The document discusses several key topics in molecular biology:
1) The central dogma of molecular biology which describes the typical flow of genetic information from DNA to RNA to protein.
2) Exceptions to the central dogma including reverse transcription by retroviruses and retrotransposons.
3) Prions which are infectious proteins that can convert normal proteins to an abnormal shape leading to neurodegenerative diseases.
4) The roles and types of RNA in cells including mRNA, tRNA, and rRNA in protein synthesis. The process of transcription is described.
5) Protein synthesis including translation, the genetic code, and post-translational modification of proteins.
This presentation is about the transcription machinery that is required for the transcription in eukaryotes. The comparison between the transcription factors involved in prokaryotes and eukaryotes. The initiation of transcription and how it helps in producing a mRNA.
This presentation is about the transcription machinery that is required for the transcription in eukaryotes. The comparison between the transcription factors involved in prokaryotes and eukaryotes. The initiation of transcription and how it helps in producing a mRNA.
This presentation deals with the ‘Central Dogma’ which is briefly the process by which the instructions in DNA are converted into a functional product. It was first proposed in 1958 by Francis Crick, discoverer of the structure of DNA.
Epigenetics is the study, in the field of genetics, of cellular and physiological phenotypic trait variations that are caused by external or environmental factors that switch genes on and off and affect how cells read genes instead of being caused by changes in the DNA sequence. -Wikipedia
This presentation deals with the ‘Central Dogma’ which is briefly the process by which the instructions in DNA are converted into a functional product. It was first proposed in 1958 by Francis Crick, discoverer of the structure of DNA.
Epigenetics is the study, in the field of genetics, of cellular and physiological phenotypic trait variations that are caused by external or environmental factors that switch genes on and off and affect how cells read genes instead of being caused by changes in the DNA sequence. -Wikipedia
Introduction to Medical Imaging, Basics of Medical Imaging, Fundamentals of Digital Image Processing, First chapter of Digital Image Processing Book by Rafael C. Gonzalez.
For More Medicine Free PPT - http://playnever.blogspot.com/
For Health benefits and medicine videos Subscribe youtube channel - https://www.youtube.com/playlist?list=PLKg-H-sMh9G01zEg4YpndngXODW2bq92w
Transcription and synthesis of different RNAs
Processing of RNA transcript
Catalytic RNA
RNA splicing and Spliceosome
Transport of RNA through nuclear pore
Translation and polypeptide synthesis
Posttranslational modification
Protein trafficking and degradation
Antibiotics and inhibition of protein synthesis.
• Define transcription• Define translation• What are the 3 steps.pdfarihantelehyb
• Define transcription
• Define translation
• What are the 3 steps of translation?
• Define the “genetic dogma”
• What is the function of Transfer RNA?
• What is the function of RNA polymerase?
• What is the function of DNA polymerase?
• Define “splicing of RNA”
• What is an exon?
• What component of the cell does the translation?
• What molecule in the cell does transcription?
• What are the functions of: operon, promotor?
• What is the difference between inducible operon and repressible operon?
Solution
• Define transcription
Transcription is the process of making an RNA copy of a gene sequence. This copy, called a
messenger RNA (mRNA) molecule, leaves the cell nucleus and enters the cytoplasm, where it
directs the synthesis of the protein, which it encodes. Here is a more complete definition of
transcription.
• Define translation
Translation is the process of translating the sequence of a messenger RNA (mRNA) molecule to
a sequence of amino acids during protein synthesis. The genetic code describes the relationship
between the sequence of base pairs in a gene and the corresponding amino acid sequence that it
encodes. In the cell cytoplasm, the ribosome reads the sequence of the mRNA in groups of three
bases to assemble the protein. Here is a more complete definition of translation:
• What are the 3 steps of translation?
Step # 1. Initiation:
Initiation of translation in E .coli involves the small ribosome subunit, a mRNA molecule, a
specific charge initiator tRNA, GTP, Mg++ and number of proteinaceous initiation factors (IFs).
These are initially part of the small subunit and are required to enhance binding affinity of the
various translational components (Table 8.1). Unlike ribosomal proteins, IFs are released from
the ribosome once initiation is completed.
