germ cell (oogonium) undergoes MEIOSIS I before the girl is born to form a primary (1º) oocyte and a polar body.
The 1º oocyte enter MEIOSIS II during puberty to become a haploid (N) secondary (2º ) oocyte and a second polar body is formed.
The 2º oocyte is ovulated before it completes Meiosis II.
The discharge of the ovum from the ovary is called ovulation.

11. three polar bodies (haploid)
first polar body (haploid)
oogonium (diploid female germ
cell)
Figure Animated! Oogenesis. The diagram is not at the same scale as Figure An egg becomes much larger than a sperm cell. It also is much larger than the polar bodies.
primary oocyte
secondary oocyte (haploid)
ovum (haploid)
a. Growth
b. Meiosis I and
cytokinesis
c. Meiosis II and cytokinesis

12. Every gamete is different
In prophase I, genes may be rearranged.
During condensation of chromosomes, homologues are stitched together tightly in a side-by-side alignment that favors crossing over.
Non-sister chromatids cross over, break, and reform; at the breaks, they exchange genetic segments.
Each gene can have alternate forms called alleles; these alleles can be swapped during crossing over to create new combinations of alleles.
Genetic recombination (exchange of alleles) is one source of genetic variation.

13. a Both chromosomes shown here were duplicated during interphase, before meiosis. When prophase I is under way, sister chromatids of each chromosome are positioned so close together that they look like a single thread.
b Each chromosome becomes zippered to its homologue, so all four chromatids are tightly aligned. If the two sex chomosomes have different forms, such as X paired with Y, they still get zippered together, but only in a tiny region at their ends.
c We show the pair of chromosomes as if they already condensed only to give you an idea of what goes on. They really are in a tightly aligned, threadlike form during prophase I.
Figure Animated! Key events during prophase I, the first stage of meiosis. For clarity, this diagram of a cell shows only one pair of homologous chromosomes and one crossover. Here, the paternal chromosome is blue and its maternal homologue is purple.
d The intimate contact encourages at least one crossover to happen at various intervals along the length of nonsister chromatids.
e Nonsister chromatids exchange segments at the crossover sites. They continue to condense into thicker, rodlike forms. By the start of metaphase I, they will be unzippered from each other.
f Crossing over breaks up old combinations of genes and puts new ones together in the cell’s pairs of homologous chromosomes.

14. Meiosis and Mitosis Compared
Mitotic cell division produces clones; this type of division provides for growth of the body and replacement of lost cells.
The chromosome number stays the same (diploid) and there is no mixing up of the genes in mitosis.
Meiosis occurs only in the germ cells used in sexual reproduction.
The chromosome number is halved and each haploid cell is genetically different. Meiosis promotes genetic variation in three ways:
Prophase I rearranges alleles by crossing over.
Metaphase I allows for random assortment of maternal and paternal chromosomes.
Chance brings together different combinations of genes through fertilization.