Download Meiosis Chromosome Structure

Gregor Mendel
Biol 202: Lecture 2
Genetics
Meiosis
• Cell division which results in halpoid “sex” cells
(i.e., egg and sperm)
• One replication of the genetic material (DNA)
during interphase, but two nuclear divisions
(meiosis I and meiosis II).
• Results in haploid (N) cells (= gametes in animals)
from an initial diploid (2N) cell
• Very similar to mitosis except that the cells
produced are not genetically identical.
Chromosome Structure
(cartoons)
sister chromatids
M phase
centromere: region of the
chromosome where
chromatids attach.
sister chromatids
1
Synthesis
represents gene A,
from one parent.
A- -A
a- -a
B- -B
b-
-b
represents the
same gene from
the other parent,
codes for the
~same protein,
may have a
slightly different
DNA sequence.
Homologous Chromosomes
Alleles
A-
Alternative forms
of the same gene.
aHeterozygous
B-
b-
C-
C-
Homozygous
Alleles occur at the same locations (loci) on homologous chromosomes.
Meiosis Prophase I
… Synapsis: the highly specific parallel
alignment of homologous chomosomes
during the first division of meiosis,
A A a a
…tetrad: the two homologous
chromosomes become attached
along their length in a structure
termed a tetrad.
B B b b
2
Meiosis Prophase I
cont.
A a A a
B B b b
Crossing Over: rearranges the genes from each parent.
Chiasmata
Meiosis Metaphase I -- Telophase I
A a Aa
B B b b
A a
A a
B B
b b
3
Meiosis Prophase II -- Telophase II
Aa
aA
no DNA synthesis
BB
bb
a
A
A
a
B
B
b
b
2n combinations of chromosomes
n = number of chromosomes
= 1,
= 2,
= 3,
= 5,
2n = 2
2n = 4
2n = 8
2n = 32
•
•
•
•
n
n
n
n
•
•
n = 23, 2n = 8,388,608 H. sapiens
n = 39, 2n = yikes! dog
Meiosis is critical for sexual
reproduction in all diploid
organisms
...meiosis leads to the formation of gametes,
– gametes (one from each parent) conjugate to
form a zygote,
...meiosis is the basis for extensive variation
among members of a population.
4
A comparison of mitosis and meiosis: summary
Gregor Mendel
Mendelian Genetics
Mendel Insight 1
• Used the pea,
5
Insight 2
• alternate forms,
Insight 3
• True breeding lines,
– “Permit me to state that, as an empirical worker, I must define constancy
of type as the retention of character during the period of observation”. Mendel
– Mendel observed his ‘true-breeding’ lines for up to 8 generations.
•
Used the pure-breeding line to form hybrid lines,
– offspring of genetically dissimilar parents.
Insight 4
• Expert plant breeder,
– carefully controlled the matings,
• prevented the intrusion of any pollen foreign to the desired mating,
– made reciprocal crosses:
• reversing the traits of the male and female parents,
– male wrinked x female smooth,
– female wrinkled x male smooth.
6
Insight 5
• Used large numbers of subjects,
– applied statistical analysis to his data!
• uncovered the patterns of transmission that we will
take for granted.
Insight 6
• Controlled for environmental factors,
– for example, when looking at the short and tall
plants, he made sure that all subjects received
equal light,
• from his studies of plant physiology, he knew that
light mediates stem elongation.
Insight Summation
•
•
•
•
•
•
Used the pea,
Identified alternate forms,
Identified and used true breeding lines,
Expert plant breeder,
Used statistical analysis,
Controlled for environmental factors.
Set up a simple ‘black and white’ system, and
then figured out how it worked.
7
Monohybrid Cross
• Mating between individuals that differ in only one
trait,
– yellow pea x green pea,
– violet flower x white flower
– tall x dwarf
– round seed x wrinkled seed
– full pod x constricted pod
– etc.
Monohybrid Cross
Generation
Parental (P)
yellow pea
(pollen)
green pea
(eggs)
x
First Filial (F1)
all yellow
Second Filial (F2)
grow plants, cross
pollinate
grow, allow to selffertilize
6022 yellow : 2001 green
3:1
8
Reappearance of Trait in F2
Generation Disproves Blending
• Blending did not occur, in fact over 2000 peas
retained the information necessary to make green
peas,
• Mendel concluded that there must be two types of
yellow peas,
– those that breed true like the parent plant,
– those that can yield some green peas, like some of the
F1 hybrids.
Dominant vs. Recessive Traits
x
P
F1
The trait that appears in the F1 generation is the DOMINANT trait.
The trait that disappears in the F1 generation is termed RECESSIVE.
Mendel’s First Postulate
Unit Factors in Pairs
• Genetic characteristics are controlled by unit
factors (Genes) that exist in pairs in individual
organisms,
– each individual receives one unit factor from each parent,
– in a monohybrid cross, three combinations of unit factors are
possible,
9
Definitions to Know
• Homozygous: the unit factors that
determine a particular trait are the same,
– YY = homozygous dominant,
– yy = homozygous recessive,
• Heterozygous: the unit factors that
determine a particular trait are different,
– Yy = heterozygous.
Mendel’s Second Postulate
Dominance/Recessiveness
• When two unlike unit factors are present in
a single individual, one unit factor is
dominant to the other, which is said to be
recessive.
