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Meiosis
Biology
2009-2010
Meiosis
• Def: type of cell division that produces 4
cells with half the number of
chromosomes as a parent cell
• Occurs in the sex organs
• Number of chromosomes= 46 or 23
pairs
Homologenous Chromosomes
• Def: chromosomes that match in
shape and size but not necessarily
same DNA patterns
Homologous
Chromosomes
• Karyotyping
– Def: arrangement of all 46 chromosomes
• Sex chromosomes
– The 23rd pair
– Determines the organisms sex
– XX= girl
– XY=boy
Diploid vs. Haploid
• Diploid
– 2 homologenous sets of chromosomes
– Forms the zygote
– 2n
• Haploid
– 1 set of chromosomes- one from each
pair
– Occurs in fertilization
–n
Number of Chromosomes
• Sex Cells each have half the number
of chromosomes (23)
• Zygote (fertilized egg) have 46
chromosomes
Steps of meiosis
• Differences between mitosis and meiosis:
– Meiosis produces 4 offspring cells
– Meiosis involves exchanging of genetic material
between the Homologous Chromosomes.
• Sister chromosomes- exact copies of DNA
• Homologenous chromosomes- DNA does not
have to be the same
Meiosis 1
• Prophase 1
– Homologenous chromosomes stick together
forming a tetrad
– Crossing over begins
• Metaphase 1
– Crossing over is complete
– Chromosomes line up in the center
• Anaphase 1
– Chromosomes migrate towards the opposite
poles
• Telophase 1
– Each pole has a haploid
Crossing Over
• DEF: Genetic materials is transferred from
one chromosome to another
Cross
ed
over
Meiosis 2
• Follows the exact same steps as
Mitosis except the end results in 4 cells
instead of 2
Chickens have 78Cats
chromosomes
have
in their body cells how
38 many
chromosomes dochromosomes
they have in
their sex cells?
in their
diploid cells.
How many do
they have in
their haploid
cells?
Birth Defects
Biology2009-2010
Numbers
• There are 200,000 people born with birth
defects every year
• To break it down, there are 25 people
born with birth defects per hour
Causes of Birth Defects
• Drugs/Alcohol/Smoking
• Radiation/Pollution
• STD’s
• Inheritance
• Malnutrition
Detection
• Blood Test
• Amniocentesis– A sample of the amniotic fluid is taken and
then the DNA is checked for Chromosomal
defects
Chromosomes
• Monosomy- loss of one chromosome
• Trisomy- gain an extra chromosome
• Sex Chromosomes- X and Y
Down Syndrome
• Occurrence-1/600 births
• Causes- trisomy 21
• Chance with increased age
– 20’s- 1/1500
– 40’s-1/44
Homozygous Recessive Traits
• Found on the X chromosome
• Passed on from mother’s to their
children
• Examples: Muscular Dystrophy and color
blindness
Mutations
• Definition- change in the DNA by
changing one or more genes
Mutagens
• Definition- Agents that cause
mutations
• Examples: Chemicals, x-rays, radiation,
genetic
Twins
• Fraternal- Two fertilized eggs are
implanted in the uterus at one time
• Identical- one fertilized egg that splits