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Transcription & Translation
From Gene to Protein
Part 1
A little history lesson
• In 1909, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions
• He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme
• Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway
Nutritional Mutants in Neurospora: Scientific
Inquiry
• George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal medium as a result of inability to synthesize certain molecules
• Using crosses, they identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine
• They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme
• However, some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein
• Many proteins are composed of several polypeptides, each of which has its own gene
• Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis
• Note that it is common to refer to gene products as proteins rather than polypeptides
Part 2
A Basic Overview
Background
• The process that describes how enzymes and other proteins are made from DNA is called protein synthesis.
• Protein Synthesis has 3 steps:– Transcription
• mRNA is created from a strand of DNA
– RNA processing• mRNA is edited
– Translation• mRNA is read by a ribosome and used to assemble amino
acids into polypeptides
Types of RNA
• mRNA– Messenger RNA
– A single strand of RNA that provides a template used for sequencing amino acids into a polypeptide.
– A triplet of 3 adjacent nucleotides on the mRNA, called a codon, codes for one specific amino acid.
– Since there are 64 possible ways that 4 nucleotides can be arranged in triplet combinations, there are 64 possible codons.
– However, there are only 20 amino acids. Thus, some codons code for the same amino acid.
Genetic Code
• A visual representation of the possible
codon combinations and the amino acids
each codon codes for.
Fig. 17-5
Second mRNA base
Fir
st
mR
NA
ba
se
(5e
nd
of
co
do
n)
Th
ird
mR
NA
ba
se
(3e
nd
of
co
do
n)
Genetic Code
Pinwheel Genetic Code
Types of RNA
• tRNA– Transfer RNA
– A short RNA molecule used for transporting amino acids to their proper place on the mRNA template.
– There are about 45 different tRNAs
– Due to interactions between various parts of the tRNA molecule, it is folded and looks like the 3 leaflets of a clover leaf.
– Contains the anticodon that allows it to bind to mRNA.
• Example: if mRNA has the codon ‘AUG’, tRNA will have the complimentary anticodon ‘UAC’. This codon-anticodon arrangement allows the tRNA to connect to the mRNA.
Types of RNA
• rRNA
– ribosomal RNA
– The building blocks of ribosomes
– In the nucleolus, various proteins are imported from the cytoplasm and assembled with rRNA to form large and small ribosomal subunits.
– Together these two subunits form a ribosome that coordinates the activities of mRNA and tRNA during translation.
Transcription
3 Parts
• 1.) Initiation• RNA polymerase attaches to a promoter region on
the DNA and begins to unzip the two strands.
• The promotor region for mRNA transcriptions often
contains the sequence T-A-T-A. called the TATA
Box.
• 2.) Elongation• Occurs as the RNA pol unzips the DNA and
assembles RNA nucleotides using one side of the
DNA as a template.
• Elongation occurs in the 5’3’ direction
• 3.) Termination• Occurs when the RNA pol reaches a special
sequence of nucleotides that serve as a
termination point.
• In eukaryotes, the termination region often
contains the DNA sequence AAAAAAA.
Fig. 17-7a-1Promoter Transcription unit
DNAStart point
RNA polymerase
553
3
Fig. 17-7a-2Promoter Transcription unit
DNAStart point
RNA polymerase
553
3
Initiation
33
1
RNAtranscript
55
UnwoundDNA
Template strandof DNA
Fig. 17-7a-3Promoter Transcription unit
DNAStart point
RNA polymerase
553
3
Initiation
33
1
RNAtranscript
55
UnwoundDNA
Template strandof DNA
2 Elongation
RewoundDNA
5
553 3 3
RNAtranscript
Fig. 17-7a-4Promoter Transcription unit
DNAStart point
RNA polymerase
553
3
Initiation
33
1
RNAtranscript
55
UnwoundDNA
Template strandof DNA
2 Elongation
RewoundDNA
5
553 3 3
RNAtranscript
3 Termination
5
5
533
3Completed RNA transcript
Fig. 17-7b
Elongation
RNA
polymerase
Nontemplatestrand of DNA
RNA nucleotides
3 end
Direction oftranscription(“downstream”) Template
strand of DNA
Newly madeRNA
3
5
5
Animation
• Transcription Animation
mRNA Processing
Editing the message
Steps of processing
• 1.) The 5’ cap (-P-P-P-G-5’)_• A cap is added to the 5’ end of the mRNA.
