Recall the Phases of the Cell Cycle
Phase
Interphase
Stage
G1
S
G2
Mitotic Phase Mitosis
cytokinesis
Description
Growth and cellular
functions; organelle
replication
Growth and chromosome
replication; organelle
replication
Preparation for cell division
Division of nucleus
Division of cytoplasm to
give 2 daughter cells
Recall the 4 Phases of Mitosis
Phase
Prophase
Metaphase
Anaphase
Telophase
Description
DNA condenses; spindle fibers begin
to form
Chromosomes line up in a single file
at the middle of the cell
Sister chromatids separate and pull to
opposite sides
Nuclear membrane reforms,
chromosomes unwind
CHAPTER 4
REPRODUCTION OF ORGANISMS
LESSON 1 – SEXUAL REPRODUCTION AND MEIOSIS
What is Sexual Reproduction?
 Observation - The individual kittens in a litter can
have different colors.
 Reason – Sexual reproduction
In sexual reproduction the genetic material (DNA) from
two different cells combine, producing an offspring.
 Sex cells are required.
 The female sex cell is called an egg (ovum).
o It is forms in an organ called an ovary.
 The male sex cell is a sperm.
o If forms in an organ called a testis.
Fertilization is the joining of an egg cell and a sperm cell.
 Fertilization produces a new cell called a zygote.
 In a litter of 4 kittens, each kitten began as a zygote.
 Each zygote had its own combination of genes in its
DNA.
Diploid Cells
Multicellular organisms that undergo sexual
reproduction have two general types of cells.
 Diploid cells are the great majority of the cells –
body cells.
 Diploid cells have pairs of chromosomes.
Chromosomes in Diploid Cells
 Recall – DNA coiled up forms a chromosome. A
duplicated chromosome has two sister chromatids
joined by the centromere.
 Page 118 shows matched pairs of chromosomes.
These are the homologous pairs in a human diploid
cell:
 Pairs of chromosomes that have genes for the same
traits arranged in the same order are called
homologous chromosomes.
 Each organism has a particular number of
chromosomes, and half of the number of
homologous pairs. (p. 118)
o One chromosome in a pair is inherited from the
mother, and the other from the father.
 Humans have 23 pairs of chromosomes in diploid
cells (body cells).
o If the number of chromosomes is different,
there are problems.
o Down syndrome occurs when there is an extra
copy of chromosome 21.
o For a male, the last pair is an X chromosome and
a Y chromosome. For a female, the last pair is
two X (long) chromosomes.
Haploid Cells
Sex cells are haploid cells.
 Haploid cells are cells that have only one
chromosome from each pair.
 To produce the haploid cells, a special cell division
called meiosis takes place.
 In meiosis, one diploid cell divides and makes four
haploid cells.
 Meiosis occurs only during the formation of sex cells.
How do diploid cells differ from haploid cells?
Comparison of Mitosis and Meiosis
Mitosis
 One cell divides once to produce 2 identical
daughter cells.
 The daughter cells have the same number of
chromosomes as the original cell.
 All the cells are diploid.
Meiosis
 One reproductive cell divides twice to produce 4
haploid cells.
 The 4 haploid cells have half the number of
chromosomes as the original cell.
 The four cells are sex cells for reproduction.
Before Meiosis
A reproductive cell goes through the interphase.
Duplicated chromosomes – two sister chromatids joined
by a centromere – are produced.
Summary of Meiosis (pages 120 – 121)
Meiosis I
The Division of One Diploid Cell into Two Cells
Stage of Meiosis I
Description
Prophase I
Nuclear membrane breaks apart.
Homologous chromosomes come
together and form pairs.
Metaphase I
Homologous chromosome pairs line
up along at the middle of the cell.
Spindle fibers attach to the
chromosomes.
Anaphase I
Chromosome pairs separate and are
pulled to opposite ends of the cell.
Sister chromatids stay together.
Telophase I
Nuclear membrane forms around
each set of duplicated chromosomes.
Cytoplasm divides during cytokinesis,
forming two daughter cells.
Meiosis II
The Division of Two Cells into Four Haploid Cells
Stage of Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Description
Nuclear membrane breaks apart
within each cell.
Sister chromatids line up at the
center of the cells.
Sister chromatids of each
duplicated chromosome begin to
separate and are pulled to opposite
ends of the cells.
A nuclear membrane forms around
each set of chromatids (again called
chromosomes). With cytokinesis
the cytoplasm of the 2 cells divides,
giving 4 haploid cells.
Advantages of Sexual Reproduction
 It gives genetic variation.
 Selective breeding is possible. Organisms with
desired traits are chosen and bred.
Disadvantages of Sexual Reproduction
 Takes time and energy
What You Must Know About Mitosis and Meiosis
Mitosis
 Begins with one diploid cell – a body cell
 Has 4 phases – prophase, metaphase, anaphase,
telophase
 After mitosis, the cell divides in two (by cytokinesis)
to give 2 identical daughter cells that are diploid.
Meiosis
 It begins with one reproductive cell that is diploid.
o The cell contains duplicated chromosomes.
 A duplicated chromosome has two identical
sister chromatids joined by a centromere.
o The duplicated chromosomes are found in pairs
called homologous chromosomes.
 Each duplicated chromosome in the pair
has genes for the same traits arranged in
the same order.
 The original reproductive cell undergoes meiosis I
with these phases:
o prophase I, metaphase I, anaphase I, telophase I
 This gives two daughter cells.
o Each daughter cell has half the number of
chromosomes as the original cell.
o The chromosomes are duplicated chromosomes.
 The two daughter cells undergo meiosis II with these
phases:
o prophase II, metaphase II, anaphase II,
telophase II
 This gives 4 new cells.
o The new cells are haploid, having half the
chromosomes of the original reproductive cell.
o The chromosomes are chromatids.
 Reason: during anaphase II the sister
chromatids of each duplicated chromosome
are separated.