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Math 4277
Computational Biology
計算生物
Arthur W. Chou 周維中
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Wiggling Hand
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Useful Learning Techniques
Explaining
• We are going to test you
on your ability to
explain the material.
• Hence, the best way of
studying is to explain the
material over and over
again out loud to
yourself, to each other,
and to your stuffed bear.
While going along with your day
Day Dream
Mathematics is not all
linear thinking.
Allow the essence of the
material to seep into your
subconscious
Pursue ideas that
percolate up and flashes of
inspiration that appear.
Be Creative
•Ask questions.
• Why is it done this way and not that way?
Guesses and Counter Examples
• Guess at potential algorithms for solving a
problem.
• Look for input instances for which your
algorithm gives the wrong answer.
• Treat it as a game between these two
players.
Refinement:
The best solution comes from a process
of repeatedly refining and inventing
alternative solutions
What is Computational Biology?

“What is Computational Biology? - The
development and application of dataanalytical and theoretical methods,
mathematical modeling and computational
simulation techniques to the study of
biological, behavioral, and social systems.”
(http://www.bisti.nih.gov/)
Why Computational Biology?
There once lived a man
who learned how to slay dragons
and give all he possessed
to mastering the art.
After three year
he was fully prepared but,
alas, he found no opportunity
to practice his skills.
- Zhuan Zhi
As a result
he began to
teach how to slay dragons.
- Rene Thom
Sequence Analysis of gp120 on HIV-1 Virus
Highly variable levels of N-linked glycosylation on the V1 loop of
HIV-1 envelope glycoproteins and their relationship to the
immunogenicity of HIV Env of primary viral isolates
by
Arthur W. Chou and Laboratory of Nucleic Acid Vaccines at UMASS Medical School
( submitted to Journal of Virology )
HIV particle and genome
envelope gp41
MHC
matri p17
x
core p24
lipid bilayer
gp120
RT
RNA
HIV vaccines approaches
recombinant protein (gp12
synthetic peptides (V3)
naked DNA
live-recombinant vectors
(viral, bacterial)
whole-inactivated virus
live-attenuated virus
HIV env genetic subtypes
Adapted from J.
Mullins
Estimated prevalence of HIV-1 env subtypes by
region
HIV-1 Envelope Glycoprotein Domains
HIV sequences
Tertiary structure of HIV-1 gp120 protein
Observation

The lengths of the V1 loop vary with the number of Nglycosylation sites; i.e, the longer the V1 loop, the
more the number of sites.
V1 loop:
Length
No. of Glycosylation sites
15 - 25
2-4
26 - 45
4-7
Questions

Does this correlation between the length of the
sequences and the number of glycosylation sites only
occur in V1?

How are the glycosylation sites distributed throughout
the sequences?
Results of Sequence Analysis

This high correlation rarely occurs at other places: the
average number of glycosylation sites for any stretch
of length 45 is ~ 2.5 .

Analysis of the distribution of Glycosylation sites show
that they are mostly concentrated on the V1 loop.
Results of the paper

Mutated Env antigen with selected removal of Nglycosylation sites at the tip of V1 loop on one of the
primary HIV-1 Env, 92US715, induced higher antibody
responses against the autologous HIV virus.
[ Contribution ] These data improve our understanding on
the structure-function relationship of HIV-1 Env
antigens, and facilitate better design of HIV vaccines
with the aim of inducing broad neutralizing antibody
responses.
Algorithm

Distance-Based method

Dynamic Programming
Time: 16:00
Wednesday September 28
Place: Department of Mathematics ST 516
Interactions
DNA Sequence
(Gene)
Protein Amino
Acid Sequence
Protein Function
Protein 3D
Structure
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Molecular Biology Primer
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Outline:
•
•
•
•
1. What Is Life Made Of?
2. What Molecule Code For Genes?
3. What Is the Structure Of DNA?
4. What Carries Information between DNA and
Proteins?
• 5. How are Proteins Made?
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Section1: What is Life made of?
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Outline For Section 1:
• All living things are made of Cells
• Prokaryote, Eukaryote
• Cell Signaling
• What is Inside the cell: From DNA, to RNA, to
Proteins
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Cells
• Fundamental working units of every living system.
