Meiosis
I.
THE LIFE CYCLE OF HUMANS
A. Meiosis
1. Occurs in the gonads (testes and ovaries) to produce gametes (sperms, egg),
which are N, or haploid (1 set of chromosomes)
2. When the sperm fertilizes the egg, a zygote (diploid or 2N) is produced
B. Mitosis
1. The fertilized egg divides mitotically for growth and repair to form the adult male
or female individual with 46 chromosomes (23 pairs) in each cell
II.MEIOSIS
A. Introduction
1. Used in sexual reproduction in animals (produces the egg and sperm) and
plants
2. Importance
a) To keep the chromosome number constant generation after generation
b) Ensure the next generation has a different genetic makeup
B. Overview of Meiosis
1. Consists of 2 cell divisions (Meiosis I and II) resulting in 4 daughter cells
a) Each daughter cell will have one-half the number of chromosomes of the
parent cell
(1) This is referred to as haploid (1N)
(2) Gametes will have one of each type of chromosome
b) Homologous chromosomes separate during meiosis so that each daughter
cell has only 23 chromosomes, one from each homologous pair of
chromosomes
2. Homologous chromosomes, each duplicated line up side by side, resulting in 4
chromatids in close proximity
a) These are referred to as tetrads
(1) Four homologous sister chromatids
3. Crossing-over occurs between non-sister chromatids (but homologous
chromosomes) during synapsis
a) This allows genetic exchange between chromosomes so as to provide new
combination of genes that are different from either parent
4. During meiosis I, homologous chromosomes separate (allows one chromosome
of every homologous pair to reach each gamete)
5. During meiosis II, sister chromatids separate
C. The Stages
1. Meiosis I
a) During prophase I, events are the same as in prophase of mitosis except
homologous chromosomes undergo synapsis and cross over
(1) Stages
(a) Leptotene stage
(i) Chromosomes begin to condense
(ii) Homology search
(b) Zygonema stage
(i)
(c) Pachynema stage
(i) Coiling and shortening continue
(ii) Synapsis
(d) Diplonema stage
(i) Chiasma -- area where crossing over occurred
(e) Diakinesis stage
(i) Nuclear membrane disintegrates
(ii) Spindle apparatus attach
b) At metaphase I, tetrads line up at the equator
(1) During mitosis, homologous chromosome do not pair up
c) During anaphase I, homologous chromosomes separate
(1) During mitosis, sister chromatids split, but in meiosis I, the chromosomes
remain intact
d) During telophase I, nuclear envelope reforms around the haploid number of
chromosomes
2. Interkinesis
a) No DNA replication during this stage
3. Meiosis II
a) No duplication of chromosomes occurs during interphase
b) At prophase II, a spindle apparatus appears and attach to the chromosomes
c) During metaphase II, chromosomes are lined up at equator and at end of
metaphase
d) During anaphase II, sister chromatids of chromosomes split into sister
chromosomes
e) During telophase II, sister chromatids separate and move toward each pole
(1) The spindle disappears, nuclear envelope reforms, and cytokinesis occurs
D. Animal Meiosis
1. Spermatogenesis
a) Characteristics
(1) Production of sperm in testes of males via meiosis results in 4 viable haploid
sperm from each original cell
b) Stages
(1) Spermatogonia divide by mitosis to form primary spermatocytes
(2) Primary spermatocytes undergo meiosis I to form secondary spermatocytes
(3) Secondary spermatocytes undergo meiosis II to form spermatids
(4) Spermatids mature into sperm or spermatozoa by a process called
spermiogenesis
2. Oogenesis
a) Production of eggs in ovaries of females via meiosis results in unequal
cytoplasmic division producing only one functional egg and 2 nonfunctional polar bodies that later degenerate
(1) Oogonium form primary oocytes which are arrested in prophase I
(a) All a females primary oocytes are produced prior to birth
(b) Prophase continues prior to ovulation
(2) Primary oocytes continue meiosis I to produce a secondary oocyte and a
smaller polar body
(3) The secondary oocytes undergoes meiosis II to form an ootid (which matures
into an oocyte) and a second polar body
(4) Meiosis II is stimulated by fertilization
3. The sperm is small and flagellated while the egg is stationary and quite large
(extra cytoplasm provides nutrients for developing embyro)
III.COMPARISON
OF MITOSIS TO MEIOSIS
A. Meiosis only occurs in the gonads that give rise to the gametes
B. Mitosis occurs in all types of cells in the body
C. Mitosis compared to Meiosis I
1. Meiosis I has homologous chromosomes pairing in Prophase I, tetrads at the
equator in Metaphase I, and homologous chromosomes separate in Anaphase I
D. Mitosis compared to Meiosis II
1. Both sister chromatids separate during anaphase, but there are 4 haploid
daughter cells in Telophase II compared to 2 diploid daughter cells in mitosis
IV.ALTERATION
OF GENERATION
A. In some organisms (i.e., plants) the product of meiosis becomes a multicellular
haploid organism
1. These are referred to as the gametophyte generation
2. The gametophytes fuse to form a diploid organism
a) The diploid organism is referred to as the sporophyte generation
(1) The sporophytes produces haploid spores to give rise to the next gametophyte
generation
3. Some plants are sporophyte dominant and others are gametophyte dominant
a) Sporophyte dominant plants spend most of their time in the diploid state,
while the gametophyte is reduced
(1) This includes ferns, gymnosperms (cone bearing plants), and angiosperms
(flowering plants)
b) Gametophyte dominant plants spend most of their time in the haploid state,
while the sporophyte is reduced
(1) This is characteristic of mosses, liverworts, and hornworts