• The cap is a guanine nucleotide with 2 additional phosphate groups, forming GTP (similar to ATP).
• This capping gives stability to the mRNA and an attachment point for the ribosome.
• 2.) The poly-A tail (-A-A-A….A-A-3’)• A poly-A tail is added to the 3’ end of the mRNA.
• The tail consists of about 200 adenine nucleotides.
• Provides stability and aids in the mRNA passing through the nuclear envelope.
• 3.) RNA Splicing• Nucleotide segments are removed from the mRNA
• DNA segments contain both coding and non-coding sequences.
• The coding segments = exons;
• The non-coding segments = introns
• The original unprocessed mRNA contains both the coding and non-coding sequences.
• The introns have to be cut out and the exons have to be spliced together in order to create an mRNA with a continuous coding sequence.
Fig. 17-10
Pre-mRNA
mRNA
Codingsegment
Introns cut out andexons spliced together
5 Cap
Exon Intron5
1 30 31 104
Exon Intron
105
Exon
146
3
Poly-A tail
Poly-A tail5 Cap
5 UTR 3 UTR1 146
• In some cases, RNA splicing is carried
out by spliceosomes
• Spliceosomes consist of a variety of
proteins and several small nuclear
ribonucleoproteins (snRNPs) that
recognize the splice sites
Fig. 17-11-1
RNA transcript (pre-mRNA)
Exon 1 Exon 2Intron
Protein
snRNA
snRNPs
Otherproteins
5
Fig. 17-11-2
RNA transcript (pre-mRNA)
Exon 1 Exon 2Intron
Protein
snRNA
snRNPs
Otherproteins
5
5
Spliceosome
Fig. 17-11-3
RNA transcript (pre-mRNA)
Exon 1 Exon 2Intron
Protein
snRNA
snRNPs
Otherproteins
5
5
Spliceosome
Spliceosomecomponents
Cut-outintron
mRNA
Exon 1 Exon 25
Animation
• RNA Splicing Animation
• Splicing Animation #2
• Another RNA Splicing Animation
Ribozymes
• Ribozymes are catalytic RNA molecules
that function as enzymes and can splice
RNA
• The discovery of ribozymes rendered
obsolete the belief that all biological
catalysts were proteins
• Three properties of RNA enable it to
function as an enzyme
– It can form a three-dimensional structure
because of its ability to base pair with itself
– Some bases in RNA contain functional groups
– RNA may hydrogen-bond with other nucleic
acid molecules
The Functional and Evolutionary
Importance of Introns• Some genes can encode more than one
kind of polypeptide, depending on which
segments are treated as exons during
RNA splicing
• Such variations are called alternative
RNA splicing
• Because of alternative splicing, the
number of different proteins an organism
can produce is much greater than its
number of genes
Translation
Making a protein
Background
• After transcription, the mRNA, tRNA and
ribosomal subunits are transported across
the nuclear envelope and into the
cytoplasm.
• In the cytoplasm, the amino acids attach to
the 3’ end of the tRNAs, forming an
aminoacyl-tRNA.
Background
• The reaction to attach the amino acid to
the tRNA requires an enzyme specific to
each tRNA and the energy from one ATP.
• As in transcription, translation is categorized into
3 steps:
– Initiation
– Elongation
– Termination
• The energy for translation is provided by several
GTP molecules.
– GTP acts as an energy supplier in the same manner
as ATP
• A ribosome has three binding sites for
tRNA:
– The P site holds the tRNA that carries the
growing polypeptide chain
– The A site holds the tRNA that carries the next
amino acid to be added to the chain
– The E site is the exit site, where discharged
tRNAs leave the ribosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Steps of translation
• 1.) Initiation begins when the small ribosomal subunit attaches to a special region near the 5’ end of the mRNA.
• 2.) A tRNA (with the anticodon UAC) carrying the amino acid methionine attaches to the mRNA at the start codon, AUG, on a spot on the ribosome called the “P” site
Fig. 17-17
3
35
5U
U
AA
C
G
GTP GDPInitiator
tRNA
mRNA
53
Start codon
mRNA binding site
Smallribosomalsubunit
5
P site
Translation initiation complex
3
E A
Largeribosomalsubunit
• 3.) Another tRNA carrying another amino
acid comes in and binds to the mRNA
at the “A”site.
• 4.) The amino acid from the tRNA in the P
site is moved to the amino acid on the
tRNA in the “A” site.• This is called “Elongation”
• 5.) The mRNA moves over one position.• The first tRNA now occupies the “E” site, the
second tRNA (with the growing amino acid chain)
now ocupies the “P” site and the “A site is open for
the next tRNA.