• Every organism is composed of one of two
radically different types of cells:
prokaryotic cells or
eukaryotic cells.
• Prokaryotes and Eukaryotes are descended from the
same primitive cell.
• All extant prokaryotic and eukaryotic cells are the
result of a total of 3.5 billion years of evolution.
An Introduction to Bioinformatics Algorithms
Cells
• Chemical composition-by weight
• 70% water
• 7% small molecules
• salts
• Lipids
• amino acids
• nucleotides
• 23% macromolecules
• Proteins
• Polysaccharides
• lipids
• biochemical (metabolic) pathways
• translation of mRNA into proteins
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Life begins with Cell
• A cell is a smallest structural unit of an
organism that is capable of independent
functioning
• All cells have some common features
An Introduction to Bioinformatics Algorithms
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Two types of cells: Prokaryotes v.s.Eukaryotes
An Introduction to Bioinformatics Algorithms
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Prokaryotes and Eukaryotes
•According to the most recent evidence, there are three main branches to the tree of life.
•Prokaryotes include Archaea (“ancient ones”) and bacteria.
•Eukaryotes are kingdom Eukarya and includes plants, animals, fungi and certain algae.
An Introduction to Bioinformatics Algorithms
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Prokaryotes and Eukaryotes, continued
Prokaryotes
Eukaryotes
Single cell
Single or multi cell
No nucleus
Nucleus
No organelles
Organelles
One piece of circular DNA Chromosomes
No mRNA post
Exons/Introns splicing
transcriptional modification
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Cells Information and Machinery
• Cells store all information to replicate itself
• Human genome is around 3 billions base pair long
• Almost every cell in human body contains same
set of genes
• But not all genes are used or expressed by those
cells
• Machinery:
• Collect and manufacture components
• Carry out replication
• Kick-start its new offspring
(A cell is like a car factory)
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Overview of organizations of life
•
•
•
•
Nucleus = library
Chromosomes = bookshelves
Genes = books
Almost every cell in an organism contains the
same libraries and the same sets of books.
• Books represent all the information (DNA)
that every cell in the body needs so it can
grow and carry out its vaious functions.
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Some Terminology
• Genome: an organism’s genetic material
• Gene: a discrete units of hereditary information located on the
chromosomes and consisting of DNA.
• Genotype: The genetic makeup of an organism
• Phenotype: the physical expressed traits of an organism
• Nucleic acid: Biological molecules(RNA and DNA) that allow organisms to
reproduce;
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
DNA the Genetics Makeup
• Genes are inherited and are
expressed
• genotype (genetic makeup)
• phenotype (physical
expression)
• On the left, is the eye’s
phenotypes of green and
black eye genes.
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
More Terminology
• The genome is an organism’s complete set of DNA.
• a bacteria contains about 600,000 DNA base pairs
• human and mouse genomes have some 3 billion.
• human genome has 24 distinct chromosomes.
• Each chromosome contains many genes.
• Gene
• basic physical and functional units of heredity.
• specific sequences of DNA bases that encode
instructions on how to make proteins.
• Proteins
• Make up the cellular structure
• large, complex molecules made up of smaller subunits
called amino acids.
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Genome Sizes
•
•
•
•
E.Coli (bacteria)
4.6 x 106 bases
Yeast (simple fungi)
15 x 106 bases
Smallest human chromosome 50 x 106 bases
Entire human genome
3 x 109 bases
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
All Life depends on 3 critical molecules
• DNAs
• Hold information on how cell works
• RNAs
• Act to transfer short pieces of information to different parts
of cell
• Provide templates to synthesize into protein
• Proteins
• Form enzymes that send signals to other cells and regulate
gene activity
• Form body’s major components (e.g. hair, skin, etc.)
An Introduction to Bioinformatics Algorithms
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DNA: The Code of Life
• The structure and the four genomic letters code for all living
organisms
• Adenine, Guanine, Thymine, and Cytosine which pair A-T and C-G
on complimentary strands.
