MEIOSIS
M
m e io s is
A t y p e o f c e l l d i v i s i o n t h a t o c c u r s i n t h e
o v a r i e s
a n d
t e s t e s
d u r i n g t h e p r o d u c t i o n
o f e g g a n d s p e r m c e l l s . D u r i n g m e i o s i s
i n h u m a n s , a c e l l c o n t a i n i n g 2 3 p a i r s o f
c h r o m o s o m e s
( 4 6
i n
t o t a l )
d i v i d e s
t o
f o r m f o u r s p e r m o r e g g c e l l s , e a c h w i t h
2 3 s i n g l e c h r o m o s o m e s .
F i r s t , e a c h c h r o m o s o m e i s d u p l i c a t e d
( m a k i n g
a
t o t a l
o f
9 2 ) ;
t h e
d o u b l e d
c h r o m o s o m e s
a r e
j o i n e d
a t
a
p o i n t
c a l l e d
t h e
c e n t r o m e r e .
M a t c h i n g
p a i r s
o f d o u b l e d c h r o m o s o m e s l i n e u p a n d
e x c h a n g e g e n e t i c m a t e r i a l . T h e c e l l t h e n
d i v i d e s
t w i c e
t o
f o r m
f o u r
d a u g h t e r
c e l l s , w i t h e a c h t a k i n g o n e c o p y o f e a c h
c h r o m o s o m e .
E g g a n d s p e r m c e l l s t h e r e f o r e h a v e 2 3
s i n g l e c h r o m o s o m e s , w h i c h i s o n l y h a l f
t h e
u s u a l
c h r o m
o s o m
e
c o n t e n t
o f
a
b o d y
c e l l ,
s o
e a c h
p a r e n t
c o n t r i b u t e s
h a l f o f t h e o f f s p r i n g ’s g e n e t i c m a t e r i a l .
T h e
e x c h a n g e
b e t w e e n
c h r o m o s o m e s
m e a n s t h a t e a c h d a u g h t e r c e l l h a s a u n i -
q u e g e n e t i c m a k e - u p . ( S e e a l s o
m i t o s i s
).
m e la e n a
B l a c k , t a r r y
f a e c e s
c a u s e d b y b l e e d i n g ,
u s u a l l y
i n
t h e
u p p e r
g a s t r o i n t e s t i n a l
t r a c t . T h e
b l o o d
i s
b l a c k e n e d
b y
t h e
a c t i o n
o f s e c r e t i o n s
d u r i n g
d i g e s t i o n .
M e l a e n a i s
u s u a l l y
c a u s e d b y a
p e p t i c
u l c e r
b u t m a y i n d i c a t e c a n c e r .
m e la n c h o lia
F o r m e r t e r m f o r
d e p r e s s i o n
.
m e la n in
T h e b r o w n o r b l a c k p i g m e n t t h a t g i v e s
s k i n , h a i r , a n d t h e i r i s o f t h e e y e s t h e i r
c o l o u r i n g . M
e l a n i n i s p r o d u c e d b y c e l l s
c a l l e d
m e l a n o c y t e s
.
E x p o s u r e
t o
s u n l i g h t
i n c r e a s e s
t h e
p r o d u c t i o n o f m e l a n i n , w h i c h p r o t e c t s
t h e
s k i n
f r o m
t h e
h a r m f u l
e f f e c t s
o f
u l t r a v i o l e t r a y s a n d c a u s e s t h e s k i n t o
d a r k e n .
L o c a l i z e d
o v e r p r o d u c t i o n
o f
m e l a n i n i n t h e s k i n c a n r e s u l t i n a p i g -
m e n t e d s p o t , m o s t c o m
m o n l y a
f r e c k l e
o r m
o l e ( s e e
n a e v u s
).
m e la n o c y te
A s p e c i a l i z e d s k i n c e l l t h a t p r o d u c e s t h e
p i g m e n t
m e l a n i n
.
T h e c e l l s c a n b e d a m -
a g e d b y
o v e r e x p o s u r e t o t h e s u n ; t h i s
d a m a g e m a y r e s u l t i n s k i n c a n c e r
( s e e
m e l a n o m a , m a l i g n a n t
).
m e la n o m a , ju v e n ile
A r a i s e d , r e d d i s h - b r o w n s k i n b l e m i s h ,
a l s o c a l l e d a S p i t z n a e v u s , w h i c h s o m
e -
t i m e s
a p p e a r s
o n
t h e
f a c e
o r
l e g s
i n
MECHANISM OF MEIOSIS
In meiosis, a cell in the
t e s t i s
or
o v a r y
containing 46 chromosomes divides to form
four germ cells (sperm or eggs), each with 23 chromosomes. Germ cells have only
half the usual chromosome content because a child can receive only half the genes
of each parent.
O r i g i n a l
---
c e l l
I
The 46 chromosomes in the original cell
duplicate themselves, then the doubled
chromosomes form 23 pairs. (Only 10 pairs
are shown in this box.) There is an exchange
of genetic material between the members in
each pair, so that each of the germ cells will
receive a unique mixofthe parental genes.
T h e c h r o m o s o m e s
f i r s t d o u b l e u p ,
t h e n t h e y f o r m
in t o p a i r s .
2
After exchange, the cell divides, the
two members of each chromosome
pair going into separate daughter cells.
i t / 7
Ä
*
*
i
n
k
Ä J
\
^
if
E a c h c e l l n o w h a s o n e
d o u b l e d - u p c h r o m o s o m e
f r o m e a c h o f t h e p a i r s .
Second division
First
division
3
Each daughter cell now
divides into two germ
cells. The doubled-up
chromosomes are pulled
apart so that each germ cell
receives a single copy of
each chromosome.
}
f
____ 1
/
ATx
^ in
0
\
S e p a r a t e g e r m c e l l s
c o n t a i n i n g d i f f e r e n t
g e n e t i c m ix e s .
%
Germ cells (sperm or eggs)
496
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