Step # 2. Elongation:
Once both subunits of the ribosome are assembled with the mRNA, binding site for two charged
tRNA molecules are formed. These are designated as the ‘P’ or peptidyl and the ‘A’ or
aminoacyl sites. The charged initiator tRNA binds to the P site, provided that the AUG triplet of
mRNA is in the corresponding position of the small subunit. The increase of the growing
polypeptide chain by one amino acid is called elongation.
Step # 3. Termination:
Termination of protein synthesis is carried out by triplet codes (UAG, UAA, UGA; stop codons)
present at site A. These codons do not specify an amino acid, nor do they call for a tRNA in the
A site. These codons are called stop codons, termination codons or nonsense codons. The
finished polypeptide is still attached to the terminal tRNA at the P site, and the A site is empty.
• Define the “genetic dogma”
A theory in genetics and molecular biology subject to several exceptions that genetic information
is coded in self-replicating DNA and undergoes unidirectional transfer to messenger RNAs in
transcription which act as templates for protein synthesis in translation
• What is the function of Transfer RNA?
The tRNA molecule, or tr.
Replication,transcription,translation complete the central dogma of life.How mRNA,tRNA,rRNA act on ribosomes for protein synthesis.Difference between eukaryotes and prokaryotes
HOT NEW PRODUCT! BIG SALES FAST SHIPPING NOW FROM CHINA!! EU KU DB BK substit...GL Anaacs
Contact us if you are interested:
Email / Skype : kefaya1771@gmail.com
Threema: PXHY5PDH
New BATCH Ku !!! MUCH IN DEMAND FAST SALE EVERY BATCH HAPPY GOOD EFFECT BIG BATCH !
Contact me on Threema or skype to start big business!!
Hot-sale products:
NEW HOT EUTYLONE WHITE CRYSTAL!!
5cl-adba precursor (semi finished )
5cl-adba raw materials
ADBB precursor (semi finished )
ADBB raw materials
APVP powder
5fadb/4f-adb
Jwh018 / Jwh210
Eutylone crystal
Protonitazene (hydrochloride) CAS: 119276-01-6
Flubrotizolam CAS: 57801-95-3
Metonitazene CAS: 14680-51-4
Payment terms: Western Union,MoneyGram,Bitcoin or USDT.
Deliver Time: Usually 7-15days
Shipping method: FedEx, TNT, DHL,UPS etc.Our deliveries are 100% safe, fast, reliable and discreet.
Samples will be sent for your evaluation!If you are interested in, please contact me, let's talk details.
We specializes in exporting high quality Research chemical, medical intermediate, Pharmaceutical chemicals and so on. Products are exported to USA, Canada, France, Korea, Japan,Russia, Southeast Asia and other countries.
Flu Vaccine Alert in Bangalore Karnatakaaddon Scans
As flu season approaches, health officials in Bangalore, Karnataka, are urging residents to get their flu vaccinations. The seasonal flu, while common, can lead to severe health complications, particularly for vulnerable populations such as young children, the elderly, and those with underlying health conditions.
Dr. Vidisha Kumari, a leading epidemiologist in Bangalore, emphasizes the importance of getting vaccinated. "The flu vaccine is our best defense against the influenza virus. It not only protects individuals but also helps prevent the spread of the virus in our communities," he says.
This year, the flu season is expected to coincide with a potential increase in other respiratory illnesses. The Karnataka Health Department has launched an awareness campaign highlighting the significance of flu vaccinations. They have set up multiple vaccination centers across Bangalore, making it convenient for residents to receive their shots.
To encourage widespread vaccination, the government is also collaborating with local schools, workplaces, and community centers to facilitate vaccination drives. Special attention is being given to ensuring that the vaccine is accessible to all, including marginalized communities who may have limited access to healthcare.
Residents are reminded that the flu vaccine is safe and effective. Common side effects are mild and may include soreness at the injection site, mild fever, or muscle aches. These side effects are generally short-lived and far less severe than the flu itself.
Healthcare providers are also stressing the importance of continuing COVID-19 precautions. Wearing masks, practicing good hand hygiene, and maintaining social distancing are still crucial, especially in crowded places.
Protect yourself and your loved ones by getting vaccinated. Together, we can help keep Bangalore healthy and safe this flu season. For more information on vaccination centers and schedules, residents can visit the Karnataka Health Department’s official website or follow their social media pages.