An Introduction to Bioinformatics Algorithms
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DNA, continued
• DNA has a double helix
structure which
composed of
• sugar molecule
• phosphate group
• and a base (A,C,G,T)
• DNA always reads from
5’ end to 3’ end for
transcription replication
5’ ATTTAGGCC 3’
3’ TAAATCCGG 5’
An Introduction to Bioinformatics Algorithms
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DNA, RNA, and the Flow of
Information
Replication
Transcription
Translation
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Overview of DNA to RNA to Protein
•
A gene is expressed in two steps
1) Transcription: RNA synthesis
2) Translation: Protein synthesis
An Introduction to Bioinformatics Algorithms
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Cell Information: Instruction book of Life
• DNA, RNA, and
Proteins are examples
of strings written in
either the four-letter
nucleotide of DNA and
RNA (A C G T/U)
• or the twenty-letter
amino acid of proteins.
Each amino acid is
coded by 3 nucleotides
called codon. (Leu, Arg,
Met, etc.)
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Section 2: What Molecule Codes
For Genes?
An Introduction to Bioinformatics Algorithms
Outline For Section 2:
• Discovery of the Structure of DNA
• Watson and Crick
• DNA Basics
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
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Discovery of DNA
•
•
DNA Sequences
• Chargaff and Vischer, 1949
• DNA consisting of A, T, G, C
• Adenine, Guanine, Cytosine, Thymine
• Chargaff Rule
• Noticing #A#T and #G#C
• A “strange but possibly meaningless”
phenomenon.
Wow!! A Double Helix
• Watson and Crick, Nature, April 25, 1953
1 Biologist
•
1 Physics Ph.D. Student
900 words
Nobel Prize
•
Rich, 1973
• Structural biologist at MIT.
• DNA’s structure in atomic resolution.
Crick
Watson
An Introduction to Bioinformatics Algorithms
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Watson & Crick – “…the secret of life”
•
Watson: a zoologist, Crick: a physicist
•
“In 1947 Crick knew no biology and
practically no organic chemistry or
crystallography..” – www.nobel.se
•
Applying Chagraff’s rules and the X-ray
image from Rosalind Franklin, they
constructed a “tinkertoy” model showing
the double helix
•
Watson & Crick with DNA model
Their 1953 Nature paper: “It has not
escaped our notice that the specific pairing
we have postulated immediately suggests
a possible copying mechanism for the
genetic material.”
Rosalind Franklin with X-ray image of DNA
An Introduction to Bioinformatics Algorithms
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DNA: The Basis of Life
• Deoxyribonucleic Acid (DNA)
• Double stranded with complementary strands A-T, C-G
• DNA is a polymer
• Sugar-Phosphate-Base
• Bases held together by H bonding to the opposite strand
An Introduction to Bioinformatics Algorithms
DNA Components
Four nucleotide types:
• Adenine
• Guanine
• Cytosine
• Thymine
Hydrogen bonds:
• A-T
• C-G
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Double helix of DNA
• James Watson and Francis Crick proposed a model for the
structure of DNA.
• Utilizing X-ray diffraction data, obtained from crystals of DNA)
• This model predicted that DNA
• as a helix of two complementary anti-parallel strands,
• wound around each other in a rightward direction
• stabilized by H-bonding between bases in adjacent strands.
• The bases are in the interior of the helix
• Purine bases form hydrogen bonds with pyrimidine.
An Introduction to Bioinformatics Algorithms
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Section 3: The Structure of DNA
An Introduction to Bioinformatics Algorithms
Outline For Section 3:
• DNA Components
• Nitrogenous Base
• Sugar
• Phosphate
• Double Helix
• DNA replication
• Superstructure
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
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DNA
• Stores all information of life
• 4 “letters” base pairs. AGTC (adenine, guanine,
thymine, cytosine ) which pair A-T and C-G on
complimentary strands.
http://www.lbl.gov/Education/HGP-images/dna-medium.gif
An Introduction to Bioinformatics Algorithms
Source: Alberts et al
The Double Helix
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
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DNA, continued
Sugar
Phosphate
Base (A,T, C or G)
An Introduction to Bioinformatics Algorithms
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DNA, continued
• DNA has a double helix structure. However,
it is not symmetric. It has a “forward” and
“backward” direction. The ends are labeled 5’
and 3’ after the Carbon atoms in the sugar
component.
