FOCUS
Cell Division
Part 2
We are going to learn about:
Sexual Reproduction
Meiosis
Central Questions
• How does sexual reproduction
work on the cellular level?
• What do chromosome number,
haploid and diploid mean?
• What are the stages of meiosis
and what happens in each one?
• What is the result of meiosis?
Sexual Reproduction
• Two types of gametes:
–Sperm- Male cells; smaller; have
flagella to move around
–Egg- Female cells; larger than
sperm
• In sexual reproduction a sperm cell
fertilizes an egg
• Fertilization produces a new cell
called a zygote, the first cell of a
new individual
Sexual Reproduction
• “Normal body cells” (somatic cells)
are formed by mitosis.
• New organisms are formed during
sexual reproduction
• Sexual Reproduction is when the
chromosomes of two parents
combine to produce new offspring
• Special cells called Gametes
combine during sexual reproduction
Sexual Reproduction
• Because two gametes join to
make a zygote, gametes must
have half the number of
chromosomes as the zygote
• Because of that gametes can’t
be made by mitosis
• Nucleus division that makes
gametes is called meiosis
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Meiosis
• Only occurs in eukaryotic cells
• Daughter cells have half the
chromosomes of the parent
cells
• Daughter cells are not all the
same
• Two divisions make 4 cells
Chromosome Number
• N = number of chromosome pairs
• We can call haploid cells N and
diploid cells 2N.
• Think Algebra! N is a variable
• Humans have 23 pairs
–Body cells have 46 chromosomes
(2N=46)
–Gametes have 23 chromosomes (N=23)
Prophase I
Chromosomes coil; nucleus disappears and
spindles appear
Chromosome Number
• Chromosomes exist as pairs which
carry the same genes (called
homologous pairs)
• Cells with two copies of each
chromosome are called diploid
• Cells with one copy of each
chromosome are called haploid
• Body cells are diploid
• Gametes are haploid
Stages of Meiosis
• In meiosis there are two sets of
divisions, Meiosis I and Meiosis II
• At the beginning of Meiosis I, just like in
mitosis, each chromosome consists of two
sister chromatids joined at a centromere
• In meiosis the homologous pairs come
together in the beginning (a tetrad)
Crossing-Over
Crossing-over is when the homologous
pairs trade a piece of themselves with
their partner. Only happens during
Prophase I
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Metaphase I
Tetrads line up along the middle of the cell
Telophase I
New nuclei form and cytokinesis proceeds.
Each new cell has one member of the
homologous pair
Anaphase I
Tetrads separate and move to opposite sides of
the cell
Meiosis II
• After Meiosis I, each cell has half the
chromosomes, but each remaining
chromosome still has two sister
chromatids (similar to prophase of
mitosis)
• In Meiosis II, the two cells from Meiosis I
split again (like in mitosis) to form a total
of 4 haploid cells
Central Questions
• How does sexual reproduction
work on the cellular level?
• What do chromosome number,
haploid and diploid mean?
• What are the stages of meiosis
and what happens in each one?
• What is the result of meiosis?
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