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Transcription & Translation
Protein Synthesis
Biology 12
Genes direct the production of proteins that
determine the phenotypical characteristics of organisms. Genes also direct the production of other physiologically
essential proteins such as antibodies and hormones. Proteins drive cellular processes such as metabolism;
determining physical characteristics and producing genetic disorders by their absence or presence in an altered form.
Metabolism is a term that is used to describe all chemical reactions involved in maintaining the living state of the cells and the organism.
Metabolism can be conveniently divided into two categories:
Catabolism - the breakdown of molecules to obtain energy
Anabolism – the synthesis of all compounds needed by cells )
The Central Dogma
An organism’s genome is housed within the nucleus. Proteins are synthesized outside the nucleus, in the cytoplasm, on ribosomes.
Since information for protein synthesis is specified by DNA (called the one gene-one polypeptide hypothesis), and DNA is not able to exist outside the nucleus, a problem exists as to how the blueprint of life is brought to the ribosomes.
The Connection Between Genes and Proteins
Nucleic acids carry information in their nucleotide sequence.Proteins carry information in their amino acid sequence.
To get from DNA (in nucleic acid language) to protein (in amino acid language) requires two steps.1.Transcription- a DNA strand provides a template for the synthesis of a complementary RNA strand. This molecule is called mRNA (messenger RNA).DNA is too valuable to be allowed to exit the nucleus. Thiscould lead to the death of the cell and possible the Death of the organism.
- use of mRNA provides protection for the Genetic information contained in DNA.
-more protein can be made simultaneously because many mRNA copies of a gene can be made than if one strand of DNA left the nucleus.
- each mRNA can be translated many times.
mRNA delivers the encoded genetic material to the ribosomes.
The ribosomes translate the message into polypeptide chains, which are processed into proteins.
This entire sequence is described as the Central Dogma of Molecular Genetics, first stated by Francis Crick in 1958.
Central Dogma
In nucleus
Produced in nucleus Travels to cytoplasm
Produced in cytoplasm
Transcription vs Translation
Transcription involves the copying of the information in DNA into mRNA.(copy from one medium to another- think of a medical or legal stenographer)
Translation involves ribosomes using the Messenger RNA as a blueprint to synthesizea protein composed of amino acids.(converting into a different language, think English to French)
Definition: Transcription
Transcription
Nucleus Location
DNATemplate
(What is read)
To change DNA into a form that can make a protein
Purpose
Messenger RNA
(mRNA)
Outcome
(End result)
Definition: Translation
Translation
Location Cytoplasm (by ribosome)
Template
(What is read)mRNA
PurposeAmino acids assembled in particular
order to make a protein
Outcome
(End result)Protein (polypeptide)
Central Dogma
RNA
RNA: How is it difference from DNA?
- contains a ribose sugar- contains the base Uracil
(not thymine)- single stranded- found in both nuclues and
cytoplasm
Purine Bases (double ring)Adenine & GuaninePyrimidine Bases (single ring)Cytosine & Uracil
Base Pairs: (purine always pairs with pyrimidine)
Adenine + UracilGuanine + Cytosine
Image: www.biologycorner.com/bio1/DNA.html
Types of RNA
Genetic information copied from DNA is transferred to 3 types of RNA:
Messenger RNA: mRNA Copy of information in DNA that is
brought to the ribosome and translated into protein by tRNA & rRNA
Varies in length , the longer the gene the longer the mRNA>
Transfer RNA: tRNA Brings the amino acid to the ribosome
that mRNA coded for.
Ribosomal RNA: rRNAMost of the RNA in cells is associated
with structures known as ribosomes, the protein factories of the cells.
Provides the construction site for the assembly of polypeptides.
It is the site of translation where genetic information brought by mRNA is translated into actual proteins.
Transcription & Translation
Transcription occurs in 3 steps: Initiation, Elongation and Termination Initiation:RNA polymerase binds to the DNA at
a specific site known as a promotor.
DNA: A T G C A A
RNA: U A C G U UThe RNA transcript is known as
elongation.
After the RNA polymerase passes the end
of the gene, it stops transcribing which is termination.
Transcription : ‘to copy’
Initiation: RNA polymerase binds to DNA at ‘promoter’ untwists the double helix 10 to 20 bases at a
time
Elongation: RNA polymerase builds mRNA
From DNA 3’ end Uses complimentary base pairing
Remember: thymine (T) is replaced by uracil (U)
Termination: RNA polymerase reaches end of gene. Stops transcribing Double helix reforms as mRNA molecule peels
away.
