Biolove genetics part 1
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Transcript of Biolove genetics part 1
M.I/ASASI/2013 Page 1
BIOLOVE
GENETICS
M.I/ASASI/2013 Page 2
Genes
– Basic unit of inheritance or information about specific traits
– Located on chromosomes
– For example, the height of pea plant is determined by a gene
Locus
– Location in chromosomes where genes are located
Allele
– If the a gene give the height
characteristics,
– Allele determine either it is tall or short
– So, Allele is the variant of the same gene
Another example
– Gene: Hair colour
Allele: Black or blonde
– Gene: Fur colour
Allele: White or yellow
Dominant Allele
– Presented by capital letter. Example: (T, P, B, A)
– Will show the effects on organism
Recessive allele
– Presented by small letter.
– No effects on organism if paired with dominant allele
For example:
If P is for blue colour, and p is for red colour
Pp = blue colour
pp = red colour
Homozygous
– The same letter. Example: tt, YY, NN
– Can be either homozygous dominant or recessive
pp Pp
M.I/ASASI/2013 Page 3
Homozygous dominant
– PP, YY, TT, MM
– Combination of two same capital letter
Homozygous recessive
– pp, tt, mm, nn
– combination of two same small letter
Heterozygous
– one small letter and one capital letter
– Example: Tt, Mn, Hh, Rr
Genotype
– Genetic composition of an organism which is not visible externally
– The genotype of plant is represented by the allele present
– Hence, if P is allele for tall plant, all tall plants will have the genotype of Pp or PP
Phenotype
– External appearance or observable features
– Example: Colour, size, structure
History of genetis
We have to know Mendel, the father of modern genetics
What he do?
– He study the inheritance of traits in pea plants
Who is he?
– An Austrian priest
M.I/ASASI/2013 Page 4
Why he chose pea plant to study about inheritance of traits?
• Advantages of pea plants for genetic study:
– There are many varieties with distinct heritable features, or characters (such as
flower color); character variants (such as purple or white flowers) are called traits
– Mating of plants can be controlled
– Each pea plant has sperm-producing organs (stamens) and egg-producing organs
(carpels)
– Cross-pollination (fertilization between different plants) can be achieved by dusting
one plant with pollen from another
• Mendel chose to track only those characters that varied in an either-or manner
• He also used varieties that were true-breeding (plants that produce offspring of the same
variety when they self-pollinate)
Inheritance of single trait is called as monohybrid
• In a typical experiment, Mendel mated
two contrasting, true-breeding varieties, a
process called hybridization
• The true-breeding parents are the P1 or
parental generation
• The hybrid offspring of the P generation
are called the F1 generation or first filial
generation
• When F1 individuals self-pollinate, the
F2 generation or second filial generation is
produced
M.I/ASASI/2013 Page 5
• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all
of the F1 hybrids were purple
• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some
had white
• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2
generation
Fertilized among F1
F1 x F1
If the dominant or
recessive trait is not
mentioned in the
question, please
refer to this table.
M.I/ASASI/2013 Page 6
Law of segregation
Two alleles of a gene seperate or segregate from each other into different gametes during the
formation of gametes.
• Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells
of an organism
• This segregation of alleles corresponds to the distribution of homologous chromosomes to
different gametes in meiosis
• The possible combinations of sperm and egg can be shown using a Punnett square, a
diagram for predicting the results of a genetic cross between individuals of known genetic
makeup
This is Monohybrid cross
Allele in a pair Separate/segregate Into different gamete
Gametes must
be circled
M.I/ASASI/2013 Page 7
• Crossing two true-breeding parents differing in two characters produces dihybrids in the F1
generation, heterozygous for both characters (YyRr)
• A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are
transmitted to offspring as a package or independently
This is not valid
for dihybrid
cross
M.I/ASASI/2013 Page 8
• Using a dihybrid cross, Mendel developed the law of independent assortment
• The law of independent assortment states that each pair of alleles segregates
independently of each other pair of alleles during gamete formation
• Strictly speaking, this law applies only to genes on different, nonhomologous
chromosomes
• Genes located near each other on the same chromosome tend to be inherited
together
• Mendel’s laws of segregation and independent assortment reflect the rules of
probability
• When tossing a coin, the outcome of one toss has no impact on the outcome of the
next toss
• In the same way, the alleles of one gene segregate into gametes independently of
another gene’s alleles
• The multiplication rule states that the probability that two or more independent
events will occur together is the product of their individual probabilities
• Probability in an F1 monohybrid cross can be determined using the multiplication rule
• Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance
of carrying the dominant allele and a chance of carrying the recessive allele
M.I/ASASI/2013 Page 9
Extension to Mendel’s Genetics
• Complete dominance occurs when phenotypes of the heterozygote and dominant
homozygote are identical
Heterozygote = Pp
Homozygote dominant = PP
• In incomplete dominance, the phenotype of F1 hybrids is somewhere between the
phenotypes of the two parental varieties
– For example:
– The third organism display both
appearance from parents
• In codominance, two dominant
alleles affect the phenotype in separate,
distinguishable ways
- Example provided in the
summary table of degree of dominance
at page 11
GG HH x
GH
OR
TT YY
TY
x
It is not necessary to write the capital
letter C when writing the genetic
diagram
Both has identical
phenotype which is
purple colour
M.I/ASASI/2013 Page 10
Multiple Alleles
• Most genes exist in populations in more
than two allelic forms
• For example, the four phenotypes of
the ABO blood group in humans are
determined by three alleles for the
enzyme (I) that attaches A or B
carbohydrates to red blood cells: IA, IB,
and i.
• The enzyme encoded by the IA allele
adds the A carbohydrate, whereas the
enzyme encoded by the IB allele adds
the B carbohydrate; the enzyme
encoded by the i allele adds neither
Epistasis
– In Epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus
– For example, in mice and many other mammals, coat color depends on two genes
– One gene determines the pigment color (with alleles B for black and b for brown)
– The other gene (with alleles C for color and c for no color) determines whether the pigment
will be deposited in the hair
If at least one allele C is exist,
the fur will have either black
or brown colour
If no allele C exist but only
allele c is exist, for example
BBcc, the fur will have no
colour. Fur will be white.
Briefly, if no dominant allele
for colour, the fur will have
no colour
M.I/ASASI/2013 Page 11
Polygenic Inheritance
• Quantitative characters are
those that vary in the population
along a continuum
• Quantitative variation usually
indicates polygenic inheritance,
an additive effect of two or more
genes on a single phenotype
• Skin color in humans is an
example of polygenic inheritance
• More than two genes will affect
the skin colour
Testcross
– Crossing an unkown genotype with a homozygous recessive
________ x rr
Summary of Extension to Mendelian Genetics
unknown
Homozygous recessive
M.I/ASASI/2013 Page 12
Useful Diagram To help you
M.I/ASASI/2013 Page 13
M.I/ASASI/2013 Page 14
Steps to write genetic diagram
Step 1: Cross the parents
BbCc x BbCc
Step 2: Obtain the gamete
– From BbCc, we make the gamete by using the arrows
– OR we can use numbering system
Number the letter as shown above
Then, combine the letter
+ = BC
BbCc
BC
bc bC
Bc
BbCc 1
3
2 4
+
2 3
2 4
1 4
+
+
=
=
=
Gamete
Note that number 1 and 2
cannot cross together.
Same goes to number 3
and 4
1 3
bC
bc
Bc
BC
M.I/ASASI/2013 Page 15
If the genotype exist as BBCC,
Then the gamete will be BC and BC. But we will take one only out of two which we’ll write BC only
instead of BC and BC because it is the same.
Step 3: Draw a punnet square to obtain the F1
Step 4: Write the Genotypic ratio and phenotypic ratio
Genotypic ratio: ________________________________________
Phenotypic ratio: _______________________________________
DNA
– Is a polymer of nucleotides, each consisting of three components: a nitrogenous base, a
sugar, and a phosphate group
BBCC BBCc BbCC BbCc
BBCc BBcc BbCc Bbcc
BbCc BbCc bbCC bbCc
BbCc Bbcc bbCc bbcc
bC
bc
Bc
BC
bC bc Bc BC
M.I/ASASI/2013 Page 16
• The nitrogenous bases
– Are paired in specific combinations: adenine with thymine, and cytosine with
guanine
• Each base pair forms a different number of hydrogen bonds
– Adenine and thymine form two bonds, cytosine and guanine form three bonds
• DNA
– Was composed of two antiparallel sugar-phosphate backbones, with the nitrogenous
bases paired in the molecule’s interior
– Twisted into helical shape
Differences between DNA and RNA
Aspect DNA RNA
Sugar Deoxyribose Ribose
Bases A-T, C-G U-A, C-G
– The base thymine, T is replaced by Uracil, U
Strand Double Single
DNA Replication
– Copying the double strand DNA
– Genetic material is passed down to daughter cells
– Happens before cell division
In prokaryotes happen through the interval between cell divisions
Eukaryotes happens during the S phase
M.I/ASASI/2013 Page 17
• In DNA replication
– The parent molecule unwinds, and two new daughter strands are built based on
base-pairing rules
Model of DNA replication
The Semiconservative model was then accepted when Meselson and Stahl carried out an experiment
to prove it.