Stay informed, stay safe, and get your flu shot today!
Report Back from SGO 2024: What’s the Latest in Cervical Cancer?bkling
Are you curious about what’s new in cervical cancer research or unsure what the findings mean? Join Dr. Emily Ko, a gynecologic oncologist at Penn Medicine, to learn about the latest updates from the Society of Gynecologic Oncology (SGO) 2024 Annual Meeting on Women’s Cancer. Dr. Ko will discuss what the research presented at the conference means for you and answer your questions about the new developments.
ARTIFICIAL INTELLIGENCE IN HEALTHCARE.pdfAnujkumaranit
Artificial intelligence (AI) refers to the simulation of human intelligence processes by machines, especially computer systems. It encompasses tasks such as learning, reasoning, problem-solving, perception, and language understanding. AI technologies are revolutionizing various fields, from healthcare to finance, by enabling machines to perform tasks that typically require human intelligence.
Couples presenting to the infertility clinic- Do they really have infertility...Sujoy Dasgupta
Dr Sujoy Dasgupta presented the study on "Couples presenting to the infertility clinic- Do they really have infertility? – The unexplored stories of non-consummation" in the 13th Congress of the Asia Pacific Initiative on Reproduction (ASPIRE 2024) at Manila on 24 May, 2024.
- Video recording of this lecture in English language: https://youtu.be/lK81BzxMqdo
- Video recording of this lecture in Arabic language: https://youtu.be/Ve4P0COk9OI
- Link to download the book free: https://nephrotube.blogspot.com/p/nephrotube-nephrology-books.html
- Link to NephroTube website: www.NephroTube.com
- Link to NephroTube social media accounts: https://nephrotube.blogspot.com/p/join-nephrotube-on-social-media.html
Ethanol (CH3CH2OH), or beverage alcohol, is a two-carbon alcohol
that is rapidly distributed in the body and brain. Ethanol alters many
neurochemical systems and has rewarding and addictive properties. It
is the oldest recreational drug and likely contributes to more morbidity,
mortality, and public health costs than all illicit drugs combined. The
5th edition of the Diagnostic and Statistical Manual of Mental Disorders
(DSM-5) integrates alcohol abuse and alcohol dependence into a single
disorder called alcohol use disorder (AUD), with mild, moderate,
and severe subclassifications (American Psychiatric Association, 2013).
In the DSM-5, all types of substance abuse and dependence have been
combined into a single substance use disorder (SUD) on a continuum
from mild to severe. A diagnosis of AUD requires that at least two of
the 11 DSM-5 behaviors be present within a 12-month period (mild
AUD: 2–3 criteria; moderate AUD: 4–5 criteria; severe AUD: 6–11 criteria).
The four main behavioral effects of AUD are impaired control over
drinking, negative social consequences, risky use, and altered physiological
effects (tolerance, withdrawal). This chapter presents an overview
of the prevalence and harmful consequences of AUD in the U.S.,
the systemic nature of the disease, neurocircuitry and stages of AUD,
comorbidities, fetal alcohol spectrum disorders, genetic risk factors, and
pharmacotherapies for AUD.
Pulmonary Thromboembolism - etilogy, types, medical- Surgical and nursing man...VarunMahajani
Disruption of blood supply to lung alveoli due to blockage of one or more pulmonary blood vessels is called as Pulmonary thromboembolism. In this presentation we will discuss its causes, types and its management in depth.
Title: Sense of Smell
Presenter: Dr. Faiza, Assistant Professor of Physiology
Qualifications:
MBBS (Best Graduate, AIMC Lahore)
FCPS Physiology
ICMT, CHPE, DHPE (STMU)
MPH (GC University, Faisalabad)
MBA (Virtual University of Pakistan)
Learning Objectives:
Describe the primary categories of smells and the concept of odor blindness.
Explain the structure and location of the olfactory membrane and mucosa, including the types and roles of cells involved in olfaction.
Describe the pathway and mechanisms of olfactory signal transmission from the olfactory receptors to the brain.
Illustrate the biochemical cascade triggered by odorant binding to olfactory receptors, including the role of G-proteins and second messengers in generating an action potential.
