Download Genetics Heredity and Variation: *Heredity is the branch of science

Genetics
BIO 221
Genetics
Heredity and Variation:
*Heredity is the branch of science explains how related organisms resemble
each other.
*Variation is a science discovers why related individuals differ from each
other.
Mendel’s work:Gregor Mendel born in 1822 and in 1843 studding natural history in Vienna
University. In 1856 studied the inheritance in pea by hybridization.
The choice of pea has 5 advantages which are :1- Presence of 22 varieties of pea.
2- Pea easy to cultivate.
3- Plant self pollinating because flower is closed = Pure breeding.
4- Easy artificial cross –breeding between varieties.
5- There are 7 characters (traits) as: stem length - seed shape – seed color - pod
shape – pod color – flower color – flower position.
Monohybrid inheritance and the principle of segregation:He work on 2 varieties one with terminal flower and the other with axial
flower. He takes pollen from axial flower to terminal flower plant and vice versa.
In all cases the resulted generation F1 was axial flower in the first generation
so axial flower called dominant.
Genetics
1
Genetics
BIO 221
The F1 plants enclosed in a bags to prevent cross-pollination and left to selfpollinated to give the 2nd generation F2, he found some flowers with axial and
other with terminal flowers
Parents
:
in ratios 3:1
Axial flowers
F1
×
Terminal flowers
All axial flowers
F2
Axial flowers
3
:
Terminal flowers
1
The separation of the pair of parental factors (one factor present in each
gamete)is known as Mendel’s first law or the principle of segregation. This stated
that: the characteristics of an organism are determined by internal factors which
occur in pairs, only one of a pair of such factors can be represented in a single
gamete.The initial letter of the dominant character (genotype) is capital.
► Monohybrid inheritance:-=First low of Mendel,=( low of segregation):
(Members of a pair of factors segregate from each other in the formation
gametes).
Example:A = Axial Flower (dominant).
Genetics
a = Terminal flower (recessive).
2
Genetics
BIO 221
Write genetic explanation of Mendel’s monohybrid crosses:Parents phenotype
Pure Axial Flower ×
Parents genotype
AA
Pure Terminal Flower
aa
Meiosis
Gametes
A
A
a
a
Aa
Aa
Aa
Aa
(n)
Randomefertilization
(F1 genotypes 2n)
►F1 Generation were self – pollinated:F1 phenotype:
Axial Flower
F1 Genotype:
Aa
×
Terminal Flower
Aa
(2n)
Meiosis
A
Gametes
(n)
A
AA
Randome
fertilization
(F2 genotypes 2n)
F2 phenotype
a
a
aa
Aa
Homozygous
Aa
Heterozygous
3Axial Flower
Homozygous
1 Terminal Flower
Q: What is the 1st and 2nd Generation of the offspring of tall plant (DD) dwarf plant
(dd) where Tall character is the dominant? Whereas the tall character is dominant
& short character is ressive.
Genetics
3
Genetics
BIO 221
Tall Plant
Parents phenotype
Parents genotype
TT
×
short Plant
X
tt
Meiosis
Gametes
T
T
t
t
Tt
Tt
Tt
Tt
(n)
Randomefertilization
(F1 genotypes 2n)
►F1 Generation were self – pollinated:F1 phenotype:
Tall Plant
F1 Genotype:
Tt
×
Tall Plant
Tt
(2n)
Meiosis
Gametes
(n)
Randome
fertilization
(F2 genotypes 2n)
F2 phenotype
T
t
T
TT
tt
Tt
Homozygous
Tt
Heterozygous
3 Tall Plant
Genetics
t
Homozygous
1 short Plant
4
Genetics
BIO 221
► Punnet quadrat:♀
♂
D
D
Genetics
D
d
DD
Dd
Tall
Tall
Dd
dd
Tall
dwarf
5
Genetics
BIO 221
»» Glossary of common genetic terms:►Gene =
Basic unit of inheritance.
►Allele =
One of a number of alternative forms of the same gene.
►Homozygous =
Diploid conditions with alleles are identical (AA).
►Heterozygous =
Diploid conditions with alleles are different (Aa).
►Genotype =
Genetic constitution with respect to alleles.
►Phenotype =
Characters resulting from interaction between genotype
and environment.
►Dominant =
Allele controls the phenotype even in the presence of other
allele (A).
►Recessive =
Allele controls the phenotype only in the presence of
another identical allele (a).
►F1 generation =
G produced by crossing homozygous parents
►F2 generation =
G produced by crossing 2 F1 organisms
Genetics
6
Genetics
BIO 221
Mendel’s second law =Dihybrid inheritance = principle of independent
assortment:
-Dihybrid inheritance means the case of two pairs of alleles
► Mendel’s second law or the principle of independent assortment:(Members of different pairs of factors segregate independently during
gametes formation).
