AP Biology Notes Outline
Chapter 13: Meiosis and the Sexual Life Cycle
SEXUAL REPRODUCTION
ADVANTAGE: results in greater genetic variation! Two parents give rise to offspring that have unique
combonations of genes inherited from both parents. Sexual reproduction involves two different types of
gametes (sex cells): oocytes and spermatozoa.
HOMOLOGOUS CHROMOSOMES
REVIEW Figure 13.4 in your textbook. Homologous chromosomes are chromosome pairs of the same length,
centromere position, staining pattern, and gene possession for the same characters: One homologous
chromosome is inherited from the organism’s father and the other from the mother (see diagram below). These
can easily be seen on a human karyotype (see karyotype of human male below). DIPLOID – means “two sets”.
This represents the cells in which the chromosomes are paired up and have a partner in size and shape. A cell
containing TWO sets of chromosomes (2n), one set inherited from each parent is referred to as a DIPLOID
CELL.
Meiosis Overview
Meiosis is the process by which the number of chromosomes per sex cell is cut in half through the separation of
homologous chromosomes in a diploid cell. In other words, meiosis is the division of sex cells that results in
the production of eggs (in females) and sperm (in males). The GOAL of meiosis is to reduce the number of
chromosomes from a diploid number to a haploid number. The new cells that result are called gametes, and can
join together with their counterparts to produce a new diploid organism (egg with sperm, sperm with egg).
HAPLOID – means “one set”; this describes the sex cells
or gametes (EGG AND SPERM) – these contain only a
SINGLE set of chromosomes. So, for a human, body cells
are diploid and have a chromosome number of 46 (23
pairs). But, sex cells are haploid, and have a chromosome
number of JUST 23.
AP Biology Notes Outline
Chapter 13: Meiosis and the Sexual Life Cycle
Phases of Meiosis:
REVIEW Figure 13.7 in your textbook. Meiosis is divided into
two distinct stages:
 Meiosis I
 Meiosis II
Meiosis I accomplishes separation of homologous chromosomes
(see diagram to right). Meiosis I is very similar to mitosis,
except that the duplicated chromosomes do not separate the
chromatids; instead, the homologous chromosomes line up and
form a TETRAD and the homologues separate.
Meiosis II accomplishes separation of sister chromatids (see
diagram to right). Meiosis II begins WITHOUT a copying of
chromosomes; thus, when division occurs, there are less
chromosomes in the resulting daughter cells that were in the
parent cell – this accomplishes reduction division.
NOTE: DNA replications ONLY ONCE during meiosis
(during meiosis I only)!!!
REVIEW Figure 13.8 in your textbook and watch the following USEFUL ANIMATION
http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
Crossing Over:
As homologous chromosomes pair up and form tetrads in meiosis I,
they may exchange portions of their chromatids. This event is called
CROSSING OVER. This results in an exchange of genetic material
that gives new gene combinations. The site at which the exchange
occurs is called the chiasma.
During prophase I of meiosis, the duplicated chromosomes pair with
their homologues, in a process called synapsis. During this process,
a protein “zipper” called the synaptonemal complex holds the
homologous chromosomes tightly together all along their lengths.
When this complex disappears in late prophase, the four closely
associated chromatids of a homologous pair are visible as a tetrad.
Crossing Over (exchanging portions of homologous chromosomes)
occurs during PROPHASE 1 OF MEIOSIS I. Crossing over gives
rise to recombinant chromosomes, individual chromosomes that
have some combination of DNA originally derived from 2 different
parents.
AP Biology Notes Outline
Chapter 13: Meiosis and the Sexual Life Cycle
Comparison of Meiosis in Males & Females:
In males, meiosis results in four equal-sized gametes – sperm.
In females, only one large egg is made. The other three cells, called polar bodies, are reabsorbed by the
organism and the materials recycled.
 The reason for this in the female is the need for more space and organelles in order to support the
development of a new organism.
 The sperm will join with the egg, but it is in the egg where continuation and development of the new
organism will actually occur.
AP Biology Notes Outline
Chapter 13: Meiosis and the Sexual Life Cycle
Mistakes in Meiosis
REVIEW the following animation:
http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.html
Mistakes in meiosis I and II can cause nondisjunction (not coming apart) of chromosomes - leads to abnormal
number of chromosomes in gametes -- trisomy 21 is the most common. In trisomy 21, individuals have 3
copies of chromosome 21 (down syndrome). Child will exhibit facial characteristics of Down syndrome.
 If the nondisjunction occurs during Meiosis I, then FOUR ABNORMAL gametes will be formed.
 If the nondisjunction occurs during Meiosis II, then two NORMAL and two ABNORMAL GAMETES
will be formed.
Parthenogensis & Cloning:
Both bypass typical meiotic processes!
 Parthenogenesis is a type of reproduction, occurring in some insects and flowers, in which the
unfertilized ovum develops directly into a new individual - Does not necessarily produce identical
clones of the parent. Rare in plants!
 Cloning occurs when an organism is descended asexually from a single ancestor, such as a plant
produced by layering or a polyp produced by budding. Offspring is IDENTICAL to parent!
AP Biology Notes Outline
Comparing Mitosis & Meiosis
BE SURE YOU CAN DO THIS!!!
Chapter 13: Meiosis and the Sexual Life Cycle