Identify different types of olfactory disorders such as anosmia, hyposmia, hyperosmia, and dysosmia, including their potential causes.
Key Topics:
Olfactory Genes:
3% of the human genome accounts for olfactory genes.
400 genes for odorant receptors.
Olfactory Membrane:
Located in the superior part of the nasal cavity.
Medially: Folds downward along the superior septum.
Laterally: Folds over the superior turbinate and upper surface of the middle turbinate.
Total surface area: 5-10 square centimeters.
Olfactory Mucosa:
Olfactory Cells: Bipolar nerve cells derived from the CNS (100 million), with 4-25 olfactory cilia per cell.
Sustentacular Cells: Produce mucus and maintain ionic and molecular environment.
Basal Cells: Replace worn-out olfactory cells with an average lifespan of 1-2 months.
Bowman’s Gland: Secretes mucus.
Stimulation of Olfactory Cells:
Odorant dissolves in mucus and attaches to receptors on olfactory cilia.
Involves a cascade effect through G-proteins and second messengers, leading to depolarization and action potential generation in the olfactory nerve.
Quality of a Good Odorant:
Small (3-20 Carbon atoms), volatile, water-soluble, and lipid-soluble.
Facilitated by odorant-binding proteins in mucus.
Membrane Potential and Action Potential:
Resting membrane potential: -55mV.
Action potential frequency in the olfactory nerve increases with odorant strength.
Adaptation Towards the Sense of Smell:
Rapid adaptation within the first second, with further slow adaptation.
Psychological adaptation greater than receptor adaptation, involving feedback inhibition from the central nervous system.
Primary Sensations of Smell:
Camphoraceous, Musky, Floral, Pepperminty, Ethereal, Pungent, Putrid.
Odor Detection Threshold:
Examples: Hydrogen sulfide (0.0005 ppm), Methyl-mercaptan (0.002 ppm).
Some toxic substances are odorless at lethal concentrations.
Characteristics of Smell:
Odor blindness for single substances due to lack of appropriate receptor protein.
Behavioral and emotional influences of smell.
Transmission of Olfactory Signals:
From olfactory cells to glomeruli in the olfactory bulb, involving lateral inhibition.
Primitive, less old, and new olfactory systems with different path
2. Central Dogma of Molecular
Biology
•
•
The flow of information in the cell
starts at DNA, which replicates to
form more DNA. Information is
then ‘transcribed” into RNA, and
then it is “translated” into protein.
The proteins do most of the work
in the cell.
Information does not flow in the
other direction. This is a
molecular version of the
incorrectness of “inheritance of
acquired characteristics”.
Changes in proteins do not affect
the DNA in a systematic manner
(although they can cause random
changes in DNA.
3. Reverse Transcription
• However, a few exceptions to the Central Dogma exist.
• Most importantly, some RNA viruses, called
“retroviruses” make a DNA copy of themselves using the
enzyme reverse transcriptase. The DNA copy
incorporates into one of the chromosomes and becomes
a permanent feature of the genome. The DNA copy
inserted into the genome is called a “provirus”. This
represents a flow of information from RNA to DNA.
• Closely related to retroviruses are “retrotransposons”,
sequences of DNA that make RNA copies of
themselves, which then get reverse-transcribed into DNA
that inserts into new locations in the genome. Unlike
retroviruses, retrotransposons always remain within the
cell. They lack genes to make the protein coat that
surrounds viruses.
4. •
•
•
•
Prionsdisease (bovine
Prions are the agents that cause mad cow
spA prion is an “infectious protein”.
ongiform encephalopathy), chronic wasting disease in deer
and elk, scrapie in sheep, and Creutzfeld-Jakob syndrome
in humans.
These diseases cause neural degeneration. In humans, the
symptoms are approximately those of Alzheimer’s
syndrome accelerated to go from onset to death in about 1
year. Fortunately, the disease is very hard to catch and
very rare, and they usually have a long incubation time. No
cure is known, and not enough is known about how it is
spread to do a thorough job of preventing it. Avoid eating
brains is a good start though.
The prion protein (PrP) is normally present in the body.
