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Genes and Proteins
• The genetic information contained in the nucleotide
sequence of DNA specifies a particular type of
protein
• Enzymes = proteins that are biological catalysts
essential for metabolic activities in the cell
• Metabolites = small molecules upon which
enzymes act
• In 1908 Archibald Garrod proposed that enzyme
defects result in inborn errors of metabolism =
hereditary diseases
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Genes and Proteins
• Garrod studied alkaptonuria and identified
abnormal excreted substance = homogentisic acid
2
http://www.ncbi.nlm.nih.gov/
Genes and Proteins
• Another defective enzyme in the same pathway, phenylalanine
hydroxylase (PAH), leads to phenylalanine accumulation which
causes the condition known as phenylketonuria (PKU)
• Incidence of PKU, characterized by severe mental retardation, is
about one in 8000 among Caucasian births.
• A defective enzyme results from a mutant gene
3
Central Dogma
Central Dogma of molecular genetics:
• DNA RNA Protein
• DNA is the informational molecule which does not
code for protein directly but rather acts through
RNA intermediate
• DNA codes for RNA = transcription
• RNA codes for protein = translation
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Fig. 1.14
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Transcription
• Transcription is the production of
an RNA strand that is
complementary in base sequence
to a DNA template = messenger
RNA (mRNA)
• RNA contains the base uracil in
place of thymine and the sugar
ribose instead of deoxyribose
• RNA is synthesized from template
DNA following strand separation of
the double helix
Fig. 1.15
6
Base pairing in DNA and RNA
• Complementary base pairing specifies the linear
sequence of bases in RNA
• Adenine pairs with uracil; thymine pairs with
adenine; guanine pairs with cytosine
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Translation
• The sequence of bases in mRNA codes for the
sequence of amino acids in a polypeptide
• The mRNA is translated in nonoverlapping group of
three bases = codons that specify the sequence of
amino acids in proteins
• Each codon specifies one amino acid
• Transfer RNAs (tRNA) contain triplet base sequences =
anticodons, which are complementary to codons in
mRNA
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Fig. 1.16
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Translation
• Translation occurs at the ribosomes which contain
several types of ribosomal RNA (rRNA)
• tRNAs participate in translation by carrying amino
acids and positioning them on ribosomes
• Translation results in the synthesis of a polypeptide
chain composed of a linear sequence of amino acids
whose order is specified by the sequence of codons
in mRNA
10
Mutations
• Mutation refers to any heritable change in a gene
• The change may be: substitution of one base pair in
DNA for a different base pair; deletion or addition of
base pairs
• Any mutation that causes the insertion of an
incorrect amino acid in a protein can impair its
function
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Genes and Environment
• One gene can affect more than one trait = pleiotropy
• Any trait can be affected by more than one gene as
well as environment
• Most complex traits are affected by multiple genetic
and environmental factors
• Often several genes are involved in genetic
disorders and the severity of a disease may depend
upon genetic status and environmental factors
12
Evolution
• All creatures on Earth share many features of the
genetic apparatus and many aspects of metabolism
• Groups of related organisms descend from a
common ancestor
• Evolution occurs whenever a population of
organisms with a common ancestry gradually
changes in genetic composition over time
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Fig. 1.21
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Evolution
• The totality of DNA in a single cell = genome
• The complete set of proteins encoded in the
genome = proteome
• Genes or proteins that derive from a common
ancestral sequence via gene duplication = paralogs
• Genes that share a common ancestral gene via
speciation = orthologs
• The molecular unity of life is seen in comparisons
among genomes and proteomes
15
Self Assessment Questions:
Question 1
 The base thymine is always paired with:
 Adenine
 Thymine
 Guanine
 Cytosine
 Thymine is never paired with another base in a DNA
molecule
Self Assessment Questions:
Question 1 Answer
 The base thymine is always paired with:
 Adenine CORRECT ANSWER
 Thymine
 Guanine
 Cytosine
 Thymine is never paired with another base in a DNA
molecule
DNA forms a
Double Helix
•The bases are projected inward whereas the sugar phosphate backbone is
on the outside
•The two strands are antiparallel
•A is paired with T through two hydrogen bonds
•G is paired with C through three hydrogen bonds
Self Assessment Questions:
Question 2
 The base cytosine is always paired with:
 Adenine
 Guanine
 Cytosine
 Thymine
 Cytosine is never paired with another base in a DNA
molecule
Self Assessment Questions:
Question 2 Answer
 The base cytosine is always paired with:
 Adenine
 Guanine CORRECT ANSWER
 Cytosine
 Thymine
 Cytosine is never paired with another base in a DNA
molecule
Self Assessment Questions:
Question 3
 Which of the following is not a base found in
DNA?
 Guanine
 Cytosine
 Thymine
 Uracil
 Adenine
Self Assessment Questions:
Question 3 Answer
 Which of the following is not a base found in
DNA?
 Guanine
 Cytosine
 Thymine
 Uracil CORRECT ANSWER
 Adenine
RNA contains the base uracil in place of
thymine.
http://caferguson.files.wordpress.com/2010/11/transcription.jpg
Self Assessment Questions:
Question 4
 Chromosomes can be found in the:
 Cytoplasm
 Endoplasmic reticulum
 Nucleus
 Cellular membranes
Self Assessment Questions:
Question 4 Answer
 Chromosomes can be found in the:
 Cytoplasm
 Endoplasmic reticulum
 Nucleus CORRECT ANSWER
 Cellular membranes
Self Assessment Questions:
Question 5
 tRNA is:
 The major structural material making up ribosomes
 The molecule that carries the genetic information from DNA
and is used as a template for protein synthesis
 The major structural component of chromosomes
 A molecule that incorporates a specific amino acid into the
growing protein when it recognizes a specific group of three
bases
 The major building block of proteins
Self Assessment Questions:
Question 5 Answer
 tRNA is:
 The major structural material making up ribosomes
 The molecule that carries the genetic information from DNA
and is used as a template for protein synthesis
 The major structural component of chromosomes
 A molecule that incorporates a specific amino acid into
the growing protein when it recognizes a specific group
of three bases CORRECT ANSWER
 The major building block of proteins
Four Basic Molecular Genetic Process: (1) Transcription,
(2) RNA processing, (3) mRNA translation, (4) DNA replication
Self Assessment Questions:
Question 6
 rRNA is:
 The major structural material making up ribosomes
 The molecule that carries the genetic information from
DNA and is used as a template for protein synthesis
 The major structural component of chromosomes
 A molecule that incorporates a specific amino acid into
the growing protein when it recognizes a specific group
of three bases
 The major building block of proteins
Self Assessment Questions:
Question 6 Answer
 rRNA is:
 The major structural material making up




ribosomes CORRECT ANSWER
The molecule that carries the genetic information from
DNA and is used as a template for protein synthesis
The major structural component of chromosomes
A molecule that incorporates a specific amino acid into
the growing protein when it recognizes a specific group
of three bases
The major building block of proteins
Self Assessment Questions:
Question 7
 mRNA is:
 The major structural material making up ribosomes
 The molecule that carries the genetic information from
DNA and is used as a template for protein synthesis
 The major structural component of chromosomes
 A molecule that incorporates a specific amino acid into
the growing protein when it recognizes a specific group
of three bases
 The major building block of proteins
Self Assessment Questions:
Question 7 Answer
 mRNA is:
 The major structural material making up ribosomes
 The molecule that carries the genetic information
from DNA and is used as a template for protein
synthesis Correct Answer
 The major structural component of chromosomes
 A molecule that incorporates a specific amino acid into
the growing protein when it recognizes a specific group
of three bases
 The major building block of proteins
Any questions?