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THE BIRDS and BEES LESSON
Everything you didn’t already know about sexual reproduction
Overview of Meiosis
Meiosis-form of cell division where there are two successive (back to back) rounds of cell division. In meiosis a……Diploid (parent) cell with (2n) or “double set” of chromosomes produces 4 haploid cells with one (n) or “single set” of chromosomes.
Diploid # in humans is 46 double (2n), the haploid # is 23 (n)
The chromosome # is reduced by ½ in two divisions:After 1 division - 23 double stranded
chromosomes (n) (interphase skipped)
After 2nd division - 23 single stranded chromatids -Meiosis occurs in our germative cells (cells that produce our gametes )
Females- Oocyte eggs (ovum)Males- Spermatocyte sperm
Body cells have 2 of each chromosome
= diploid (2n)
How many total chromosomes? 46
Sperm/egg cells have 1 of each chromosome
= haploid (1n)
How many chromosomes? 23
Why do we need meiosis?
-Process keeps the chromosomal number in organisms from doubling from one generation to the next during sexual reproduction.
Egg Sperm Zygote
from mom from dad child
toomuch!
46 chromosomes + 46 chromosomes = 92 Chromosomes
Why Meiosis is Needed
Meiosis reducesgenetic content by half
23 chromosomes + 23 chromosomes = 46 Chromosomes
Egg Sperm Zygote
- to bring two haploid gametes together to form a diploid zygote.
-n (mom) + n (dad) = 2n (offspring)
HOMOLOGOUS CHROMOSOMES
The members of each pair are called homologous because they are similar in genetic content
Each member of each pair came from a parent during sexual reproduction
One chromosome is maternal (from the mother)
One is paternal (from the father)
Homologous chromosomes are equivalent and contain the same kinds of genes in the same order .
They may have different versions of genes or “alleles” because they come from different parents
Overview of Meiosis continued..
• SAME SIZE• SAME SHAPE• CARRY GENES for
the SAME TRAITS• BUT ______________!• Don’t have to have
the same versions(alleles)
http://sps.k12.ar.us/massengale/genetics%20tutorial.htm
NOT IDENTICAL
AB
C
D
E
F
AB
C
D
E
F
abc
de
f
a
bc
de
f
What is a Homologous Chromosome?
Chromosomes are made of sister chromatids.Same genes with the identical alleles (versions)
MeiosisDuring Meiosis, three important things occur that did not occur in mitosis:•Chromosomes pair up prior to division forming Homologous pairs of chromosomes•Homologous chromosomes exchange genetic information in an event called “Crossing Over”•The newly scrambled chromosomes separate and go into different daughter cells so that each contains only one chromatid of each pair of homologous chromosomes•The result is 4 cells each having unique DNA with gene combinations
Metaphase of Mitosis
Metaphase of Meiosis I
Homologous chromosomes “pair-up” prior to division forming a (grouping 4 chromatids) known as a TETRAD (homologous pairs of chromosomes)
1st Major Differences between Mitosis in Meiosis
Chromosomes line up individually along the equatorial plane
What happens to the pairs?
Homologous chromosomes separate in meiosis I and therefore different alleles also separate.
During Meiosis II individual chromatids separate
In meiosis the homologous pairs of chromosomes will separate in anaphase of the 1st division (Meiosis I), and in the second division (Meiosis II) individual chromatids will separate
Maternal Chromosome
Paternal Chromosome
Single Paternal Chromatid
Prophase I of Meiosis
Dyad pair or “tetrad”
2nd Major Differences between Mitosis in Meiosis
During Meiosis I homologous chromosomes “pair up” forming a Tetrad (grouping of 4 chromatids)
Homologous chromosomes exchange genetic information in an event called “Crossing Over”
CROSSING OVER (recombination)
•The resulting Chromatids have DNA that is unique or “recombinant”
a
b
c
d
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f
A
B
C
D
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F
A
B
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F
a
b
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d
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f
• Occurs in prophase of meiosis I
• Generates diversity by reordering the alleles of genes within homologous chromosomes
CROSSING OVER (recombination)
• Occurs in prophase of meiosis I
• Generates diversity by reordering the alleles of genes within homologous chromosomes
Letters denote genes Case denotes alleles (versions)
•Creates chromosomes with new combinations of alleles for genes A to F.
A
B
C
D
E
F
a
b
c
d
e
f
c
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e
f
A
B
a
b
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CROSSING OVER (recombination)
• Occurs in prophase of meiosis I
• Generates diversity by reordering the alleles of genes within homologous chromosomes
Letters denote genes Case denotes alleles
A
B
C
D
E
F
a
b
c
d
e
f
c
d
e
f
A
B
a
b
C
D
E
F
•The resulting Chromatids have DNA that is unique or “recombinant”
What is different between Meiosis and Mitosis?
TWO divisions resulting in Four CellsResult: Each new daughter cell contains only One chromatid (DNA) of each of the original homologous pairs.
Diploid germ cell
Meiosis I Meiosis II
Haploid gametes
PMATC
Single chromosome
chromatid
PMATC
Homologous pairs
Meiosis I : the reduction division
Prophase I(early)
(diploid)
Prophase I(late)
(diploid)
Metaphase I(diploid)
Anaphase I(diploid)
Telophase I(diploid)
Nucleus Spindlefibers
Nuclearenvelope
Tetrad
Prophase I
Early prophase
•Chromosomes condense.•Spindle forms.•Nuclear envelope/membrane fragments and disappears.