Like all proteins, it is folded into a specific conformation, a
state called PrPC. Prion diseases are caused by the same
protein folded abnormally, a state called PrPSc. A PrPSc can
bind to a normal PrPC protein and convert it to PrPSc. This
conversion spreads throughout the body, causing the
disease to occur. It is also a form of inheritance that does
not involve nucleic acids.
5. RNA
• RNA plays a central role in the life of the cell. We are
mostly going to look at its role in protein synthesis, but
RNA does many other things as well.
• RNA can both store information (like DNA) and catalyze
chemical reactions (like proteins).
• One theory for the origin of life has it starting out as RNA
only, then adding DNA and proteins later. This theory is
called the “RNA World”.
• RNA/protein hybrid structures are involved in protein
synthesis (ribosome), splicing of messenger RNA,
telomere maintenance, guiding ribosomes to the
endoplasmic reticulum, and other tasks.
• Recently it has been found that very small RNA
molecules are involves in gene regulation.
6. RNA Used in Protein Synthesis
• messenger RNA (mRNA). A copy of the gene that is
being expressed. Groups of 3 bases in mRNA, called
“codons” code for each individual amino acid in the
protein made by that gene.
– in eukaryotes, the initial RNA copy of the gene is called the
“primary transcript”, which is modified to form mRNA.
• ribosomal RNA (rRNA). Four different RNA molecules
that make up part of the structure of the ribosome. They
perform the actual catalysis of adding an amino acid to a
growing peptide chain.
• transfer RNA (tRNA). Small RNA molecules that act as
adapters between the codons of messenger RNA and
the amino acids they code for.
7. RNA vs. DNA
• RNA contains the sugar ribose; DNA
contains deoxyribose.
• RNA contains the base uracil; DNA
contains thymine instead.
• RNA is usually single stranded; DNA is
usually double stranded.
• RNA is short: one gene long at most; DNA
is long, containing many genes.
8. Transcription
•
•
•
•
•
•
Transcription is the process of making an RNA copy of a single gene.
Genes are specific regions of the DNA of a chromosome.
The enzyme used in transcription is “RNA polymerase”. There are several
forms of RNA polymerase. In eukaryotes, most genes are transcribed by
RNA polymerase 2.
The raw materials for the new RNA are the 4 ribonucleoside triphosphates:
ATP, CTP, GTP, and UTP. It’s the same ATP as is used for energy in the
cell.
As with DNA replication, transcription proceeds 5- to 3’: new bases are
added to the free 3’ OH group.
Unlike replication, transcription does not need to build on a primer. Instead,
transcription starts at a region of DNA called a “promoter”. For proteincoding genes, the promoter is located a few bases 5’ to (upstream from) the
first base that is transcribed into RNA.
Promoter sequences are very similar to each other, but not identical. If
many promoters are compared, a “consensus sequence” can be derived.
All promoters would be similar to this consensus sequence, but not
necessarily identical.
9. Process of Transcription
•
•
•
•
•
•
Transcription starts with RNA polymerase binding to the promoter.
This binding only occurs under some conditions: when the gene is
“on”. Various other proteins (transcription factors) help RNA
polymerase bind to the promoter. Other DNA sequences further
upstream from the promoter are also involved.
Once it is bound to the promoter, RNA polymerase unwinds a small
section of the DNA and uses it as a template to synthesize an exact
RNA copy of the DNA strand.
The DNA strand used as a template is the “coding strand”; the
other strand is the “non-coding strand”. Notice that the RNA is
made from 5’ end to 3’ end, so the coding strand is actually read
from 3’ to 5’.
RNA polymerase proceeds down the DNA, synthesizing the RNA
copy.
In prokaryotes, each RNA ends at a specific terminator sequence.
In eukaryotes transcription doesn’t have a definite end point; the
RNA is given a definitive termination point during RNA processing.
11. After Transcription
• In prokaryotes, the RNA copy of a gene is messenger
RNA, ready to be translated into protein. In fact,
translation starts even before transcription is finished.
• In eukaryotes, the primary RNA transcript of a gene
needs further processing before it can be translated.
This step is called “RNA processing”. Also, it needs to be
transported out of the nucleus into the cytoplasm.
• Steps in RNA processing:
– 1. Add a cap to the 5’ end
– 2. Add a poly-A tail to the 3’ end
– 3. splice out introns.
