Download Genetics and Protein Synthesis

Genetics and Protein Synthesis
Felix Hernandez, M.D.
The Monk and his peas
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Gregor Mendel, developed the
fundamental principles that would become
the modern science of genetics.
Mendel demonstrated that heritable
properties are parceled out in discrete
units, independently inherited.
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These eventually were termed genes.
Mendel's experimental organism was a
common garden pea (Pisum sativum),
which has a flower that lends itself to selfpollination
The Pea Plant Studies
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The garden peas were planted and studied
for eight years.
Each character studied had two distinct
forms, such as tall or short plant height, or
smooth or wrinkled seeds.
Mendel's experiments used some 28,000
pea plants.
Characteristics Studied
Characteristics Studied
Mendel’s Contribution
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Mendel was able to demonstrate that traits were
passed from each parent to their offspring
through the inheritance of genes.
Mendel's work showed:
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Each parent contributes one factor of each trait
shown in offspring.
The two members of each pair of factors segregate
from each other during gamete formation.
The blending theory of inheritance was discounted.
Males and females contribute equally to the traits in
their offspring.
Acquired traits are not inherited.
Principle of Segregation
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A cross involving only one trait is referred to as a
monohybrid cross.
Mendel crossed pure-breeding (also referred to
as true-breeding) smooth-seeded plants with a
variety that had always produced wrinkled seeds
To help with record keeping, generations were
labeled and numbered.
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The parental generation is denoted as the P1
generation.
The offspring of the P1 generation are the F1
generation (first filial).
The self-fertilizing F1 generation produced the F2
generation (second filial).
Principle of Segregation
Mendel's Results
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Summary of Mendel's Results:
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The F1 offspring showed only one of the two
parental traits, and always the same trait.
Results were always the same regardless of
which parent donated the pollen.
The trait not shown in the F1 reappeared in
the F2 in about 25% of the offspring.
Traits remained unchanged when passed to
offspring: they did not blend in any offspring
but behaved as separate units.
Reciprocal crosses showed each parent made
an equal contribution to the offspring.
Mendel's Conclusions
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Evidence indicated factors could be hidden or
unexpressed, these are the recessive traits.
The term phenotype refers to the outward
appearance of a trait, while the term genotype
is used for the genetic makeup of an organism.
Male and female contributed equally to the
offspring's' genetic makeup: therefore the
number of traits was probably two (the
simplest solution).
Upper case letters are traditionally used to
denote dominant traits, lower case letters for
recessives.
Principle of Independent
Assortment
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Principle of Independent Assortment -that when gametes are formed, alleles
assort independently.
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We now interpret the Principle of Independent
Assortment as alleles of genes on different
chromosomes are inherited independently
during the formation of gametes
Punnett squares
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Step 1 - definition of alleles and
determination of dominance.
Step 2 - determination of alleles present in
all different types of gametes.
Step 3 - construction of the square.
Step 4 - recombination of alleles into each
small square.
Step 5 - Determination of Genotype and
Phenotype ratios in the next generation.
Step 6 - Labeling of generations, for
example P1, F1, etc.
Punnett squares
Genetic Terms
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Gene - a unit of inheritance that usually is directly
responsible for one trait or character.
Allele - an alternate form of a gene. Usually there are
two alleles for every gene, sometimes as many a three
or four.
Homozygous - when the two alleles are the same.
Heterozygous - when the two alleles are different, in
such cases the dominant allele is expressed.
Dominant - a term applied to the trait (allele) that is
expressed irregardless of the second allele.
Recessive - a term applied to a trait that is only
expressed when the second allele is the same (e.g. short
plants are homozygous for the recessive allele).
Phenotype - the physical expression of the allelic
composition for the trait under study.
Genotype - the allelic composition of an organism.
Punnett squares - probability diagram illustrating the
possible offspring of a mating.
Incomplete dominance
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Incomplete dominance is a condition when
neither allele is dominant over the other.
The condition is recognized by the
heterozygotes expressing an intermediate
phenotype relative to the parental
phenotypes.
If a red flowered plant is crossed with a
white flowered one, the progeny will all be
pink.
Polygenic Inheritance
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Polygenic inheritance is a pattern responsible for
many features that seem simple on the surface.
Many traits such as height, shape, weight, color,
and metabolic rate are governed by the
cumulative effects of many genes.
Polygenic traits are not expressed as absolute or
discrete characters
polygenic traits are recognizable by their
expression as a gradation of small differences (a
continuous variation).
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The results form a bell shaped curve, with a mean
value and extremes in either direction.
Polygenic Inheritance
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Usually polygenic traits are distinguished
by
Traits are usually quantified by
measurement rather than counting.
Two or more gene pairs contribute to the
phenotype.
Phenotypic expression of polygenic traits
varies over a wide range.
Pleiotropy
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the effect of a single gene on more than
one characteristic
Sickle-celled individuals suffer from a
number of problems, all of which are
pleiotropic effects of the sickle-cell allele.
Chromosome Abnormalities
Chromosome Abnormalities
The physical carrier of inheritance
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four nitrogenous bases: cytosine, thymine,
adenine, and guanine; deoxyribose sugar;
and a phosphate group.