5’ AATCGCAAT 3’
3’ TTAGCGTTA 5’
DNA always reads 5’ to 3’ for transcription
replication
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
DNA Components
•
Nitrogenous Base:
N is important for hydrogen bonding between bases
A – adenine with T – thymine (double H-bond)
C – cytosine with G – guanine (triple H-bond)
•
Sugar:
Ribose (5 carbon)
Base covalently bonds with 1’ carbon
Phosphate covalently bonds with 5’ carbon
Normal ribose (OH on 2’ carbon) – RNA
deoxyribose (H on 2’ carbon) – DNA
dideoxyribose (H on 2’ & 3’ carbon) – used in DNA sequencing
•
Phosphate:
negatively charged
An Introduction to Bioinformatics Algorithms
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Basic Structure
Phosphate
Sugar
An Introduction to Bioinformatics Algorithms
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Basic Structure Implications
• DNA is (-) charged due to phosphate:
gel electrophoresis, DNA sequencing (Sanger method)
• H-bonds form between specific bases:
hybridization – replication, transcription, translation
DNA microarrays, hybridization blots, PCR
C-G bound tighter than A-T due to triple H-bond
• DNA-protein interactions (via major & minor grooves):
transcriptional regulation
• DNA polymerization:
5’ to 3’ – phosphodiester bond formed between 5’ phosphate
and 3’ OH
An Introduction to Bioinformatics Algorithms
The Purines
www.bioalgorithms.info
The Pyrimidines
An Introduction to Bioinformatics Algorithms
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Double helix of DNA
• The double helix of DNA has these features:
• Concentration of adenine (A) is equal to thymine (T)
• Concentration of cytidine (C) is equal to guanine (G).
• Watson-Crick base-pairing A will only base-pair with T, and C with G
• base-pairs of G and C contain three H-bonds,
• Base-pairs of A and T contain two H-bonds.
• G-C base-pairs are more stable than A-T base-pairs
• Two polynucleotide strands wound around each other.
• The backbone of each consists of alternating deoxyribose and
phosphate groups
An Introduction to Bioinformatics Algorithms
Double helix of DNA
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Double helix of DNA
• The DNA strands are assembled in the 5' to 3' direction
• by convention, we "read" them the same way.
• The phosphate group bonded to the 5' carbon atom of one deoxyribose is
covalently bonded to the 3' carbon of the next.
• The purine or pyrimidine attached to each deoxyribose projects in toward the
axis of the helix.
• Each base forms hydrogen bonds with the one directly opposite it, forming
base pairs (also called nucleotide pairs).
An Introduction to Bioinformatics Algorithms
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DNA - replication
• DNA can replicate by
splitting, and rebuilding
each strand.
• Note that the rebuilding
of each strand uses
slightly different
mechanisms due to the
5’ 3’ asymmetry, but
each daughter strand is
an exact replica of the
original strand.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/DNAReplication.html
An Introduction to Bioinformatics Algorithms
DNA Replication
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
Source: Alberts et al
DNA Organization
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
Superstructure
Lodish et al. Molecular Biology of the Cell (5th ed.). W.H. Freeman & Co., 2003.
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
Superstructure Implications
• DNA in a living cell is in a highly compacted and
structured state
• Transcription factors and RNA polymerase need
ACCESS to do their work
• Transcription is dependent on the structural
state – SEQUENCE alone does not tell the
whole story
An Introduction to Bioinformatics Algorithms
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Section 4: What carries information
between DNA to Proteins
An Introduction to Bioinformatics Algorithms
Outline For Section 4:
• Central Dogma Of Biology
• RNA
• Transcription
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
• Central Dogma
(DNARNAprotein)
The paradigm that DNA
directs its transcription
to RNA, which is then
translated into a protein.
• Transcription
(DNARNA) The
process which transfers
genetic information from
the DNA to the RNA.
• Translation
(RNAprotein) The
process of transforming
RNA to protein as
specified by the genetic
code.
www.bioalgorithms.info
How Do Genes Code for Proteins?