12. Capping
•
•
•
RNA is inherently unstable,
especially at the ends. The
ends are modified to protect it.
At the 5’ end, a slightly
modified guanine (7-methyl G)
is attached “backwards”, by a
5’ to 5’ linkage, to the
triphosphates of the first
transcribed base.
At the 3’ end, the primary
transcript RNA is cut at a
specific site and 100-200
adenine nucleotides are
attached: the poly-A tail. Note
that these A’s are not coded in
the DNA of the gene.
13. Introns
•
•
•
•
Introns are regions within a gene that don’t code for protein and
don’t appear in the final mRNA molecule. Protein-coding sections of
a gene (called exons) are interrupted by introns.
The function of introns remains unclear. They may help is RNA
transport or in control of gene expression in some cases, and they
may make it easier for sections of genes to be shuffled in evolution.
But , no generally accepted reason for the existence of introns
exists.
There are a few prokaryotic examples, but most introns are found in
eukaryotes.
Some genes have many long introns: the dystrophin gene (mutants
cause muscular dystrophy) has more than 70 introns that make up
more than 99% of the gene’s sequence. However, not all eukaryotic
genes have introns: histone genes, for example, lack introns.
14. Intron Splicing
• Introns are removed from
the primary RNA
transcript while it is still in
the nucleus.
• Introns are “spliced out”
by RNA/protein hybrids
called “spliceosomes”.
The intron sequences are
removed, and the
remaining ends are reattached so the final RNA
consists of exons only.
15. Summary of RNA processing
•
•
•
•
•
In eukaryotes, RNA polymerase produces a “primary transcript”, an exact RNA copy of the gene.
A cap is put on the 5’ end.
The RNA is terminated and poly-A is added to the 3’ end.
All introns are spliced out.
At this point, the RNA can be called messenger RNA. It is then transported out of the nucleus
into the cytoplasm, where it is translated.
16. Proteins
• Proteins are composed of one or more polypeptides,
plus (in some cases) additional small molecules (cofactors).
• Polypeptides are linear chains of amino acids. After
synthesis, the new polypeptide folds spontaneously into
its active configuration and combines with the other
necessary subunits to form an active protein. Thus, all
the information necessary to produce the protein is
contained in the DNA base sequence that codes for the
polypeptides.
• The sequence of amino acids in a polypeptide is known
as its “primary structure”.
17. Amino Acids and Peptide Bonds
•
•
•
There are 20 different amino
acids coded in DNA.
They all have an amino group
(-NH2) group on one end, and
an acid group (-COOH) on the
other end. Attached to the
central carbon is an R group,
which differs for each of the
different amino acids.
When polypeptides are
synthesized, the acid group of
one amino acid is attached to
the amino group of the next
amino acid, forming a peptide
bond.
18.
19. Translation
• Translation of mRNA into protein is accomplished by the
ribosome, an RNA/protein hybrid. Ribosomes are
composed of 2 subunits, large and small.
• Ribosomes bind to the translation initiation sequence on
the mRNA, then move down the RNA in a 5’ to 3’
direction, creating a new polypeptide. The first amino
acid on the polypeptide has a free amino group, so it is
called the “N-terminal”. The last amino acid in a
polypeptide has a free acid group, so it is called the “Cterminal”.
• Each group of 3 nucleotides in the mRNA is a “codon”,
which codes for 1 amino acids. Transfer RNA is the
adapter between the 3 bases of the codon and the
corresponding amino acid.
20. Transfer RNA
•
•
•
•
Transfer RNA molecules are short RNAs
that fold into a characteristic cloverleaf
pattern. Some of the nucleotides are
modified to become things like
pseudouridine and ribothymidine.
Each tRNA has 3 bases that make up the
anticodon. These bases pair with the 3
bases of the codon on mRNA during
translation.
Each tRNA has its corresponding amino
acid attached to the 3’ end. A set of
enzymes, the “aminoacyl tRNA
synthetases”, are used to “charge” the
tRNA with the proper amino acid.
Some tRNAs can pair with more than one
codon. The third base of the anticodon is
called the “wobble position”, and it can form
base pairs with several different
nucleotides.
