Chapter 4 Introduction
Lesson 1
Sexual
Reproduction
and Meiosis
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What is sexual reproduction?
• Sexual reproduction is a type of
reproduction in which the genetic
materials from two different cells
combine, producing an offspring.
• The female sex cell is called an egg
• The male sex cell is called sperm
What is sexual reproduction? (cont.)
During a process
called fertilization,
an egg cell and a
sperm cell join
together to create a
zygote.
Diploid Cells
• Organisms that reproduce sexually
form body cells and sex cells.
• Diploid cells are body cells that have
pairs of chromosomes.
Diploid Cells (cont.)
Different organisms have different numbers
of chromosomes.
Diploid Cells (cont.)
Pairs of chromosomes
that have genes for the
same traits arranged in
the same order are
called homologous
chromosomes.
Because one
chromosome is inherited
from each parent, the
chromosomes are not
identical.
MOM
DAD
46
sperm
Egg
92
46
chromosomes!
chromo
somes
Zygote
• Both sperm
and eggs have
only ½ the
normal number
of
chromosomes
• Human sperm
and eggs have
23
chromosomes
• No matching
pairs!!!
Haploid Cells
Haploid cells are cells that have only
one chromosome from each pair of
chromosomes.
haploid
from Greek haploeides, means
“single”
Haploid Cells
In meiosis, one diploid cell divides and
makes four haploid sex cells.
• Meiosis occurs during the formation of sex
cells only.
• Sex cells (sperm & egg cells)
– Different from ordinary body cells
– Each have 1 chromosome from each
homologous pair
• Only 23 chromosomes in human sex cells
23
sperm
Egg
46
23
chromosomes!
chromos
omes
Zygote
Why is meiosis important?
• Meiosis forms sex cells with the correct
haploid number of chromosomes.
• When haploid sex cells join together
during fertilization, they make a diploid
zygote, or fertilized egg.
• Meiosis also creates genetic variation
by producing haploid cells.
Why is meiosis important? (cont.)
The fertilized egg, formed when sex cells join
together, divides by mitosis to create a diploid
organism.
How do mitosis and meiosis differ?
• During mitosis and cell division, a
body cell and its nucleus divide once
and produce 2 identical cells.
• The two daughter
cells produced by
mitosis and cell
division have the
same genetic
information.
How do mitosis and meiosis differ? (cont.)
• During meiosis, a reproductive cell and
its nucleus divide twice and produce 4
cells—each with half the # of
chromosomes as the parent cell.
• Meiosis forms sex
cells used for sexual
reproduction.
Mitosis: The nucleus
splits once
resulting in 2 new
cells
Meiosis: The nucleus
splits twice resulting
in 4 new cells
• Sex chromosomes carry
genes that determine
whether the offspring is male
or female
• Humans have 46
chromosomes (23 pairs)
• Pair number 23 determines
your sex
• XX= female
• XY= male
Advantages of Sexual Reproduction
• Sexual reproduction produces
offspring that have a new combination
of DNA.
• Genetic variation occurs in all
organisms that reproduce sexually.
• Due to genetic variation, individuals
within a population have slight
differences that might be an advantage
if the environment changes.
Advantages of Sexual Reproduction
(cont.)
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Selective breeding has been used to
develop many types of plants and
animals with desirable traits.
Disadvantages of Sexual
Reproduction
• Organisms have to grow and develop
until they are mature enough to
produce sex cells.
• Searching for a mate takes time and
energy and might expose individuals to
predators, disease, or harsh
environmental conditions.
• Fertilization occurs when an egg cell
and a sperm cell join together.
• Organisms produce sex cells through
meiosis.
• Sexual reproduction results in
genetic variation among individuals.
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