Example of dihybrid cross:He used pea seed shape and pea seed colour, he crossed plant with (round
seed with yellow colour) and (wrinkled seed with green colour).
F1 = Round and yellow it is dominant.
R = Round seed (dominant).
r = wrinkled (recessive).
Y = Yellow (dominant).
y = green (recessive).
Parents phenotype
Parents genotype
Round seed and yellow ×
Wrinkled and green
RRYY
rryy
RY
ry
Meiosis
Randomefertilization
(F1 genotypes 2n)
Genetics
All RrYy (Hetero) Round and Yellow
7
Genetics
BIO 221
F1 phenotype:
Round seed and yellow
×
Round seed and yellow
RrYy
F1 Genotype:
RY
Ry
RrYy
rY
ry X
RY
Ry
rY
ry
► Punnet quadrat:♀
♂
RY
Ry
rY
ry
RY
Ry
RRYY
RRYy
round
round
yellow
yellow
RRYy
* RRyy
round
round
yellow
green
RrYY
RrYy
round
round
yellow
yellow
RrYy
rY
ry
RrYY round
RrYy round
yellow
yellow
RrYy round
*Rryy round
yellow
green
#rrYYwrinkled #rrYywrinkled
yellow
yellow
*Rryy
#rrYy
@ rryy
round
round
wrinkled
wrinkled
yellow
green
yellow
green
The Ratio of 2nd generation F2 in dihybrid inheritance .9:3:3:1
The Proportions of each phenotype to ratio 9:3:3:1 is known as dihybrid ratio.
Allelomorphic characters = Phenotypes determined by different alleles.
Genetics
8
Genetics
BIO 221
► Polyhybrid inheritance:- more than 2 characters
Crossing of parents have 3 pairs of characters.
The characters are:
» Tall
=
D
=
Dominant
» Round seed
=
R
=
Dominant
» Yellow seed
=
Y
=
Dominant
» Dwarf
=
d
=
Recessive
» Wrinkled seed
=
r
=
Recessive
» Green seed
=
y
=
Recessive
DDRRYY
×
Tall Round yellow
DRY
ddrryy
Dwarf wrinkled green
×
Dry
Parents
Parents Gametes
DdRrYy
F1
All Tall round yellow
1st generation
Crossing of this generation
Genetics
9
Genetics
BIO 221
Formation of Gametes:-
R
D
r
d
R
Y
DRY
y
DRy
Y
DrY
y
Dry
Y
dRY
y
dRy
Y
DrY
y
dry
r
♂
DRY
♀
DRY
DRy
DDRRYY DD
RR
Yy
DrY
DD
Rr
YY
Dry
DD
Rr
Yy
dRY
Dd
RR
YY
dRy
Dd
RR
Yy
drY
Dd
Ry
YY
dry
Dd
Ry
Yy
DRy
DrY
Dry
dRY
dRy
Genetics
10
Genetics
BIO 221
drY
dry
27
Tall round yellow.
9
Tall round green.
9
Tall wrinkled yellow.
3
Dwarf wrinkled yellow.
9
Dwarf round yellow.
1
Dwarf wrinkled green.
3
Tall wrinkled green.
Genetics
11
Genetics
BIO 221
Gene interaction:
Variation of Mendelian ratios,:
A)
Incomplete dominance:-F2 (1:2:1 ).
1)
Flower color in snapdragon:When cross Red flower × white flower we found the F1 all pink but
F2 has Red : Pink : White (1:2:1 ).
Red
=
W
Parents
White
=
×
ww
(white)
WW
( Red )
F1
w
Ww (Pink)
Crossing
between F1
Ww
×
WW
(Red)
Ww
(Pink)
Ww
Ww
ww
(Pink) (white)
B) Co dominance:-F2 (1:2:1 ).
1) Feather color in hens:White Feather
W1W1
F1
F2
×
Black Feather
W2W2
W1W2
All Blue Feather
W1W1
White
F2 in Co dominance has
W1W2
Blue
W1W2
Blue
W2W2
Black
White : Blue : Black (1:2:1 ).
We found that blue color in Co dominance condition due to presence of both
black and White in the same feather.
Genetics
12
Genetics
BIO 221
C) Lethal genes:- F2 in Lethal Genes =2 : 1
Lethal genes change the Mendel’s percentages from 3:1 to 2:1
F2 in Lethal Genes =2 : 1
Example: Yellow color in Rates:- In some living rats yellow color never occur
in pure homozygous because Rats carrying YY are die.