Transcription
RNA
DNA
cbio course, spring 2005, Hebrew University
Translation
Protein
Transcription
sequences can be transcribed to RNA
Source: Mathews & van Holde
 Coding
 RNA
nucleotides:
 Similar to DNA, slightly different backbone
 Uracil (U) instead of Thymine (T)
cbio course, spring 2005, Hebrew University
RNA Editing
cbio course, spring 2005, Hebrew University
Translation
cbio course, spring 2005, Hebrew University
Translation
Translation
is mediated by
the ribosome
Ribosome is a complex of
protein & rRNA molecules
The ribosome attaches to the mRNA at a translation
initiation site
 Then ribosome moves along the mRNA sequence and in the
process constructs a poly-peptide
 When the ribosome encounters a stop signal, it releases the
mRNA. The construct poly-peptide is released, and folds
into a protein.

cbio course, spring 2005, Hebrew University
Source: Alberts et al
Translation
cbio course, spring 2005, Hebrew University
Source: Alberts et al
Translation
cbio course, spring 2005, Hebrew University
Source: Alberts et al
Translation
cbio course, spring 2005, Hebrew University
Source: Alberts et al
Translation
cbio course, spring 2005, Hebrew University
Source: Alberts et al
Translation
cbio course, spring 2005, Hebrew University
Genetic Code
cbio course, spring 2005, Hebrew University
The Central Dogma
Experiments
Genes
Transcription
RNA
DNA
cbio course, spring 2005, Hebrew University
Translation
Protein
An Introduction to Bioinformatics Algorithms
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Central Dogma of Biology
The information for making proteins is stored in DNA. There is
a process (transcription and translation) by which DNA is
converted to protein. By understanding this process and how it
is regulated we can make predictions and models of cells.
Assembly
Protein
Sequence
Analysis
Sequence analysis
Gene Finding
An Introduction to Bioinformatics Algorithms
www.bioalgorithms.info
RNA
• RNA is similar to DNA chemically. It is usually only
a single strand. T(hyamine) is replaced by U(racil)
• Some forms of RNA can form secondary structures
by “pairing up” with itself. This can have change its
properties
dramatically.
DNA and RNA
can pair with
each other.
tRNA linear and 3D view:
http://www.cgl.ucsf.edu/home/glasfeld/tutorial/trna/trna.gif
An Introduction to Bioinformatics Algorithms
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RNA, continued
• Several types exist, classified by function
• mRNA – this is what is usually being referred
to when a Bioinformatician says “RNA”. This
is used to carry a gene’s message out of the
nucleus.
• tRNA – transfers genetic information from
mRNA to an amino acid sequence
• rRNA – ribosomal RNA. Part of the ribosome
which is involved in translation.
An Introduction to Bioinformatics Algorithms
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Transcription
• The process of making
RNA from DNA
• Catalyzed by
“transcriptase” enzyme
• Needs a promoter
region to begin
transcription.
• ~50 base pairs/second
in bacteria, but multiple
transcriptions can occur
simultaneously
http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/nucleic/chpt15/transcription.gif
An Introduction to Bioinformatics Algorithms
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DNA  RNA: Transcription
• DNA gets transcribed by a
protein known as RNApolymerase
• This process builds a chain of
bases that will become mRNA
• RNA and DNA are similar,
except that RNA is single
stranded and thus less stable
than DNA
• Also, in RNA, the base uracil (U) is
used instead of thymine (T), the
DNA counterpart
An Introduction to Bioinformatics Algorithms
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Transcription, continued
• Transcription is highly regulated. Most DNA is in a
dense form where it cannot be transcribed.
• To begin transcription requires a promoter, a small
specific sequence of DNA to which polymerase can
bind (~40 base pairs “upstream” of gene)
• Finding these promoter regions is a partially solved
problem that is related to motif finding.
• There can also be repressors and inhibitors acting in
various ways to stop transcription. This makes
regulation of gene transcription complex to
understand.