• 6.) The first tRNA is ejected from the “E”
site and goes into the cytoplasm to get
another amino acid.
Fig. 17-16b
P site (Peptidyl-tRNAbinding site) A site (Aminoacyl-
tRNA binding site)E site(Exit site)
mRNAbinding site
Largesubunit
Smallsubunit
(b) Schematic model showing binding sites
Next amino acidto be added topolypeptide chain
Amino end Growing polypeptide
mRNAtRNA
E P A
E
Codons
(c) Schematic model with mRNA and tRNA
5
3
Fig. 17-18-1
Amino endof polypeptide
mRNA
5
3E
Psite
Asite
Fig. 17-18-2
Amino endof polypeptide
mRNA
5
3E
Psite
Asite
GTP
GDP
E
P A
Fig. 17-18-3
Amino endof polypeptide
mRNA
5
3E
Psite
Asite
GTP
GDP
E
P A
E
P A
Fig. 17-18-4
Amino endof polypeptide
mRNA
5
3E
Psite
Asite
GTP
GDP
E
P A
E
P A
GDP
GTP
Ribosome ready fornext aminoacyl tRNA
E
P A
Termination of Translation
• Termination occurs when a stop codon in the mRNA
reaches the A site of the ribosome.
• The A site accepts a protein called a release factor.
• The release factor causes the addition of a water molecule
instead of an amino acid.
• This reaction releases the polypeptide, and the translation
assembly then comes apart.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-19-1
Releasefactor
3
5
Stop codon(UAG, UAA, or UGA)
Fig. 17-19-2
Releasefactor
3
5
Stop codon(UAG, UAA, or UGA)
5
3
2
Freepolypeptide
2 GDP
GTP
Fig. 17-19-3
Releasefactor
3
5
Stop codon(UAG, UAA, or UGA)
5
3
2
Freepolypeptide
2 GDP
GTP
5
3
Animation
• Translation Animation
Mutations revisited
Types of mutations
• Point mutation:
– Examples:
• insertion,
• deletion,
• substitution,
• frameshift (results from insertion or deletion)
Effects of mutations
• 1.) Silent mutation• Has no effect, because the new codon codes for
the same amino acid as the old codon.
– Example: CUU, CUG, CUA, CUC all code for the amino
acid Leucine. So long as the 3rd nucleotide is the only
one that is changed, the effect is zero.
• 2.) Missense Mutation• The mutation causes a new codon that codes for a
new amino acid.
• This may have only a minor effect or it may result
in the production of a protein that is unable to form
into its proper 3-D shape and, therefore, is unable
to carry out its normal function.
• The hemoglobin protein that causes sickle-cell
disease is caused by a missense mutation.
• 3.) Non-sense Mutation• Occurs when the new codon is a stop codon.
DNA Organization
Background
• In eukaryotes, DNA is packaged with
proteins to form a matrix called chromatin
• The DNA is coiled around bundles of 8-9
histone proteins to form DNA-histone
complexes called nucleosomes.• Microscopically, nucleosomes look like beads on a
string.
Fig. 16-21a
DNA
double helix
(2 nm in diameter)
Nucleosome
(10 nm in diameter)
HistonesHistone tail
H1
DNA, the double helix Histones Nucleosomes, or “beads on a string” (10-nm fiber)
Fig. 16-21b
30-nm fiber
Chromatid (700 nm)
Loops Scaffold
300-nm fiber
Replicated chromosome (1,400 nm)
30-nm fiber Looped domains (300-nm fiber)
Metaphase chromosome
• During cell division, DNA is compactly organized into chromosomes.
• When the cell is not dividing, there are 2 types of chromatin:– 1.) Euchromatin
– DNA is loosely bound to histones.
– DNA here is actively being transcribed.
– 2.) Heterochromatin– DNA is tightly bound to the histones.
– DNA is inactive in these regions.
What in the wide, wide world of
sports is a’ goin’ on here?!!?
Jumping genes?
Yep…
• Some DNA segments within genes are
able to move to new locations.
• This isn’t really a good thing….
• These transposible genetic elements,
called transposons (or jumping genes)
can move to a new location on the same
chromosome or to a different
chromosome.
History
• Discovered in the 1940s by Barbara McClintock.
– She also discovered that crossing over in meiosis
was a thing.
