Biology
Meiosis and Sexual Reproduction
Chapter 7 Notes
Section 1 terms:
Meiosis-a form of cell division that halves the number of
chromosomes when forming gametes
Crossing-over- when portions of a chromatid are broken and
exchanged with the corresponding chromatid in a homologous pair
Independent assortment- the random distribution of homologous
chromosomes during meiosis
Spermatogenesis- sperm production in male animals
Sperm- male gamete
Oogenesis- egg production in female animals
Ovum- female gamete
I. Meiosis
A. Haploid Cell Formation
1. gametes are haploid
2. meiosis is the process by which gametes are formed
3. the chromosome number is reduced to one set
B. The 8 stages of Meiosis:
Stage 1: Prophase I (Starting off with one cell that is diploid
and has homologous chromosomes)
1. chromosomes become visible
2. the nuclear envelope breaks down
3. crossing over occurs
Stage 2: Metaphase I
1. homologous pairs move to the equator of the cell
Stage 3: Anaphase I
1. homologous chromosomes move to opposite ends
Stage 4: Telophase I and cytokinesis (Ending with 2 cells that
each contain one chromosome as sister chromatids)
1. chromosomes gather at the poles
2. cytoplasm divides
Stage 5: Prophase II (starting with 2 cells that each contain
one chromosome as sister chromatids)
1. new spindle forms around the chromosomes
Stage 6: Metaphase II
1. chromosomes line up at the equator
Stage 7: Anaphase II
1. centromeres divide
2. chromatids move to opposite poles of the cell
Stage 8: Telophase II and Cytokinesis
(ending with 4 haploid cells)
1. nuclear envelope forms around each set of chromosomes
2. the cytoplasm divides
II. Meiosis and Genetic Variation
A. Independent assortment (remember it is the random
distribution of homologous chromosomes)
1. this allows for genetic variation
2. it is by chance as to which of the 2 pair of
chromosomes that an offspring receives
3. this process gives about 8 million gametes with
different genetic possibilities from just one cell
B. Crossing-Over and Random Fertilization
1. crossing-over gives an almost infinite number of
genetic possibilities
2. in random fertilization, each gamete is produced
independently and when an egg is fertilized by sperm,
the number of genetic possibilities is 64 trillion
C. Importance of Genetic Variation
1.The pace of evolution is sped up by genetic recombination
a. read examples in book and discuss (p. 147)
2. the combination of genes from 2 organisms results in a
type that is different from the 1st two.
III. Meiosis and Gamete Formation
A. Meiosis in males
B. Meiosis in Females
Section 2 terms:
Asexual reproduction- single parent passing copies of all of its
genes to its offspring
clone- an organism that is genetically identical to its parent
Sexual Reproduction- 2 parents each giving ½ of its genes to an
its offspring
Life Cycle- the entire span of an organisms life
Fertilization- the process of gametes joining together resulting
in a zygote
Sporophyte- the diploid phase in a plants life cycle –that
produces spores
Gametophyte- the haploid phase in a plants life cycle—produces
Gametes by mitosis
I. Types of Asexual Reproduction
A. Fission
1. this is the separation of a parent into 2 or more
individuals of about equal size
2. amoebas
B. Fragmentation
1. this is when the body breaks into several pieces and
adults will grow from this
2. sea stars
C. Budding
1. new individuals split off from existing ones
2. hydra, yeast
D. Asexual reproduction is the most simple and primitive
means of reproduction
1. does not use a lot of energy to produce gametes
2. little genetic variation
II. Sexual Reproduction
A. Haploid Life Cycle
1. Some protists, Algae, and some Fungi
2. the zygote is the only diploid cell
a. it undergoes meiosis immediately after it is
formed creating new haploid cells
b. the haploid cells give rise to haploid
multicellular individuals that make gametes
c. gametes are formed by mitosis NOT meiosis
d. the gametes fuse to produce a diploid zygote
B. Diploid Life cycle
1. adult individuals are diploid
2. offspring receive chromosomes from 2 parents
3. haploid gametes formed by meiosis (sperm & egg)
4. sperm and egg join by fertilization resulting in
a diploid zygote
a. the one celled zygote gives rise to whole
individual
C. Alternation of Generations--multicellular
1. Plants, Algae and some protists undergo a life cycle
that alternates from between haploid and diploid
phases
Diploid phase
Haploid phase
Makes haploid spores called
The gametophyte produces
sporophytes by meiosis
gametes by mitosis
Spores can grow into adult
The gametes fuse and give rise
without fusing with another cell
to the diploid phase
This new multicellular organisms
is called a gametophyte
D. Similarities between all 3 types of sexual reproduction
1. all involve alternation of haploid and diploid phases
E. Differences between the 3 types of sexual reproduction
1. differ in the phases that become multicellular