Meiosis to Mendel
Chapter 9
Read first few sections.
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Reproduction

Asexual reproduction – mitosis
clones – genetically identical
individuals
 Produces

What would happen if the environment
changed?
 Genetic
variability helps the population as a
whole survive changes
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WHY SEX ?
Sexual reproduction combines the DNA
from two different individuals
 A gene is a section of a chromosome that
carries instructions for a specific trait
(protein)

 An

allele is a different version of a gene
The greater the number of different
combinations of genes the more variation
among individuals, and the greater the
chance of survival of the species.
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Sex cell formation
Sexually reproducing organisms need to
produce specialized reproductive cells or
gametes.
 Produced from germ cells in organs called
gonads
 In females ovaries produce eggs or ova
 In males testes produce spermatozoa
 A mutation in one of your cells won’t be
passed on unless it is in the germ line cells

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Diploid vs. Haploid
In animals, most plants, and many other
organisms, each cell contains 2 sets of
chromosomes.
This is the diploid (2n) number of
chromosomes.
A pair of homologous chromosomes, one
from each parent.
Gametes contain only one member of each
pair or the haploid (n) number of
chromosomes
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When a sperm and egg combine in the
process of fertilization, or syngamy, a
new diploid cell or zygote is formed.
 The gametes must be haploid, that is,
have n number of chromosomes. If the
gametes were diploid, every generation
would have twice the number of
chromosomes.
 To form haploid gametes, there needs to
be a process other than mitosis – this is
called meiosis.

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Meiosis
This is a two part process : meiosis I and
meiosis II, features TWO cell divisions.
 However, the DNA is only replicated once
 Meiosis I and II both use the same four
stages of mitosis: prophase, metaphase,
anaphase and telophase

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Major differences between mitosis
and meiosis
Meiosis has two cell divisions.
 Meiosis has a step where homologous
chromosomes swap pieces

 Increases
genetic variation, new combos.
In the first division, sister chromatids don’t
split apart! Homologs separate instead.
 In the second division, sister chromatids
separate like in mitosis.

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During prophase I the homologous chromosomes pair
up in synapsis. This is the longest phase of meiosis.
Crossing over may occur further increasing genetic
variation.
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Homologs don’t hang together
In metaphase I the tetrads migrate to the
center of the cell.
 In Anaphase I the centromeres do not
break and one member of each
homologous pair (2 sister chromatids)
move to opposite ends of the cell
 Which homolog goes to which end of the
cell occurs at random.
 Telophase occurs as in mitosis.

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Meiosis II
There is no replication of chromosomes
between telophase I and prophase II
 Meiosis II proceeds just like mitosis –
during anaphase the centromeres break
and the two sister chromatids go to
opposite poles.

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With only 2 pairs of
chromosomes, a gamete
could have any one of 4
different combinations
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Cytokinesis
Varies by which type of cell is being made
 If we are producing sperm, each of the
four cells produced by meiosis II become
sperm.

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If we are making ova, cytokinesis is
uneven and one cell takes nearly all the
cytoplasm, leaving the other cell merely a
package of discarded DNA called a polar
body.
 In humans, the cell again divides
unevenly, so at the end of meiosis II we
have formed one ovum (egg) and three
polar bodies. The polar bodies
disintegrate.

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The average woman produces one ovum
every 28 days
 Males produce 300 million sperm/ day
 If less than 20 million / ml, a man is
considered infertile.
 Fertilization is a group effort, but only one
sperm penetrates the ovum.

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Sometimes things go wrong.
 In anaphase I the separation of
homologous chromosomes is called
disjunction.
 When they do not separate it is called
nondisjunction and the resulting gametes
contain one too many or one too few
chromosomes.
 Fertilization results in a zygote with 45 or
47 chromosomes. This is an aneuploid
(vs. euploid) number of chromosomes

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Chromosomal Abnormalities per 100,000 recognized human pregnancies
15,000
85,000
spontaneous abortions
live births
--------------------------------------------------------------------------------------------------------Trisomy
A Group
1
2
3
0
159
53
0
0
0
B Group
4
5
95
0
0
0
C Group
6-12
G group
21
22
561
350
424
0
113
0
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
Three copies of a chromosome is called
trisomy – Down syndrome is trisomy 21

A zygote with one too few chromosomes does
not usually develop. (monoploid)
Extra copies of the sex chromosomes (vs. the
autosomes) do not cause as much of a problem
XX is female in humans (male in birds)
XY is male in humans
XXY , XYY, XXX, XO all happen
Plants do much better with multiple
chromosomes

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
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15,000
spontaneous abortions
85,000
live births
sex chromosomes
XYY
4
46
XXY
4
44
XO
1350
8
XXX
21
44
14
164
225
52
1275
0
450
0
280
49
7500
550
Translocations
Balanced
Unbalanced
Polyploid
Triploid
Tetraploid
Other (mosaics, etc)
Total
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Genetics

A gene is a section of DNA that codes for :
Proteins – for structures such as muscles
- or for enzymes
RNA molecules like rRNA and tRNA
Genes have sections of DNA next to them
that control whether they get used or not:
regulatory regions
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DNA is like magnetic recording
tape; song information is
encoded in the structure of the
tape magnetically. Many different
songs are recorded and each
one can be played individually
(or not played).
DNA is very long and thin like
recording tape.
DNA is like digital media also,
because each song can be
specifically accessed at the beginning.
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Each cell of an organism that reproduces
sexually has two copies of each
chromosome, and therefore has two
copies of every gene – one on each
member of each pair of chromosomes
(exception is the Y chromosome, which is
smaller than the X).
 The two versions of each gene are called
alleles. Alleles may be the same or
different, depending on the traits of the
parents.

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All the genes that are contained on all the
chromosomes of an individual make up
that individual’s Genome.
 The Genotype is all the genetic
information the individual has for a
particular trait.

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Not all genes are expressed.
 Those traits that are expressed: can be
seen (physical traits) or measured
(chemical traits) are the individual’s
phenotype.
 The phenotype is influenced by both the
genotype and the environment.
 In diploid organisms (like us) an allele from
one parent can mask the appearance of
the other. So often genotype ≠ phenotype.
 We can still pass on a hidden trait and see
it in our children.

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