7 gametes

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SPECIALIZED SEX CELLS
WHY ARE THEY NECESSARY?
Why do diploid organisms need
to have specialized sex cells?
 Sex cells (gametes: sperm or egg) allow traits to be combined
from two organisms
2n (46)
+
4n = 92
too many
2n (46)
Sexual Reproduction
 Gametes have only one of each chromosome
 Requires special cell division: Meiosis
Diploid cells (2n)
Gametes (n)
•Takes place in gonads (testis, ovary)
n (23)
+
2n = 46
n (23)
How is Meiosis different from Mitosis?
Meiosis
Cells divide 2x
Mitosis
Cells divide 1x
2n
2n
2n
2n
n
n
Diploid Cells
n
n
n
n
Haploid Cells
Mitosis
Identical cells
Meiosis
Different cells
Mitosis
Cell division is over
Done
Meiosis
Cells divide again;
sister chromatids line up
How many unique gametes can a cell with two
pairs of chromosomes make?
Why bother?
•For humans with 23 pairs of chromosomes?
•(2)23
> 8 million different possible gametes
•For a couple
•possible unique offspring
•(8 million) x (8 million) = (64,000,000,000,000)
But wait! There’s more!
Crossing over enables even
greater variety
• Exchange of equivalent sections between homologous
chromosomes.
• Occurs at random locations along chromosome.
• Creates new versions of chromosomes.
CQ#1: The cell to the right shows a diploid organism with
two chromosomes (2n=2). The pictures below show some
of the steps this cell may go through
during mitosis or meiosis.
A.
D.
B.
E.
F.
C.
G.
H.
Place the appropriate steps in order for a cell going through
mitosis. (Note: you may use just some or all of the steps.)
CQ#2: The cell to the right shows a diploid organism with
two chromosomes (2n=2). The pictures below show some
of the steps this cell may go through
during mitosis or meiosis.
A.
D.
B.
E.
F.
C.
G.
H.
Place the appropriate steps in order for a cell going through
meiosis. (Note: you may use just some or all of the steps.)
CQ#3: The different copies
of chromosome 21 in John
and Jane are shown to the
right.
Parent #2
Parent #1
Which of the following is a normal gamete that might
be produced by either John or Jane?
A
B
C
D
E
KING HENRY VIII
 King Henry married six times in hopes of producing a strong line
with male heirs.
1. Katherine of Aragon
2. Anne Boleyn
3. Jane Seymour
4. Anne of Cleves
5. Katherine Howard
6. Katherine Parr
CHILDREN
 Katherine of Aragon had six pregnancies:
3 sons, 3 daughters, only one daughter survived
 Anne Boleyn had three pregnancies: 1 son, 1 daughter, and 1
unknown, only 1 daughter survived.
 Jane Seymour had one pregnancy: 1 son,
who survived
 None of Henry’s remaining wives had pregnancies
WHO WAS AT FAULT?
 Henry divorced Katherine, and beheaded Anne
due to his belief that his wives were denying him
male heirs.
 CQ#4: Was it in fact the fault of his wives?
YES
NO
WHY?
 Females can
provide only
X’s to their
children
 Males can
provide either
an X or a Y
Gamete Formation –
Sperm
 This process is called spermatogenesis
 Takes place in the testes of the male
 The process begins with a diploid cell called the
spermatogonium
 The spermatogonium enlarges and undergoes
meiosis I and II.
 The final product is 4 haploid sperm cells
 Each sperm cell has the same number of
chromosomes and the same amount of cytoplasm
 Each sperm cell also has a long, tail-like flagellum
used for movement
Gamete Formation - Eggs
 This process is called oogenesis
 Takes place in the ovary of the female
 The process begins with a diploid cell called the
oogonium
 The oogonium enlarges and undergoes meiosis I
and II
 At the end of meiosis one, however, the
cytoplasm is not equally divided between the two
daughter cells.
 The cell that receives most of the cytoplasm is
called the primary oocyte, the other cell is
called the polar body
 The polar body is not a viable gamete
 As the primary oocyte undergoes meiosis II, the
cytoplasm again divides unevenly.
 The cell that receives most of the cytoplasm is
called the Ovum or egg.
Why the unequal division?
 The unequal division of the cytoplasm during
oogenesis is to provide the ovum with enough
nutrients to support the developing zygote for a
few days after fertilization
What is the zygote?
 The zygote is the new cell formed through the
process of fertilization.

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