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WHY ARE THEY NECESSARY?
SPECIALIZED SEX CELLS
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)
2n (46)+
4n = 92
too many
Sexual ReproductionGametes have only one of each chromosomeRequires special cell division: Meiosis
2n = 46
+n (23)
n (23)
Diploid cells (2n) Gametes (n) • Takes place in gonads (testis, ovary)
Mitosis Cells divide 1x
MeiosisCells divide 2x
How is Meiosis different from Mitosis?
2n
2n 2n
2n
n n
n n n n
Diploid Cells
Haploid Cells
Mitosis Meiosis
Identical cells Different cells
Mitosis Meiosis
Cell division is over
Cells divide again; sister chromatids line up
Done
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. B. C.
D. E. F. 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. B. C.
D. E. F. 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 #1 Parent #2
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 Aragon2. Anne Boleyn3. Jane Seymour4. Anne of Cleves5. Katherine Howard6. 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?
YESNO
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 oogenesisTakes place in the ovary of the
femaleThe 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.