Chapter 9.5 - Meiosis
College Prep Biology
Mr. Martino
I. Introduction
• Germ Cells:
develop into sex
cells (sperm and
egg)
• Gametes: mature
sex cells – sperm
and egg
III. Sexual and Asexual
Reproduction
• Sexual Reproduction:
reproductive process that
includes the union of sperm
and egg
– Two parents – each
donate a complete set of
genes
• Asexual Reproduction:
production of offspring
without the participation of
sperm and egg
– Single parent
• Allele: unique form of the
same gene (a gene)
IV. Meiosis Halves Chromosome
Number
• Meiosis: nuclear division
that divides chromosomes
twice
• Diploid number: (2n) pair
of each type of chromosome
– Homologous chromosomes:
both chromosomes of a pair
• Same length and shape
• Genes are for same traits
• Haploid number: (n)
results from the second
division of meiosis (sex
cells)
• Sister chromatids: two
attached chromosomes
– Once they pull apart, they
are considered
chromosomes
• Meiosis consists of 2
divisions:
– Meiosis I: DNA is
replicated and divides
– Meiosis II: another cell
division without more
chromosome doubling
V. Stages of Meiosis
• Meiosis: the process
that produces haploid
gametes (sex cells)
– 2 consecutive divisions:
meiosis I and meiosis II
• Meiosis II is
essentially the same
as mitosis
– 4 daughter cells
produced
– Only 1 duplication of
chromosomes, second
division halves
chromosome number
Interphase I
• Like interphase in
mitosis
• Each pair
replicates
forming a 4
chromatid
structure
Prophase I
• Most complex phase of
meiosis
• Lasts 90% of meiosis
• homologous chromosomes
form tetrads (4
chromatids)
Structure of a Tetrad
• Tetrad = structure
consisting of 2 pairs
of chromosomes
– 2 chromosomal
pairs = 4 sister
chromatids
– Visible during
Prophase I only
Metaphase I
• Tetrads align on
metaphase plate
Anaphase I
• Begins as homologous
chromosomes (still
attached at centromere)
separate and move towards
opposite poles
• Only tetrads are separated
Telophase I and Cytokinesis
• Chromosomes arrive at
opposite poles
• Cytokinesis results in 2
diploid cells
• No DNA replication
occurs between meiosis I
and meiosis II
Prophase II
• Chromosomes must
condense again
since there was a
pause prior to
meiosis II
Metaphase II
• Chromosomes are
aligned at
metaphase plate
Anaphase II
• Begins when
centromeres of
sisters chromatids
separate
Telophase II and Cytokinesis
• Nuclei form at
opposite poles
• Cytokinesis
occurs
• Forms 4 haploid
daughter cells
VI. From Gametes to Offspring
• Spermatogenesis: sperm
formation
– results in 4 viable
sperm
• Oogenesis: egg formation
– First meiotic division
results in a polar body and
a secondary oocyte
– Second division results in
3 polar bodies and 1 egg
– Polar bodies are
reabsorbed – egg is viable
VII. Meiosis and Mitosis Compared
• Chromosomes of both
replicate only once
• Mitosis consists of one
cell division and two
diploid cells
• Meiosis entails two cell
divisions and four
haploid cells
• Mitosis and meiosis II
are virtually identical
9.6 Meiosis Increases Genetic
variation among offspring
• Genetic variation – differences among the
gene combinations in the offspring due to
sexual reproduction
• Assortment of Chromosomes – the aligning of
chromosomes during metaphase I is random;
which results in many possible chromosome
combinations
• Crossing Over – the exchange of genetic
material in between homologous
chromosomes; during prophase I when
tetrads are present