• Experiments performed by Meselson and Stahl
– Supported the semiconservative model of DNA replication
M.I/ASASI/2013 Page 18
• The copying of DNA
– Is remarkable in its speed and accuracy
• More than a dozen enzymes and other proteins
– Participate in DNA replication
M.I/ASASI/2013 Page 19
• The replication of a DNA molecule
– Begins at special sites called origins of replication, where the two strands are
separated
• A eukaryotic chromosome
– May have hundreds or even thousands of replication origins
Replication occurs in the direction from 5’ to 3’
– Continuous repliation on 3’ to 5’ template
– Discontinuous on 5’ to 3’ template. Replication form short segments
DNA replication starts when the DNA is unwind by DNA Helicase at area known as replication
fork
More replication bubbles – faster the process of copying the DNA strand
Once Helicase has unwind the strand, single strand binding protein will bind to the single
strand DNA and stabilize it.
Here are the summary of action by the molecules
M.I/ASASI/2013 Page 20
Primase will synthesize RNA primer
RNAase H removes the RNA primer
DNA polymerase I replace the RNA primer with DNA nucleotides
RNA primer is the start point for DNA polymerase
• Elongation of new DNA at a replication fork
– Is catalyzed by enzymes called DNA polymerases, which add nucleotides to the 3
end of a growing strand
• DNA polymerase I add nucleotides
– Only to the free 3 end of a growing strand
• Along one template strand of DNA, the leading strand
– DNA polymerase III can synthesize a complementary strand continuously, moving
toward the replication fork
• To elongate the other new strand of DNA, the lagging strand
– DNA polymerase III must work in the direction away from the replication fork
• The lagging strand
– Is synthesized as a series of segments called Okazaki fragments, which are then
joined together by DNA ligase
DNA polymerase I
M.I/ASASI/2013 Page 21
Synthesis of leading and lagging strands during DNA replication
• DNA polymerases cannot initiate the synthesis of a polynucleotide
– They can only add nucleotides to the 3 end
Figure 16.15 showing Replication in Lagging strands
RNAase H remove
the RNA primer
M.I/ASASI/2013 Page 22
6. RNAase H removes the primer
from 5’ end of second fragment. DNA
polymerase I replace the primer with
DNA nucleotides that it adds one by
one to the 3’ end of the 3rd fragment.
The replacement of the last RNA
nucleotide with DNA leaves the sugar
phosphate backbone with a free 3’
end
M.I/ASASI/2013 Page 23
Protein Synthesis
The two stgaes of protein synthesis is Transcription and Translation
• Transcription
– Is the synthesis of RNA under the direction of DNA
– Produces messenger RNA (mRNA)
• Translation
– Is the actual synthesis of a polypeptide, which occurs under the direction of mRNA
– Occurs on ribosomes
• In prokaryotes
– Transcription and translation occur together
• In eukaryotes
– RNA transcripts are modified before becoming true mRNA
M.I/ASASI/2013 Page 24
• Genetic information
– Is encoded as a sequence of nonoverlapping base triplets, or codons
• How many bases correspond to an amino acid?
