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Meiosis
Reduction
Division
Genetics Vocabulary
• Gene:
– a discrete unit of hereditary information consisting
of a specific nucleotide sequence in DNA (or RNA,
in some viruses) at a specific locus (location)
• Allele:
– Any of the alternative versions of a gene that
produce distinguishable phenotypic (visible)
effects
• Genome:
– The genetic material of an organism or a virus;
the complete complement of an organism’s or
viruses genes along with its noncoding nucleic
acid sequences
Meiosis Vocabulary
• Diploid cell:
– a cell that contains all (2n) of the genetic info it should.
• Haploid cell:
– a cell that contains half (n) of the genetic info it should.
• Homologous chromosomes:
– “paired”(at the same locus) chromosomes that have the
same traits arranged in the same order but the are not
identical
• Meiosis:
– The process that creates cells with a haploid number of
genetic info.
Meiosis Vocabulary
• Gametes:
– The only cells in the body not produced by mitosis
– Made in the gonads
– Sex cells
• Sperm: The male sex cell
• Egg: The female sex cell
• Each one represents 1 in 8.4 million possible genetic
combinations
• Zygote:
– The result of sperm fertilizing egg.
– Combo of sperm and egg makes a 1 in 70 trillion possible
combinations
• Somatic cell:
– Any other cell other than those involved in gamete formation
Miscellaneous Factoids
• Typically, a thousand or more genes are
arranged on each chromosome
• Humans have 46 chromosomes
• Why meiosis?
• Offspring get information from mom and
dad
– Genetic diversity is key!!
• If that info wasn’t split, you would get
twice as much information as they would
have (reduction division)
Phases
•
•
•
•
Interphase
The same as interphase in mitosis
Chromosomes replicate
Each chromosome then has two
identical sister chromatids held together
by a centromere
Prophase I
• Chromosomes coil
• Spindle forms
• Tetrads are formed
– Joining of two homologous chromosomes
with two sister chromatids each (called
synapsis)
– Bind so tightly that information is exchanged
• Now called Chiasmata (Chiasma)
– Called Crossing over (remember the
purpose of introns!!!)
• Forms recombinant chromosomes
Metaphase I
• Tetrads line up at midline
• Homologous chromosomes line up in
pairs
Anaphase I
• Homologous chromosomes separate
and migrate towards poles
• Centromeres do not split
– This ensures that each new cell gets only
one chromosome
Telophase I
•
•
•
•
Spindle breaks down
Chromosomes uncoil
Cytoplasm divides (cytokinesis)
Cell has half of original cells genetic info
but still double the amount it should
Interphase II**
• Very short or non-existent
• Chromosomes do not replicate
Meiosis II
• Prophase II
– Spindle forms
• Metaphase II
– Sister chromatids line up at the equator
• Anaphase II
– Centromeres split
– Sister chromatids split
• Telophase II
– Spindle breaks down
– New nuclei form
– Cytokinesis occurs
Meiosis in Men and Women
• Male version of meiosis
• Spermatogenesis
• 4 sperm cells for every 1 cell that divides
• Female version of meiosis
• Oogenesis
• 4 egg (ova) cells for every 1 cell that divides
• 3 die and 1 survives
Meiosis is the same all around the world!
Alternation of Generations
The adult generation produces spores, while the spore generation
produces sex cells. Only seen in plants.
Errors
• DNA mutations
– Point
• Base-pair substitution
– Silent mutations
» Harmless
» Occur within non-coding areas or don’t affect the
coded polypeptide
– Missense mutations
» Amino acid works but not the way it’s supposed
to (i.e., “close but no cigar”)
– Nonsense mutations
» Functionless amino acid
» Example: a premature stop code
• Base-pair insertions/deletions
– Almost always lead to frameshift mutations
Errors
• Chromosomal
– Nondisjunction
• Leads to aneuploidy (abnormal # of chromsomes)
– Trisomic (2n+1)
– Monosomic (2n-1)
• Polyploidy
– Triploidy (3n)
– Tetraploidy (4n) (gray tree frog)
– Chromsomal structure alterations
• Deletion
• Duplication
• Inversion
• translocation
Examples of Aneuploidy
• Trisomy 21:
Down’s syndrome
Examples of Aneuploidy
Trisomy 13: Patau syndrome
(heart and kidney defects
among problems)
Examples of
Aneuploidy
• Trisomy 18: Edwards
syndrome (body wide effects)
Examples of Aneuploidy
• Klinefelter
syndrome: XXY
Examples of Aneuploidy
• XYY syndrome:
– Not defined as syndrome
– taller than average
– Violent tendencies? (Chino
prison study)
Examples of Aneuploidy
• XXX syndrome:
relatively normal
Karyotyping
Chorionic villus
sampling
&Amniocentesis