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)