Meiosis
Cuts chromosome number in half
Forms sex cells or gametes
(sperm and egg)
Mitosis
equal division
forms body or somatic cells
Slide 1
Human’s have 23
homologous pairs
of chromosomes
Slide 2
One DNA double helix,
one chromosome,
one chromatid
Old half
of DNA
Old half
of DNA
Review from
DNA
Replication
Lecture
New nucleotides
Two DNA double helices,
one chromosome,
two chromatids
Review from
Mitosis Lecture
Chromosome with two chromatids
Slide 3
Ploidy = number of chromosome sets in a cell
Haploid (1N) = one set of chromosomes
Diploid (2N) = two sets of chromosomes
Slide 4
Each member of
the homologous
pair was inherited
from either a
person’s male and
female parent.
Slide 5
Sperm, 1N
23
chromosomes
23 + 23 = 46
homologous
chromosomes
egg, 1N
23
chromosomes
Zygote = 2N cell
Slide 6
Sperm, 1N
egg, 1N
1
1’
2
2’
1 and 1’ =
homologous
chromosomes
1’
1 Eye
Eye
light
Dark
Same cell after
DNA replication
1’
1
1’
1
Zygote = 2N cell
2
2’
2
2’
Slide 7
Zygote (2n)
Human =
46 chromosomes
Mitosis
Adult Cells (2n)
Human = 46
Meiosis
Fertilization
23 + 23
Meiosis
Sperm (1n gamete) Egg (1n gamete)
Sperm =
23 chromosomes
Egg =
23 chromosomes
Slide 8
Meiosis =
One 2N cell forms four 1N cells
Mitosis =
One 2N cell forms two 2N cells
Slide 9
Mitosis
2N
1
1
1’
2
2’
2N
2
Meiosis
1’
1
2’
1N
2
2’
1
1’
2N
1’
1N
2
2’
2
2’
1 1’
2N
1 2
1N
1 2
1N
2’ 1’
1’ 2’
1N
1N
Slide 10
Crossing over =
Exchange of genetic material between
chromatids on homologous chromosomes
…... Only in meiosis
Slide 11
1
1’
1
Homologous
Chromosomes
Crossing over
1’
Recombinant
Chromatids
Slide 12
Chiasmata = cross-overs
Slide 13
Mitosis
2N
Which one
can have
crossing
over?
1
2
1’
1N
2’
1
1’
2N
1’
2
2’
2N
1
2’
2
1
Meiosis
1’
1N
2
2’
2
2’
1 1’
2N
1 2
1N
1 2
1N
2’ 1’
1’ 2’
1N
1N
Slide 14
Crossing over occurs in meiosis
because homologous chromosomes
are next to each other.
Review
Slide 15
Summary
Review
Mitosis – 1, 2N cell makes 2, 2N Cells
Meiosis – overall; 1, 2N cell makes 4, 1N cells
Slide 16