The basic unit (nucleotide) was composed
of a base attached to a sugar and that the
phosphate also attached to the sugar
The Structure of DNA
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DNA is a double helix, with bases to the
center (like rungs on a ladder) and sugarphosphate units along the sides of the
helix (like the sides of a twisted ladder).
The strands are complementary (deduced
by Watson and Crick from Chargaff's data,
A pairs with T and C pairs with G, the
pairs held together by hydrogen bonds).
The Structure of DNA
DNA Replication
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Nucleotides have to be assembled and available
in the nucleus, along with energy to make bonds
between nucleotides.
DNA polymerases unzip the helix by breaking
the H-bonds between bases.
Once the polymerases have opened the
molecule, an area known as the replication
bubble forms (always initiated at a certain set of
nucleotides, the origin of replication).
New nucleotides are placed in the fork and link
to the corresponding parental nucleotide already
there (A with T, C with G).
DNA Replication
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Since the DNA strands are antiparallel, and
replication proceeds in the 5' to 3'
direction on EACH strand,
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one strand will form a continuous copy, while
the
other will form a series of short Okazaki
fragments.
DNA Replication
One-gene-one-protein
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One gene codes for the production of one
protein.
"One gene one enzyme" has since been
modified to "one gene one polypeptide"
since many proteins (such as hemoglobin)
are made of more than one polypeptide.
RNA
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Ribonucleic acid (RNA) was discovered
after DNA
Information flow (with the exception of
reverse transcription) is from DNA to RNA
via the process of transcription, and
thence to protein via translation.
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Transcription is the making of an RNA
molecule off a DNA template.
Translation is the construction of an amino
acid sequence (polypeptide) from an RNA
molecule
Uses of RNA
Types of RNA
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Messenger RNA (mRNA) is the blueprint for
construction of a protein.
Transfer RNA (tRNA) is the truck delivering the
proper amino acid to the site at the right time.
RNA has ribose sugar instead of deoxyribose
sugar.
The base uracil (U) replaces thymine (T) in RNA.
Most RNA is single stranded, although tRNA will
form a "cloverleaf" structure due to
complementary base pairing.
Transcription
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making an RNA copy of a DNA sequence
RNA polymerase opens the part of the
DNA to be transcribed. Only one strand of
DNA (the template strand) is transcribed.
RNA nucleotides are available in the region
of the chromatin and are linked together
similar to the DNA process.
Transcription
Translation
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RNA code into protein
The code consists of at least three bases
To code for the 20 essential amino acids a
genetic code must consist of at least a 3-base
set (triplet) of the 4 bases
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64 possibilities
The genetic code consists of 61 amino-acid
coding codons and three termination codons,
which stop the process of translation.
The genetic code is thus redundant (degenerate
in the sense of having multiple states amounting
to the same thing), with,
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for example, glycine coded for by GGU, GGC, GGA,
and GGG codons.
Translation
Protein Synthesis
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Promoters are sequences of DNA that are the start
signals for the transcription of mRNA.
Terminators are the stop signals
Transfer RNA (tRNA) is basically cloverleaf-shaped. tRNA
carries the proper amino acid to the ribosome when the
codons call for them.
At the top of the large loop are three bases, the
anticodon, which is the complement of the codon.
There are 61 different tRNAs, each having a different
binding site for the amino acid and a different anticodon.
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For the codon UUU, the complementary anticodon is AAA.
Amino acid linkage to the proper tRNA is controlled by
the aminoacyl-tRNA synthetases.
Energy for binding the amino acid to tRNA comes from
ATP conversion to adenosine monophosphate (AMP).
Protein Synthesis
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Translation is the process of converting the
mRNA codon sequences into an amino acid
sequence.
The initiator codon (AUG) codes for the amino
acid N-formylmethionine (f-Met). No
transcription occurs without the AUG codon. f
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Met is always the first amino acid in a polypeptide
chain, although frequently it is removed after
translation.
The initiator tRNA/mRNA/small ribosomal unit is
called the initiation complex.
The larger subunit attaches to the initiation
complex.
After the initiation phase the message gets
longer during the elongation phase.
Protein Synthesis
Protein Synthesis
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New tRNAs bring their amino acids to the open
binding site on the ribosome/mRNA complex,
forming a peptide bond between the amino
acids.
The complex then shifts along the mRNA to the
next triplet, opening the A site.
The new tRNA enters at the A site.
When the codon in the A site is a termination
codon, a releasing factor binds to the site,
stopping translation and releasing the ribosomal
complex and mRNA.
Protein Synthesis
Protein Synthesis
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Often many ribosomes will read the same
message, a structure known as a
polysome forms.
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In this way a cell may rapidly make many
proteins.
The Eukaryotic Genome
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We use the term genome to refer to all of the
alleles possessed by an organism
Almost half the DNA in eukaryotic cells is
repeated nucleotide sequences. Protein-coding
sequences are interrupted by non-coding
regions.
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Non-coding interruptions are known as intervening
sequences or introns.
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Most, but not all structural eukaryote genes contain introns.
Although transcribed, these introns are excised (cut out)
before translation
Coding sequences that are expressed are exons.
The Eukaryotic Genome