1. Meiosis
How
Reproductive
Cells Divide &
Pass on DNA
2. Why do Cells Divide?
• To replace old/worn
out or damaged cells
• skin, red blood cells,
intestines
• To fight disease
• white blood cells
identical copies
• To reproduce
• gametes (sex cells)
• male sperm cell
• female ova cell
variation
3. DNA: Secret Code of the Cell
Traits are characteristics inherited from parents
• i.e.: leaf shape, eye/hair color, tree height, etc.
Instructions that determine these traits are found in
sections of DNA called genes
• most cells have two genes that control each trait
• one gene comes from the mother (maternal)
• one gene comes from the father (paternal)
This gene might give
instructions for nose
width in a human.
4. Mama Genes & Papa Genes
Homologous chromosomes
• every body (somatic) cell has a set of two similar chromosomes
• one paternal, one maternal
• genes of homologous chromosomes code for the same traits
• genes for similar traits are usually found at the same location on
the two different homologous chromosomes
5. Homologues
Same length
Same centromere position
Carry genes that control the
same inherited traits
• i.e.: both carry the gene for hair
color - one is for brown hair and
one is for red hair
One is from mother, one is
from father = a matching set
NOT identical
6. Haploid vs. Diploid Cells
Cells with only one set of chromosomes
(one gene for each trait) are haploid
• symbolized by N
• all gametes (sex cells) are haploid
• haploid cells are the result of meiosis
Cells having homologous chromosomes
(two genes for each trait) are diploid
• symbolized by 2N
• all somatic cells (body cells) are diploid
• diploid cells are created through mitosis
7. Sexual Reproduction
The nuclei of haploid gametes combine in fertilization,
resulting in a diploid zygote
haploid gametes
diploid
zygote
8. Does Chromosome # determine complexity?
Comparative Number of Chromosomes
Organism Body Cell (2n) Gamete (n)
Fruit fly 8 4
Garden pea 14 7
Flatworm 16 8
Corn & Green algae 20 10
Frog 26 13
Apple 34 17
Cat 38 19
Monkey 42 21
Human 46 23
Potato & Chimpanzee 48 24
Dog 78 39
Adder's tongue fern 1260 630
11. Meiosis
Interphase
• Chromosomes replicate
• Chromatin condenses
Prophase I Interphase
• Replicated homologous
chromosomes pair up (synapsis)
• Chromosomes each consist of two
identical sister chromatids
• Nuclear membrane breaks down
• Spindles form
• Crossing over begins
Prophase I
12. Meiosis
Crossing over
• chromosomal segments are
exchanged between pairs of
homologous chromosomes
• produces variation in genetic
information passed to offspring Prophase I
13. Meiosis
Metaphase I
• Chromosome kinetochores
attach to spindle fibers.
• Homologous chromosome
pairs line up at the Metaphase I
equatorial plate.
14. Meiosis
Anaphase I
• Homologues separate
and move to opposite
poles of the cell
• Sister chromatids Anaphase I
remain attached!
15. Meiosis
Telophase I
• Spindles break down
• Chromosomes uncoil
• Two nuclei form Telophase I
• Cytoplasm divides
(cytokinesis)
Cells have only half as many chromosomes
(they are now haploid)
• homologous pairs have separated into two cells
• chromosomes have already replicated
17. Meiosis
Prophase II
• A second set of phases
begins in both haploid
daughter cells
• Spindle apparatus forms
Prophase II
• Chromosomes
condense
• Nuclear membrane
breaks down
No DNA replication occurs between Meiosis I and
Meiosis II (interphase is skipped)
18. Meiosis
Metaphase II
• A haploid number of
chromosomes line up at
the equatorial plate
• Spindle fibers attach to
Metaphase II
kinetochores
19. Meiosis
Anaphase II
• Sister chromatids are
pulled apart at the
centromere
• Spindle fibers pull Anaphase II
chromatids toward the
opposite poles of the cell
Telophase II
• chromosomes reach the
poles
• nuclear membranes and
nuclei reform
Telophase II
20. Meiosis
Cytokinesis
• Plasma membranes and
cytoplasm of cells divide
• Four gametes are the
result, each with n
number of
chromosomes Cytokinesis
23. Meiosis - summary
Germs cells (reproductive cells produced in the
testes & ovaries in humans) divide by meiosis
In meiosis, a cell divides twice to produce four
daughter cells
Cells arising from meiosis are haploid, which
means they half the number of chromosomes
Provides genetic variability, because daughter
cells are not identical (as in mitosis)
What determines a cell's shape? function? DNA!
Where would you expect Humans to land? What about a plant, like an apple? How many chromosomes do you think cats and dogs have?
Modified by Liz LaRosa 2011
...and this is WITHOUT crossing over, and ONLY TWO alleles! Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.
A. Chromosome number of various organisms: Man = 46 , Penicillium = 4, Mosquito = 6, Fruit fly = 8, Garden pea = 14, Adder's tongue fern = 1262,Frog = 26, Vampire bat 28, Chimpanzee = 48, Duck = 80.