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Protein Synthesis - Transcription and Translation

Review

Central Dogma of Molecular Biology

Protein synthesis requires two steps: transcription and translation.

DNA contains codes

Three bases in DNA code for one amino acid. The DNA code is copied to produce mRNA. The order of amino acids in the polypeptide is determined by the sequence of 3-letter codes in mRNA.

DNA vs RNA

DNA

RNA

Sugar:

deoxyribose

ribose

Bonds with Adenine:

thymine

uracil

# of Strands:

two

one

 

Kinds of RNA

Messenger RNA (mRNA)

Messenger RNA contains genetic information. It is a copy of a portion of the DNA.

It carries genetic information from the gene (DNA) out of the nucleus, into the cytoplasm of the cell where it is translated to produce protein.

Ribosomal RNA (rRNA)

This type of RNA is a structural component of the ribosomes. It does not contain a genetic message.

Transfer RNA (tRNA)

Transfer RNA functions to transport amino acids to the ribosomes during protein synthesis.

Transcription

Transcription is the synthesis of mRNA from a DNA template.

It is like DNA replication in that a DNA strand is used to synthesize a strand of mRNA.

Only one strand of DNA is copied.

A single gene may be transcribed thousands of times.

After transcription, the DNA strands rejoin.

Steps involved in transcription

RNA polymerase recognizes a specific base sequence in the DNA called a promoter and binds to it. The promoter identifies the start of a gene, which strand is to be copied, and the direction that it is to be copied.

RNA polymerase unwinds the DNA.

RNA polymerase assembles bases that are complimentary to the DNA strand being copied. RNA contains uracil instead of thymine.

A termination code in the DNA indicates where transcription will stop.

The mRNA produced is called a mRNA transcript.

Processing the mRNA Transcript

In eucaryotic cells, the newly-formed mRNA transcript (also called heterogenous nuclear RNA or hnRNA) must be further modified before it can be used.

A cap is added to the 5’ end and a poly-A tail (150 to 200 Adenines) is added to the 3’end of the molecule.

The newly-formed mRNA has regions that do not contain a genetic message. These regions are called introns and must be removed. Their function is unknown.

The remaining portions of mRNA are called exons. They are spliced together to form a mature mRNA transcript.

The Nucleus

DNA is located in an organelle called the nucleus.

Transcription and mRNA processing occur in the nucleus.

The nucleus is surrounded by a double membrane. After the mature mRNA transcript is produced, it moves out of the nucleus and into the cytoplasm through pores in the nuclear membrane.

Translation

Translation is the process where ribosomes synthesize proteins using the mature mRNA transcript produced during transcription.

Overview

The diagram below shows a ribosome attach to mRNA, and then move along the mRNA adding amino acids to the growing polypeptide chain.

Translation - Details

A mature mRNA transcript, a ribosome, several tRNA molecules and amino acids are shown. There is a specific tRNA for each of the 20 different amino acids.

Below: A ribosome attaches to the mRNA transcript.

A tRNA molecule transports an amino acid to the ribosome. Notice that the 3-letter anticodon on the tRNA molecule matches the 3-letter code (called a codon) in the mRNA. The tRNA with the anticodon "UAC" bonds with methionine. It always transports methionine. Transfer RNA molecules with different anticodons transport other amino acids.

A second tRNA molecule bonds to the mRNA at the ribosome. Again, the codes must match.

A bond is formed between the two amino acids.

The tRNA bonded to methionine drops off and can be reused later.

The ribosome moves along the mRNA to expose another codon (GAU) for a tRNA molecule.

The only tRNA molecule that can bond to the GAU site is a molecule with a CUA anticodon. Transfer RNA molecules with CUA anticodons are specific for asparagine.

Asparagine is now added to the growing amino acid chain.

Animation of Translation on the Internet

The link below will take you to an animation that summarizes transcription and translation.

http://www.wisc-online.com/objects/index_tj.asp?objID=AP1302

Initiation and Termination Codes

An initiation code signals the start of a genetic message. As the ribosome moves along a mRNA transcript, it will not begin synthesizing protein until it reaches an initiation code.

Termination codes signal the end of the genetic message. Synthesis stops when the ribosome reaches a terminator codon.

Genetic Code

The table below can be used to determine what amino acid corresponds to any 3-letter codon in the mRNA.

First

Base

Second Base

Third

Base

U

C

A

G

 

U

 

 

UUU

phenylalanine

UCU

serine

UAU

tyrosine

UGU

cysteine

U

UUC

phenylalanine

UCC

serine

UAC

tyrosine

UGC

cysteine

C

UUA

leucine

UCA

serine

UAA

stop

UGA

stop

A

UUG

leucine

UCG

serine

UAG

stop

UGG

tryptophan

G

 

C

 

 

CUU

leucine

CCU

proline

CAU

histidine

CGU

arginine

U

CUC

leucine

CCC

proline

CAC

histidine

CGC

arginine

C

CUA

leucine

CCA

proline

CAA

glutamine

CGA

arginine

A

CUG

leucine

CCG

proline

CAG

glutamine

CGG

arginine

G

 

A

 

 

AUU

isoleucine

ACU

threonine

AAU

asparagine

AGU

serine

U

AUC

isoleucine

ACC

threonine

AAC

asparagine

AGC

serine

C

AUA

isoleucine

ACA

threonine

AAA

lysine

AGA

arginine

A

AUG (start)

methionine

ACG

threonine

AAG

lysine

AGG

arginine

G

 

G

 

 

GUU

valine

GCU

alanine

GAU

aspartate

GGU

glycine

U

GUC

valine

GCC

alanine

GAC

aspartate

GGC

glycine

C

GUA

valine

GCA

alanine

GAA

glutamate

GGA

glycine

A

GUG

valine

GCG

alanine

GAG

glutamate

GGG

glycine

G

Mutation

Mutations are changes in the DNA.

Frameshift

A frameshift mutation is usually severe, producing a completely nonfunctional protein.

The priniciple of a frameshift can be explained using the sentence below. If the letters are read three at a time and one is deleted, the second sentence becomes meaningless.

Original DNA:
Frameshift mutation: 

THE BIG RED ANT ATE ONE FAT BUG
THB IGR EDA NTA TEO NEF ATB UG?

Point Mutation

Point mutations involve a single nucleotide, thus a single amino acid.

In the sentence below, eliminating one letter does not change in the remaining three-letter words and therefore may not cause a significant change in the meaning of the sentence.

Original DNA:
Frameshift mutation:

THE BIG RED ANT ATE ONE FAT BUG
THA BIG RED ANT ATE ONE FAT BUG

Silent, Missense, and Nonsense Mutations

 Three kinds of point mutations can occur. A mutation that results in an amino acid substitution is called a missense mutation. 

A mutation that results in a stop codon so that incomplete proteins are produced, it is called a nonsense mutation. 

A mutation that produces a functioning protein is called a silent mutation.

Exercise - Understanding DNA, mRNA, tRNA, and protein.

Exercise 1.

Each column in the table below represents three nucleotides. In each column, fill in the information in the cells that are blank by using the information in the cell that is given. As you complete the table below, you will need to look up mRNA codes in the table in the Genetic Codes section above.

Coding (anti-sense) strand   

GGG

 
Non-coding strand

TAC

    
mRNA    

CCU

tRNA 

UCG

   
Amino Acid  

Leucine

  

 Exercise 2.

List several characteristics that a chemical ought to have if it is to be used as genetic material.