An Introduction to Bioinformatics Algorithms
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Definition of a Gene
•
Regulatory regions: up to 50 kb upstream of +1 site
•
Exons:
protein coding and untranslated regions (UTR)
1 to 178 exons per gene (mean 8.8)
8 bp to 17 kb per exon (mean 145 bp)
•
Introns:
splice acceptor and donor sites, junk DNA
average 1 kb – 50 kb per intron
•
Gene size:
Largest – 2.4 Mb (Dystrophin). Mean – 27 kb.
An Introduction to Bioinformatics Algorithms
Splicing
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
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Splicing and other RNA processing
• In Eukaryotic cells, RNA is processed
between transcription and translation.
• This complicates the relationship between a
DNA gene and the protein it codes for.
• Sometimes alternate RNA processing can
lead to an alternate protein as a result. This
is true in the immune system.
An Introduction to Bioinformatics Algorithms
Splicing (Eukaryotes)
• Unprocessed RNA is
composed of Introns and
Extrons. Introns are
removed before the rest is
expressed and converted
to protein.
• Sometimes alternate
splicings can create
different valid proteins.
• A typical Eukaryotic gene
has 4-20 introns. Locating
them by analytical means
is not easy.
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
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Section 5: How Are Proteins Made?
(Translation)
An Introduction to Bioinformatics Algorithms
Outline For Section 5:
•
•
•
•
•
mRNA
tRNA
Translation
Protein Synthesis
Protein Folding
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
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Terminology for Ribosome
• Codon: The sequence of 3 nucleotides in DNA/RNA that
encodes for a specific amino acid.
• mRNA (messenger RNA): A ribonucleic acid whose
sequence is complementary to that of a protein-coding
gene in DNA.
• Ribosome: The organelle that synthesizes polypeptides
under the direction of mRNA
• rRNA (ribosomal RNA):The RNA molecules that constitute
the bulk of the ribosome and provides structural scaffolding
for the ribosome and catalyzes peptide bond formation.
• tRNA (transfer RNA): The small L-shaped RNAs that
deliver specific amino acids to ribosomes according to the
sequence of a bound mRNA.
An Introduction to Bioinformatics Algorithms
mRNA

Ribosome
• mRNA leaves the nucleus via nuclear pores.
• Ribosome has 3 binding sites for tRNAs:
• A-site: position that aminoacyl-tRNA
molecule binds to vacant site
• P-site: site where the new peptide bond
is formed.
• E-site: the exit site
• Two subunits join together on a mRNA
molecule near the 5’ end.
• The ribosome will read the codons until
AUG is reached and then the initiator tRNA
binds to the P-site of the ribosome.
• Stop codons have tRNA that recognize a
signal to stop translation. Release factors
bind to the ribosome which cause the
peptidyl transferase to catalyze the addition
of water to free the molecule and releases
the polypeptide.
www.bioalgorithms.info
An Introduction to Bioinformatics Algorithms
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Terminology for tRNA and proteins
• Anticodon: The sequence of 3 nucleotides in
tRNA that recognizes an mRNA codon
through complementary base pairing.
• C-terminal: The end of the protein with the
free COOH.
• N-terminal: The end of the protein with the
free NH3.
An Introduction to Bioinformatics Algorithms
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Purpose of tRNA
• The proper tRNA is chosen by having the
corresponding anticodon for the mRNA’s codon.
• The tRNA then transfers its aminoacyl group to the
growing peptide chain.
• For example, the tRNA with the anticodon UAC
corresponds with the codon AUG and attaches
methionine amino acid onto the peptide chain.
An Introduction to Bioinformatics Algorithms
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Translation: tRNA
mRNA is translated in 5’ to 3’ direction
and the from N-terminal to C-terminus of
the polypeptide.
Elongation process (assuming
polypeptide already began):
 tRNA with the next amino acid in
the chain binds to the A-site by
forming base pairs with the codon
from mRNA
• Carboxyl end of the protein is released from the tRNA at the Psite
and joined to the free amino group from the amino acid attached to
the tRNA at the A-site; new peptide bond formed catalyzed by
peptide transferase.
• Conformational changes occur which shift the two tRNAs into the
E-site and the P-site from the P-site and A-site respectively. The
mRNA also shifts 3 nucleotides over to reveal the next codon.
• The tRNA in the E-site is released
• GTP hydrolysis provides the energy to drive this reaction.