• A super important scientist most people have NEVER heard
of.
• She was studying maize.
– Findings:
• Parts of chromosomes can move randomly to
other locations on the chromosome, affecting
phenotypic expression.
• Genes can be turned on/off by environmental
factors.
• Genetic disorders can be reversed (in maize).
• Some transposons consist only of DNA that codes for an enzyme that enables it to be transported.
• Other transposons contain genes that invoke the replication of the transposon.
• After replication, the new transposon copy is transported to the new location.
• Wherever they are inserted, transposons have the effect of a mutation. – They may change the expression of a gene, turn on or off its
expression, or have no effect at all.
Stats:
• McClintock found that approx. 90% of
maize DNA consists of transposons.
– 44% in humans
In humans…
• Most common transposon is the Alu
sequence.
– 300 bp long.
– Occurs over 1 million different times in human
genome.
• Very common sequence (approx. 17% of total
genome).
– Insertions in Alu sequence in humans
generally have no effect b/c most of the
sequences occur in introns.
In humans…
• ACE gene (Angiotensin Converting
Enzyme)
– Comes in 2 varieties, one WITH an Alu
insertion, and one WITHOUT.
• Variation is linked to sporting performance:
– With = better at endurance events (distance running,
biking, distance swimming, etc).
– Without = better at strength/power events (weightlifting,
wrestling, etc).
In humans…
• Opsin gene duplication in Old World
Primates (including humans) is
hypothesized for the regaining of
trichromacy (3-color vision).
– Birds/fish have 4-color vision.
– Most other mammals are dichromatic.
The Molecular Genetics of
Viruses
Background
• Viruses are parasites of cells.
• Typical mode of infection:
1.) A typical virus penetrates a cell,
2.) it takes over the cell’s metabolic machinery,
3.) The virus (using the cell’s machinery) assembles
hundreds of new viruses that are copies of itself.
4.) Viruses then leave the cell (usually by destroying the
host cell) and infect other cells.
• Viruses are specific for the kinds of cells
they will parasitize.
– Some viruses only attack one type of cell
within a single host species.
– Others attack similar cells from a range of
closely related species.
– Bacteriophages, or phages, are viruses that
attack only bacteria.
Viral Structure
• Viruses consist of the following structures:
– A Nucleic Acid• Either DNA or RNA (not both)
• Contains the hereditary info of the virus
• May be double stranded (dsDNA or dsRNA) or single stranded (ssDNA or ssRNA)
– A Capsid• A protein coat that encloses the nucleic acid.
• Identical protein subunits, called capsomeres, assemble to form the capsid.
Viral Structure
• Some viruses have an envelope that
surrounds the capsid.
• The envelopes incorporate phospholipids
and proteins obtained from the cell
membrane of the host.
– Why would this be advantageous to the virus?
Viral StructureAnimal Virus StructureBacteriophage Structure
Enveloped Virus Structure
Types of Viruses
Viral Replication
• Two ways:
• 1.) Lytic Cycle• The virus penetrates the cell membrane of the host
cell and takes over the host cell.
• Once the viral particles have been replicated, the
host cell ruptures, releasing the viral particles.
• 2.) Lysogenic Cycle• viral DNA is temporarily incorporated into the host
cell DNA.
• A virus in this dormant stage is called a provirus
or if a bacteriophage a prophage.
• As the cell goes through mitosis, it copies the virus
as well.
• The virus remains inactive until some
environmental trigger causes the virus to begin the
destructive lytic cycle.
– Triggers = radiation, chemicals.
Retroviruses
• ssRNA viruses that use an enzyme called reverse transcriptase to make a DNA complement of their RNA.
• The DNA complement can then be transcribed immediately to manufacture mRNA (to make new viral proteins) or it can begin the lysogenic cycle.
• HIV works this way.
• Life cycle of HIV
The Molecular Genetics of
Bacteria
Background
• Bacteria have
– Cell walls
– Cell membranes
– Ribosomes
– DNA
• In a single circular
chromosome
• Bacteria lack
– Nucleus
– Specialized organelles
– Histones
Bacterial Chromosome
• Is often called a “naked chromosome”
because it lacks the histones and other
proteins associated with eukaryotic
chromosomes.
Bacterial Plasmids
• Bacteria contain short, circular DNA molecules outside the chromosome called plasmids.
• Plasmids carry genes that are beneficial but not normally vital for survival of the organism.
• Plasmids replicate independently of the chromosome.
