Chapter 4 Section 2 Sexual Reproduction and Meiosis

1. Chapter 4: Section 2
Sexual Reproduction and
Meiosis

2. II. Sexual Reproduction
A. Introduction to Sexual Reproduction
1. In sexual reproduction, a new organism is produced
when sex cells from two parents combine.
a. Each sex cell is produced by a different parent.
1. Sperm are sex cells produced by the male
parent.
a. Sperm are usually small with whiplike
tails.
2. Eggs are sex cells produced by the female
parent.
a. Eggs are usually large and contain food
material.
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3. B. Production of Sex Cells
1. Human body cells have 23 pairs of
chromosomes, but human sex cells
have only 23 total chromosomes.
2. The process of nuclear division that
produces sex cells is called Meiosis.
a. Meiosis takes place in cells of
reproductive organs in both plants and
animals.
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4. C. Importance of Sex Cells
1. In body cells, chromosomes are found in
pairs.
a. A cell that has two of every kind of
chromosome is said to be diploid.
b. Diploid is written as 2n in Biology.
2. Sex cells contain only one chromosome
from each matched pair.
a. A cell that contains only one chromosome
is said to be haploid.
b. Haploid is written as 1n in Biology.
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5. Haploid Cells Diploid Cells
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6. D. Comparing Haploid and Diploid Cells
1. A human body cell is considered
diploid.
a. Human body cells contain 23 pairs of
chromosomes.
2. A human sex cell is considered to be
haploid.
a. A human sex cell contains 23 total
chromosomes.
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7. E. Fertilization
1. Sexual reproduction starts with the
formation of sex cells and ends when
one sex cell joins with another; a
new organism is formed.
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8. E. Fertilization
2. The joining of an egg and a sperm is
called fertilization.
a. The cell that forms in fertilization is
called a zygote.
b. If an egg (1n) with 23 chromosomes
joins with a sperm (1n) that has 23
chromosomes, a zygote forms that has
46 chromosomes.
1. The zygote is considered to be diploid.
(2n)
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9. 9

10. F. Chromosomes in Species
1. Each species has a fixed number of
chromosomes.
a. Humans have 46 chromosomes.
b. A donkey has 66 chromosomes.
c. A horse has 60 chromosomes.
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11. III. Meiosis
A. Introduction to Meiosis
1. In Meiosis, there are two divisions of
the nucleus, Meiosis I and Meiosis II.
a. Mitosis only has one division!!!
2. Meiosis deals only with sex cells.
a. Sperm and Egg
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12. Diagram of Meiosis I
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13. III. Meiosis
B. Meiosis I
1. Interphase 1: Cells undergo a round of
DNA replication, forming duplicate
chromosomes.
2. Prophase 1:
a. Double stranded chromosomes and
spindle fibers appear.
b. The nuclear membrane and nucleolus
disappear.
c. Like chromosomes come together in
matching pairs to form a TETRAD.
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14. III. Meiosis
B. Meiosis I
3. Metaphase 1
a. Pairs of chromosomes line up in the
center of the cell.
b. Centromeres become attached to the
spindle fibers.
4. Anaphase 1
a. The spindle fibers pull the homologous
(like chromosomes) toward opposite
ends of the cell.
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15. III. Meiosis
B. Meiosis I
5. Telophase 1
a. The cytoplasm
divides and two
cells form.
b. Each
chromosome is
still double
stranded!!
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16. Diagram of Meiosis II
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17. III. Meiosis
C. Meiosis II (follows Meiosis I)
1. Prophase 2
a. The double stranded chromosomes
and spindle fibers reappear in each
new cell.
2. Metaphase 2
a. The double stranded chromosomes
move to the center of the cell.
b. Centromeres attach to the spindle
fibers.
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18. III. Meiosis
C. Meiosis II (follows Meiosis I)
3. Anaphase 2
a. The centromere divides, and two
strands of each chromosome separate
and move to opposite ends of the cell.
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19. III. Meiosis
C. Meiosis II (follows Meiosis I)
4. Telophase 2
a. The spindle fibers disappear, and a
nuclear membrane forms around the
chromosomes at each end of the cell.
b. Each nucleus contains only half the
number of chromosomes that were in
the original nucleus.
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20. Meiosis I and Meiosis II
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21. III. Meiosis
D. Facts about Meiosis
1. A cell with 46 chromosomes at the
beginning of Meiosis 1 divides to
produce cells that each only have 23
single stranded chromosomes at the
end of Meiosis 2.
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22. 22

23. III. Meiosis
E. Mistakes in Meiosis
1. Mistakes are more common in plants
than animals.
2. Mistakes can produce sex cells with too
few or too many chromosomes.
a. Sometimes, zygotes produced from these
sex cells die.
b. If the zygote lives, the organism may not
grow normally because cells will have the
wrong number of chromosomes.
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24. III. Meiosis
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