21. Initiation of Translation
• In prokaryotes, ribosomes bind to specific translation
initiation sites. There can be several different initiation
sites on a messenger RNA: a prokaryotic mRNA can
code for several different proteins. Translation begins at
an AUG codon, or sometimes a GUG. The modified
amino acid N-formyl methionine is always the first amino
acid of the new polypeptide.
• In eukaryotes, ribosomes bind to the 5’ cap, then move
down the mRNA until they reach the first AUG, the
codon for methionine. Translation starts from this point.
Eukaryotic mRNAs code for only a single gene.
(Although there are a few exceptions, mainly among the
eukaryotic viruses).
• Note that translation does not start at the first base of the
mRNA. There is an untranslated region at the beginning
of the mRNA, the 5’ untranslated region (5’ UTR).
22. More Initiation
• The initiation process
involves first joining the
mRNA, the initiator
methionine-tRNA, and the
small ribosomal subunit.
Several “initiation
factors”--additional
proteins--are also
involved. The large
ribosomal subunit then
joins the complex.
23. Elongation
•
•
•
The ribosome has 2 sites for tRNAs, called P and A. The initial
tRNA with attached amino acid is in the P site. A new tRNA,
corresponding to the next codon on the mRNA, binds to the A site.
The ribosome catalyzes a transfer of the amino acid from the P site
onto the amino acid at the A site, forming a new peptide bond.
The ribosome then moves down one codon. The now-empty tRNA
at the P site is displaced off the ribosome, and the tRNA that has the
growing peptide chain on it is moved from the A site to the P site.
The process is then repeated:
– the tRNA at the P site holds the peptide chain, and a new tRNA binds to
the A site.
– the peptide chain is transferred onto the amino acid attached to the A
site tRNA.
– the ribosome moves down one codon, displacing the empty P site tRNA
and moving the tRNA with the peptide chain from the A site to the P
site.
25. Termination
•
•
•
Three codons are called “stop
codons”. They code for no amino
acid, and all protein-coding
regions end in a stop codon.
When the ribosome reaches a
stop codon, there is no tRNA that
binds to it. Instead, proteins
called “release factors” bind, and
cause the ribosome, the mRNA,
and the new polypeptide to
separate. The new polypeptide is
completed.
Note that the mRNA continues on
past the stop codon. The
remaining portion is not translated:
it is the 3’ untranslated region (3’
UTR).
26. Post-Translational Modification
• New polypeptides usually fold themselves spontaneously
into their active conformation. However, some proteins
are helped and guided in the folding process by
chaperone proteins
• Many proteins have sugars, phosphate groups, fatty
acids, and other molecules covalently attached to certain
amino acids. Most of this is done in the endoplasmic
reticulum.
• Many proteins are targeted to specific organelles within
the cell. Targeting is accomplished through “signal
sequences” on the polypeptide. In the case of proteins
that go into the endoplasmic reticulum, the signal
seqeunce is a group of amino acids at the N terminal of
the polypeptide, which are removed from the final protein
after translation.
27. The Genetic Code
•
•
•
•
•
Each group of 3 nucleotides on the mRNA is a codon. Since
there are 4 bases, there are 43 = 64 possible codons, which must
code for 20 different amino acids.
More than one codon is used for most amino acids: the genetic
code is “degenerate”. This means that it is not possible to take a
protein sequence and deduce exactly the base sequence of the
gene it came from.
In most cases, the third base of the codon (the wobble base) can
be altered without changing the amino acid.
AUG is used as the start codon. All proteins are initially translated
with methionine in the first position, although it is often removed
after translation. There are also internal methionines in most
proteins, coded by the same AUG codon.
There are 3 stop codons, also called “nonsense” codons.
Proteins end in a stop codon, which codes for no amino acid.
28.
29. More Genetic Code
• The genetic code is almost universal. It is used
in both prokaryotes and eukaryotes.
• However, some variants exist, mostly in
mitochondria which have very few genes.
• For instance, CUA codes for leucine in the
universal code, but in yeast mitochondria it
codes for threonine. Similarly, AGA codes for
arginine in the universal code, but in human and
Drosophila mitochondria it is a stop codon.
• There are also a few known variants in the code
used in nuclei, mostly among the protists.