– 3 base will form codon which will specify amino acid
Each codon consist of three nucleotides (base)
During transcription
– The gene determines the sequence of bases along the length of an mRNA molecule
4 characteristics of Genetic code
1. Triplet:
– 3 nucleotides(codon) will specify one amino acid
Each amino acid may have more than one codon. But one codon only specify for an
amino acid
– Overall 61 codons + 3 stop codons
– AUG is the start codon (AUG codes for methionine) for transcription
Stop codons
– UAA, UAG, UGA
M.I/ASASI/2013 Page 25
2. Redundant:
– More than one codon for an amino acids
3. Unambigous
– One codon can only specify an amino acid only
– Which means if AAA is for Lysine, it is strictly for Lysine and cannot form other amino acid
4. No spaces or punctuation
Transcription
- Happens when the DNA strands separate
- One strand of DNA is used as the pattern to produce RNA using specific base pairing
- Catalyze by RNA polymerase
- Takes place in nucleus
Flow of transcription
Start: promotercoding regionterminator:End
• RNA synthesis
– Is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks
together the RNA nucleotides
– Follows the same base-pairing rules as DNA, except that in RNA, uracil(U) substitutes
for thymine(T)
• The stages of transcription are
– Initiation
– Elongation
– Termination
M.I/ASASI/2013 Page 26
Initiation
- RNA polymerase bind to promoter
- DNA double helix unwind
- RNA polymerase transcribe the coding region
- No primer is needed for transcription because starts at the promoter
What is promoter?
- TATA box
TATA box is recognize by transcription factors
Transcription factors(TF) are proteins involve in the regulation of gene expression
TF has two domains(base)
At TATA box
For RNA polymerase to bind
M.I/ASASI/2013 Page 27
- TF binds to DNA strand
- RNA polymerase II bind to DNA at TF
- TF + RNA polymerase + TATA box = transcription initiation complex
Elongation
M.I/ASASI/2013 Page 28
- RNA polymerase elongate transcription from 5’ to 3’
- Transcribed DNA will reform the double helix
- New RNA dissociate from template
Termination
- RNA polymerase transcribes a terminator
- Terminator is one of the three stop codons (UAA, UAG, UGA)
- RNA strands released
- RNA polymerase dissociates from the template strand
• The mechanisms of termination
– Are different in prokaryotes and eukaryotes
• Eukaryotic cells modify RNA after transcription
• Enzymes in the eukaryotic nucleus
– Modify pre-mRNA in specific ways before the genetic messages are dispatched to the
cytoplasm
• Each end of a pre-mRNA molecule is modified in a particular way
– The 5 end receives a modified nucleotide cap (capped with Guanine, G)
– The 3 end gets a poly-A tail (Adenine, A)
M.I/ASASI/2013 Page 29
Why mRNA ends need to be altered?
- Protects the RNA from degradation
- Helps the ribosome to attach to the mRNA
- Facilitates the transport out of the RNA from the nucleus
RNA splicing
– Pre-mRNA contains Introns (non coding region) and Exons (coding region)
– Removes introns and joins exons
• RNA splicing Is carried out by spliceosomes in some cases
• Ribozymes
– Are catalytic RNA molecules that function as enzymes and can splice RNA
• The presence of introns
– Allows for alternative RNA splicing
M.I/ASASI/2013 Page 30
Translation
• A cell translates an mRNA message into protein
– With the help of transfer RNA (tRNA) and ribosomes
• Consist of three stages
- Initiation, elongation and termination
Translation: the basic concept
• Molecules of tRNA are not all identical
– Each carries a specific amino acid
on one end
– Each has an anticodon on the
other end
• Which means it has two attachment sites
- Amino acids
- Anticodon
M.I/ASASI/2013 Page 31
• Ribosomes
– Facilitate the specific coupling
of tRNA anticodons with mRNA
codons during protein
synthesis
• The ribosomal subunits
– Are constructed of proteins
and RNA molecules named
ribosomal RNA or rRNA
• The ribosome has three binding sites for tRNA at the large subunit
– The P site
– The A site
– The E site
Small subunit is for mRNA binding site
E site – hold discharged amino acid
P site – holds the tRNA growing polypeptide chain
A site – holds the tRNA carrying the next amino acid
M.I/ASASI/2013 Page 32
Initiation of Translation
Elongation
• In the elongation stage of translation
– Amino acids are added one by one to the preceding amino acid
M.I/ASASI/2013 Page 33
Termination
• The final stage of translation is termination
– When the ribosome reaches a stop codon in the mRNA
• A number of ribosomes can translate a single mRNA molecule simultaneously
- Forming a polyribosome
Summary of transcription and translation