MEIOSIS
REPRODUCTION
• Asexual • splitting
• budding
• parthenogenesis (egg develops w/o fertilization) • occurs naturally in some invertebrate animal species
(e.g., water fleas, aphids, nematodes, some bees, some scorpion species, and parasitic wasps) and a few vertebrates (e.g., some fish, amphibians, reptiles, and very rarely birds)
2
SEXUAL REPRODUCTION
• Fusion of two gametes to produce a single zygote.
• Introduces greater genetic variation, allows genetic
recombination.
• With exception of self-fertilizing organisms (e.g.
some plants), zygote has gametes from two
different parents.
3
REVIEW
• What happens in each of the phases of mitosis?
• Prophase
• Metaphase
• Anaphase
• Telophase
WHAT WOULD HAPPEN IF CELLS ONLY DID MITOSIS?
Cycle Number Stage Number of
chromosomes
1 Mitosis 2
1 Interphase
1 Fertilization
2 Mitosis
2 Interphase
2 Fertilization
3 Mitosis
3 Interphase
3 Fertilization
MEIOSIS
• Occurs in reproductive organs
• Meiosis produces haploid gametes
• ½ the number of chromosomes of regular
cells
CHROMOSOMES AND CHROMOSOME NUMBER
• DNA in chromosomes is arranged into
segments called genes
• Control production of proteins
• Each chromosome has hundreds of genes
HOMOLOGOUS CHROMOSOMES
• Human Somatic cells each have 46 chromosomes
• 23 from each parent
• 23 pairs of chromosomes
• Chromosomes that make up a pair = homologous
chromosomes
• Have same length and same centromere position
• Carry genes that control the same trait
• Don’t necessarily code for the same expression
of gene
HOMOLOGOUS CHROMOSOMES
9
HAPLOID & DIPLOID CELLS
• Gametes – sex cells
• Have half the number of chromosomes as somatic
cells
• Humans – 23 individual chromosomes (not pairs)
• Haploid – n
• Sex cells
• Diploid – 2n
• Somatic cells
MEIOSIS I
• Interphase – same as always
• Prophase 1
• Chromatin condenses into chromosomes
• Homologous chromosomes pair up
• Crossing over occurs
• Spindle forms
CROSSING OVER
• Process in which chromosomal segments are
exchanged between a pair of homologous
chromosomes
• Metaphase I • Homologous
chromosomes line up in the middle of the cell
• Anaphase I • Homologous
chromosomes are pulled to opposite ends of the cell
• Telophase I
• Chromosome uncoils
• Nucleus reforms
• Cells split apart
• Results in 2 diploid
daughter cells
MEIOSIS II
• DNA IS NOT REPLICATED AGAIN!!!
• Prophase II
• Spindle forms
• Chromosomes condense
• Nucleus disappears
• Metaphase II
• Chromosomes line up in the middle
• Anaphase 2
• Sister chromatids are pulled to opposite ends of
the cell
• Telophase II
• Chromosomes reach poles
• Nuclear membranes reform
• Results in 4 haploid daughter cells
IMPORTANCE OF MEIOSIS
• Results in genetic variation
• How homologous pairs line up in metaphase I is
random
Meiosis Mitosis
Number of
divisions
Number of cells
formed?
Type of cells
formed
(haploid/diploid)
Where does it
occur?
Why does it
happen?
Unequal distribution of
chromosomes during
meiosis
Resulting gametes zero
or two copies of a
chromosome instead of
a single copy
25
26
27