In F1 Crossing yellow × Black give ½ yellow and ½ black
parents
Yy (yellow) ×
Gametes
Y
yy (black)
y
y
y
F1: Yy( yellow) : Yy ( yellow) : yy (Black) : yy (Black) :
Crossing between F1
YY
:
Pure yellow
Die
Genetics
Yy
×
Yy
Yy : Yy
:
yy
yellow
:
black
2
:
1
13
Genetics
BIO 221
Gene interaction
1- Linkage
Linkage means some genes are situated on the same chromosome & pass
into the same gamete and inherited together).
Linked genes do not conform to Mendel's principle of independent
assortment and do not give the ratio 9:3:3:1 in dihybrid inheritance
In Drosophila genes of color and wing length are allelomorphs.
* Grey
and long wing dominant
G and L
* Black
and short wing recessive
g and l
In case of linked genes = on the same chromosome. (We will not
found gametes containing G l or gL because GL are Linked.)
Grey and long
G
g
L
l
Black and short
×
g
g
l
l
heterozygous
G
g
L
l
g
×
l
G
g
g
g
L
l
l
l
1
Grey and long
:
1
Black and short
We were
depicts the
Genetics
14
Genetics
BIO 221
following
Gg Ll × ggll
♀
GL
Gl
gL
gl
GgLl
Ggll
ggLl
ggll
Grey
Grey
Black
Black
long
short
long
short
♂
gl
Crossing-over At( synapsis) is an exchange of genetic material between nonsister
chromatids of a bivalent during meiosis I.
Crossing over: Means the formation of chiasmata during prophase I of meiosis
. the breakage and recombination of homologous chromosomes.
Meiosis
G
g g
g
L
l
l
l
Grey
Black
long
short
G
G g
g
L
L
l
showing
crossing-over
G
l
G
g
g
Gametes
×
L
Genetics
g
l
L
l
l
15
Genetics
BIO 221
g
G
Offspring
g
g
g
g
G
l
frequency %
l
L
g Recombination
L
l
l
l
l (Crossing over Value
(CoV)
= X/Y ×100
Grey long grey short blank long
blank short
Gene mapping
Gene mapping meaning relative positions of genes on chromosomes
Calculating (CoV) crossing over value help us to produce maps for gene
position on the chromosomes, by converting CoV this value into hypothetical
distances along the chromosome.
Ex: a (CoV) of 4% between genes A and B means that those genes are
situated 4 units apart on the same chromosome.
A
4
B
Sex Determination
In drosophila we found one pair of chromosomes always shows difference
between sexes called sex chromosomes.
Sex chromosomes called heterosomes all other non sex chromosomes
called autosomes.
Drosophila has 4 pairs of chromosomes 3 identical in both sexes and one not
which known as X and Y
Genetics
16
Genetics
BIO 221
In Human ♀female =XX
In some birds and butterflies
,
♂ male = XY
♀ =XY
as human.
♂ = XX.
,
Genetic explanation of the sex ratio in humans.
♀ =XX
♂ = XY
×
Meiosis Gametes
X
X
X
Y
Offspring
XX
XY
XX
XY
♀
♂
♀
♂
Ratio
1♂
:
1♀
Multiple alleles =A character controlled by a one gene but this gene appeared
in many forms alleles ex: Inheritance of blood groups
O, B, A …etc
Gene of blood group is I symbol and has 3 alleles A, B & O ,
A and B equally dominant but O recessive.
Presence of any dominant alleles make blood produce antibody called
agglutinin. Ex: I
A
Io form agglutinogen A on membrane but plasma will contain
agglutinin anti B.
Genetics
17
Genetics
BIO 221
Genotype
Blood group (Phenotype)
I A IA
A
I A Io
A
I B IB
B
Human
Blood
IBIo
IAIB
IoIo
Genetics
B
group
AB
O
18
Genetics
BIO 221
The Cell Cycle
The cell cycle is a set of stages between the time a cell divides and the time the
resulting daughter cells also divide.
The events that occuresWhen a cell is going to divide:
1-Cell grows larger,
2- Number of organelles doubles,
3- Amount of DNA doubles by DNA replication.
Cell cycle consists of two portions
1- interphase, 3 stages (G1 + S + G2)
2- Mitotic stage(M stage) includes mitosis & cytokinesis.
1- Interphase. (Preparations for mitosis).
Interphase consists of 3 stages= G1, S, & G2.
G1 Stage
G1 is a stage before DNA replication,
Events occurs During G1 stage:
1-cell increases in size,
2- Cell doubles its organelles
3- Cell accumulates materials of DNA synthesis.
