Chapter 23

Chapter 23
Meiosis and
Sexual
Reproduction

Asexual Reproduction
(review)
• Single parent gives rise to new
offspring by mitotic cell division
• Each new individual receives a set of
chromosomes identical to the parent
chromosomes
• No variation of hereditary information

(I) Sexual Reproduction
• Two parents give rise to new offspring by
the fusion of nuclear materials from two
different cells
• Offspring are not identical to the parent
• Variations exist, which increases the species
ability to adapt to the changing
environment

(A) Gonads
• Sex organs
1. Males – Testes
2. Females – Ovaries

(A) Gametes
• Sex cells
1. Males – sperm
2. Females – ovum

(B) Fertilization
• The fusion of the nuclei of one
sperm with one ova (egg) to
produce a zygote
• Sperm (n) + Ovum (n) =2n
• Sperm (23) + Ovum (23) = 46

Fertilization and the Stages of
Mitosis

(C) Haploid
• Also known as monoploid
• Represented by: n
• The gametes contain half the number of
chromosomes
• Remember, gametes are sex cells that
combine to form new offspring. Therefore
gametes are haploid and once they fuse
(combine), they form a zygote that is diploid

(D) Diploid
• Contain the full number (set)
of chromosomes
• Represented by: 2n

Homologous Chromosomes
• Pairs of similar chromosomes
• Humans have 46 chromosomes:
22 pairs of homologous chromosomes
1 pair of sex chromosomes

(II) Meiosis
• Known as Reduction Division
• Meiosis is a type of cell
division in which the daughter
cell receives only half the
number of chromosomes
present in the parent cell

23 Pairs of chromosomes of a human cell
• The chromosomes
labeled X and Y
are the sex
chromosomes
• XX = female
• XY = male

(III) Stages of Meiosis
• In meiosis, each cell divides twice:
1. The first stage is very similar to mitosis
2. The second stage is also like mitosis
with the exception that there is
NO replication of chromosomes in the
second phase (meiosis 2)

Phases of Meiosis
First Division
Prophase I
Metaphase I
Anaphase I
Telophase I
Second Division
Prophase II
Metaphase II
Anaphase II
Telophase II

Second Division

Meiosis

Key
SS = single-stranded
DS = double-stranded

First Meiotic
Division

Prophase I
• Replication of SS chromosomes
into DS chromosomes has
already occurred.
• Each chromosome then pairs up
with it’s homologous
chromosome.
• This is known as synapsis.

Prophase I
• Each chromosome pair consists
of 4 chromatids and thus is a
tetrad.
• Crossing over occurrs when
segments of DNA are exchanged
between the chromatids found in
tetrads.

Metaphase I
• Alignment of the tetrads (4
chromatids) in the middle of the
cell (equatorial plane).

Telophase I
• Cytokinesis occurs as in Mitosis
however each of the daughter cells
has half the # of chromosome
compared to the parent.
• After this point, the chromosomes
WILL NOT replicate.
• Cells usually start prophase 2
immediately after telophase 1.

Second
Meiotic Division

Prophase II
• Chromosomes DO NOT
replicate.
• DS chromosomes move
towards the equatorial plane.

Metaphase II
• Tetrads line up in the
MIDDLE of the cell.
• This is sometimes called the
equatorial plane or the
metaphase plate.

Anaphase II
• DS chromosomes
separate.
• SS chromosomes move
to opposite ends.

Telophase II
• Both daughter cells undergo
cytokinesis forming 4 haploid
cells.
• Thus, each cell has ½ the number
of chromosomes as the parent.

Meiosis
• Diploid (2n) cells divide
haploid (n) cells
These cells mature into specialized
reproductive cells (sperm and ova).

(III) Sexual Reproduction
in
Animals

(A) Reproductive System
• Gonads- specialized organs that
produce gametes
a) ovaries - female gonad
- produce ova (egg)
b) testes - male gonad
- produce sperm cell

(B) Hermaphrodite
• Contain both male and female
reproductive structures

(C) Gametogenesis
• Process by which gametes are
produced in the gonads
• Two types:
Spermatogenesis
Oogenesis

Spermatogenesis
• Primary spermatocyte
goes through first meiotic
division and form two
daughter cells of equal
size
• Each daughter cell
(secondary spermatocyte)
goes through the second
meiotic division forming
four motile sperm cells of
equal size

• The primary oocyte goes
through first meiotic
division and the
cytoplasm of the cell is
divided unequally
• The larger daughter cell
is called the secondary
oocyte and the smaller
daughter cell is called the
polar body
• During second meiotic
division, 1 mature ovum
is produced and 3 polar
bodies
Oogenesis

Comparison of Ova and Sperm
• Egg cells contain stored food in the
form of yolk.
• Egg cells are larger than sperm.
• Sperm are motile, egg cells sessile
(can’t move on their own).

Fertilization
• Union of a haploid
sperm nucleus with a
haploid egg nucleus
• This results in a
diploid zygote with the
full number of
chromosomes

Fertilization
• Union of a haploid
sperm nucleus with a
haploid egg nucleus
• This results in a
diploid zygote with the
full number of
chromosomes

Two types of Fertilization
Internal Fertilization
External Fertilization

Internal Fertilization
• Takes place inside the body of
the female
• Less eggs are required
Ex: mammals and birds

Internal Fertilization

External Fertilization
• Eggs are fertilized outside the body of
the female
• Large number of eggs are required
• This type of fertilization is found in
an aquatic environment
Ex: fish, frogs, & salamanders.

External Fertilization
Typical amphibian habitat

External Fertilization
Eastern spotted newt

External Fertilization
Eastern spotted newt
spots
newt

External Fertilization
An example of an “egg sac.”
“egg sac’s” actually contain
developing embryos.

External Fertilization
Multiple “egg” sacs
Near Bull’s Hill , NY
“egg sac”
Saccone

External Fertilization
A frog and multiple “egg” sacs
Near Bull’s Hill , NY

External Fertilization
Another “egg” sac
found near New Paltz, NY

External Fertilization
Close-up of individual “frogs”
developing (already fertilized)