Meiosis
Quick Review of MITOSIS
• What happens to the DNA BEFORE mitosis?• DNA synthesis (replication)
• Why? • Needs an exact copy for the new “daughter
cells”
2 types of cells:
• Somatic Cell = body cell– Example: skin, muscle,
lung cells– Diploid cells
• Gamete = sex cell– Example: egg, sperm,
pollen– Haploid cells
Haploid Cells vs. Diploid Cells
• Haploid = have 1 of EACH kind of chromosome (n)
• Diploid = have 2 of EACH kind of chromosome (2n)
• Meiosis = cell division that produces ½ the chromosomes of the parent cell; makes gametes
What does meiosis produce?
• SEX CELLS • (aka: gametes, haploid
cells)
Why do we need Meiosis?
How is MITOSIS different
from MEIOSIS?
Meiosis Vocabulary
• Homologous Chromosome = 2 pairs of sister chromatids (form a tetrad); same order and letter of gene
How do we get diploid cells?
Mitosis!
How do we get haploid cells?
MEIOSIS!
Will Meiosis or Mitosis occur in these cells?
• Skin cells• Pollen • Ovum/ovules• Muscle cells
What is genetic variation?
• Genetic variation = the differences between genes of individuals of a species or populations of different species.
• Which two individuals will have the least genetic variation in their genes?
A. Brother and sisterB. Brother and cousinC. Sister and cousinD. Mother and nephew
• Which two species will have the greatest genetic variation between species.
1. Lancelet and Lamprey2. Lancelet and Leopard3. Salamander and Leopard4. Turtle and Leopard
Meiosis provides genetic variation among organisms
• 2 sources of genetic variation:
1. Random alignment of chromosomes during metaphase
2. Crossing Over
Chromosome alignment can vary!
• Crossing Over = exchange of genetic information during prophase I of meiosis– Provides MORE
variation– Basis for evolution– This is a form of genetic
recombination (or the recombining of genes!)
Chromosome Action Explains Mendel’s Work
• Law of segregation– Traits are separately inherited from parents
• Law of independent assortment– Traits are inherited independently of each other
• Rule of dominance– If its dominant it will appear; recessive will only
appear if there is a “double dose”
• http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
QUIZ TIME!
• What are some examples of gametes?• Egg• Sperm
Haploid or Diploid?
• Sex cells• Somatic cells• Body cells• Sperm• Egg
QUIZ TIME!
• How can you tell if a homologous chromosome will be homologous?
Mitosis or Meiosis?
• Gametes produced• Diploid cells produced• Haploid cells produced• Half the number of chromosomes produced• Same number of chromosomes produced
QUIZ TIME!
• During what phase of meiosis does crossing over occur?
QUIZ TIME!
• If a cell starts with 20 chromosomes how many chromosomes will be present in the cells following meiosis?
• How many chromosomes would be present following mitosis in the same original cell?
QUIZ TIME!
• How many cells are produced during mitosis?• 1 2
• How many cells are produced during meiosis?• 1 4
Mistakes in Meiosis
What do the chromosomes have to do in meiosis in order for it to be successful?
• One chromosomes from each homologous pair moves to each pole of the cell…
What if the chromosomes fail to separate?
• Nondisjunction = the failure of homologous chromosomes to separate properly during meiosis
• What does nondisjunction do to the chromosome count?
• One cell has too many chromosomes• Another cell has too few chromosomes
When would you notice the effects of nondisjunction?
• Seen after gametes fuse fertilization
• When a gamete with an extra chromosome is fertilized by a normal gamete, the zygote will have an extra chromosome…
• It should have 2, but has three!• This is called trisomy
What if an organism is lacking chromosomes?
• Organisms with an extra will often survive BUT organisms lacking one or more, usually do not survive
• When a gamete with a missing chromosome fuses with a gamete with normal chromosome number, you get monosomy
What if BOTH gametes have extra sets of chromosomes?
• Produces offspring with 4 sets of chromosomes
• This is called a tetraploid• Organisms with more than the usual number
of chromosomes are called polyploids
Polyploids in plants
• Very useful commercially– Tetraploid wheat (4n)– Triploid apples (3n)
• Plant breeders use chemicals to artificially cause nondisjunction to get these polyploid plants.
• Why could this be an area of concern for humans?
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