{Nerves do not complete the cell cycle & entered a G0.}
S Stage
Stage of DNA synthesis or replication.
Events occur During S stage:
1-chromosome will duplicate & will consist of two sister chromatids instead of one
chromatid.
G2 Stage
G2 is the stage from the end of DNA replication to the beginning of mitosis.
Events occurs During G2 stage:
1- Cell synthesizes proteins .
2- Formation of spindles.
Mitotic Stage ( M stage):
Genetics
19
Genetics
BIO 221
This stage includes mitosis & cytokinesis.
1-mitosis means nuclear division.
2-cytokinesis means division of the cytoplasm.
Control of the Cell Cycle
The cells arrested in G0 & finish the cell cycle if stimulated to do so by growth
factors.
Significance of mitosis
1- Genetic stability:
2- Growth: Increasing the cell number
3- Asexual replication, regeneration and cell replacement.
Genetics
20
Genetics
BIO 221
Meiosis (Meio = to reduce)
Meiosis is a nuclear division involving a reduction from diploid number (2n)
of chromosomes to the haploid number (n).
Meiosis occurs during the formation of sperm and ova in animals
(gametogenesis) and formation of spores in plants.
The behaviour of chromosomes during meiosis is shown in Fig. which
shows a nucleus containing 2 chromosomes (pairs of chromosomes.)
chromosome from
maternal
n
2n
1st meiotic Meiosis I
chromosome from
paternal
n
Meiosis II
Haploid
gametes
Genetics
21
Genetics
BIO 221
Biochemistry of heredity:
In principle: Genes are made of nucleic acids.
The structure of Deoxyribonucleic Acid (DNA).
The Watson-Crick structure for Deoxyribonucleic acid (DNA) (1953) a
double- stranded helix
1-Sugar-phosphate backbone outside.
2-Nitrogenous bases (A, C, G, T) inside.
3-Nitrogenous bases held together by hydrogen bonds.
4-Nitrogenous bases pair occurs according to specific rules:
A with T
G with C
Building blocks of nucleic acids (DNA and RNA).
I-Nitrogenous bases
1-Pyrimidines (single ring) are:
Cytosine (C)
thymine (T)
& {uracil in RNA (U)}
" Pyramids were CUT from stone"
The pyrimidines in DNA are C and T; but in RNA, U replaces T.
[ thymine = 5- methyl-uracil]
Genetics
22
Genetics
BIO 221
2- Purines (double ring) are:
Adenine (A) & guanine (G)
" AGs are PURe"
II- The Nucleoside = Nitrogenous base + sugar
Types of Sugars:
1- Deoxyribose sugar occurs in DNA (-H on 2,-C)
2- Ribose sugar occurs in RNA (-OH on 2,-C)
The base is connected to the sugar through the 1,- carbon of the sugar
The differences between Ribose sugar & Deoxyribose sugar ?
Deoxy-ribose sugar characterized by the absence of the (-OH) group on the
C.2
3- Nucleotide = nucleoside + phosophate [PO4]
The molecule shown as a nucleoside monophosphate.
Note that the phosphate group is attached to the 5, -carbon of the sugar.
4-Polynucleotide = nucleotide + nucleotide + nucleotide + etc
Nucleotides are linked by 3,
Genetics
5, phosphodiester bonds
23
Genetics
BIO 221
Types of structures of DNA
1-A-DNA : not groovy,
2-B-DNA: Right -handed helix
3-Z-DNA : left-handed helix
Structure of B-DNA
1- Two twisted right-handed polynucleotide
2- Helices antiparallel in 5,
3, orientation.
3- Strands held by hydrogen (H-) bonds between bases.
H-bonds form according to specific base-pairing rules.
A pairs with T by two H-bonds A=T
G pairs with C by three H-bonds G -=C
4- Base pairs interval = 3.4 A
5- Period of helix is 10 pb (base pairs) =34 A
6- 3-D structure of B DNA.
Genetics
24
Genetics
BIO 221
Characters of RNA (Ribonucleic Acid):
1-uracil ( U ) Substitute thymine ( T )
2- single-stranded
3- The sugar is Ribose
Types of RNAs
1-mRNA (messenger RNA): long, single-strand.
2-rRNA (ribosomal RNA): medium-sized, complex "stem and loop"
3- t RNA (transfer RNA): small, "cloverleaf" structure.