End Result: mRNA breaks away from DNA mRNA exits nucleus If there is a high demand for a
protein, the cell can have several RNA
polymerases transcribing the gene at the same
time to produce several mRNA’s.
Translation: ‘new language’Initiation: Ribosome binds at a specific sequence on
the mRNA. The ribosome moves along the mRNA three
nucleotides at a time. This is called a codon.
Each set of three (a codon) codes for an amino acid. Why?
There are only 4 bases but 20 amino acids.
41 = 4 (1 base=1 acid) 42 = 16 43 = 64
The codon AUG not only codes for the amino acidMethionine, but it also indicates the start of a translation.
Some amino acids are coded for by two or more codons but a given codon ALWAYS only codesfor one amino acid. GAA and GAG both code for glutamic acid, but never mean any other amino acid.
Elongation: Ribosome moves along mRNA
From mRNA 5’ end 3 nucleotides of mRNA = codon = amino acid The “interpreter” tRNA delivers the proper complimentary base to the
ribosome. Anticodons are blocks of 3 tDNA bases that actually attach to the correct protein.
The anticodon( tRNA) binds by complimentary base pairing to the nucleotides of the codon.
Example: if the codon on a mRNA is UUU,
a tRNA with an AAA anticodon will bind to it.
The ribosome links adjacent amino acids with a peptide bond, causing the amino acid to let go of it tRNA.
The finished protein has a sequence of amino acids that have been determined by the mRNA base sequence which has been translated by the tRNA.
The ribosome then adds each amino acid
and the polypeptide chain is elongated. Elongation occurs until a stop signal occurs.
Termination: Ribosome reaches stop codon Stops translating
End Result: Ribosome falls off mRNA Protein (polypeptide chain) is released
Start and Stop Codons
Start Codon: Begins translation
AUG (universal start codon) ALSO Codes for methionine (Met)
Sometimes GUG or UUG
Stop Codon: Ends translation
UGA, UAA, UAG
Next amino acid to be added topolypeptide
Growingpolypeptide
mRNA
tRNA
The Whole
Picture
Example
DNA template:
3’ TAC ACA CGG AAT GGG TAA AAA ACT 5’
Complimentary DNA Read from DNA template (start reading at 3’)
mRNA codon Read from DNA template (start reading at 3’)
tRNA anticodon Read from mRNA
Amino Acids (protein) Read from mRNA
Task A:#2 – Central Dogma
DNA makes RNA (mRNA) through transcription
RNA makes proteins through translation
#4 – RNA types
mRNA Messenger RNA End product of
transcription Takes message from
DNA into cytoplasm Used by ribosome to
make protein
tRNA Transfer RNA Delivers amino acid
to ribosome rRNA
Ribosomal RNA Helps form and
maintain ribosomes
#5 – DNA vs. RNA
DNA Sugar – deoxyribose Double stranded Base pair – thymine Stays in nucleus Can replicate itself Longer strands
RNA Sugar – ribose Single Stranded Base pair – uracil Can leave nucleus Cannot replicate
itself Shorter strands
#6 – Transcription/Translation
Transcription Purpose:
To make mRNA from DNA
Location: Nucleus
Translation Purpose:
To make a specific protein from mRNA
Location: Cytoplasm
(ribosome)
#9 – Stop vs. Start Codon
Start Codon mRNA code Tells ribosome to
begin translation Example:
AUG Also codes for
methionine And: UUG, GUG
Stop Codon mRNA code Stops translation of
that specific amino acid chain
Examples: UAA, UAG, UGA
#10 – Transcribe to mRNA
DNA:
GGA TCA GGT CCA GGC AAT
TTA GCA TGC CCC AA
*mRNA*:
CCU AGU CCA GGU CCG UUA
AAU CGU ACG GGG UU
#11 – Translate to Amino Acids
mRNA sequence divided into codons:
GGC AUG GGA CAU UAU UUU GCC CGU UGU GGU GGG GCG UGA
*Protein translation*:
Gly Met(start) Gly His Tyr Phe Ala
Arg Cys Gly Gly Ala (stop)
Task B:#2 – Transcribe to mRNA
DNA:
TAC TAC GGT AGG TAT A
*mRNA*:
AUG AUG CCA UCC AUA U
Task C:#3 – Anticodons
#4 – Change in 3rd Base May Not Result in Error
Why not? Amino acids have more than one codon
Example: proline Codons CCU, CCC, CCA, and CCG CC - always codes for proline Third base/nucleotide does not matter
#6 – Translate to Amino Acids
mRNA:
GGC CCA UAG AUG CCA CCG GGA AAA GAC UGA GCC CCG
*Protein translation*:
Met (start) Pro Pro Gly Lys Asp (stop)