Unlike Unit Factors
=
Alternate Forms of the Same Gene
=
Alleles
10
When Unit Factors Separate
Two Unit Factors = Diploid
One Unit Factor = Haploid
During Gamete formation, Unit Factors Separate
More Definitions to Know
• Phenotype: an observable trait,
• Genotype: the actual composition of alleles
present in an individual.
11
Monohybrid Cross
P:
GG
F1:
x
gg
Gg
Gametes:
G
1/2
F2:
g
1/2
(GgxGg)
G
1/2
g
1/2
GG
Gg
gG
gg
1/2 x 1/2
1/2 x 1/2
1/2 x 1/2
1/2 x 1/2
1/4
1/4
1/4
1/4
Random Segregation
F2:
GG
1/4 GG
1/4 GG
Gg
gG
gg
1/4 Gg
1/4 gG
1/4 gg
1/2 Gg
1/4 gg
Punnett Squares
Y
y
Y
YY
Yy
Y
YY
Yy
gametes
Parent 2
gametes
Parent 1
Predicted
Offspring In
Squares
12
Mendel’s Third Postulate
Segregation
• During the processes of heredity, the paired unit
factors separate so that the offspring receives one unit
factor from each parent,
• The unit factors segregate to offspring randomly.
Postulates 1-3 Applied
F1 Generation
P1:
Gametes:
Yellow
YY
Green
yy
Y
y
Phenotype
Genotype
Yellow
Yy
F1:
Phenotype
Genotype
Postulates 1-3 Applied
F2 Generation
Yellow
Yy
F1:
F1 Self-Cross:
Gametes:
F2:
YY
Yy
Yy
Y or y
Y or y
Yy
Yy
yy
13
Dihybrid Crosses
• Monohybrid Cross,
– one set of contrasting traits,
• Y (yellow) versus y (green).
• S (smooth) versus s (wrinkled).
• Dihybrid Cross,
– SSYY x ssyy
Mendel’s Forth Postulate
Independent Assortment
• How do two traits segregate in the offspring
of an individual that is heterozygous for
both traits?
14
Watson and Crick
On the last day of February
1953, Francis Crick
announced to the patrons
of the Eagle pub in Cambridge
“We have discovered the
secret of life”
Figure 16.5 The double helix
Genetic Code
• Three DNA letters are transcribed (process of
transcription) into three mRNA letters called a
Codon
• A specific codon will code for a specific amino
acid = subunit building block of proteins that are
covalently linked together by peptide bonds
(amino group to carboxyl group).
• Polypeptide - a molecule made up of amino
acids.
• Protein - a molecule (gene product) made of one
or more polypeptides, 3D structure, specific
function.
15
Genetic Code
•
•
•
•
In DNA there are 4 bases: A,C,G,T
In a codon there are 3 bases
Thus, 64 possible codons (43)= 64
However, only 20 different amino acids, plus one
start codon and three stop codons
• Genetic code = nucleotide triplets
16
Characteristics of the Code
•
•
•
•
•
•
•
Written in linear form in mRNA letters (A,C,G,U)
mRNA transcribed from 3’-5’ template strand
Each mRNA “word” contains three letters
Each group of three letters = codon
One codon specifies one amino acid
Code = Triplet
Code is degenerate (more than one codon can specify a
given amino acid)
• Code is non-overlapping
• Code is universal/nearly universal
• Amino acid is a “sub-unit” building block of protein
Autosomal Recessive Traits
• AA=normal, Aa=normal, aa=affected
• 90% of all autosomal genetic defects
• Trait not carried on sex chromosomes, but on other 22
pairs (autosomes)
• Many arise from relatives mating - increased chance
for recessive alleles to pair
• Two affected parents will always produce affected
offspring
• Parents can be carriers (Aa) and have a 25% chance of
producing affected offspring
• Trait often skips a generation
• Males and females affected equally
Autosomal Dominant Traits
•
•
•
•
•
•
•
•
•
AA=affected, Aa=affected, aa=normal
~10% of autosomal genetic defects (chin dimple)
Not carried on sex chromosomes
Two normal parents can only have normal offspring
Two affected parents who are heterozygotes (Aa) have
a 25% chance of producing normal offspring
An affected offspring must have at least one affected
parent
Traits will not skip a generation
Trait should appear in almost equal numbers
When an affected person mates with a normal person,
50% of the offspring are expected to be affected
17
X-Linked Recessive Traits
XB=normal,
•
Xb=affected
• Affects more males (50% chance of inheriting
recessive allele from mother = hemizygous)
• Traits can skip a generation
• Affected mother produces affected sons
• Normal mother produces normal daughter,
unless mother is a carrier and father is affected
then daughter could be affected
• Two affected parents will have affected children
X-Linked Dominant Traits
XB=affected,
•
Xb=normal
• Affects more females.
• Traits does not skip a generation
• Affected males must come from affected
mothers
• Two normal parents will have normal children
• All the daughters, but none of the sons, of an
affected father are affected
• Approximately 50% of the children of an
affected heterozygous female are affected
Y-linked Traits
• Only males affected
• If one male in pedigree is affected then all
related males must be affected
• If one male is normal then all related males are
normal
18