Genetics
25
Genetics
BIO 221
DNA Replication and Transcription
Replication means : Duplication of a double- DNA
(An exact "copy" of the existing DNA molecule)
Transcription means : Synthesis of a new single- DNA
)"copy" of an existing strand (
Types of DNA replication:
1- conservative
2- semi-conservative
3- dispersive
In principle: DNA replication is semi-conservative
DNA is not the " Genetic Code" for proteins information, DNA must first
be transcribed into RNA messenger
RNA transcript is base-complementary to template strand of DNA and therefore
homologous with sense strand of DNA.
5„-
A
T
G
C
A
T
G
C
- „3
3„-
T
A
C
G
T
A
C
G
- „5
5„-
A
U
G
C
A
U
G
C
- „3
Genetics
Sense DNA (Partner)
Template DNA
(Antisense)
Messenger RNA
26
Genetics
BIO 221
DNA Synthesis in prokaryotes:
1- Formation of replication fork
Provides two single-stranded DNA template (ssDNA)
2- Synthesis of RNA primer.
3- Addition of nucleotides (dNTPs by DNAPol III) at 3‘ end forming the
continuous synthesis known as leading strand.
4- Discontinuous synthesis on lagging strand forming Okazaki fragments
5- Excision of RNA primer by DNAPol I
6-Ligation (connection) of fragment ends at gaps by DNA ligase.
Genetics
27
Genetics
BIO 221
Protein Synthesis
Transcription: synthesis of messenger RNA (mRNA):
RNA transcribed from DNA by RNA Polymerase (RNA Pol I)
Steps of transcription:
1- Recognition of transcriptional unit:- " Gene " by promoters (sequences that
regulate transcription)
2- Initiation and Elongation
mRNA synthesized 3„
5„ from DNA template strand
mRNA sequences therefore homologous to DNA sense strand.
3- Termination.
Regulation of transcription:
In prokaryotes, transcription and translation may occur simultancously.
In eukaryotes, transcription occurs in nucleus translation occur in
cytoplasm,
RNA must cross nuclear membrane:
Primary RNA transcript is processed from heterogeneous nuclear RNA
(hnRNA) into mRNA.
Processing of heterogeneous nuclear RNA (hnRNA) means (intron DNA
sequences removed from hnRNA, exon DNA sequences represented in mRNA:
“expressed" in protein)
Genetics
28
Genetics
BIO 221
The “Central Dogma ”in prokaryotic cells
The “Central Dogma ”in prokaryotic cells
In prokaryotes, in the absence of nuclear membrane, DNA Transcription
and RNA Translation are not physically separated, the RNA Transcript can directly
.
Genetics
29
Genetics
BIO 221
Protein synthesis needs 2 steps:1-Transcription.
2- Translation.
1- Cell gets message to make protein.
2- Strands of DNA unwinds exposing the gene of this portion.
3- Formation of template DNA followed by the formation of mRNA Sense:
Template:
mRNA:
4- Many mRNA leave nucleus and enter cytoplasm from pore.
mRNA
5- mRNA binds with Ribosome which read each 3bp at each time (Triplet
code or codon)
Genetics
30
Genetics
BIO 221
6- Each codon is specific for one amino acid.
7-
tRNA has 2 ends
end of binding amino acid (binding
site)
end of anticodon which base pair
………………………………………with codon on mRNA.
8- tRNA bind with amino acid and carries it to Ribosome, its anticodon bases
with codon on mRNA.
9- A second tRNA brings its amino acid to Ribosome.
10- The 1st tRNA go and bind its amino acid to the second amino acid.
Genetics
31
Genetics
BIO 221
11- The process repeats until a entire message read forming poly peptide chain
= protein.
-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-
Genetics
32
Genetics
BIO 221
The central dogma Of Protein synthesis in prokaryotes
Replication
DNA
Transcription
mRNA
Translation
Protein.
The central dogma Of Protein synthesis in Eukaryotes
heterogenous nuclear RNA
DNA
Transcription
hnRNA
Processing
mature
mRNA
Replication
Transport
………………………………………………
to
…………………………………………………………………………cytoplasm
In nucleus
mRNA
Amino acid chain
Translation
Protein
In Cytoplasm
Genetics
33
Genetics
BIO 221
DNA packaging
1- DNA is a double stranded helical.
2- DNA is complex with histones to form nucleosomes .
3- Nucleosomes core = 8 histone = 2{H2A + H2B + H3 + H4}.
4- chromatosome = Nucleosome +
H1 histone
5- Each 6 nucleosomes fold up forming a fiber or solenoid.
Solenoid = 6 nucleosomes
Genetics
34
Genetics
BIO 221
6- Fibers forming loops.
7- Fibers loops compressed forming wide fiber
8- Coiling of wide fiber forming a chromatide of Chromosome.
Genetics
35
Genetics
BIO 221
Genetics
36