H
What is rneiosis?
—F
/
a-
/DG
-
(® 3it)
KEY TERMS
gametes: reproductive cells
meiosis: process by which gametes form
125
1
C
In asexual reproduction there is only one parent and one set of chromo
somes. The chromosomes are duplicated. The offspring are just like the
pa rent.
Sexual reproduction is different. In sexual reproduction, there are two
parents—two sets of chromosomes. A new organism is formed with one
set of chromosomes from each parent. The offspring inherits traits from
both parents.
Think of yourself, for example. In some ways you are like your mother.
In other ways you are like your father. You have inherited traits from
both your parents.
How are chromosomes exchanged during sexual reproduction? The
chromosomes of body cells are paired. The chromosomes of sex cells are
not paired. Chromosomes of sex cells are single chromosomes. Therefore,
a sperm or an egg cell has only half as many chromosomes as a body
cell.
When fertilization takes place, the sperm chromosomes join the egg
chromosomes. Together, they add up to the full number of chromosomes
found in body cells.
The fertilized egg, or zygote, now has chromosomes from both parents.
It also has traits from both parents.
Reproductive cells also are called gametes [GAM-eetsl. Gametes develop
from special cells in the body. The process by which gametes form is
called meiosis [my-OH-sisj. You can see the process of meiosis on the
next page.
126
(
1. Original cell with two pair of chromosomes. One
member of each chromosome pair is from one parent.
The other chromosome is from the other parent.
fl
p flJ
4
2. First, the amount of DNA doubles. Then the members
of every pair of similar chromosomes pair with one
another. Frequently, paired chromosomes will exchange
segments in a process called crossing over.
3. Spindle fibers form in the cell. The pairs of similar
chromosomes attach to the spindle fibers.
4. The members of the similar pairs of chromosomes
separate.
5. The cell splits in two, with each cell
containing one double chromo
some from each pair.
6. Spindle fibers form in each new cell.
Double chromosomes attach to the
spindle fibers.
7. The double chromosomes splil
One chromosome goes to each
side of the cell. Then, each cell
splits.
8. Four cells are produced. Each
cell has one chromosome from
each pair.
Figure A
127
Body cells are produced by mitosis. But sperm and egg cells do not form this way.
Reproductive cells are formed by meiosis. Each gamete has only half the usual num
ber of chromosomes. But when the sperm and egg join, the zygote has the full number of
chromosomes.
46 chromosomes
(23 pairs)
A human body cell has 46 chromosomes.
The chromosomes are paired. So there are
23 pairs.
body cell
Figure B
Each human sperm cell has 23 single
chromosomes.
ngIe chromosomes
Figure C
C
23 single
Each human egg cell has 23 single chromo
somes.
egg
Figure D
23 chromosomes
Fertilization links the gamete chromo
somes.
23 chromosomes
Figure E
+
23 chromosomes
23 chromosomes
46 chromosomes
zygote
Figure F
126
The zygote, then, has a total of 46 chromo
somes. 23 are from the mother, 23 are from
the father.
The zygote starts to divide after fertiliza
tion. It divides by mitosis. It divides over
and over again as it develops.
In the space provided, write “true” if the sentence is true. Write “false” if the sentence is false.
1. The chromosomes of body cells are paired.
2. The process by which gametes form is meiosis.
3. A human body cell has 23 chromosomes.
4. In sexual reproduction, an offspring inherits traits from only one parent.
5. Fertilization links the chromosomes of gametes.
6. Every organism has the same number of chromosomes.
7. Spindle fibers form twice during meiosis.
8. A gamete has the same number of chromosomes as a body cell.
9. A gamete has twice the number of chromosomes as a body cell.
10. A frog gamete has 13 chromosomes. Every frog body cell, then, has
26 chromosomes.
Complete each statement using a term or terms from the list below. Write your answers in
the spaces provided.
just like
paired
parent
traits
meiosis
one set
half
two
1. In asexual reproduction there is only one
2. In asexual reproduction,
parent to offspring.
of chromosomes is passed on from
the parent.
3. In asexual reproduction, offspring are
parents. Offspring inherit
4. In sexual reproduction, there are
from both parents.
5. Gamete cells are produced by cell division called
6. A sperm or egg cell has only
as many chromosomes as a body cell.
7. Chromosomes in a body cell are
129
Scientists often study fruit flies because they have large chromosomes whose genes are
easy to see.
• Every body cell of a fruit fly has 8 chromosomes.
• Every fruit fly gamete (sperm or egg) has 4 chromosomes.
4.
1. Body cells reproduce by a process
8 chromosomes
..
4 chromosomes
called
body cell
2. Gamete cells are formed by a process
sper
4 chromosomes
.._1!![
egg
called
3. a) How many chromosomes does the
Figure C
egg cell of a fruit fly have?
b) Sperm cells?
4. What do chromosomes control?
c
4 chromosomes
+
4 chromosomes
zygote
5. How many chromosomes does a fruit
fly
Figure H
A sperm fertilizes an egg.
zygote
6. How many chromosomes will each
body
8chromosomes
have?
cell
have?
7. The offspring will have traits of the
mother and the father. Why?
B chromosomes
zygote dividing
new
body calls
8 chromosomes
Figure I The zygote divides. Then each
new cell divides.
Why must a gamete have only one half the number of chromosomes found in body cells?
130