Download Meiosis

I told you before that we categorize human cells in
two categories: body cells and sex cells, what do
you THINK makes them so different?
HOW MANY CHROMOSOMES ARE FOUND
IN EACH HUMAN BODY CELL?
• Genes do not exist free in the nucleus of a cell; they
are lined up on chromosomes.
Diploid Cells
• In the body cells of animals and most plants,
chromosomes occur in pairs.
• In each pair, one you inherited from your mom, the
other from your dad.
From
Mom
From
Dad
Homologous Chromosomes
• The two chromosomes of a pair are called
homologous chromosomes.
• Each pair of homologous chromosomes has genes for
the same traits.
Diploid Cells
• A cell with two of each kind of chromosome
is called a diploid cell.
• All of your body cells are diploid.
• Body cells (diploid) are different from sex cells.
•
Each person can only pass on ONE of their homologous
chromosomes to their young.
HUMANS = 46 CHROMOSOMES
IN THEIR BODY CELLS
Each parent passes 23 to their young, so
that when the sperm and egg combine- it
will equal 46 Chromosomes.
Haploid Cells
• So, sex cells CANNOT be diploid.
•A cell containing only one of each kind of
chromosome is called a haploid cell.
Sexual Reproduction
Zygote
Diploid and Haploid Cells
Chromosome Numbers of Common Organisms
Organism
Body Cell
Sex Cell
Fruit fly
8
4
Garden pea
14
7
Corn
20
10
Tomato
24
12
26
13
Apple
34
17
Human
46
23
Leopard Frog
Chimpanzee
Dog
Adder’s tongue fern
48
24
78
39
1260
630
In Humans, how many chromosomes would each of
these cells have…..
CHEEK CELL
46
SPERM CELL
23
EGG CELL
23
NERVE CELL
46
Since they have different amounts of chromosomes, the way they are made differ.
● Body cells are made by the division process MITOSIS.
● Sex cells are made by the division process MEIOSIS.
Before a cell can divide by either method, it must first prepare for
cell division.
The cell cycle (or entire life cycle of the cell) consists of:
INTERPHASE: The longest/busiest stage in a cell’s life
1. Chromosomes (DNA) replicates
2. Cell grows bigger
CELL DIVISION: Mitosis or Meiosis
CYTOKINESIS: When the cytoplasm divides and the
cell completely separates.
Mitosis:
It starts with a DIPLOID cell
cell division that makes BODY CELLS.
46
Chromosomes Replicate
92
46
46
It ends with TWO IDENTICAL DIPLOID cells
The kind of cell division that produces sex cells (haploid)
containing half the number of chromosomes as a body cell, is
called meiosis.
Meiosis
I
Meiosis
II
Meiosis:
cell division that makes SEX CELLS.
46
It starts with a DIPLOID cell
Chromosomes Replicate
92
46
23
46
23
23
23
It ends with FOUR HAPLOID cells (not identical)
Crossing-Over
In Meiosis:
Before the sex cells are created,
Homologous chromosomes
actually break and exchange traitsin a process known as crossing
over.
FROM
MOM
This
chromosome
inherited by
mom could code
for straight hair,
brown hair, and
green eyes…
FROM
DAD
After crossing-over
This
chromosome
inherited by dad
could code for
curly hair,
black hair, and
brown eyes…
Now one chromosome
passed on to offspring
could have traits from
both homologous… curly
hair, brown hair, and
green eyes…
Crossing-Over
Crossing over can occur at any location on a chromosome, and
it can occur at several locations at the same time.
This causes the traits found in each gamete varied. This is
why siblings of the same parents can look so different from
each other.
Meiosis
• These haploid cells are called sex cells—
gametes.
• Male gametes are called sperm.
• Female gametes are called eggs.
Meiosis
• When a sperm fertilizes an egg, the
resulting zygote once again has the
diploid number of chromosomes.
• This chart of chromosome pairs is called a karyotype,
and it is valuable in identifying unusual chromosome
numbers in cells.
Nondisjunction
• The failure of homologous chromosomes to separate
properly during meiosis is called nondisjunction.
•This would occur when homologous chromosomes
are suppose to separate from each other.
Down syndrome: Trisomy 21
• Down syndrome is the only autosomal
trisomy in which affected individuals
survive to adulthood.
Down syndrome: Trisomy 21
• Extra chromosome on 21
• 1 in every 1100 live births
• Range from mild to severe
physical and mental disabilities
• The incidence of Down syndrome births is higher in
older mothers, especially those over 40.
Question 1
A cell with two of each kind of chromosome is
__________.
A. diploid
B. haploid
C. biploid
D. polyploid
Homologous Chromosome 4
a
A
Terminal
Inflated
Tall
Axial
D
d
T
t
Constricted
Short
The answer is A. The
two chromosomes of
each pair in a diploid
cell are called
homologous
chromosomes. Each
has genes for the same
traits.
Question 2
Meiosis
Haploid gametes
(n=23)
Why do organisms need
meiosis for
reproduction?
Sperm Cell
Meiosis
Egg Cell
Fertilization
Diploid zygote
(2n=46)
Mitosis and
Development
Multicellular
diploid adults
(2n=46)
Meiosis is cell
division that
produces haploid
gametes. If meiosis
did not occur, each
generation would
have twice as many
chromosomes as
the preceding
generation.
Meiosis
Haploid gametes
(n=23)
Sperm Cell
Meiosis
Egg Cell
Fertilization
Diploid zygote
(2n=46)
Mitosis and
Development
Multicellular
diploid adults
(2n=46)
Question 2
Trisomy usually results from _______.
(TX Obj 2; 6C)
A. polygenic inheritance
B. incomplete dominance
C. nondisjunction
D. twenty-two pairs of chromosomes
The answer is C.