• Some plasmids, called episomes, can become incorporated into the bacterial chromosome.
Genetic Variation in Bacteria
• Bacteria can alter their genome in 3 ways:
• 1.) Conjugation• DNA exchange between bacteria.
• A donor bacterium produces a tube, called a Sex
Pilus, that connects to another bacterium.
• The donor sends chromosomal or plasmid DNA to
the recipient through the pilus.
• In some cases, large portions of a donor’s
chromosome are sent, thus allowing recombination
with the recipient’s chromosome.
Genetic Variation in Bacteria
• Two plasmids in conjugation:
– 1.) F Plasmid– Contains the genes needed to produce pili (plural for
pilus).
– When the recipient bacterium receives the F plasmid, it
too can become a donor cell.
– 2.) R plasmid– Provide bacteria with antibiotic resistance.
Genetic Variation in Bacteria
• Bacteria can alter their genome in 3 ways:
• 2.) Transduction• Occurs when new DNA enters the bacterial
genome by way of a virus (bacteriophage).
• When a virus is assembled during the lytic cycle, it
is sometimes assembled with some bacterial DNA
in place of some viral DNA.
• When the new virus particles infect another cell,
the bacterial DNA they carry can recombine with
the resident DNA.
Genetic Variation in Bacteria
• Bacteria can alter their genome in 3 ways:
• 3.) Transformation• Occurs when bacteria absorb DNA from their
surroundings and incorporate it into their genome.
• Some bacteria have specialized proteins on their
cell membranes that allow this to occur.
Just for fun…
Regulation of Gene
Expression
Background
• Every cell in a human contains exactly the same sequences of DNA. Yet, some cells become muscle cells and other cells become nerve cells.
• One way that cells with identical DNA become different is by regulating gene expression through transcription of only selected genes.
• This is called “epigenetics”
Prokaryotic Gene Expression
• In prokaryotes, an operon is a unit of DNA
that controls gene transcription.
• Operons contain the following parts:
– 1.) Promoter
– 2.) Operator
– 3.) Structural Gene
– 4.) Regulatory Gene
• Promoter
– A sequence of DNA to which the RNA
polymerase attaches to begin transcription.
• Operator
– a region of DNA that can block the action of
RNA polymerase if the region is occupied by
a repressor protein.
• Structural Gene
– Contain DNA sequences that code for
several related enzymes that direct the
production of a particular end product.
• Regulatory Gene
– Is outside the operon region
– Produces repressor proteins and activator
proteins
• Repressor Protein
– Substances that occupy the operator region
and block the action of RNA polymerase.
• Activator Protein
– Assist the attachment of RNA polymerase to
the promotor region
Examples of Operons
• 1.) lac operon• In E. coli
• Controls the breakdown of lactose.
• Enzymes produced by the operon are inducible enzymes.
• 2.) trp operon • In E. coli
• Controls the production of the amino acid tryptophan
• Enzymes produced by the operon are repressible enzymes.
Eukaryotic Gene Expression
• 3 methods:
• 1.) Regulatory Proteins• Operate similarly to those in prokaryotes.
• Influence how readily the RNA pol will attach to the
promoter region.
• 2.) Nucleosome Packing• Influences whether a section of DNA will be
transcribed.
• DNA segments are tightly packed by methylationof the histones, which often makes the region un-transcribable.
– Barr Bodies are highly methylated
• DNA segments are loosely packed by acetylationof the histones, which allows the DNA to be transcribed.
• Heterochromatin & Euchromatin
• 3.) RNA Interference• Occurs when short interfering RNAs (siRNAs)
block mRNA transcription or translation.
– Under certain circumstances, an RNA molecule will fold
back and base pair with itself, forming dsRNA.
– An enzyme then cuts the dsRNA into short pieces, which
then base pair to complimentary DNA regions,
preventing further transcription of the gene.
– Sometimes the siRNAs will bind to existing mRNA
strands, thereby preventing ribosomes from binding to
the mRNA. This effectively inactivates the mRNA.
• 4.) Alternative RNA Splicing• Certain codons are copied from a gene when other
codons in same gene are not.
• This allows the same gene to code for many
different proteins.
• We’ve discussed this before.
• 5.) Protein degradation • As proteins age, they lose functionality due to bond
interferences.
• Proteins that need to be recycled due to non-
function are marked for destruction with a protein
called ubiquitin (so called because it is ubiquitous,
found in all eukaryotic cells).