Download Meiosis

9/26/05 Biology 321
Answers to optional challenge probability questions posed in 9/23/05lecture notes are included at
the end of these lecture notes
The history of
Life on Earth
reflects an unbroken chain of genetic
continuity and transmission of genetic
information:
How many years of genetic continuity
does this unbroken chain represent?
1
The processes of mitosis and meiosis are
common to all eukaryotic cells suggesting
that these processes evolved in very
ancient eukaryotic cells that were the
ancestors to all present-day eukaryotic
cells
How old are these processes then?
2
Transmission or propagation of any genetic
program requires two basic steps:
1. Copying over step (replication of the DNA
molecule)
2. Parcelling out step (getting the
duplicated information properly
distributed to the progeny)
In the nucleus of eukaryotic cells, the
parcelling out process occurs in the form of 2
different types of cell division:
3
Mitosis is a conservative propagation of
genetic information: the daughter cells
have the same genetic composition as the
mother cell
Meiosis is a non-conservative
propagation of genetic information: the
products of meiosis have a different
genetic composition than the mother cell
4
Review cell cycle and stages of
mitosis
label the stages: prophase, metaphase,
anaphase, telophase
5
Stages of mitosis in the cells of the Indian muntjak, a deer species where 2n=6.
This small number of chromosomes allows clear visualization of the condensation
(yellow to orange), separation (red to turquoise) and decondensation (green to
yellow) of chromosomes throughout mitosis.
6
• At the end of prophase of mitosis each
duplicated chromosome is maximally
condensed
• Each chromosome consists of 2 identical
halves known as sister chromatids
• Each chromatid contains one double-stranded
DNA molecule coiled up
• Sister chromatids are held together at a
constriction called the centromere
scanning electron micrograph of human chromosomes
metaphase chromosome spread
Note the size variation and that the centromere is located at
different positions in different chromosomes
7
Diploid cell
n = 2, 2n = 4
homologs
G1, S, G2
sister
chromatids
centromere
(= kinetochore + DNA)
OR
Meiosis I
Mitosis
Meiosis II
8
2n=2 in this example
n= number of chromosomes in one copy of the
(nuclear) genome
1n = haploid 2n = diploid 3n= triploid
4n = tetraploid
the number preceding the n indicates the
number of genome copies (or number of copies
of each chromosome)
Chromosome number (n) and genome size do
not necessarily correlate
9
Connecting the products of semi-conservative DNA
replication with sister chromatids:
Each sister chromatid is composed of a single,
continuous polymer of double-strand DNA coiled
into a compact form::
HOW LONG ARE THESE POLYMERS?
The next figure diagrams the connection between semiconservative replication of DNA and the sister chromatids
10
11
Meiosis is a non-conservative
propagation of genetic
information: the products of
meiosis have a different genetic
composition than the mother
cell
Meiosis I (reductional division)
Homologous chromosomes are separated
into different cells: each daughter cell
contains one duplicated copy of each type
of chromosome (the number of
chromosomes is cut in half)
Meiosis II (equational division)
Sister chromatids are separated into
different cells (like a mitotic division)
12
What events occur during prophase of
meiosis that do not occur during
prophase of mitotis?
13
synapsis: close pairing of homologs
during prophase of Meiosis I
What is a homolog?
14
Homologous chromosomes
• chromosomes that pair with each other
during meiosis
• chromosomes that possess the same
genes at corresponding positions
Diploids have two copies or two homologs
of each different type of chromosome
15
crossing over:
• a precise breakage and reunion event
that occurs between two non-sister
chromatids
• pieces of the DNA strands in the two
chromatids are exchanged
16
note: we will look at crossing over in detail later in the
quarter
17
Diploid cell
n = 2, 2n = 4
homologs
G1, S, G2
sister
chromatids
centromere
(= kinetochore + DNA)
OR
Meiosis I
Mitosis
Meiosis II
18
Walter Sutton was a Kansas farm boy and in
1902 was the first person to point out that:
http://gnn.tigr.org/timeline/1902_Boveri_Sutton.shtml
?????
19
Chromosomes obey Mendel’s rules of
segregation and independent assortment
Interested in the History of genetics?
check out this web site:
http://www.genomenewsnetwork.org/timeline/timeline_home.shtml
Mendel’s principle of segregation: Paired hereditary factors
(R and r alleles in this diagram) segregate into different
gametes
20
MENDEL’S PRINCIPLE OF SEGREGATION:
During gamete formation the members of each gene
pair segregate (that is, separate) into different gamete
cells.
Segregation of alleles into different gametes is ensured by
• the pairing of homologs during prophase of the first meiotic
division
• and the separation of homologs into different daughter cells in
anaphase of the first meiotic division
21
What events in meiosis explain Mendel’s
principle of independent assortment?
22
In metaphase of the first meiotic division, the
paired homologs (the bivalents) line up randomly
with respect to each other.
Meiosis generates gametes
with many genetic combinations
Diploid cell
n = 2, 2n = 4
Independent assortment
OR
Meiosis I
Meiosis II
Crossing over
Meiosis I
Meiosis II
23
NOTE that allele symbols are different here from what we used in class
G= yellow g= green W = round w=wrinkled
yellow round x green wrinkled
P: GGWW x ggww
gamete
gw
gamete
GW
F1 zygote
GgWw
F1 x F1
G
g
OR
W
w
g
G
w
G
g
W
w
G
W
W
Meiosis I
d
g
w
T
Gametes
male gametes
G
W
G
w
g
W
d
g
w
T
G
W
female
gametes
G
w
g
W
d
g
w
T
On quiz Friday must indicate location of all alleles
for Meiosis I --shorthand in this diagram not
acceptable. See also answer sheet--question 7.)
[If time: Mendel and independent assortment -- table --genomic location
of pea traits]
24
State a general rule of thumb to
answer this question:
What organisms will show Mendelian
patterns of inheritance?
25
 Since meiosis provides the physical
basis for Mendel’s genetic principles, all
organisms that have meiosis as part of
their lifecycle will show Mendelian
patterns of inheritance
Does this include haploid orgnanisms?
26
Do mitosis and meiosis account
for the propagation of all
genetic information represented
by Life on Earth?
What are the exceptions?
27
Prokaryotes: binary fission
Eukaryotes: mitochondrial and
chloroplast DNA
Viruses: parasitize cells
Genes not propagated by meiosis will not
show Mendelian patterns of inheritance
28
What happens during meiosis in an
organism that has an odd number of
genome copies?
The problems with bananas……who
haven’t had sex in decades-- see article
on 321 web site
29
Answers to challenge problems
Problem 1
(3/4)10 = 0.056=1 in 18 hets will produce only dominant progeny and be
miscored as homozygotes for the dominant allele
Problem 2
Homozygotes for the dominant allele will breed true and always be
scored correctly: 0.33 of F2 with dominant phenotype scored correctly
For each of the ten progeny of a heteozygote:
3/4 chance it shows the dominant phenotype
1/4 chance it shows the recessive phenotype (which would reveal the
genotype of the parent)
(3/4)10 = 0.056= chance that all ten progeny of a heterozygous plant will
show the dominant phenotype
1- 0.056 = probability that at least one progeny of a heterozygous plant
is homozygous recessive = 0.944
2/3 [fraction of F2 with dominant phenotype that are het] [(3/4)10 ]=
0.0375 scored incorrectly
scored as RR = 0.33 (correct) + 0.0375 (incorrect) = 0.37
scored as Rr = 0.67 (0.944) = 0.63
0.63 Rr to 0.37 RR = 1.7 to 1
30