Late prophase
•Homologous chromosomes pair•Tetrads form with four chromatids .•Crossing over occurs- an event unique only to meiosis.
Metaphase I
Homologous pairs/Tetrads alignalong the equator of the cell.
Anaphase I
Homologues/tetrads separate andmove to opposite poles.
Sister chromatids remain Attached at their centromeres.
Telophase I
Nuclear envelopes reassemble.Spindle disappears.Chromosomes decondenseCytokinesis divides cell into two daughter cells.
Meiosis II : the equational division
Prophase II*(haploid)
Metaphase II(haploid)
Anaphase II(haploid)
Telophase II(haploid)
Four nonidentical
haploid daughter cells
Prophase II
Chromosomes CondenseNucleus disappearsSpindle apparatus forms
Haploid Daughter cells
Metaphase II
Chromosomes line-upalong equator of cell.Two sister chromatids attached at the centromeres
Anaphase II
Sister chromatids separateand move to opposite poles.
Telophase II
Nucleus reorganizes and nuclear envelope/membrane reforms.
Chromatids which are now called “chromosomes” decondense.
Spindle disappears.
Cytokinesis divides each cell into two forming four haploid cells.
Results of meiosis
Gametes
Four haploid cells
One copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome
Mitosis Meiosis
Number of divisions 12
Number of daughter cells
2 4
Genetically identical?
Yes No
Chromosome # Same as parent Half of parent
Where Somatic cells Germ cells
When Throughout life At sexual maturity
Role Growth and repair Sexual reproduction
____________________________:Change in the ______________ or ____________ of chromosomes
CHROMOSOMAL MUTATIONS
structurenumber
Deletion
Duplication
Inversion
Translocation
Figure 12–20 Chromosomal Mutations
Section 12-4
Homologous chromosomes ________________ during MEIOSIS
= _________________________
One cell gets 2 copies of the chromosome the other cell gets none.
NONDISJUNCTION
fail to separate
Mistakes in Meiosis
Normal Meiosis Nondisjunction
http://www.tokyo-med.ac.jp/genet/anm/domov.gif
http://web.udl.es/usuaris/e4650869/docencia/gen_etica/meioferti2.html
Nondisjunction
NondisjunctionSince it happens to a
sperm or egg, the new
baby can end up
with _____________ of a
chromosome
= __________________
OR
only ___________ of a
chromosome
= ___________________
TRISOMY
MONOSOMY
3 copies
one copy
Body cells have 2 of each chromosomeexcept one pair
= Monosomy (2n-1)How many chromosomes?
45
Body cells have 2 of each chromosomeexcept one pair has 3 copies
= Trisomy (2n+1)How many chromosomes?
47
Human Abnormalities caused by Non-Disjunction______________________________________________________
Down syndrome
Klinefelter syndrome
Turner syndrome
A __________ is a picture of an organism’s chromosomes
KARYOTYPE
Karyotype(need cells from baby)
Can tell __________________ chromosomes__________Some _____________________Can’t see _______________ mutations
Image from: http://members.tripod.com/~yenial/chromosome.html
MISSING/EXTRAGENDER
DELETIONS/INSERTIONSsingle gene
SEX DETERMINATIONXX =
Xy =
female
male
Chromosomes that determine the sex of an organism = _________________
All other chromosomes = _________________
Sex chromosomes
autosomes
Humans have two sex chromosomes
and _____ autosomes
X y44
http://www.angelbabygifts.com/
Who decides?
Mom can give X
Dad can give X or y
SO ____ determines sex of the baby.
If dad gives X with mom’s X = girlIf dad give y with mom’s X = boy
X X
X
y
X X X X
X y X y
Dad
Nondisjunction of Autosomal chromosomes
Normal female Normal male
Down syndrome (= ____________)TRISOMY 21
Nondisjunction of Autosomal Chromosomes
Male with trisomy 21 (Downs
syndrome)
Down syndrome (Trisomy 21)
• 1 in 800 births
• Similar facial features
• Slanted eyes
• Protruding tongue
Down syndrome (Trisomy 21)
Simian line on palm
Down syndrome (Trisomy 21)• Most common chromosomal
abnormality
• 50% have heart defects that need surgery to repair
• Mild to severe mental retardation
• Increases susceptibility to many diseases
• Risk of having a child with Down syndrome increases with age of mom
Sex Chromosomal Nondisjunction
• Failure of chromosomes to separate properly during meiosis
• Can lead to cells with extra genetic material (chromosomes)
• Can lead to cells without any chromosomes (bar bodies)
If having extra chromosomes causes genetic problems, how
come having two X chromosomes in females and
one X in males is not a problem?
Turner syndrome
Turner syndrome ____
• 1 in 5000 births• Females have only one X chromosome• Small size• Slightly decreased intelligence• 35% have heart abnormalities• Hearing loss common• Broad chest• Reproductive organs don’t develop at puberty• Can’t have children
http://medgen.genetics.utah.edu/photographs/diseases/high/611.gif
XO
Klinefelter syndrome XXy
Klinefelter syndrome
• 1 in 1000 births
• Males have extra X chromosomes
(Can be XXy, XXXy, or XXXXy)
• Average to slight decrease in intelligence
• Small testes/can’t have children
• Usually not discovered until puberty when don’t mature like peers