An Introduction to Bioinformatics Algorithms
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Uncovering the code
• Scientists conjectured that proteins came from DNA;
but how did DNA code for proteins?
• If one nucleotide codes for one amino acid, then
there’d be 41 amino acids
• However, there are 20 amino acids, so at least 3
bases codes for one amino acid, since 42 = 16 and
43 = 64
• This triplet of bases is called a “codon”
• 64 different codons and only 20 amino acids means that
the coding is degenerate: more than one codon sequence
code for the same amino acid
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Revisiting the Central Dogma
• In going from DNA to proteins,
there is an intermediate step where
mRNA is made from DNA, which
then makes protein
• This known as The Central
Dogma
• Why the intermediate step?
• DNA is kept in the nucleus, while
protein sythesis happens in the
cytoplasm, with the help of
ribosomes
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The Central Dogma (cont’d)
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RNA  Protein: Translation
• Ribosomes and transfer-RNAs (tRNA) run along the
length of the newly synthesized mRNA, decoding
one codon at a time to build a growing chain of
amino acids (“peptide”)
• The tRNAs have anti-codons, which complimentarily match
the codons of mRNA to know what protein gets added next
• But first, in eukaryotes, a phenomenon called
splicing occurs
• Introns are non-protein coding regions of the mRNA; exons
are the coding regions
• Introns are removed from the mRNA during splicing so that
a functional, valid protein can form
An Introduction to Bioinformatics Algorithms
Translation
• The process of going
from RNA to
polypeptide.
• Three base pairs of
RNA (called a codon)
correspond to one
amino acid based on a
fixed table.
• Always starts with
Methionine and ends
with a stop codon
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Translation, continued
• Catalyzed by Ribosome
• Using two different sites,
the Ribosome
continually binds tRNA,
joins the amino acids
together and moves to
the next location along
the mRNA
• ~10 codons/second,
but multiple translations
can occur
simultaneously
http://wong.scripps.edu/PIX/ribosome.jpg
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Protein Synthesis: Summary
• There are twenty amino
acids, each coded by threebase-sequences in DNA,
called “codons”
• This code is degenerate
• The central dogma
describes how proteins
derive from DNA
• DNA  mRNA  (splicing?)
 protein
• The protein adopts a 3D
structure specific to it’s amino
acid arrangement and
function
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Proteins
• Complex organic molecules made up of amino acid
subunits
• 20* different kinds of amino acids. Each has a 1
and 3 letter abbreviation.
• http://www.indstate.edu/thcme/mwking/aminoacids.html for complete list of chemical structures
and abbreviations.
• Proteins are often enzymes that catalyze reactions.
• Also called “poly-peptides”
*Some other amino acids exist but not in humans.
An Introduction to Bioinformatics Algorithms
Protein Folding
• Proteins tend to fold into the lowest
free energy conformation.
• Proteins begin to fold while the
peptide is still being translated.
• Proteins bury most of its hydrophobic
residues in an interior core to form an
α helix.
• Most proteins take the form of
secondary structures α helices and β
sheets.
• Molecular chaperones, hsp60 and hsp
70, work with other proteins to help
fold newly synthesized proteins.
• Much of the protein modifications and
folding occurs in the endoplasmic
reticulum and mitochondria.
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Protein Structure
 Proteins
are polypeptides of 70-3000
amino-acids
 This
structure is
(mostly) determined
by the sequence of
amino-acids that
make up the protein
cbio course, spring 2005, Hebrew University
Protein Structure
cbio course, spring 2005, Hebrew University
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Protein Folding
• Proteins are not linear structures, though they are
built that way
• The amino acids have very different chemical
properties; they interact with each other after the
protein is built
• This causes the protein to start fold and adopting it’s
functional structure
• Proteins may fold in reaction to some ions, and several
separate chains of peptides may join together through their
hydrophobic and hydrophilic amino acids to form a polymer
An Introduction to Bioinformatics Algorithms
Protein Folding (cont’d)
• The structure that a
protein adopts is vital to
it’s chemistry
• Its structure determines
which of its amino acids
are exposed carry out
the protein’s function
• Its structure also
determines what
substrates it can react
with
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